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Plant-induced changes of soil-carbon and nitrogen dynamics in lowland Amazonia, Brazil

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Plant-induced changes of soil-carbon and nitrogen dynamics in lowland Amazonia, Brazil
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Smith, Charles Kenneth, 1961-
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viii, 198 leaves : ill. ; 29 cm.

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Forest litter ( jstor )
Forest plantations ( jstor )
Forest soils ( jstor )
Forests ( jstor )
Plant litter ( jstor )
Plant roots ( jstor )
Plantations ( jstor )
Soils ( jstor )
Species ( jstor )
Trees ( jstor )
Carbon cycle (Biogeochemistry) ( fast )
Dissertations, Academic -- Forest Resources and Conservation -- UF ( lcsh )
Forest Resources and Conservation thesis, Ph. D ( lcsh )
Nitrogen cycle ( fast )
Soil ecology ( fast )
Brazil ( fast )
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theses ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 171-197).
Additional Physical Form:
Also available online.
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Typescript.
General Note:
Vita.
Statement of Responsibility:
by Charles Kenneth Smith.

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PLANT-INDUCED CHANGES OF SOIL CARBON AND NITROGEN
DYNAMICS IN LOWLAND AMAZONIA, BRAZIL












By

CHARLES KENNETH SMITH

















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

1996













ACKNOWLEDGMENTS


I gratefully thank my major advisor, Dr. Henry Gholz, for his helpful guidance,

editing, and efforts in procuring financial support for this study. I would also like to

acknowledge the contributions of the other members of my supervisory committee, Drs. P.K.

Nair, John J. Ewel, Nick Comerford, and Willie Harris, for their constructive criticism and

advice during the development of my proposal and the writing of this dissertation. Drs. Nick

Comerford and Willie Harris also provided laboratory space for some of the soil analyses

presented here. In addition to my committee members, Dr. Ken Clark gave me useful insights

on conducting field research in a remote location, and his assistance with my laboratory

analyses was deeply appreciated.

Financial support for this study was received through a competitive grant from the

USDA National Research Initiative Program (Forest, Crop, and Rangeland Program), the

USDA International Institute of Tropical Forestry (Puerto Rico), the Tropical Conservation

and Development Program (University of Florida), and a fellowship from the National

Security Exchange Program. The School of Forest Resources and Conservation also

provided a research assistantship through Dr. Henry Gholz.

Special thanks are extended to Professor Francisco de Assis Oliveira, Ms. Anadilza

Baima, Ms. Dagma Costa, and Ms. Nailza Pedroso de Souza of the Faculdade de Cidngias

Agrarias do Para for their assistance in Belem and at the CuruA-Una Forest Reserve in Brazil.


ii








The Faculdade de Ciengias Agririas do Para also provided valuable logistical support with

customs and housing in Brazil. In addition, the Centro de Tecnologia Madeireira of the

SuperintendEngia do Desenvolvimento da Amazonia provided housing and logistical support

in Santar6m and at the Curua-Una Forest Reserve. In particular, Dona Fatima Meckdece, Mr.

Sebastiao Castro de Almeida, Mr. Antonio de Souza Pereira, and Mr. Jos6 Nildo Morais da

Rocha were invaluable to the success of this study. Also, thanks to the people of Barreirinha

who invited me into their homes and made living in a remote site much more enjoyable.

In conclusion, I have to give another big thank you to my wife, Deborah McGrath,

for her endless patience, support, good cooking, and assistance with my fieldwork. I would

also like to acknowledge my remaining grandparents, Mrs. Betty Sullivant and Mrs. Jackie

Smith, who have always supported my efforts despite all my travels and travails. Finally, I

thank my parents, Charles and Alice Smith, for all those camping, fishing, and skiing trips that

inspired my choice of a career.





















iii














TABLE OF CONTENTS


12ag

ACKNOW LEDGM ENTS ........................................ . ii

ABSTRACT .............................................. ........ vii

CHAPTERS

1 IN TR O D U CTIO N ...................................... ........ 1

Literature R eview .............. ......... ... ... ...... .......... 3
Conclusions and the Primary Objectives and Hypotheses of this Study ..... 18

2 STUDY SITE DESCRIPTION AND PLANTATION SELECTION ........... 21

Introduction . ........ .. ....... 21
Climate and Forest Composition ................................... 23
Soils ......... ...... .. ......... ... .... .. . ..... 26
Tree Plot Establishm ent ............................. ....... 27
Plantation Selection ................................ ...... . 29
Summary ......... ..................... ...... 32

3 SOIL HOMOGENEITY ................... .............. .......... 33

Introduction ........... ........................................ 33
Materials and Methods...... . . . .................... ........ 35
R esults and D iscussion .................................... 36
C onclusions ..................... ... ... .. ....... ... ..... .. .... 46

4 LITTERFALL AND NITROGEN-USE EFFICIENCY OF
PLANTATIONS AND PRIMARY FOREST IN THE
BRAZILIAN AMAZON .... . ....... 48

Introduction ................ ............................... 48
Materials and Methods ..................... . ........ 50


iv








R esults ..................................................... 53
Discussion ................................. ....... ...... 61
C onclusions ....................................... ......... 7 1

5 THE RELATIONSHIP BETWEEN FINE ROOT AND
MICROBIAL BIOMASS, ABOVEGROUND LITTERFALL,
AND CO2 EVOLUTION UNDER TREE MONOCULTURES
AND TERRA FIRME FOREST IN THE BRAZILIAN AMAZON ............ 73

Introduction ........... ........................................ 73
M aterials and M ethods .................................... 76
R esults ..................................................... 79
D iscussion .................. .... ............. ...... .. . . 87
C onclusions ....................................... ........ . 95

6 FINE LITTER CHEMISTRY, DECAY, AND NITROGEN DYNAMICS
UNDER PLANTATIONS AND PRIMARY FOREST IN LOWLAND
A M A Z O N IA .......................................... ....... . 97

Introduction ....................................... ........ 97
M aterials and M ethods . . . . . .......... ............ ..... 99
Results ............................................. ..... 104
D iscu ssio n . . . . . . . . . . . . . . . . . . . . . . . . . 1 15
C onclusions .......................................... . .. 125

7 SOIL NITROGEN DYNAMICS AND PLANT TO SOIL FEEDBACK
MECHANISMS UNDER MONOCULTURES AND PRIMARY FOREST
IN LOWLAND AMAZONIA .................................. 126

Introduction .............................. . ............... 126
M aterials and M ethods ................................... 128
Results ................... ............................... 133
D iscu ssio n . . . . . . . . . . . . . . . . . . . .. . .. . . 14 2
C onclusions ......................................... . . .. 16 1

8 A FRAMEWORK OF PLANT-INDUCED CHANGES OF SOIL CARBON
AND NITROGEN DYNAMICS IN A LOWLAND TROPICAL
ENVIRONMENT AND CONCLUSIONS ............................. 162

Plant to Soil Feedback Mechanisms A Framework .......... ...... .. 162
C onclusions ....... ................. .... .. .. .. .. ....... . .. 169




V









LIST OF REFERENCES ................................ ....... . 171

BIOGRAPHICAL SKETCH. . . . ............................ 198

















































vi













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


PLANT-INDUCED CHANGES OF SOIL CARBON AND NITROGEN
DYNAMICS IN LOWLAND AMAZONIA, BRAZIL

By

CHARLES KENNETH SMITH

December 1996


Chairperson: Dr. Henry L. Gholz
Major Department: School of Forest Resources and Conservation

The principal objective that guided this study was to determine if plantations

consisting of tree species with contrasting phenologies, resource requirements, and

biochemical characteristics had significantly changed carbon (C) and nitrogen (N) dynamics

in a lowland tropical ecosystem when planted on an initially uniform soil. The replicated,

monospecific plots consisted of Pinus caribaea var. hondurensis, Euxylophora paraensis

Huber, Carapa guianensis Aubl., and a Leguminosae combination (Dalbergia nigra Allemao

ex Benth., Parkia multijuga Bent., Dinizia excelsa Ducke). The experimental plots were

established in the CuruA-Una Forest Reserve in the east-central Amazon Basin, and replicated

plots in adjacent, undisturbed native forest were used as a control. Soils under the

experimental units were determined to be homogeneous based on mineralogical, textural, and



vii








chemical analyses of surface and subsurface soils. During the year examined (September

1994 to October 1995), the plantations and the native forest control produced widely varying

quantities ofaboveground litterfall (8 to 10.3 t ha"' yr'1), had wide differences in aboveground

N inputs from litterfall (43 to 115 kg ha-1 yr'), and also had significant differences in forest

floor mass (8 to 11 t ha'l) and turnover (0.78 to 1.2 years). In addition, annual litter N inputs

were related to root mat and mineral soil fine root C allocation (R2 = .84). Leaf litter decay

differed significantly among the treatments, but no single chemical constituent was found to

be a good predictor of leaf litter decay despite large differences in initial leaf litter chemistries.

There were large differences among the treatments for annual net N mineralization rates (195

to 328 kg ha"' yr'l), and annual net N mineralization rates were related to N allocation to fine

roots (R2 = .96) and total fine litterfall N inputs (R2 = .81). These results suggest that at

Curui-Una, plant-to-soil feedbacks existed because the treatments with the highest N

concentrations in foliage, litterfall, and N concentrations in fine roots supported the highest

rates of soil N transformations.



















viii














CHAPTER 1
INTRODUCTION


Vegetation is a primary factor in soil genesis (Jenny 1980) and tree species have been

shown to alter soil chemical and physical properties in several temperate forest ecosystems

(e.g., Mergen and Malcolm 1955, Challinor 1968, Alban 1969). A trend frequently reported

in earlier work was that conifers acidified surface soils and reduced soil fertility compared to

hardwood species (Handley 1954). Stone (1975) dismissed the results of many studies which

found deleterious effects of trees on soil properties because they lacked plot replication and

had confounding effects of land-use history (e.g., intensive agriculture).

In forest ecosystems, the largest pools of ecosystem carbon (C) and nitrogen (N) are found

in the forest floor and mineral soil (Cole and Rapp 1981, Edwards and Grubb 1982, Jordan

1985), and changes in above- and belowground quantity and quality of plant litter should alter

rates of litter and soil C and N transformations. Work conducted in replicated experimental

units has demonstrated that groups of plants with different ecological characteristics, such as

leaf and root litter quantity and quality, can influence soil C and N dynamics (Binkley 1994).

Feedback loops between soil and vegetation in a system may develop if plants control soil N

supply rates by varying the quantity and quality of above- and belowground litter inputs

(Wedin and Tilman 1990).





1








2

Plant-induced soil changes are important to understand because of their effects on

ecosystem nutrient dynamics and competition between plant species, and the implications for

vegetation management. For example, if a successional plant community or plant species

occupying a site affects soil N availability, then this plant-driven influence on N supply could

control competition and succession (Ewel 1986, Vitousek and Walker 1989). High within-

stand nutrient-use efficiency is characterized by high dry mass:nutrient ratios in plant litter

(Vitousek 1982) and often results in efficient within-stand cycling of nutrients that limit plant

production (Vitousek et al. 1982). Plant communities growing on infertile sites generally

have higher nutrient-use efficiencies than those found on fertile sites (Chapin 1980, Vitousek

1982), and many lowland tropical forests circulate high quantities of N and have low within-

stand nitrogen-use efficiency or NUE (Vitousek 1984, Vitousek and Sanford 1986). If soil

nutrient mineralization rates are controlled by litter decomposition rates (McClaugherty et al.

1985), litter quality (Gower and Son 1992), or any other factor directly linked to the specific

character of the vegetation occupying a site, then plant-soil-plant feedback loops may stabilize

forest species composition over time (Pastor et al. 1984).

As Binkley (1994) summarized in a review of plant induced soil changes, questions related

to soil degradation or improvement are not useful unless they address the specific mechanisms

behind observed changes in soil processes or properties. Thus, the objectives of this

introduction are to 1) review previous work about plant-induced soil changes, 2) briefly

discuss the mechanisms behind these changes, and 3) outline the objectives and principal

hypotheses of this study.








3

Literature Review

Plant Effects on Soil Moisture. Temperature, and Structure

Plants can alter soil moisture and temperature through differences in canopy leaf area that

attenuates incoming shortwave and outgoing longwave radiation (Binkley 1994). For

example, soil temperatures under deciduous trees in seasonally dry tropical forests might be

expected to rise during the dry season, which generally is the time of heaviest litterfall

(Murphy and Lugo 1995), although this temperature rise could also result from an increased

number of warm and cloudless days. On the other hand, the buildup of litter layers under

individual species insulates the mineral soil from temperature and moisture fluctuations (Lal

et al. 1980), and these off-setting factors may explain the ambiguity of past results. For

example, in a study conducted on afforested sand dunes in California, Amundson and

Tremback (1989) found no differences in mineral soil temperatures among stands of

Eucalyptus globulus Labill., Pinus radiata D. Don, and Quercus agrifola N&e. despite large

differences in 0 horizon thicknesses under each stand. In contrast, Qashu and Zinke (1964)

reported lower mean annual soil temperatures at a 60 cm depth under 13-year-old

monospecific stands of scrub oak (Quercus dumosa Nutt.) compared to Coulter pine (Pinus

coulterii B. Don) in southern California.

Tree species affect soil moisture primarily through canopy rainfall interception and

transpiration which are influenced by the specific conductance of leaves, leaf area, and

phenology (Kaufmiann 1985). In general, evergreen tree species that maintain high leaf areas

throughout the year transpire more water annually per unit land area than species that remain

leafless for extended periods. In modeling canopy water interception and transpiration in








4

monospecific stands with identical basal areas, Kaufmann (1985) determined that the annual

transpiration of aspen forests, (Populus tremuloides Michx.), which is deciduous, was 50%

of that from forests of lodgepole pine (Pinus contorta Dougl. Ex Loud.), which is evergreen,

thus resulting in higher soil moisture under the aspen. Gower and Son (1992) reported

significantly higher mineral soil moisture content under red oak (Quercus rubra L.) compared

to four conifer species in a replicated common garden experiment in Wisconsin. In addition,

Nihlgard (1971) found soil water content was higher under beech (Fagus silvatica L.)

compared to adjacent stands of Norway spruce (Picea abies (L.) Karst.).

Plant-induced changes in soil structure may result from differences in plant rooting

densities and depths combined with the effects of plant litter quantity and quality on the size

and activity of soil macrofaunal communities. Soil fauna influence soil bulk densities and

particle size through their tunneling and nest building activities, and they frequently use root

channels as conduits through the soil profile (Lavelle et al. 1992). In Florida, crayfish

tunneling and their burrows under pine plantations were found to influence surface lateral

water flow and soil mixing (Stone 1993). Plant litter chemical composition, especially

nitrogen and lignin contents, is also known to influence soil faunal activity (Tian et al. 1993).

In one of the few studies to examine how tree species influence soil structure indirectly

through effects on soil faunal communities, Graham and Wood (1991) indicated that soils

under 41-year-old Coulter pine lacked earthworms while the A horizons under similarly aged

scrub oak and ceanothus (Ceanothus crassifolia Torr.) were composed of earthworm casts

and contained higher clay contents than soils under pine. These authors also stated that








5

earthworms were moving clay-sized particles to the surface soils, while earthworm

depauperate soils under pine were experiencing illuvial accumulation of clay in the B horizon.

Plant Effects on Soil Faunal and Microbial Communities

Early research comparing soil communities in "mor" soils (consisting of an acidic litter

layer with little mixing of forest floor and mineral soil) and "mull" soils (less acid forest floor

well mixed into A horizon) led Rommell (1935) to conclude that mor soils were dominated

by acid-producing fungi and mull soils by "neutrophilous" bacteria. These microbial

communities and their by-products were thought to regulate forest floor characteristics (e.g.,

pH), thus mor and mull forming tree species could directly influence microbial populations.

Broadfoot and Pierre (1939) were among the first to relate variations in leaf chemistry to

decomposition rates and differences in microbial populations colonizing the forest floor under

different tree species.

Considering more current work, Theodorou (1984) found that the surface soils under

Pinus radiala had higher fungal but significantly lower total microbial biomass-N than

adjacent eucalypt woodlands. Keinzler (1986) sampled soils under the same stands studied

by Perala and Alban (1982) in Minnesota and determined that surface soils under aspen had

more invertebrates, ten times more bacteria, and twice as much fungal growth than surface

soils under pine and spruce.

Under similar climatic regimes, plant litter quantity and quality are also thought to regulate

soil macrofaunal activity (Cuendet 1984, Tian et al. 1993). Gast (1937) reviewed previous

research from the Harvard Forest and the observed preferences of earthworms for aspen,

white ash (Fraxinus americana L.), and basswood (Tilia glabra Vent.) leaves over red oak








6

leaves. In particular, Gast (1937) commented on the importance of the mixing of organic

material and mineral soil by earthworms, particularly with leaves from tree species with higher

mineral nutrient contents. This division of organic material (comminution) and mixing with

mineral soil and stimulates soil microbial activity by increasing the surface area of leaf

particles (Swift et al. 1979, Luxton 1982). Thus, tree species with more palatable leaf litter

can increase soil macrofaunal populations, which influence microbial activity and leaf

decomposition. In support of this theory, Zou (1993) found that Paraserianthesfalcataria

(L.) Nielson plantations in Hawaii (where earthworms are not indigenous) had significantly

higher earthworm densities than adjacent Eucalyptus saligna Sm. plantations, and he

attributed this finding to the higher litter quality of Paraserianthes. Also in Hawaii, Aplet

(1990) determined that earthworm biomass was increased by the presence of high N litter

under the colonizing, N2-fixing shrub Myricafaya.

Plant Effects on Soil pH

Much of the early research on plant-induced soil changes focused on the forest floor and

mineral soil pH, with the prevalent view that conifers both acidified and degraded soils

compared to hardwoods (Ovington 1953). In a review of early European work, Miles (1985)

observed that the establishment of conifers and ericaceous shrubs led to the formation of mor

soils while hardwoods developed mull soil. Handley (1954) summarized the thinking on

plant-induced soil changes during the first half of the century by stating that when climate,

geology, and topography were held constant, a change in vegetation will bring a change in

soil properties, with conifers generally causing a decline in soil pH.








7

In contrast to Ovington (1953) and Handley's (1954) earlier work, Rennie (1955)

expressed the view that there was no clear evidence to indicate that conifers caused rapid

changes in soil pH over short time periods. Later, Stone (1975) pointed out that the spatial

heterogeneity of soil properties and inadequate sampling in non-replicated plots led some

researchers to wrongfully attribute short-term changes in soil chemical properties to tree

influences. In support of Stone's argument, Tamm and Hallbacken (1986) sampled for soil

pH under Norway spruce (Picea abies (L.) Karst.), beech (Fagus silvatica), and oak

(Quercus sessiliflora Salisb.) stands in Sweden in 1927 and 1982, and they found no

differences among species at either time period, although all the stands experienced a decline

in pH over time. In addition, Erickson and Rosen (1994) found no significant differences in

surface soil pH among replicated blocks of four conifers after 36 years.

More recent work has documented changes in soil pH attributable to plant influences, but

the generalization that conifers always acidify surface soils has not withstood the test of time.

For example, Alban (1982) determined that four tree species altered the top 30 cm of a soil

profile in Minnesota, and a hardwood species (aspen), accumulated more exchangeable acidity

compared to two pine species. Binkley and Richter (1987) attributed Alban's finding to

aspen's higher base cation uptake and concurrent release of H' from roots in the upper mineral

soil. In addition, France et al. (1989) found no significant differences in pH or exchangeable

acidity among two conifers and two hardwoods in the top 15 cm of soils beneath replicated,

27-year-old plantations in Ontario, Canada. In North Carolina, Richter et al. (1994) reported

an increase in KCl-exchangeable acidity over 28 years in the surface soils of a loblolly pine

(Pinus taeda L.) forest. Finally, Binkley and Valentine (1991) found surface soil pH under








8

replicated, 50-year-old plots of green ash (Fraxinuspennsylvanica Marsh.) were higher than

eastern white pine (Pinus strobus L.) and Norway spruce.

In the Pacific Northwest of the United States, there are well established effects of N-fixing

tree species such as red alder (Alnus rubra Bong.) on soil pH. These derive from NH'4

mineralized from N-enriched litter and the subsequent leaching from surface soils of NO'3 with

a base cation, which leaves behind H' to displace cations on soil exchange sites (Van

Miegroet and Cole 1988). Nitrification processes in soils under red alder in Washington

represented an internal H' source ten times greater than input by precipitation (Stuanes et al.

1992). In addition, decreases in soil pH under red alder are attributed to the greater acid

strength of soil organic matter found under alder stands (Binkley and Sollins 1990).

In the tropics, Sanchez et al. (1985) indicated that plantations of Gmelina arborea Roxb.

and Pinus caribaea had surface pH that varied as much as 1 unit in Jari, Brazil. In Costa

Rica, surface soil pH declined under three types of successional plant communities and a

monoculture 5 years after clearing and burning, and soil pH values under all treatments were

observed returning close to pre-burn levels (Ewel et al. 1991). In the dry tropics, Mailly and

Margolis (1992) found a 0.8 unit decline in soil pH (7.3 to 6.5) under 34-year-old Causuarina

equisetifolia Forst.& Forst. plantations in Senegal.

Individual trees within a stand have also been shown to affect soil pH in the temperate

zone. Riha et al. (1986) indicated that soil pH variability between individual trees in

monospecific stands of red pine (Pinus resinosa Ait.), Norway spruce, and sugar maple (Acer

saccharum Marsh.) was greater than that encountered between pure stands of the three

species. Boettcher and Kalisz (1990) found lower levels of surface soil pH under crowns of








9

Liriodendron tulipifera L.compared to soils under Tsuga canadensis (L). Carr. in Kentucky.

In contrast, Zinke and Crocker (1962) sampled soils under a giant sequoia (Sequoiadendron

giganteum (Lindl.) Buchholz) estimated to be 1500- to 3000-years-old and found pH did not

vary with distance from the stem.

In drier habitats that do not support continuous tree cover, individual perennial plants such

as bluebunch wheatgrass (Pseudoroegneria spicata (pursh) A Love) and sagebrush

(Artemisia tridentata (Rydb.) Beetle) greatly influence soil pH, which was shown to vary by

as much as 1.3 units in 0.5 meters (Jackson and Caldwell 1993). In addition, Charley and

West (1975) reported a species influence in surface soil pH under Artemisia tridentata and

Atriplex confertifolia (Torr.& Frem.) in the Great Basin, and isolated trees in tropical

savannas in West Africa have been shown to increase soil pH under their canopies (Mordelet

et al. 1993).

The possible mechanisms of plant-induced changes in soil pH have been summarized by

Binkley et al. (1989) and Binkley and Richter (1987). These include the following:

1. Tree canopy effects on forest floor depositional rates of acidic atmospheric chemicals.

2. Humified organic matter from plant species increase the quantity of weak acids stored in

the soil, and these weak acids increase H' concentrations in the soil solution.

3. Increasing the acid saturation of soil exchange complex or decreasing base saturation

allows more H' to move into solution.

4. Contributing soil humus that is strongly acidic that will donate more H' to the soil

solution.








10

5. Biological nitrogen fixation which leads to increased N in litterfall, increased

nitrification, and increased nitrate leaching with a base cation.

6. Plant uptake of cations with a concurrent release of H' to the soil solution.

Plant Effects on Soil Cations

Tree species can significantly alter levels of extractable base cations in surface soils, for

example, Binkley and Valentine (1991) observed higher concentrations of K1, Ca2", Mg2+ at

a 0-15 cm depth under 50-year-old, replicated plots of green ash (Fraxinus pennsylvanica

Marsh) and white pine (Pinus strobus L.) compared to Norway spruce. Son and Gower

(1992) also reported a wide range of extractable cations among tree species in surface soils

of 28-year-old plots. In contrast, France et al. (1989) found no significant differences for

extractable cations among four tree species (two hardwoods and two conifers) in 27-year-old

replicated plots in Canada.

At Jari, Brazil, Russell (1983) found 8.5-year-old plantations of Gmelina arborea

increased Ca' concentrations in surface soils to a level similar to adjacent native forest. In

addition, Pinus caribaea Mor. lowered base concentrations 9.5 years after establishment

relative to Gmelina and native forest. Belsky et al. (1989) sampled soils under isolated trees

in tropical savanna and detected higher total K' and Ca2" under trees compared to adjacent

grasslands. These authors stated that higher concentrations under tree canopies resulted

from nutrient uptake from surrounding soils and deposition of nutrients in litter and in dung

by animals that rest and feed near the tree.










Plant Effects on Soil Phosphorus

Over time, tree species have been clearly shown to affect various phosphorus (P) pools.

Polglase et al. (1992) demonstrated an age affect with labile inorganic P under Eucalyptus

regnans F. Muell. which declined from 34 ug g-' soil at stand establishment to 2.3 ug g'i soil

at 16 years. Gholz et al. (1985) observed a decrease in double-acid-extractable P over time

in replicated stands ofPinus elliottii Engelm.(2 to 35 years old), with the most rapid change

occurring the first 18 years (to a 1 meter depth). In replicated plantations in Hawaii, Zou et

al. (1995) found labile organic P concentrations and soil acid phosphatase activity were higher

in Paraserianthesfalcataria (L.) Nielson plantations compared to Eucalyptus saligna Sm.

In paired plantations and secondary forests in Puerto Rico, Lugo (1992) reported higher total

P concentrations (1 meter depth) under Pinus caribaea compared to secondary forest. In

other paired stands, secondary forest had higher total P than two mahogany (Swietenia

macrophylla Jacq.) stands.

In Brazil, Russell (1983) observed higher total P contents (kg ha"') under Gmelina

plantations (8.5 years old) compared to Pinus caribaea (9.5 years old) or native forest. Isichei

and Muoghalu (1992) determined that available phosphorus (Bray extraction) was higher

under tree canopies than grassland in northern Nigerian savannas. Tergas and Popenoe (1971)

found that pure stands of Heliconia sp. and Gynerium sp. accumulated four times more

extractable P in surface soils as mixed fallows of the same age in P deficient Inceptisols of

Guatemala.

Mechanisms behind plant-induced changes in soil phosphorus are not only linked to subsoil

mining of plant available P and the return of this P through litter (Comerford et al. 1984), but








12

also can be attributed to plant associations with fungi and bacteria (Binkley and Richter

1987). Plant roots, bacteria, and fungi can produce many low-molecular weight organic acids

(Fox and Comerford 1990) including phosphatases which aid in the mineralization of organic

P (Fox and Comerford 1992), and plant species have been shown to influence soil acid

phosphatase activity (Zou et al. 1995).

Plant Effects on Carbon and Nitrogen Dynamics

Plant-induced changes in soil C and N dynamics have recently received much attention,

with studies conducted in homogeneous forest patches or natural stands, semi-arid grasslands,

and in common garden experiments. It is convenient to study C and N dynamics jointly

because they are covalently bonded in organic matter (McGill and Cole 1981), and thus they

are linked together in soil organic matter dynamics. There are also no mineral sources of C

and N from forest soils.

Much of the interest in plant-induced changes in C and N dynamics arose as a result of the

concept of feedback loops between plant and soil because plants occupying infertile sites were

observed to return less N to soil in litterfall while species on fertile sites shed litter with higher

N concentrations, thus maintaining higher soil N concentrations (Hobbie 1992). In addition,

work conducted in semi-arid grasslands and savannas identified plant-induced "islands" of

high nutrient concentrations under isolated trees and shrubs (Charley and West 1975,

Tiedemann and Klemmedson 1973). Nevertheless, Gower and Son (1992) and Binkley

(1994) signaled that many previous studies confused site effects with species effects, and they

recommended that the best way to identify plant influences on soil properties was to establish

a common garden consisting of plants with different characteristics on a homogeneous soil.








13

The Return of N in Litterfall and NUE

If positive N feedback loops exist in nature, plant species with high N concentrations in

litter (low nitrogen-use efficiency or NUE) would influence site fertility by increasing the soil

N supply (Vitousek 1982). Theoretically, this feedback loop reinforces the dominance of

species that are well adapted to productive sites (Tilman 1988). Traits typical of such species

include higher tissue N concentrations and high N allocation to aboveground plant material

(Vitousek 1982). Another type of feedback loop would include vegetation that maintained

low N concentrations in litter (high NUE) combined with high root biomass to outcompete

potential site successors for limited N supply (Wedin and Tilman 1990).

Experimental evidence demonstrating the correlation between site quality and litter NUE

include Birk and Vitousek's (1986) study of loblolly stands growing on sites with varying N

availabilities. These authors determined that within-stand NUE decreased in sites with high

N availability, and that changes in NUE were attributed to a decrease in nutrient absorbing

efficiency in low N sites. In contrast, Gower and Son (1992) found that Norway spruce, a

species frequently claimed to be a "site degrader" in Europe, had a lower NUE than red oak

and white pine, two species that are typically found on moderately fertile and fertile sites

(Harlow et al. 1979).

The quantity and quality of root litter and mycorrhizal turnover directly affects soil N

supply rates, yet belowground litter dynamics are poorly understood and have been estimated

in few ecosystems (Vogt et al. 1986). Most studies of fine root production and turnover have

been conducted in litter layers and surface soils (Attiwill and Adams 1993), and few have

related root dynamics to nutrient availability (Nadelhoffer et al. 1985). In the tropics, fine








14

roots in a lowland Amazonian forest at San Carlos, Venezuela had 25% turnover per month

in surface soils, and fine root N inputs in this terra firme forest were estimated to be

approximately six times higher than aboveground fine litter N inputs (Vitousek and Sanford

1986).

Forest Floor Mass and Turnover

The early studies of plant-induced soil changes focused on the pH and mass of the forest

floor, the two factors that were believed to control the formation of mull and mor soil types

(Handley 1954). In contrast to the early dogma that forest floor mass under conifers always

exceeded that under hardwoods, Perala and Alban (1982) found no differences in forest floor

mass between 40-year-old hardwood and conifer stands in Minnesota. Gower and Son

(1992) did find that total forest floor biomass under red oak was much lower (8.7 t ha'1) than

forest floor mass under four conifers (24 43 t ha-') in 28-year-old plots in Wisconsin.

In the tropics, some pine plantations have been shown to develop high forest floor mass

compared to other species planted in monocultures or adjacent natural forest. For example,

Lugo et al. (1990) estimated forest floor litter mass to be 27 t ha"1 under a 26-year-old Pinus

caribaea plantation in Puerto Rico, and total forest floor N contents (kg ha"') under Pinus

caribaea were higher than total forest floor N contents under nine other monocultures. In

addition, Cuevas et al. (1991) reported significantly higher standing stocks of total litter under

11-year-old Pinus caribaea (10.5 t ha-') compared to adjacent secondary forest (5.0 t ha-').

The turnover rate of forest floor mass has been estimated for forests around the world by

first assuming steady state conditions and then by dividing annual aboveground fine litterfall

by forest floor mass. This turnover ratio (KL) for lowland tropical forests is generally greater








15

than one because the decomposition of forest floor litter is faster than inputs from

aboveground litterfall. Lowland tropical forests have KL values ranging from 1.0 to 3.3

(Anderson and Swift 1983), and one lowland Amazon forest was reported on the lower end

of this range at 1.1 (Klinge 1973).

Plant Effects on C Release and Microbial Biomass

Annual plant species can affect microbial composition and population size (Chanway et

al. 1991), and individual plants in agricultural fields have been shown to alter mycorrhizal

populations (McGonigle and Fitter 1990). In an experiment with four perennial grass species,

Bever (1994) reported that negative feedback effects existed between grass species and their

native soil communities. The growth and survival of grasses in their own soil inocula were

lower than for species grown in "exotic" inocula. Thus, the potential long-term influences

of a plant species on microbial biomass and their activity are great.

Plant-induced changes in leaf area will impact temperature and moisture regimes under

forest canopies which directly affect soil microbial populations and their activities (Singh and

Gupta 1977, Gupta and Singh 1981). Also, plant-induced changes in release of soil C and in

microbial biomass may result from differences in fine-root biomass and turnover, and

variations in above- and belowground litter quantity and quality. Soil respiration rates are

known to vary widely across natural forests and plantation types (Raich and Nadelhoffer

1989, Raich and Schlesinger 1992), and soil C release attributed to plant root respiration

ranges from 35 to 90 % of total soil C release (Edwards and Harris 1977, Ewel et al. 1987,

Bowden et al. 1993, Nakane et al. 1995, Thierron and Laudelout 1996).








16

Changes in plant life forms or plant species that affect soil C availability may also influence

soil N cycling. Soil carbon availability has been found to be an important control on N cycling

rates in soil, and C release measurements may be an index of soil N cycling (Hart et al. 1994).

In mineral soil, no relation between microbial respiration and net N mineralization has been

reported (Johnson and Edwards 1979, Johnson et al. 1980), but gross N mineralization and

immobilization are known to be correlated with microbial respiration in incubated samples

(Schimel 1986, Hart et al. 1994).

Plant Effects on Litter Quantity, Quality. and Decomposition

Plant species have evolved with a wide range of leaf chemistries, and rates of

decomposition and mineralization of nutrients from plant litter have been shown to be affected

by initial N, lignin, holocellulose, and polyphenol concentrations. In British Columbia, soil

organic matter (SOM) formation is affected by different plant associations (deMontigny et

al. 1993), and the availability of forest floor N is influenced by tree species (Prescott et al.

1993). Simulations of interactions between plant tissue chemistry, herbivores, and litter

decomposition have also shown that plant tissue chemistry influences ecosystem C and N

dynamics (Pastor and Naiman 1992).

Leaf litter lignin/N (L/N) ratios of several temperate zone species have been shown to be

good predictors of decay rates (Melillo et al. 1982), and initial leaf chemistries can be used

to predict the length of time required to convert fresh litter into soil organic matter (Aber et

al. 1990). In contrast, Edmonds (1980) found that litter C/N ratios were better predictors

of leaf decay rates than initial lignin concentrations. In microcosm tests, Taylor et al. (1989)

found C/N ratios were better predictors of mass loss than L/N ratios for leaf material








17

containing low initial lignin concentrations. Despite wide differences in initial carbon fractions

and C/N ratios, long-term decomposition studies have found that the chemical constituents

of leaf litter converge to similar values over time (Berg et al. 1984, Aber et al. 1990).

Other C fractions in leaf litter may regulate decomposition, and polyphenolic contents have

been found to control N release rates from tropical plant material with high N concentrations

(Palm and Sanchez 1990, 1991). Constantinides and Fownes (1994) incubated litter from 12

species with a wide range of initial leaf chemistries and determined that N loss was best

predicted by initial N and polyphenol concentrations. In Pinus muricata (D. Don) stands in

California, polyphenol concentrations in litter inhibited N mineralization from dissolved

organic nitrogen encased in tannin complexes (Northup et al. 1995).

Plant Effects on N Mineralization Rates

In replicated common gardens, four of Jenny's (1980) five soil forming factors (i.e.,

climate, vegetation, relief, parent material, time) can be held constant with only vegetation

varying across experimental units. These experiments give a more precise estimate of plant-

induced affects on soil N mineralization, although pre-establishment soil homogeneity is rarely

quantified. In a 28-year-old common garden experiment in Wisconsin, Gower and Son

(1992) found significant differences between five species for mineral soil nitrate and

ammonium concentrations, annual net N mineralization, and annual nitrification. Binkley and

Valentine (1991) reported significantly higher net N mineralization rates under eastern white

pine compared to green ash in another common garden trial in Connecticut.

In a number of studies, leaf litter chemistries have been correlated with net N

mineralization rates. In mixed species stands in Wisconsin, McClaugherty et al. (1985)







18

reported that N mineralization rates were correlated with decomposition rates of leaf litter

originating from the stands' dominant tree species. Prescott and Preston (1994) stated there

was a connection between litter and nutrient supply in cedar-hemlock forests, and they

attributed low N in the forest floor under western red cedar (Thuja plicata Donn) to its

recalcitrant litter. In addition, Gower and Son (1992) indicated that initial leaf litter L/N

ratios for five species were related to annual net N mineralization in surface soils in

Wisconsin, and Stump and Binkley (1993) determined that leaf litter L/N ratios were better

predictors of net N mineralization than initial leaf N concentrations alone.

Conclusions and the Primary Objectives and Hypotheses of this Study

Researchers have identified plant-induced soil changes in homogeneous forest patches,

natural stands, savannas, grasslands, and in replicated common gardens. In a review of the

influence of tree species on soil properties, Binkley (1994) stated that initial leaf chemistry,

SOM dynamics in mineral soil, and soil biota activity were the key variables when considering

how species influence soil nutrient availability. Most previous studies were conducted in

temperate environments, and little is know about plant-induced changes in Oxisols, although

this soil order is one of the most widespread in the world, covering approximately 8.9 % of

Earth's land area, mostly in the tropics (Brady 1990).

Previous work has demonstrated that physical and chemical soil properties change after

short-term exposure to monocultures of tree species with varying leaf litter quantities and

qualities. Soil moisture, temperature, physical structure, microbial and faunal communities,

nutrient status, and mineralization rates are influenced by plant species composition, and

plant-soil-plant feedback loops may influence forest ecosystem dynamics. Plant-induced soil








19

changes are important to understand because of their effects on nutrient dynamics and plant

succession, and their implications for forest management.

In this context, the overall objective guiding this study is to determine if four tree

monocultures consisting of species with contrasting phenologies, resource requirements, and

biochemical characteristics have significantly changed carbon and nitrogen dynamics in a

lowland tropical ecosystem when planted on an initially uniform soil. The study used

replicated plots of four monocultures and undisturbed native forest as a control, and the four

monocultures consist of Pinus caribaea var. hondurensis, Euxylophora paraensis Huber,

Carapa guianensis Aubl., and a Leguminosae combination (Dalbergia nigra Allemao ex

Benth., Parkia multijuga Bent. Dinizia excelsa Ducke). All the plots were established in

the Curui-Una Forest Reserve in the east-central Amazon Basin.

The specific objectives of this study are the following:

1. To determine the effects of above-ground fine litter quality and quantity on litter storage

and turnover rates, decomposition, soil respiration, soil carbon storage, N mineralization, and

nitrification under the monocultures and undisturbed native forest, and

2. To determine the factors controlling the processes of soil organic matter formation,

composition, and degradation under each monoculture and undisturbed native forest.

The hypotheses that relate to these objectives are the following:

1. Surface soils under monocultures with low leaf litter quality (high L/N ratios) will have

higher carbon and lower N contents than under monocultures with higher leaf litter quality.

2. Leaf litter with lower L/N ratios will decompose faster resulting in higher net N

mineralization rates in the forest floor and surface soil.








20

3. Higher nitrogen-use efficiency will be associated with lower net N mineralization rates in

surface soils.

4. CO2 evolution, inorganic N pools, and net N mineralization will be highly seasonal in the

natural forest control and the four monocultures.

5. Monospecific stands with higher NUE will have less soil total N and inorganic pools

relative to the control and other treatments with higher NUE, thus creating a feedback loop

from plant to soil to plant.

This dissertation is divided into eight chapters, including this introduction. The second

chapter describes the study area, its climate, and the history of the experimental plots. The

third chapter examines the assumption of initial soil homogeneity under the monocultures and

native forest. The fourth chapter discusses differences in litterfall N contents,

litterfall/standing crop quotients, and NUE among the plantations. The fifth chapter examines

differences among the plantations and forest for soil C release, fine root biomass, and

microbial biomass, along with their interrelationships. The sixth chapter explores differences

in leaf litter chemistry among the plantations and the forest, and the controls on first-year

mass loss and long-term litter decomposition. The seventh chapter describes patterns of N

mineralization among the plantations and the forest, including an attempt to explain the

mechanisms behind observed differences in N transformations among the treatments. The

final chapter is a synthesis of how tree species with different ecological characteristics can

influence soil properties in lowland Amazonia, and I will present a framework of plant-soil

interactions at Curu&-Una.













CHAPTER 2
STUDY SITE DESCRIPTION AND PLANTATION SELECTION


Introduction

The world's largest contiguous tropical moist forest is located in the Amazon Basin of

South America, and it covers 5.5 million km2 that extend through nine countries (Browder

1988). Approximately 66 percent of lowland Amazonia (below 1000 m.a.s.l.) is located in

Brazil (Eden 1990), and 49 percent of Brazil's portion of Amazonia is covered by dense

tropical forest, primarily in the northern states of Para, Amazonas, Amapa, Roraima, and

Maranhao (Browder 1988).

In the 1950's, foresters working for the Food and Agricultural Organization (FAO) and

the Brazilian government postulated that relatively few tree species would be utilized to

supply Brazil's future needs for timber. They therefore established plantation trials of

potentially commercial native and exotic tree species from 1959 to 1975 at the Curua-Una

Experimental Station (Dubois 1971, Palmer 1977), on a site selected as representative of the

terra firme (upland, non-flooded) forests that cover over 80% of the Amazon Basin (IBGE

1990, Uhl et al. 1981).

The field studies presented here were carried out at the Curua-Una Experimental Station

in eastern Para, at the confluence of the Amazon and Curui-Una Rivers (2S, 54W), 110

km east of Santar6m (Figure 2.1). Curua-Una is a 72,000 ha forest reserve, and it is the



21








22
























55 500
W W

Santarem






Curua-Una
\ Reserve .



SW w












Figure 2.1. Location of the Curud-Una Reserve.








23

oldest forest research station in the Amazon Basin. The life zone in this region is classified

as tropical moist forest (Holdridge 1978) and the native forest has both deciduous and

evergreen broadleaf tree species in the overstory. Most terra firme forest at the reserve is on

theplanalto, which is a plateau that rises to 180 m.a.s.l. approximately 6 km from the Curui-

Una River.

Climate and Forest Composition

Average yearly rainfall at the station is 1900 mm, most of which falls from December to

July, and the average daily temperature is 26C (SUDAM 1979a, b). During the course of

this study, which took place from September 1994 to October 1995, most of the monthly

rainfall totals fell within 3 standard deviations of ten year monthly averages (1983 to 1993)

from Belterra, the nearest agriculture station (150 km) that records monthly rainfall (Figure

2.2). Rainfall during April and May in 1995 was higher than the 10-year average for these

months at Belterra, but a review of long-term weather data from this region indicated that

unusually high rainfall totals have been recorded during these months in past years. Average

daily temperatures during the 1994-1995 field season remained between 26 and 280C, and

the warmest temperatures were recorded during the dry season months from August to

November (Figure 2.3).

At CuruA-Una, inventories of trees over 25 cm diameter breast height (DBH) in 1 ha plots

in terra firme forest on the planalto estimated timber volumes of 157 to 170 m3 hda

(Heindsjik 1958, Pitt 1961, Glerum and Smit 1965). Data from these inventories indicated

that there were 33 to 37 trees over 25 cm diameter per ha and 103 to 140 tree species per ha.







24





500-

450 1983- 1993
---- 1994-1995 --a

400 -

350 /

300-

250-
.200-
a 200 -/

150 -

100 -

50 -

0-
S O N D J F M A M J J A S O
Month






Figure 2.2. Average monthly rainfall from 1994 to 1995 at the Curua-Una Reserve compared
to 10-year averages (+ 3 sd) from 1983 to 1993 from the Belterra Agricultural Station.







25






38-

36- Maximum
S-A-- Minimum r
--v-- Average /A
34-

32- "-

30 -

I. 28-

26 -V

24 -

22 A AA

20 I I I I I 1
N D J F M A M J J A S O
Month





Figure 2.3. Mean daily maximum and minimum temperatures from 1994 to 1995 at the
Curui-Una Forest Reserve.








26

The families most frequently represented in these inventories on the Curua-Unaplanalto were

the Leguminosae, Sapotaceae, Lecythidaceae, and Lauraceae (SUDAM 1971).

Soils

Sombroek (1966) described the planalto soils at CuruA-Una as a 15 to 20 m thick,

uniform, yellow, heavy textured, kaolinitic, sedimentary clay with no stratification. He

classified this soil as an Oxisol and named this soil type "Belterra clay", which is the

predominant soil type on the planalto stretching from the Tapaj6s to the Xingu River. IBGE

(1990) estimated that this soil type covers 29% of the Brazil's Amazonian lowlands. The

origin of this kaolinitic clay is widely disputed with several authors claiming it is of lacustrine

origin from the late Pliocene early Pliestocene (Sombroek 1966, Putzer 1984, Klammer

1984). Irion (1984) believes the Oxisol was formed in situ because the clays contain quartz

sand which would not appear together with the fine clays in lake deposited sediments.

Soil surveys and analyses were carried out on the planalto soils at Curua-Una by Day

(1961) and Cate (1960) before the removal of approximately 10 hectares of primary forest

in 1958. Day and Cate sampled soils in the center of the tree plots utilized in this research

project, and also from forest adjacent to the research plots. Although the number of samples

analyzed is unknown, their results indicated that these soils had a high clay fraction and low

extractable phosphorus (Table 2.1).

X-ray diffraction of the samples detected a high quantity of kaolinite and low levels of

gibbsite in surface soils and at 1 meter at this site (Cate 1960, Sombroek 1966). Cate (1960)

proposed that resilication of surface soils was occurring in this region because a high silica

clay (kaolinite) is found above the more silica depleted gibbsite. This theory has been recently








27

Table 2.1. Analyses of soils by Cate (1960) and Day (1961) prior to tree plot establishment
at Curui-Una Forest Reserve.
pH Total P Bulk Density
Depth (cm) C (%) N (%) (H20) (mg kg-') clay (%) (g cm"3)
0 30cm 3.56 0.26 4.4 3 73 0.96
90 130cm 0.56 0.04 4.9 3 84 1.06


revived by Lucas et al. (1993) who proposed that the soil mineral composition of these

ecosystems is maintained by the cycling of silica by vegetation.

Tree Plot Establishment

The 12 experimental tree plots used in this study were planted between 1959 and 1961 and

in 1973, and these plots were selected from the approximately sixty species trials that were

established on theplanalto (Figure 2.4). The objective of these plantings at the reserve was

to identify which native or exotic species grew well in plantations in this environment (Dubois

1971). In addition to the species trials, several experiments with selective cutting and natural

regeneration were carried out in km2 blocks located near the species trials (Figure 2.4).

The plots that were established as species trials were cleared of natural vegetation, burned,

and planted with seedlings raised in a nursery at the research station. All native species have

their provenance from native forest at Curua-Una and Pinus caribaea seed came from Belize

(Dubois 1971). At the time of planting the plots received "one-half tablespoon of phosphate"

per seedling which had no effect on growth or survival (Pitt 1961). The only treatment each

plot received since establishment was the clearing of understory growth (stems cut at base),

and the Pinus caribaea plots were thinned (50% reduction of stems) in 1980.











Amazon River A
N
2 km
Experimental Plots and
/ Former Species Trials
Foe / T s Reserve Boundary (72,000 ha)
e Li Village -


-D / Road
5.

Species Trials
Selective Cutting Experiments






Figure 2.4. The layout of the CuruA-Una Experimental Station and Forest Reserve.








29

The three native forest control plots (Figure 2.5) have not been disturbed by fire or cutting

since 1955 when United Nations personnel first arrived at the site. Workers who have lived

at the reserve since 1960 indicated that the terra firme forest did not show any signs of

disturbance when they first arrived. The encampment of the reserve was located next to the

Curua-Una River on a site that was once occupied by Native Americans, so, it is possible that

the forest plots may have been disturbed by humans in the past. Indeed, charcoal was found

during the course of this study in surface soils and at a 100 cm depth in the forest plots.

Sombroek (1966) also reported finding charcoal in undisturbed forest at the reserve down to

150 cm, which he attributed to Indian fires that burned roots deep into the soil profile.

Anthropogenic black earth or terra preta is widespread along the banks of the Curua-Una

River which is 6 km from the research plots.

Plantation Selection

The plantations were chosen according to the following three criteria: 1) The presence of

three plots (replicates) of a species or combinations of species that had similar foliar and leaf

litter characteristics, 2) The replicates could not have been dominated by native, successional

vegetation, and 3) As wide a range of phenologies, resource requirements, and biochemical

characteristics were to be included. Based on this screening, three replications each of Pinus

caribaea, Euxylophora paraensis, Carapa guianensis, and the Leguminosae combination

(one replicate each of Dalbergia nigra, Parkia mullijuga, and Dinizia excelsa) were selected

out of the sixty species that were originally planted at this site.

Pinus caribaea was chosen because it is known to have high C/N ratios in leaf litter, high

litterfall, high standing crop mass, and low N concentrations in litter (Lugo 1992). Carapa









30











A

N







NF
PC
PC
CG
CG NF
LEG
LEG LEG
NF
PC CG pP'N

EP EP EP &


KKEY
NF = Native Forest
PC ='Pinus caribaea
CG = Carapa gulanensis
LEG = Legumlnosae
EP = Euxylophora paraensis












Figure 2.5. Layout of the replicates used for this study at the Curua-Una Forest Reserve.








31

guianensis is native to this region, had leaf litter quality close to that of the mixed native

forest control, and had high survivorship in three plots. The Leguminosae treatment does not

contain three plots of the same species, but was selected for study to include a treatment with

high N concentrations in aboveground litterfall relative to the native forest control. Since

there were no replicated plots of any known N-fixing species, I combined three separate

unreplicated plots of species that had pinnately compound leaves and high N concentrations

in foliage and leaf litter to form this treatment. The inclusion of the combined Leguminosae

treatment does not allow me to attribute any effects on soils directly to a N-fixing species

because none of theses are reported to fix atmospheric-N (Allen and Allen 1978). Including

a treatment with high N inputs in leaf litterfall provided a good contrast with the native forest

control and the other three plantations that had lower litter quality.

The Euxylophora paraensis stand was younger than the other treatments (23 years

compared with 36 years), but I included this species because the plantations were mature at

23-years-old, and I believed any soil effects would be evident after this amount of time. In

addition, Euxylophora paraensis is native to this region and has leaf litter chemical

characteristics intermediate between native forest and Pinus caribaea, thus its inclusion

helped achieve my objective of including trees with a range of biochemical characteristics in

leaf litter.

The native forest control plots were located in a block of undisturbed forest adjacent to

the plantations. These plots were established to test for differences between soils under

undisturbed natural forest and the plantations at the initial phase of this project, and to








32

contrast and compare seasonal C and N transformations between natural forest and the

plantations over a one year period.

Summary

Replicated plots were selected from previously established plantations and seasonally dry,

lowland Amazonian forest to test the hypothesis that trees with varied ecological

characteristics will affect soil dynamics. The plantations were selected primarily to span a

range of leaf litter characteristics and because they were replicated. Recently undisturbed

native forest was selected as an experimental control to compare and contrast with the

monocultures. The replicates were not originally established with a randomized experimental

design, but the 15 plots (4 plantations + 1 control = 5 treatments x 3 replicated plots) will be

analyzed as a completely randomized design with treatment effects fixed. All the experimental

plots were within 1.5 km from one another, received approximately 1900 mm of precipitation

annually, and were growing on a soil previously identified as an Oxisol.













CHAPTER 3
SOIL HOMOGENEITY


Introduction

Previous studies examining plant-induced soil changes frequently encountered the

confounding effects of site variability, soil heterogeneity, and land-use history (Stone 1975).

For a study to accurately assess how plants influence soil properties, more precise estimates

can be made if experimental material is planted on an initially homogeneous substrate (Wedin

and Tilman 1990). If soils are initially uniform then any differences in soil properties that arise

over time among experimental treatments may be attributed to plant effects (Gower and Son

1992).

Common garden tree experiments are used to examine the influence of tree species on soil

properties because they offer researchers older, replicated plots of tree monocultures located

in close proximity (Binkley 1994). In many previous, long-term retrospective studies

examining changes in soil properties under tree monocultures, initial soil sampling was not

carried out because the objective of most common garden experiments was to compare

genotypic and phenotypic responses of tree species grown on a common site (Zobel and

Talbert 1984). Initial soil homogeneity was frequently assumed based on indirect evidence

such as the proximity and similar slope position of experimental units, block randomization,

and similar soil textures (Gower and Son 1992 ).



33








34

Although soil properties are spatially heterogeneous (Robertson et al. 1988), soils are

classified into orders according to characteristics such as effective cation exchange capacity

(ECEC), mineralogy, texture, and horizonation (Soil Survey Staff 1992). Previous soil

sampling in the upland, terra firme forest at the CuruA-Una Reserve indicated that the forest

was growing on an Oxisol (Sombroek 1966), the predominate soil order in lowland Amazonia

(IBGE 1990). These old and highly weathered soils are a good substrate on which to

examine plant influences because they are deep, well aerated, and highly uniform. Compared

to other soil orders, Oxisols cover the largest portion of the tropics (i.e., 22%, Van Wambeke

1992), and 9% of the earth's land area (Brady 1990), yet little is known of plant influences

on their chemical and physical properties.

Since the majority of aboveground detritus and a large proportion of belowground

turnover occurs in surface soils, subsurface soils are less influenced by plants and may

maintain physical and chemical characteristics despite changes in plant cover. Although

lowland forests may affect soil mineralogy in the long-term (Lucas et al. 1993), short-term

changes in vegetation are less likely to change mineralogy. The comparison of surface soil

texture and mineralogy and subsoil chemical properties may be a reliable indicator of initial

soil homogeneity under adjacent, replicated experimental units. Thus, the objectives of this

chapter are to 1) compare and contrast physical and chemical soil properties between the

forest control and each monoculture, and 2) determine if soils under the forest control and

each treatment were initially uniform.








35

Materials and Methods

Soil Sampling and Statistical Analysis

For the determination of soil physical and inorganic properties, four randomly located

cores were taken at 0 to 20 cm and 95 to 105 cm depths in each of the 15 plots (4

monocultures + 1 control = 5 treatments x 3 replicated plots = 15 plots) and combined by plot

and depth. All soils were passed through a 2 mm sieve, had fine roots removed by hand, were

air dried, and then stored at approximately 4o C in sealed plastic bags until analysis.

Soil particle size was analyzed in triplicate from pooled samples from each plot by the

pipette method (< 2mm fraction) and volume weight determination for the fraction > 2 mm

(Soil Survey Laboratory Staff 1992). Soil mineralogy was determined by x-ray diffraction

of the fine earth fraction (<2 mm), using samples from each depth combined by treatment

(i.e., 0 -20 cm and 100 cm). One sample per treatment was analyzed for mineralogy at each

depth using one KCL and one magnesium saturated tile per treatment (Whittig and Alardice

1986).

Effective cation exchange capacity (ECEC), pH, total C, N, P, and base cations were

determined from four randomly located samples per plot that were combined and subsampled

in triplicate at each depth (0 20 cm and 100 cm). ECEC was estimated as the sum of 1 N

NH4OAc (pH 7) extractable base cations and 1 N KCI extractable aluminum (Thomas 1982).

Soil pH was determined in a 2:1 water suspension (McLean 1982) Total N and C were

measured with a Carlo-Erba nitrogen autoanalyzer 1500, and extractable P, Al, and base

cations were determined by Inductively Coupled Argon Plasma Spectroscopy (ICAP) after

Mehlich I extraction.








36

Significant differences for mean soil physical and chemical properties between the forest

and the four plantations were determined by analyzing replicate means (n = 3) with a one-way

analysis of variance with treatment (plantations + forest control) as a fixed effect. A

completely randomized design (CRD) was used because the treatments were not randomly

assigned to blocks when the original species trials were installed. Because the plantations

were 23 to 36-years-old, I also could not randomly assign the treatments to the experimental

units (replicates), but I chose to use a CRD because I assumed the experimental units were

similar and any error attributable to differences among the replicates was small. In fact,

precipitation, temperature, relief, slope, soil parent material, understory cleaning, and fertilizer

applications were the same for all 12 of the monospecific plots. If soil texture, mineralogy,

and subsoil chemical characteristics could be shown to be similar, then the any variation in

surface soil properties among the treatments should be attributable to the species or group

of species that was established upon it. Finally, significant differences between soil properties

under the forest and each of the plantations were analyzed with Dunnett's t-test following the

analysis of variance to identify which plantations had changed soil properties compared to

undisturbed forest soils.

Results and Discussion

Soil Properties

Only the percentage of clay and sand in soils under E. paraensis differed significantly from

the soils under the forest; the physical properties of soils under the other plantations did not

significantly differ from the forest soil (Table 3.1). Although the clay and sand contents of

surface soils under E. paraensis were different than the forest control, the total fine earth








37

fraction (clay + silt) of surface soils were similar (85 vs 95%). The mineralogy of surface

soils and subsurface soils was similar under all the plots, and kaolinite was the predominate

mineral at both depths (Figures 3.1 3.5). In addition, quartz was present in all the samples

from surface soils and absent at 100 cm, which was evidence that the soils under the

plantations and the forest had been influenced by similar pedogenic processes (Figures 3.1 -

3.5). These findings also indicated that the principal inorganic components of the soils under

the plantations and the forest were similar at the time of plantation establishment.


Table 3.1. Mean physical soil characteristics ( se) at the 0 20 cm depth, Curua-Una Forest
Reserve. Values followed by (*) are significantly different than the forest at p .05.
Soil property
Treatment clay (%) silt (%) sand (%) Mineralogy
Forest 59.6 (1.5) 35.9 (1.4) 4.5 (0.5) Kaolinite
P. caribaea 54.5 (6.6) 34.5 (3.7) 11.0 (3.3) Kaolinite
C. guianensis 51.5 (1.5) 39.8 (2.1) 8.7(1.4) Kaolinite
Leguminosae 57.2 (4.0) 36.5 (3.5) 6.3 (0.6) Kaolinite
E. paraensis 40.3 (3.2)* 44.4 (2.7) 15.3 (2.1)* Kaolinite


The ECEC of surface soils from the monocultures ranged from 1.11 to 1.73 cmol, kg-1

soil, and there were no significant differences between the control and any of the

monocultures (Table 3.2). Considering this soil's mineralogy, ECEC, depth, and moisture

regime, this soil is a Typic Haplustox according to the USDA criteria (Soil Survey Staff

1992). These surface soils have a lower ECEC than several Amazonian Oxisols analyzed by

Motavalli et al. (1994), which ranged from 2.93 to 4.00 cmol, kg-' soil. The lower levels of

ECEC from soils at Curua-Una may result from deeper sampling (0 20 cm) compared to









38
ID: NATIVE FOREST(0-15CM) MG(GLY) (35kV. 20nmA)
File: 23M MDI Scan: 2-32/.03/1/#1001. Anode CU




KYolinite

Kaolinite
O- 10





250 .Goethite | Quartz






2-Theta
Figure 3. la. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under
native forest at Curua-Una.



ID: NATIVE FOREST 100CM MG(GLY) (35kV, 20mA)
File 24M MDI Scan 2-32/ 03/1/#1001. Anode: CU
7.18

3.59
750- Kaolinite
Kntolinite







Goethite
250 41




o o "" 20 "2 "D
2-Theta
Figure 3.lb. Mineralogy as detected by x-ray diffraction from subsurface soils (1 m) under
native forest at Curui-Una.









ID: PINE PLANTATION 0-15CM(MG(GLY) (35kV. 20mA) 3 9
File: 25M MDI Scan: 2-32/.03/1/#1001. Anode: CU

7.18
750-

3.59


500-




250-
.4.4



o- 10 15 20' o 25 3 "
2-Theta
Figure 3.2a. Mineralogy as detected by x-ray diffraction from surface soils (0 20cm) under
Pinus caribaea at Curui-Una.


ID: PINE PLANTATION 100CM(Mg(GLY)) (35kV, 20mA)
File: 26M.MDI Scan: 2-32/.03/1/#1001. Anode: CU


750-







8


250-




b10 15 2b 3b
2-Theta

Figure 3.2b. Mineralogy as detected by x-ray diffraction from subsurface soils (1 m) under
Pinus caribaea at Curui-Una.










ID: Ken Smith CG Mg-sat tile (35kV. 20mA)
File: 0027.MDI Scan: 2-32/.03/1/#1001. Anode: CU




750-














010 l 1 25 30









1000-
250"





























10 1 2 a
2-Theta

Figure 3.3b. Mineralogy as determined by x-ray diffraction of subsurface soils (100 cm)
under Carapa guianensis at Curu--Una.
under Carapa guianensis at Curub.-Una.









41
ID: Ken Smith -20cm Mg-sot tile (35kV, 20mA)
File: 0019.MDI Scan: 2-32/.03/1/1001, Anode: CU
1000-




750-




Ssat,








0 1 15 20
2-Theta
Figure 3.4a. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under
the Leguminosae plantations at Curua-Una.


ID: Ken Smith -100cm Mg-sat tile (35kV, 20mA)
File: 0029.MDI Scan: 2-32/.03/1/11001. Anode: CU




750-



500










0 10 3
2-Theta


Figure 3.4b. Mineralogy as determined by x-ray diffraction of subsurface soils (100 cm) under
the Leguminosae plantations at Curui-Una.









42
ID: Ken Smith EP-20cm Mg-sat tile (35kV. 20mA)
File: 0023.MDI Scan: 2-32/03/1/#1001, Anode: CU

1000-




750i



S500-




250-



0 31
l0 10 2 20 25 30a
2-Theta
Figure 3.5a. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under
Euxylophora paraensis at CuruA-Una.


ID: Ken Smith EP-100cm Mg-sat tile (35kV, 20mA)
File: 0021 MDI Scan: 2-32/.03/1/#1001. Anode: CU




750-




8









10 15 .. ..

2-Theta


Figure 3.5b. Mineralogy as determined by x-ray diffraction of subsurface soils (100 cm) under
Euxylophoraparaensis at Curua-Una.







Table 3.2. Mean soil chemical characteristics (se) at a 0 20 cm depth, Curui-Una Forest Reserve (n = 3).
Values followed by (*) are significantly different than the forest control at p .05.
Soil Property
ECEC pH Total N Total C Extract. P Total Base cations
Species (c mol, kg-') (HO20) (g kg-') (g kg-') (mg kg') (mg kg1)
Forest 1.60(0.2) 4.48 (0.05) 3.57 (0.1) 57.03 (1.3) 3.77 (0.6) 56.4 (4.4)
P. caribaea 1.11(0.1) 4.55 (0.1) 2.68 (0.1)* 50.09 (2.7) 3.83 (0.2) 45.8 (2.6)
C. guianensis 1.20 (0.1) 4.93 (0.1) 3.13 (0.1)* 49.62 (1.6)* 3.64(0.6) 62.8 (4.0)
Leguminosae 1.73 (0.3) 4.89 (0.07) 3.13 (0.05)* 50.90 (2.1) 3.75 (0.3) 49.9 (5.1)
E. paraensis 1.55 (0.1) 4.66 (0.08) 4.10 (0.06)* 62.94 (1.6) 3.75 (0.1) 71.1(11.5)



Table 3.3. Mean soil chemical characteristics (se) at a 100 cm depth, Curui-Una Forest Reserve (n = 3).
Values followed by (*) are significantly different than the forest control at p < .05.
Soil Property

pH Total N Total C Extract. P Total Base Cations
Species (H20) (g kg"') (g kg") (mg kg')) (mg kg-')
Forest 4.51 (0.04) 0.54 (.02) 8.8 (0.4) 1.53 (0.7) 21.4(2.5)

P. caribaea 4.36 (0.03) 0.43 (.03) 8.5 (0.9) 1.87 (0.4) 23.4 (9.0)
C. guianensis 4.62 (0.08) 0.52 (.04) 8.3 (0.5) 1.44 (0.1) 19.57 (2.9)
Leguminosae 4.66 (0.05) 0.46 (.03) 7.4 (0.6) 1.69 (0.3) 19.19(0.9)
E. paraensis 4.68 (0.11) 0.69 (.09) 14.1 (2.8)* 1.50 (0.01) 43.75 (23.1)








44

samples analyzed by Motavalli et al. (1994), which came from depths of 0 8 and 0 12 cm.

Soil organic matter provides negative charge in highly weathered, variable charged soils

(Gillman 1985), and the lower organic matter contents of the Curui-Una samples from 12 -

20 cm may have diluted the ECEC.

The concentrations of Mehlich I extractable P in these surface soils ranged from 3.64 to

3.83 mg kg-' dry soil and these values are similar to those reported for tropical lowland

forests in Sabah, Malaysia (Proctor et al. 1988) and montane forest in Puerto Rico (Cuevas

et al. 1991). The concentrations of Mehlich I extractable P in surface soils at Curua-Una were

much lower than estimates of acid extractable P reported from a wide range of tropical

lowland forests in a review by Silver (1994). Mehlich I extractable P and base cations under

the plantations did not significantly differ with the forest at either depth, and each decreased

dramatically in concentration with depth (Tables 3.2, 3.3). Although these measures do not

give an indication of what was available for plant uptake, they did indicate that the plantations

had not influenced Mehlich I extractable P and base cation concentrations in the soil.

Surface soil total N concentrations ranged from 2.68 to 4.10 g kg'' dry soil, values similar

to those from lowland forests in Costa Rica (Heaney and Proctor 1989) and Sarawak (Proctor

et al. 1983). These values are also similar to a mean value of 3.2 g N kg'' soil reported for

Oxisols in a review by Sanchez et al. (1982). Total N concentrations decreased with depth

under all the plantations and the forest (Tables 3.2, 3.3), and surface soils under the forests

had higher total N concentrations than all the plantations except for E. paraensis (Table 3.2).

In contrast, there were no significant differences between the forest and the plantations for

subsurface total N concentrations (Table 3.3). If my assumption of initial soil homogeneity








45

under the plantations and the forest was correct, these findings indicated that the plantations

had influenced surface soil N dynamics.

Although C and N are linked in forest soil biogeochemical cycling (McGill and Cole 1981),

surface soil total C concentrations under the forest were only significantly higher than under

C. guianensis (Table 3.2). Surface soil total C concentrations ranged from 49.62 to 62.94

g kg'' dry soil (Table 3.2), and decreased greatly in subsoils (Table 3.3). The surface soil total

C concentrations reported here are higher than the mean value of 38.00 g kg-' from surface

soils (0-15 cm) of 19 Oxisols reviewed by Sanchez et al. (1982). Only subsurface total C

concentrations under E. paraensis were significantly higher than under the forest (Table 3.3).

The C/N ratios of surface soils at Curuai-Una ranged from 15.33 (E. paraensis) to 18.60

(P. caribaea), and these are much higher than the mean Oxisol surface soil C/N ratio of 11.87

reported by Sanchez at al. (1982). The subsurface C/N ratios ranged from 16.0 (legumes)

to 18.9 (E. paraensis), and these ratios are higher than those reported in a worldwide study

of C and N storage by Post et al. (1985), who determined that tropical dry and moist forest

soil profiles to a 1 m depth had C/N ratios of 13 to 15.

Surface soil pH ranged from 4.48 to 4.98 (Table 3.2) and decreased with depth under the

plantations while increasing slightly under the forest (Table 3.3). These data indicate that

P. caribaea did not acidify surface soil pH, contrary to results reported from temperate zone

sites that were reforested with conifers (Brand et al. 1986). Forest sites occupied by N-fixing

species have decreased surface soil pH in temperate environments (Van Miegroet and Cole

1984), and this phenomenon has been attributed to Ht production resulting from increased

nitrification rates in combination with greater acid strength of soil organic matter under N-








46

fixers (Binkley and Sollins 1990). At Curua-Una, the Leguminosae treatment did not acidify

surface soils compared to the forest, and surface soil pH under the legumes was higher than

in the forest control (Table 3.2). I did not note any nodulation on fine roots under the

legumes, thus perhaps these species did not fix atmospheric-N or they fixed N only when they

were younger. These findings suggest that soil nitrification rates were higher under the forest,

which had a wide number of potential N-fixing legumes growing in it, or perhaps that the

inputs of a wide range of litter types into the forest control maintained a higher acid strength

of soil organic matter.

Conclusions

Several lines of evidence support the conclusion that initial soil physical and chemical

properties under each monoculture were not significantly different than those observed under

present day primary forest at the Curua-Una Reserve, and these include the following:

1. There were no significant differences in surface soil texture between plantations of P.

caribaea, C. guianensis, and the Leguminosae combination and native forest. Only surface

soils under E. paraensis differed from the forest control in their percentage of clay and sand.

2. The dominant mineral constituent under all the plantations and the forest control in both

the surface soils (0 20 cm) and subsurface soils (100 cm) was kaolinite.

3. There were no significant differences between any of the monocultures and the forest for

total C and N, or extractable P, and base cations at the 1 m depth.

4. All the research plots shared the same slope and altitudinal position on the plateau (slope

was 0 to 1 %, altitude 180 m), and they were exposed to the same rainfall and temperature

regimes.








47

Although there were no significant differences between the forest and the plantations for

total N at a 1 m depth, this variable differed significantly in surface soils. The physical

properties between native forest and the plantations were similar and these soils had only been

exposed to a change in vegetative cover. Therefore, this indicates that the plantations may

have influenced N dynamics of these surface soils. The primary objective guiding the

remainder of this work is to identify the mechanisms of these plant-induced soil changes, with

an emphasis on the altered soil N dynamics.













CHAPTER 4
LITTERFALL AND NITROGEN-USE EFFICIENCY OF PLANTATIONS
AND PRIMARY FOREST IN THE BRAZILIAN AMAZON



Introduction

Aboveground fine litterfall and decomposition are critical processes for transferring

nutrients from aboveground forest biomass to soils (Golley et al. 1975, Swift et al. 1979).

In seasonally dry tropical forests (< 100 mm rainfall for four consecutive months), peak

litterfall usually coincides with the dry season (Wright and Cornejo 1990). In the east-central

Brazilian Amazon, which has a distinct dry season ranging from August to November (Klinge

and Rodrigues 1968, Nepstad et al. 1994), peak litterfall occurs at the onset of the dry season

in riverine and upland forests (Franken et al. 1979). Forest floor mass also increases during

the dry season in this region (Klinge 1977).

In forest ecosystems worldwide, foliar nitrogen (N) concentrations, fine litterfall N

contents, and within-stand nutrient-use efficiency (NUE) have been used as indices of N

availability and soil fertility (Van den Driessche 1974, Vitousek 1982). In a review of tropical

forests receiving more than 1500 mm of precipitation annually, Vitousek and Sanford (1986)

noted that Amazonian terra firme forests had foliar N concentrations intermediate to the

other tropical forests examined, which indicated that N may not be limiting to forest growth

in this region. Nitrogen return in litterfall of lowland tropical forests is higher than for



48








49

temperate forests (Proctor 1984), and in lowland Amazonia, Vitousek (1984) suggested that

N availability did not limit growth of terra firme forests growing on Oxisols because they had

relatively low within-stand NUE (represented by the litterfall dry mass:nitrogen ratio).

Differences in surface soil nutrient concentrations due to species or stand composition may

be identified if climate, species composition, successional status, and soil type are controlled

(Vitousek and Sanford 1986). If N concentrations in Oxisols supporting Amazonian terra

firme forest do not limit aboveground litter production, forest conversion to tree plantations

that consist of species with varied nutrient requirements and aboveground litter quantities and

qualities might change N dynamics in these soils. After native forest removal, site

preparation, and reforestation, the N status of these soils should recover to varying degrees

based on the characteristics of the plants that dominate a particular site. For example, I

demonstrated in Chapter 3 that total N concentrations in surface soils under plantations and

terra firme forest have changed significantly over a 20 to 35 year period. I therefore

hypothesize that mature plantations which are planted on an initially uniform soil will have

varying litterfall N inputs, foliar N concentrations, and NUE, and that these three factors will

be related to differences in surface soil N concentrations. Differences in litterfall N inputs,

foliar N concentrations, and NUE among the treatments (the four plantations and the forest

control) may also indicate that the plantations are influencing N availability in these surface

soils.

The specific objectives of this chapter are to 1) describe the physical environment of each

plantation and the adjacent undisturbed forest, 2) determine the timing and quantity of

aboveground litterfall, 3) determine the turnover quotients and N contents of aboveground








50

litterfall, 4) estimate NUE and foliar N concentrations, and 5) relate foliar N concentrations,

within-stand NUE, and litterfall N contents with total N concentrations in surface soils.

Materials and Methods

Plot Establishment and Inventory

Three of the plantations were established between 1959 and 1961, with one replicate of

each installed each year, and the Euxlyophora paraensis plots were planted in 1973. Despite

its younger age, this monoculture was included because it was mature and its leaf litterfall had

characteristics different from the other plantations. The forest control has not been disturbed

recently, although charcoal was found at soil depths from 5 cm to 100 cm, indicating that it

has been influenced by fire in the past.

All the overstory trees in each replicate 10 cm diameter at breast height (DBH) were

measured for height and diameter, but the exterior row of trees in each replicate was not

included in estimates of basal area because the vegetation surrounding each plot was variable.

For each treatment, basal area (m2 ha"') was estimated from trees with diameters at breast

height (DBH) 2 10 cm. Survival was calculated based on original stocking levels of 1600

trees ha'1 (2.5 m spacing). Understory vegetation was sampled with five, randomly located

.001 ha circular subplots located in each main plot.

Vegetation Area Index

Vegetation area index (VAI) was obtained with a LAI-2000 Plant Canopy Analyzer (LI-

COR Inc., Lincoln, Nebraska), which integrates readings of above- and below-canopy diffuse

sky radiation from five zenith angles. The LAI-2000 measures all light-blocking objects, thus

VAI is a more appropriate term than leaf area index (Strachan and McGaughey 1996). In








51

broadleaf canopies, trees are assumed to have randomly distributed hand horizontal foliage,

and the LAI-2000 output approximates single-sided leaf area index (LAI). The LAI-2000

output is usually adjusted for conifer stands because needles are not arranged randomly

(LICOR 1991).

To obtain VAI in each plot, I took an initial reading in a road cut nearest to each plot, then

entered the plot and took ten random measurements that were compared to each initial

reading. I sampled each plot from 7-9 AM during one week in July and one week in October.

I sampled at this hour because at these times of the year skies were uniformly clear, and the

solar disc was not visible. I used the 2700 view cap for all measurements, and the LAI-2000

device was pointed in the same direction for the initial reading and for all ten readings in each

plot. VAI was estimated for each plot based on an average of the individual ten readings as

compared to the initial reading (which I substituted for above-canopy conditions).

Litterfall. Forest Floor Mass. and NUE

Aboveground fine litterfall was estimated by placing six 1 m2 traps in each replicate of

each treatment (3 replicatess x 6 traps = 18 traps per treatment). Traps were placed

systematically to provide even coverage over each plot, always one row inside the boundary

of each replicate. They were suspended 40 cm above the forest floor, and fine litter was

collected in nylon screen with 1 mm mesh. Litter was collected every 14 to 20 days from

October 1994 to October 1995. Fine litter was separated into leaves, fruits, cones, flowers,

bark, and branches (< 2 cm diameter), and dried at 1050C until a constant weight. During

every collection period the trash fraction was partitioned into each of the previous categories

according to what it most resembled, and insect parts and feces were discarded. Fine litter








52

that was kept for nutrient analysis was dried to a constant weight at 600C and refrigerated

until analysis. Every four months, fine litterfall was separated into overstory and understory

components to estimate the impact of non-overstory species. Monthly litterfall totals were

calculated by determining the average daily litterfall rate for each collection period.

To estimate forest floor mass, one 50 cm2 quadrat was randomly placed on the forest floor

of each plot every three months, and all fine forest floor material inside the quadrat to mineral

soil was removed. Thus for 12 months, 4 quadrats x 3 plots = 12 quadrats analyzed per

treatment. Forest floor material was separated into leaves, fruit plus flowers, and bark plus

branches (< 2 cm diameter), and dried at 105C until a constant weight.

Nitrogen-use efficiency (NUE) is the ratio of the rate of dry matter production:rate of

nitrogen taken up by a stand of trees (Hirose 1971, Grubb 1989). I did not estimate either

of these rates, thus I calculated NUE using Vitousek's (1982, 1984) practical definition of

NUE which is the ratio of the dry matter:nitrogen content of litterfall or the inverse of litter

N concentrations. I also assumed that the plantations and the forest were at steady-state,

aboveground net primary productivity was equal to litterfall, and that the N lost in litter was

equal to N taken up by the stands.

Nutrient and Statistical Analysis

Fine litterfall material kept for nutrient analysis was ground through a 1 mm sieve, then

finely ground with a ball grinder and subsampled for C and N concentrations in a Carlo Erba

Nitrogen 1500 Analyzer in triplicate. Estimates of total fine litterfall and leaf litter N were

derived from samples collected during September 1994, January 1995, and April 1995. To

estimate total fine litterfall N, samples were bulked by plant fraction (leaves, fruit plus flower,








53

bark plus branch) and treatment and analyzed in triplicate. Total fine litterfall N contents (kg

ha"') were derived by multiplying litterfall N concentrations of each plant fraction by the total

weight (t ha"1) of each fraction that fell from during the study period. Total fine litterfall N

concentrations were then estimated by dividing total litterfall N (kg ha-') by total litterfall

(t ha-'). Leaf litter samples from the three months were analyzed separately for N

concentrations by month and treatment. Leaf litter N contents (kg ha-') were estimated by

multiplying total leaf litterfall (t ha'1) by average leaf litter N concentrations from leaf litter

collected in September, January, and April.

Foliar N concentrations were estimated from foliage removed from the mid-canopy of

each plantation and the forest in September 1994. Foliage was shot out of the canopy with

a shotgun, oven dried at 600C until a constant weight, and refrigerated until analysis. The age

of the foliage collected was unknown.

Significant differences among experimental treatment means (n = 3) for litterfall, forest

floor mass, and leaf litter nitrogen concentrations were determined with Tukey's HSD

multiple comparison following one-way analysis of variance using PROC GLM in SAS (SAS

1988) with treatment effect fixed. Relationships between litterfall, forest floor mass, nitrogen

contents in litterfall and soil N concentrations were determined with PROC REG in SAS.

Results

Tree Density, Basal Area. and Species Diversity

Stem density (2 10 cm DBH) ranged from a low of 463 per ha in the forest control to

1095 per ha in the C. guianensis plots (Table 4.1). Only one liana > 10 cm DBH was rooted

in the forest plots, although smaller lianas were numerous and larger lianas rooted outside the







Table 4.1. Characteristics of the forest and the plantations at the Curua-Una Forest Reserve.

Pinus Carapa Euxylophora
Plot Characteristic Native Forest caribaea guianensis Leguminosae paraensis

Mean Plot Size 1000 m2 905 m2 497 m2 515 m2 1000 m2

Stand Age (years) ? 35-37 35-37 35-37 23

# Trees 10cm dbh ha"' 463 588 1095 1027 625

# Species 1l0cm dbh 57 1 1 1 1

# Families a 10cm dbh 27 1 1 1 1

Basal Area (m'ha*') 26.2 39.7 43.9 46.1 17.9

VAI (July) 3.6 2.5 2.9 2.1 3.7

VAI (Oct.) 3.1 2.4 2.5 2.1 2.8

Original stems (%) ---- 35 72 64 39

# Stems and seedlings < 10 60,067 21,080 22,571 34,571 10,000
cm dbh ha'

# Species < 10 cm dbh 102 52 26 41 16

# Families < 10cm dbh 29 31 22 27 14














om








55

plots entered them through the canopy. Average basal area (trees > 10 cm DBH) ranged

from 17.9 m2 ha-' (E. paraensis) to 46.1 m 2 ha -' (legumes), and average diameters were

highest for P. caribaea (28.7 cm). The families most represented with trees 10 cm DBH

in the forest included the Leguminosae, Sapotaceae, Chrysobalanaceae, and Melastomataceae.

Previous inventories in the reserve, which covered much larger areas, found the most widely

represented taxonomic families were the Leguminosae, Sapotaceae, Lecythidaceae, and

Lauraceae (SUDAM 1971). The pine overstory had the lowest stem density of all the

plantations because it was thinned in 1980. Although the sample size used to estimate species

richness under the plantations and the forest were identical, direct comparison of understory

species richness was not possible because the plot sizes of the plantations and the forest

ranged from 0.05 ha to 10 ha (Table 4.1). Nevertheless, it is interesting to note that the pine

understory supported more families than the forest, although the total number of tree

seedlings and saplings in the pine plantation was much lower than in the forest.

Vegetation Area Index

In July, which is the peak litterfall season for the forest control and three of the plantations

(Figure 4.1), VAI ranged from 2.1 (Leguminosae) to 3.75 (E. paraensis). Except for the

Leguminosae treatment, VAI decreased for each treatment and the control in October, which

is in the middle of this region's dry season.

The LAI-2000 underestimates LAI in conifer canopies (Gower and Norman 1991), but

I did not use a correction factor to estimate VAI because broadleaf understory vegetation also

contributed to VAI in these plots. Trees in the replicates of P. caribaea were also widely

spaced due to thinning, and I believed application of a correction factor would have








200 200 56

160- A. "' 160- B.

120- 120-

80 800-

J 40 40 -


NDJFMAMJJASO NDJFMAMJJASO

200 Month 200 -Month

160- C. 160- D.

120- 120-

80 80-

S40- 40-
0-

NDJFMAMJJASO NDJFMAMJJASO

Month Month
200

7 160- E. -- Leaf Litter
1 --- Bark + Branches
120 ~ Fruit + Flowers

80-

40 -

0-
NDJFMAMJ JASO

Month
Figure 4.1. Fine litterfall ( se) at the Curudi-Una Forest Reserve from 1994 1995 under A)
the native forest control, B) P. caribaea, C) C. guianensis, D) the Leguminosae, and E) E.
paraensis.








57

overestimated VAI. The legume plantations had the lowest VAI of all the treatments despite

having the second highest stem density.

Litterfall and Forest Floor Mass

Total fine litterfall ranged from 8.09 t ha' yr"' under E paraensis to 10.35 t ha' yr" under

P. caribaea (Table 4.2). Mean total litterfall differed significantly among treatments (p< .05)

primarily because of differences for total litterfall between E. paraensis and P. caribaea

(Table 4.2). The percentage of total fine litterfall contributed by the understory and adjacent

vegetation in the plantations ranged from 0.1 (E. paraensis) to 6 % (P. caribaea), therefore

the contribution of other species to litterfall inputs and forest floor mass was minimal in each

plantation.


Table 4.2. Total fine litterfall, forest floor mass, and mean
residence times (inverse of litterfall: standing crop quotient) for
fine litter for native forest and four plantations at the CuruA-Una
Reserve ( se). Values followed by different letters are significant
at p < .05.
Total fine litter Standing crop l/KL
Treatment (t ha-' yr-I) (t ha-1) (yr)
Forest 9.76 (.58) ab 7.28 (1.0) b 0.77
P. caribaea 10.35 (.43) a 11.06 (1.2) a 1.06

C. guianensis 8.81 (.74) ab 10.26 (0.7) ab 1.16

Leguminosae 10.19 (1.3) ab 8.02 (1.3) ab 0.78
E. paraensis 8.09 (.63) b 7.77 (1.0) ab 0.96


Total monthly litterfall for each category was similar for the plantations consisting of

species native to the region, with peaks in leaf and branch litterfall in June and July (Figure

4.1). Peak leaf fall for P. caribaea occurred during dry season, with heaviest branch fall in








58

December (Figure 4. 1b). The forest had the highest fruit and flower fall starting at the

commencement of the rainy season (December) and lasting until March (Figure 4.1 a). The

plantations of species native to the region had similar peaks in fruit and flower fall, and the

legumes had two distinct peaks when P. multijuga and D. excelsa flowered at different times.

Pinus caribaea displayed steady fruit and flower production all year (Figure 4. b).

Total fine litterfall N contents ranged from 43 kg ha' to 134 kg ha (Table 4.3). These

values were not analyzed for significant differences because they were derived from pooled

samples of fine litterfall over three months, and I did not obtain replicate means from which

to estimate sample variance. Within-stand NUE was highest for P. caribaea and lowest for

the legume plantation.


Table 4.3. Total fine litterfall N concentrations and contents and NUE for
native forest and the four plantations at the CuruA-Una Reserve.
Standard errors are not reported for litter because values are from one
pooled sample (per treatment) from litter collected in September 1994,
January 1995, and April 1995. Nitrogen-use efficiency (NUE) is the ratio of litterfall
mass:nitrogen content of litter.
Total fine litterfall Total fine litterfall
Treatment N (mg g-1) N (kg ha1-) NUE
Forest 11.79 115.1 84.7
P. caribaea 4.70 43.1 240.1

C. guianensis 10.36 91.2 96.6

Leguminosae 13.19 134.4 75.8
E. paraensis 7.34 59.4 136.2


Forest floor mass ranged from 7.28 to 11.06 t ha-' and mean residence times for fine litter

ranged from 0.77 to 1.16 years (Table 4.2). Significant differences among treatments for








59

mean forest floor mass were detected (p< .05) primarily because P. caribaea had much higher

forest floor mass than the forest (Table 4.2). The contribution of understory and adjacent

vegetation to forest floor mass ranged from 0% under E. paraensis to 8.5% under P.

caribaea (data not shown).

Total Leaf Litterfall, Leaf Litter Standing Crop. and Leaf Turnover

Total leaf litterfall ranged from 6.45 t ha-' yr (E. paraensis) to 8.09 t ha' yr"' (P.

caribaea), and there were no significant differences among treatments for total mean leaf

litterfall (Table 4.4). Leaf litter standing crop varied widely and ranged from 3.06 t ha-'

(Leguminosae) to 8.19 ha (C guianensis), and significant differences among means of leaf

litter forest floor mass were detected at p .05 (Table 4.4). Leaf turnover quotients were

highest for the legume plantations, and mean leaf residence time was shortest for the legumes

(0.38 years) and the forest (0.5 years). Leaf material (including rachis) of C. guianensis had

the longest turnover time (Table 4.4).


Table 4.4. Leaf litterfall, leaf standing crop, and mean residence
time for leaf litterfall for native forest and plantations at the
Curui-Una Reserve ( se). Values followed by different letters
significantly different at p .05.
Total leaf litter Leaf standing crop 1/KL
Treatment (t ha-' yr-I') (t hal-) (yr)
Forest 6.81 (.55) a 3.42 (.52) c 0.50
P. caribaea 8.09 (.37) a 7.85 (.99) ab 0.97
C. guianensis 7.46 (.71) a 8.19 (.48) a 1.09
Leguminosae 7.92 (1.3) a 3.06 (.62) c 0.38
E. paraensis 6.45 (.59) a 3.97 (.46) be 0.61








60

Foliar and Leaf Litter N Concentrations

As might be expected, the plantations of the Leguminosae had the highest foliar and leaf

litter N concentrations while P. caribaea had the lowest N concentrations. Leaf litter from

the legumes had the highest N concentrations at 15.6 mg g-' dry leaf matter, and P. caribaea

had the lowest concentrations at 4.5 mg g'' dry leaf matter (Table 4.5). Nitrogen

concentrations of live foliage were also highest in the Leguminosae plantations (18.8 mg g"

dry leaf matter) and lowest in P. caribaea (9.0 mg g-' dry leaf matter). As with leaf litter,

foliar N concentrations in the legumes and the forest did not differ significantly, but foliar N

concentrations for the other three plantations were significantly lower than both the forest and

the legumes (Table 4.5). The N content of total leaf litterfall was highest under the legumes

and lowest under P. caribaea (Table 4.5). The foliar N: leaf litter N ratio was highest for P.

caribaea and lowest for the legume plantations.


Table 4.5. Nitrogen contents and concentrations of leaf litterfall, and foliar nitrogen
concentrations of leaf litter from native forest and four plantations at the CuruA-Una Forest
Reserve ( se).
Foliar N Leaf Litter N N content Foliar N
Treatment (mg g'" ) (mg g' ) (kg ha-') :Litter N
Forest 17.98 (0.7) a 13.97 (0.2) a 95.34 22.3
Pinus 9.00 (0.2) c 4.50 (0.2) c 36.40 50.0
Carapa 13.26 (0.2) b 10.87 (0.6) b 80.50 18.0
Leguminosae 18.82 (1.6) a 15.65 (0.4) a 123.94 16.8
Euxylophora 14.54(0.1) b 8.60 (0.4) b 55.47 40.8








61

Discussion

Forest Tree Density. Basal Area, and Species Richness

Previous studies in lowland Amazonian primary forest in Brazil have found tree densities

(2 10 cm DBH) in natural forest ranging from 230 to 460 per ha, and these densities are much

lower than those for terra firme forest at San Carlos in Venezuela (Table 4.6). Tree density

in the undisturbed forest at Curua-Una is also in the lower range of the 414 957 trees per

ha reported for tropical forests worldwide (Phillips and Gentry 1994). Basal area for the

forest control was 26.2 m2 ha'1, which was similar to other Amazonian terra firme forests

(Table 4.6). Larger scale inventories in Curud-Una region (100% inventories of I ha plots)

have found a total of 103 140 tree species per ha in this region with 38 42 species > 25 cm

DBH per ha (Heindsjiik 1958, Pitt 1961, Glerum and Smit 1965).

Tree species diversities of lowland Amazonian forests are difficult to compare because

researchers have inventoried plots of variable sizes and used variable minimum diameters

(Table 4.7). Species richness in the forest at Curua-Una was similar to values reported for

lowland Amazonian forest near Manaus and in Roraima (Table 4.7). For example, Klinge et

al. (1975) reported 50 tree species (stems 15 cm DBH) and 20 families contributing to

overstory diversity in a 0.2 ha plot near Manaus with the Leguminosae, Sapotaceae,

Euphorbiaceae, Lecythidaceae, and Vochysiaceae the most widely represented families with

stems over 15 cm DBH. Terra firme forest in the Peruvian Amazon is the most diverse in the

world with 283 tree species represented in 580 stems per ha > 10 cm DBH (Gentry 1988).

According to Gentry (1995), tropical dry forests have 50 to 70 tree species > 2.5 cm DBH

per 0.1 ha and moist semi-evergreen forests have 100 to 150 species > 2.5 cm DBH per 0.1








62

Table 4.6. Tree densities and basal areas (trees a 10 cm DBH) of lowland Amazonian forests.
Tree Density Basal Area
Location and Forest Type (# ha-') (n' ha-') Reference

Brazilian terra firme

Curui-Una 463 26.2 this study

Roraima 419 23.8 Thompson et al. 1992

Pari 230, 448 Black et al. 1950

Para 594 Pires et al. 1953
Para 393 -460 28.2 32.1 Campbell et al. 1986

Peruvian terra firme

Yanamono 580 Gentry 1988

Mishana 842 Gentry 1988

Venezuelan terra firme

San Carlos 670 786 27.8 Uhl and Murphy 1981

San Carlos 748 23.1 Jordan and Uhl 1978

Columbian river terraces

CaquetA 610 30.0 Duivenvoorden 1996

Suriname lowland forest

Kabo 477 25.2 Jonkers and Schimdt
1984


ha. Lugo and Murphy (1986) found that tropical dry forest has 35 to 90 tree species per ha

(210cm DBH) and a basal area between 17 and 40 m2 ha"'. These authors also indicated that

wet tropical forest has 50 to 200 tree species > 10 cm DBH and basal areas between 20 and

75 m2 ha"1. Although my figures for species richness in the forest were summed over three 0.1

ha plots, tree species richness and basal area for terra firme forest at Curua-Una falls in

between classifications for dry and moist lowland tropical forest.








63

Table 4.7. Tree species diversity in plots of various sizes in lowland Amazonia.
Minimum Plot Size
Location and Forest Type Diameter (ha) # Species Reference

Brazilian terra firme

Curui-Una 10 cm 0.3 57 this study

Curua-Una > 25 cm 1 38 -42 Pitt 1961

Para 10 cm 1 118- 162 Campbell et al. 1986

Para 10 cm 3.5 179 Pires et al. 1953

Para 2 10 cm 1 87 Black et al. 1950

Amazonas > 10 cm 1 79 Black et al. 1950

Amazonas 2 15 cm 0.2 50 Klinge et al. 1975

Amazonas 2 2.5 cm 0.1 92 Gentry 1978

Roraima a 10 cm 0.25 33 47 Thompson et al. 1992

Peruvian terra firme

Yanamono 2 10 cm 1 283 Gentry 1988

Mishana 2 10 cm 1 275 Gentry 1988

Venezuelan terra firme

San Carlos > 10cm 0.5 63-79 Uhl and Murphy 1981

San Carlos 10 cm 1 83 Uhl and Murphy 1981

Columbian river terraces

Caquetai 2 10 cm 0.1 37 Duivenvoorden 1996

Suriname

Kabo 5 cm 1 108 Jonkers and Schimdt
1984


Vegetation Area Index

My estimates of VAI for the forest control are lower than previously reported values of

LAI in other Amazonian forests, perhaps because researchers have used different techniques

to estimate LAI. For example, LAIs of 5.2 and 6.7 estimated from leaf dry weight/area








64

relationships have been reported for terra firme forests in Venezuela (Jordan and Uhl 1978,

Saldarriaga and Luxmoore 1991). McWilliam et al. (1993) determined leaf area by similar

methods for terra firme forest near Manaus and reported a LAI of 5.7. These authors

sampled this forest during October, and the climate and temperature regime in this region is

similar to that at Curua-Una. VAI of the forest stands at Curua-Una was estimated during

clear days using the LICOR-2000, and it is possible that VAI was underestimated due to

reflections of sunlight off leaf surfaces (LICOR 1991). These estimates for Amazonian forest

leaf area are at the lower end of values reported worldwide for tropical forests, which have

LAIs ranging from 6 to 17 (Leith 1975).

Total Fine Litterfall and Seasonality of Litterfall

Total fine litterfall (9.7 t ha' yr') in the forest from October 1994 to October 1995 is

higher than most values reported from Brazilian terra firme forest, which range from 6.9 to

9.9 t ha"' yr'1 (Table 4.8). Dry season intensity and the number of deciduous trees located

in relatively small forest plots greatly influence total fine litterfall in this region; thus, variation

in fine litterfall among sites is to be expected. Total fine litterfall in the forest control was

greatest during the onset of the dry season (June August), so, if N concentrations in fine

litterfall do not vary greatly over the year, aboveground N additions to the forest floor were

also highest at this time.

Seasonality of total fine litterfall has been widely reported in Brazilian terra firme forests,

with peak litterfall in this forest type occurring at the onset of the dry season (Franken et al.

1979). Other neotropical lowland forests with distinct dry and wet seasons also have peak

litterfall during the driest months of the year including semi-evergreen forest in Panami








65

Table 4.8. Total fine litterfall and forest floor mass in lowland Amazonian forests.
Total Fine Forest Floor
Litterfall Mass
Location and Forest Type (t ha' yr') (t ha-') KL Reference

Brazilian terra firme
Curui-Una 9.7 7.2 1.34 This study
Roraima 9.3 4.6 2.02 Scott et al. 1992
Roraima 6.9 3.1 2.22 Scott et al. 1992

Manaus 7.6 7.2 1.05 Klinge 1977
Manaus 7.3 Klinge and Rodrigues
1968
Manaus 7.9 Franken et al. 1979
Pari 9.1 Uhl et al. 1988

Pari 9.9 Klinge 1977

Pari 8.0 Dantas and Phillipson
1989


(Wieder and Wright 1995), and deciduous forest in dry forest in Chamela, Mexico (Martinez-

Yrizar and Sarukhan 1990) and Costa Rica (Borchert 1994).

Total fine litterfall under P. caribaea slowed only during February (Figure 4. ib), and was

consistently high compared to the other plantations throughout the year. Pinus caribaea

planted in plantations produces more litterfall than similarly aged plantations of broadleaf tree

species (Table 4.9) and secondary forests in Puerto Rico (Cuevas et al. 1991, Lugo 1992).

Previous studies have found that forest leaf litter contributes approximately 60 to 75% of

total fine litterfall (Bray and Gorham 1964, Proctor 1984). Leaf litter in the forest fell within

this range at 69.5% of total fine litterfall (Figure 4. l1a). The 13.9% contribution of

reproductive parts in the forest control is a higher percentage than previously reported values








66

Table 4.9. Total fine litterfall, forest floor mass, and litterfall:standing crop quotients (KL) for
plantations at Curua-Una and other tropical sites.
Total Fine Forest Floor
Litterfall Mass
Location and Forest Type Age (t ha-' yr') (t ha-') KL Reference

Curua-Una

P. caribaea 36 10.35 11.06 0.93 this study

C. guianensis 36 8.81 10.26 0.85 this study

Leguminosae 36 10.19 8.02 1.16 this study

E. paraensis 23 8.09 7.77 1.04 this study

Puerto Rico

P. caribaea 7 7.5 9.1 0.82 Lugo 1992

P. caribaea 12 12.1 10.5 1.15 Cuevas et al.
1991

P. caribaea 21.5 14.5 18.8 0.77 Lugo 1992

P. caribaea 26 27.2 Lugo et al.
1990

Swietenia macrophylla 17 12 13.2 0.90 Lugo 1992

S. macrophylla 26 12.5 Lugo et al.
1990

S. macrophylla 49 14.1 9.76 1.44 Lugo 1992

Nigeria

P. caribaea 10 19.7 Egunjobi and
Bada 1979
Costa Rica

Stryphnodendron 5 11.7 Montagnini et
microstachyum al. 1993

Hyeronima 5 8.2
alchorneoides

Vochysia guatemalensis 5 12.6

Vochysiaferruginea 5 9.5








67

in Amazonian terrafirme forests. Reproductive parts in other lowland forests in Brazil have

contributed 2.5 % (Klinge and Rodrigues 1968) to 13 % (Scott et al. 1992) of total fine

litterfall. The year studied may have been a mast year in the forest; other lowland tropical

forests have occasional years of exceptionally high fruit production (Stocker et al. 1995).

Forest Floor Mass and Turnover Quotients

The forest floor mass in the forest (7.28 t ha&) is almost identical to Klinge's (1973)

standing crop estimate in a terra firme forest near Manaus, but much smaller than the estimate

from Uhl et al. (1988) in a terra firme forest in Paragominas, Para (Table 4.8). My estimate

for forest floor mass is also much higher than estimates for terra firme forest in Roraima

(Table 4.8). Other seasonally dry neotropical lowland forest have similar estimates of forest

floor mass, including a semi-evergreen forest in Panama (7.8 t ha-1, Wieder and Wright 1995),

and dry forests in Indian Church, Belize (7.2 t ha"-, Lambert et al. 1980) and Chamela,

Mexico (7.6 t ha1", Martinez-Yrizar 1995).

Forest floor mass for P. caribaea and C. guianensis are at the high end of world estimates

for forest floor mass in tropical and temperate forest ecosystems (Anderson and Swift 1983).

Pinus caribaea planted in plantations in Puerto Rico and Nigeria have similarly high standing

crops of litter (Table 4.9). Pinus caribaea at Curua-Una is not as productive as similarly

aged stands in Puerto Rico, possibly due to lower stand densities and slower growth at

CuruA-Una. Montagnini et al. (1993) indicated that broadleaf species planted in plantations

in Costa Rica had statistically different forest floor mass after only 4.5 years after

establishment, and total fine litterfall in these young plantations was equal to litterfall in the

36-year-old plots at Curua-Una (Table 4.9).








68

Fine litter turnover quotients (KL) in Curua-Una terra firme forest were slower than for

a forest in Roraima, possibly due to Roraima's longer wet season (Table 4.8). Fine litter

turnover quotients in the forest and legume plantations were similar to values reported from

other lowland forests in Sarawak, New Guinea, and Malaysia which ranged from 1.0 to 1.7

(Anderson and Swift 1983). Seasonally dry lowland forest in Panama has a similar fine litter

turnover quotient, ranging from 1.5 to 1.7 (Weider and Wright 1995).

Fine litter in the monospecific plots of P. caribaea, E. paraensis, and C. guianensis had

slow turnover rates which were similar to values reported for P. caribaea plantations in

Puerto Rico (Table 4.9). Turnover quotients below one are infrequent in tropical lowland

forests because fine litter decomposition is accelerated by warm temperatures and moist

conditions (Anderson and Swift 1983). These low turnover quotients indicated that

aboveground fine litter, particularly leaf and branch litter, in these plantations was relatively

recalcitrant to decomposition.

Litterfall N Inputs and Their Relationship with Soil N

Total fine litterfall N inputs and leaf litter N concentrations from the forest were similar

to those previously reported for terra firme forest in Brazil and Venezuela (Table 4.10).

These values for aboveground litterfall N inputs and leaf litter N concentrations are

intermediate compared to other tropical forests on a global scale (Vitousek and Sanford

1986). Total fine litterfall N inputs and leaf litter N concentrations under the legume

plantations at Curua-Una fell within the range of values for terra firme forest, but the other

plantations had values below that of undisturbed forest (Table 4.10).








69

Table 4.10. Total fine litterfall N contents and leaf litter N concentrations for Amazonian
terra firme forest and tropical plantations.
Total Fine
Litterfall N Leaf Litter N
Location and Forest Type (kg N ha-' yr') (mg g',) Reference

Brazilian terra firme

CuruA-Una 115 13.97 this study

Roraima 118 13 Scott et al. 1992

Roraima 85.2 12 Scott et al. 1992

Manaus 106 15 Klinge and
Rodrigues 1968

Manaus 151 18 Luizao 1989

Manaus 109 14 Luizao 1989

Para 157 17 Klinge 1977

Para 115 Dantas and
Phillipson 1986
Venezuelan terra firme

San Carlos 123.8 16.3 Cuevas and
Medina 1986
CuruA-Una plantation

P. caribaea 43.1 4.5 this study

C guianensis 91.2 10.87 this study

Leguminosae 134.4 15.65 this study

E. paraensis 59.4 8.6 this study

Puerto Rico plantation

P. caribaea (6-yr) 14 3 Lugo 1992

P. caribaea (20.5-yr) 86 4.5 Lugo 1992








70

Aboveground Litterfall and Total N in Surface Soils

Total litterfall N contents, leaf litter N contents, within-stand NUE, and foliar N

concentrations were not significantly related to total surface soil N concentrations under the

forest and the plantations at Curua-Una. Total leaf litterfall (t ha' yr-') was inversely related

to total surface soil N (R' = .90, p < .01), with the lowest values for total soil N found in

stands with higher leaf litterfall (Figure 4.2). Total soil N is composed of organic- and

inorganic-N, thus the significance of this finding is limited because it does not indicate if

variations in leaf litter inputs have impacted soil N transformation rates or N availability.

Also, belowground litter contributes greatly to total soil N, but I have not quantified these

rates. In the next chapters, I will present data that examines seasonal net N mineralization

rates, and I will attempt to determine the relationships between litterfall and N dynamics in

the forest floor and mineral soil.

6

e 5 Y = 8.64 -.724
R2=.90, p=.02
S4- E





1- I3I

5 6 7 8 9 10
Total Leaf Litterfall (t ha yr )
Figure 4.2. The relationship between leaf litterfall and total N in surface soils under the forest
(F) and plantations (P = Pinus, C = Carapa, E = Euxylophora, L = legumes) at the Curuai-
Una Reserve.








71

Conclusions

The plantations and forest represented a wide range of physical environments with basal

areas, VAIs, tree densities, and tree species diversities varying widely. The forest is similar

to other Brazilian terra firme forests with respect to litterfall N contents, the number of trees

per hectare, and foliar N concentrations. Low within-stand NUE in the forest suggests that

this lowland terra firme forest was not severely limited by nitrogen availability. The

functioning of the plantations differed in varying degrees from the forest that they replaced,

and at the time of this study these mature plantations were producing widely varying

quantities of litterfall, had wide differences in aboveground N inputs from litterfall, and also

had differences in forest floor mass and turnover.

Within-stand NUE was lowest for the legume plantations and the forest, and highest for

the P. caribaea plantation; NUE was not correlated with total N in surface soils. Other

indices of site fertility such as foliar N concentrations and total litterfall N contents were also

not related to total surface soil N among the treatments. The only significant relationship

between aboveground litter production and total soil N was derived with total leaf litterfall

(R2= .90), indicating that greater leaf litter production was related to smaller amounts of total

N in surface soils.

Relating litterfall and foliar characteristics to total soil N does not give an indication of the

rates of N turnover in these surface soils and the forest floor, or how N transformation rates

and N availability have been altered by the plantations. In addition, observations of

unconfined litter turnover has shown that aboveground litterfall was decomposing at different

rates under the forest and plantations, and that total fine litter and leaf litter N concentrations








72

were not related to turnover. Therefore, other leaf chemical components must be controlling

decomposition, and the influence of initial litter chemistry on leaf and needle decomposition

rates should be determined under the plantations and the forest control.

The differences observed among the stands for litterfall and aboveground N inputs

demonstrated that the species comprising these plantations have different genotypic and

phenotypic responses to this lowland Amazonian site. Although differences in surface soil

total N concentrations signified that the plantations altered N dynamics in this environment,

further evidence is needed to justify the claim that these plantations induced changes in soil

chemical properties. Several questions that should lead to identifying the existence of plant-

induced soil changes include the following: 1) Are there differences in fine root and microbial

biomass among the treatments, and are they related to aboveground litterfall inputs?, 2) How

have these observed differences in aboveground C and N inputs, forest floor mass, and litter

turnover rates influenced forest floor and mineral soil C and N transformation rates?, and 3)

How does initial leaf litter chemistry influence soil organic matter formation under each

treatment? These questions will guide the work presented in the following chapters.













CHAPTER 5
THE RELATIONSHIP BETWEEN FINE ROOT AND MICROBIAL BIOMASS,
ABOVEGROUND LITTERFALL, AND CO2 EVOLUTION UNDER TREE
MONOCULTURES AND TERRA FIRME FOREST IN THE BRAZILIAN AMAZON


Introduction

Plants species with different nutritional requirements, phenologies, and litter chemical

characteristics may induce changes in soil chemical properties and processes over time

(Gower and Son 1992). A primary mechanism regulating plant-induced soil changes is the

feedback effect that litter quality and quantity have on forest floor and soil nutrient

mineralization and subsequent cycling (Binkley 1994). Characteristics of plants that dominate

infertile environments (e.g., low N availability) include relatively high carbon (C) allocation

to roots, low tissue N concentrations, low litter quality (i.e., high C/N ratios), and slow litter

turnover (Chapin 1980, Tilman 1988). These plant characteristics may induce plant to soil

feedbacks because they create conditions of low soil N availability and aid in the exclusion

of plants that require higher soil N concentrations for establishment and growth (Tilman and

Wedin 1990). In contrast, high tissue N concentrations, high quality litter (i.e., low C/N

ratios), and rapid litter turnover characterize plants that dominate fertile environments

(Vitousek 1982). The rapid cycling of soil N through high quality litter and root turnover is

considered a positive feedback mechanism for species requiring high soil N availability to





73








74

maintain N-rich tissues and to support the higher net primary productivity (NPP) that is

characteristic of more fertile sites.

Plant species may also influence soil nutrient cycling by altering belowground turnover

through their impacts on microbial populations and activities. For example, plant-induced

changes in leaf area will impact temperature and moisture regimes under forest canopies,

thereby directly affecting the soil microbial biomass (Singh and Gupta 1977, Gupta and Singh

1981). Annual plant species influence microbial composition and population size by altering

root exudates and through infection specificity between microbes and plant species (Chanway

et al. 1991). Individual plants in agricultural fields have been shown to alter mycorrhizal

populations through preferential mycorrhizal infection of grasses and forbs (McGonigle and

Fitter 1990). In an experiment with four perennial grass species, Bever (1994) reported that

negative feedback effects existed between grass species and their native soil communities, and

survival of grasses in their own soil inocula was lower than for species grown in "exotic"

inocula. Van Veen et al. (1989) indicated that microbial activity was also related to how a

plant or group of plants impacted nutrient availability through root uptake, and these changes

in soil nutrient status directly influenced microbially mediated processes.

If plant species or groups of similar species influence root biomass and turnover in

conjunction with microbial biomass and microbial activity, then the influence of a plant species

on soil respiration and C storage potentially are great. The primary sources of C release from

the forest floor and soils are microbially mediated decomposition of above- and belowground

litterfall and root respiration (Raich and Nadelhoffer 1989). Soil C release attributed to plant

root respiration ranges from 35 to 90 % of total soil C release (Edwards and Harris 1977,








75

Ewel et al. 1987, Bowden et al. 1993, Nakane et al. 1995, Thierron and Laudelout 1996), and

root respiration plus heterotrophic respiration of C from dead roots generally contributes

approximately 75% to total soil respiration in forest soils (Raich and Nadelhoffer 1989).

Soil CO2 evolution rates are temperature dependent, particularly in climates that are

seasonally cold (Lloyd and Taylor 1989), and the highest soil evolution rates recorded were

from moist, lowland tropical forests that had a warm climate, high annual precipitation, and

large annual litter inputs (Raich and Schlesinger 1992). Soil CO2 evolution rates contribute

greatly to ecosystem C fluxes, and in terra firme forests in Brazil, the largest flux of

ecosystem C release comes from the forest floor and soil (Fan et al. 1990, Jarvis et al. 1996).

Experimental evidence suggests that groups of plants or plant species with similar

characteristics influence soil processes through feedback effects involving both above- and

belowground litter quality and turnover. I decided further evaluate the hypothesized plant-to-

soil feedback effects by examining differences in fine root biomass, microbial biomass, and

soil C release among plantations and native forest in a lowland tropical environment. The

primary hypothesis of this work was that the treatments that developed the smallest fine root

and microbial biomass in mineral soil would also have had the slowest annual soil C release.

The objectives were the following: 1) To determine if monocultures have altered soil C

release compared to the forest and to identify seasonal patterns of soil C release among the

treatments, 2) To determine if soil C release was related to litterfall inputs, forest floor mass,

fine root biomass, or microbial biomass, and 3) To determine if fine root C allocation in the

surface soil was related to litterfall inputs.








76

Materials and Methods

Soil Temperature. Moisture Contents. and CO, Evolution

The tree plots and soils previously described in Chapters 2, 3, and 4 were utilized for this

portion of the study except that soil temperatures and CO2 evolution rates were not estimated

under E. paraensis because of logistical constraints. Surface soil temperatures (2 cm depth)

were estimated monthly under the forest control, P. caribaea, the Leguminosae treatment,

and C. guianensis by randomly placing two dial thermometers in each replicate once a month

during the CO2 evolution experiment. Soil moisture contents were estimated gravimetrically

by collecting four randomly located samples per plot per month (0 20 cm), combining two

samples, and drying the two combined samples per plot at 105C until a constant weight.

Forest floor and soil CO2 evolution rates were estimated with the static chamber soda lime

technique (Anderson and Ingram 1993) and following the recommendations of Raich and

Nadelhoffer (1989). This technique underestimates CO2 efflux, particularly when evolution

rates are above 300 mg m'2 hr' (Cropper et al. 1985, Ewel et al. 1987, Nay et al. 1994).

However, soil CO2 emission rates determined with static chambers using a soda lime

absorbent were correlated with emission rates in dynamic chambers in Massachusetts over a

range of 150 370 mg m"2 hr"1 (Raich et al. 1990). The soda lime technique was chosen to

estimate CO2 evolution rates at CuruA-Una because it was easy to operate at the remote field

site, and allowed me to make at least a qualitative comparison of evolution rates among the

plantations and the forest.

CO2 evolution rates were estimated monthly from September 1994 to October 1995, and

each month, three buckets were randomly located in each plot of the forest, P. caribaea, C.








77

guianensis, and the legume treatment. Each bucket was 23 cm diameter at the open end and

had a chamber space of 6835 cm3. Buckets were placed 2 cm into mineral soil 72 hours

before estimating CO2 evolution rates, and seedlings or other live vegetation were removed

from the sampled area before the buckets were installed. After 72 hours passed, 40 g of 6-12

mesh soda lime were placed into aluminum cans 8 cm in diameter and 5 cm deep, and the cans

were placed under the inverted buckets for 24 hours. The soda lime was dried immediately

for 12 hours at 105 C, and CO2 evolution rates were estimated following correction for water

uptake (Anderson and Ingram 1993). Soil CO2 evolution was not estimated in October 1994.

Root and Microbial Biomass and Statistical Analysis

Root biomass (0 10 mm diameter) of the surface root mat was estimated by random

sampling during May (the height of the rainy season) and October (height of the dry season).

I sampled three randomly located points in each plot by inverting a bucket 23 cm in diameter

and cutting around the bucket edge. I then cut the roots entering the surface soil and

removed the sample from the surface organic layer. The sample was washed with deionized

water successively through a 2 mm and .425 mm soil sieve, debris was removed by hand, and

roots were hand sorted into three diameter classes (0 -2 mm, 2.1 5.0 mm, 5. 1 10 mm). All

roots (dead and living) were oven-dried to a constant weight at 600C and weighed. Nitrogen

and carbon concentrations were determined on the finest fraction (0 2 mm diameter) with

a Carlo Erba 1500 Nitrogen Analyzer after grinding the tissue.

Root biomass and nutrient contents from surface soils (0 20 cm) were also estimated

during May and October. During each of these months, I sampled six randomly chosen

locations in each plot with a soil auger. Samples were washed with deionized water through








78

a 2 mm and .425 mm soil sieve, and hand sorted into the three diameter classes. All roots (0 -

10 mm diameters) were oven-dried to a constant weight at 600C and weighed. Nitrogen and

carbon concentrations of the finest root fraction (0 2 mm diameter) were determined by

combining the 6 samples from each plot.

Microbial biomass-C and -N were estimated from surface soil samples taken during June

1995. Four randomly located soil cores (0 20 cm) from each plot were combined and one

50 g sample per replicate was kept for analysis. Soils were then refrigerated at field moisture

contents for seven days until analysis. Estimates of microbial biomass-C were made following

the ninhydrin-reactive nitrogen technique of Amato and Ladd (1988). Microbial biomass-N

was calculated by multiplying ninhydrin-reactive N released from fumigated soils by 3.1 as

recommended by Amato and Ladd (1988).

The experiment was analyzed as a completely randomized design with treatment and

month effects fixed. Each plantation and the control (forest) had three replicates, thus

replicate means (n=3) of soil moisture, soil temperature, and CO2 efflux were analyzed with

a repeated measures ANOVA over the thirteen months sampled (September 1994 to October

1995). Estimates of CO2 evolution rates were log transformed prior to analysis to

homogenize variance. Seasonal means (dry season vs. wet season) of root biomass were also

log transformed prior to analysis with a repeated measures ANOVA. Microbial biomass was

only estimated once during the year, thus this experiment was analyzed as a one-way ANOVA

with treatment effect fixed. Relationships between CO2 efflux and the other variables were

determined with PROC REG in SAS (SAS 1988).








79

Results

Surface Soil Temperatures. Moisture Contents. and CO2 Evolution

Mean monthly surface soil temperatures (2.5 cm) ranged between 25 and 28 C throughout

the year (Figure 5.1) and were a full degree higher under P. caribaea compared to C.

guianensis (Table 5.1). Repeated measures ANOVA of mean monthly surface soil

temperatures detected significant differences among the treatments (p = .0001). Mean

monthly soil moisture contents (gravimetric) peaked during the height of the rainy season in

April, May, and June, and surface soils under the forest maintained the highest moisture

contents throughout the year (Figure 5.1). Mean annual surface soil moisture contents were

lowest under E. paraensis and highest under the forest (Table 5.1). Repeated measures

ANOVA of mean monthly soil moisture contents detected significant differences among the

treatments (p = .01). Pinus caribaea maintained the highest forest floor and soil CO2

evolution rates throughout the year, while the forest consistently maintained the lowest rates

(Figure 5.2). Mean annual soil CO2 evolution rates under the forest were much lower than

under the plantations (Table 5.1). Repeated measures ANOVA of log transformed estimates

of monthly soil CO2 evolution rates detected significant differences among the treatments (p

= .003).

Root and Microbial Biomass

Estimates of mean fine root biomass (0 10 mm) from the surface root mat were lowest

under P. caribaea and highest under the forest (Table 5.2). The forest and the legume

plantations allocated the highest C and N contents to the 0 2 mm diameter class in the root

mat (Table 5.2). Repeated measures ANOVA of log transformed data detected no treatment







80

38--

36 -
34 -


S-28


Co 26-

24-
22 11111111111iiiii_ i i
SNDJ FMAMJ JASO -- Forest
Month --P- Pinus
3-A-- Carapa
--v-- Legumes

29.- Euxylophora

28 -

27 -

26 -'/-

25 --"

24 r i1 i I I I
SNDJFMAMJ JASO
Month
Figure 5.1. Mean monthly soil temperature and moisture (+ 1 sd) from September 1994 to
October 1995 at the Curui-Una Forest Reserve. Soil parameters were not estimated during
October 1994.







81



800 -- 800-
A. B.
600- 600-

c 400- 400

S200 \ 200 -

0 *T" i i i O 0 II I
SNDJFMAMJJASO SNDJFMAMJJASO
Month Month


800 800- --,
C. D.
S600 600-


o CO

g 200- 200-

0 I I I i I i I I I 0 I I I I I I I
SNDJFMAMJJASO SNDJFMAMJJASO
Month Month






Figure 5.2. Mean monthly forest floor and soil CO2 evolution rates ( se) from September
1994 to October 1995 under A. The forest control, B. P. caribaea, C. C. guianensis, and D.
the Leguminosae at the Curua-Una Forest Reserve. Soil CO2 evolution was not estimated
during October 1994.








82

Table 5.1. Average monthly soil temperatures (o C), soil moisture
(gravimetric, 2.5 cm), and CO2 evolution rates (n = 13) from mineral soil under
native forest and plantations at the CuruA-Una Reserve ( se).
Soil temperature Soil moisture CO2 efflux
Treatment (0C) (%) (mg m'2 hr1)
Forest 25.6(0.12) 31.4(0.8) 350.0 (17.5)
P. caribaea 26.5 (0.24) 29.2 (0.8) 495.9 (20.5)
C. guianensis 25.2 (0.32) 29.9 (0.6) 450.9 (17.8)
Leguminosae 26.3 (0.16) 29.3 (0.6) 430.2 (17.8)
E. paraensis 28.9 (0.6)


effect (p = .63), and no season effect (p = .59) for total root biomass (0 10 mm) in this

surface layer despite large differences among treatment means. In the surface organic layer,

there were also no treatment effects (p = .61) detected for mean biomass in the fine root class

(0 2 mm diameter) following the repeated measures analysis of log transformed data.

Estimates of mean root biomass (0 10 mm) from surface soils (0 20 cm) were lowest

under P. caribaea and highest under the legume plantations (Table 5.3). In the finest root

class (0 2 mm), E. paraensis had the largest biomass and C and N contents, while P.

caribaea had the smallest biomass and allocated the least C and N in surface soils of all the

treatments (Table 5.3). Repeated measures analysis of log transformed data did not detect

a significant treatment effect ( p = .07) for total root biomass (0 10 mm), but a significant

treatment effect was detected for the 0 2 mm root class (p = .002). In addition, no

significant season effect was detected among the treatments for the 0 -2 mm diameter class

(p = .37). Total root C allocation (0 10 mm diameter) in the surface organic layer and

surface soils was related to aboveground litter N inputs (Figure 5.3).








Table 5.2. Total root biomass (0 10 mm) and C and N contents of 0 2 mm root diameter class
from surface organic layers in native forest and plantations at the Curua-Una Reserve ( se). Estimates
are from sampling in May and October 1995 (n = 3).
Root diameter class
0-2 mm 0 2 mm 0 2 mm 2.1 5.0 mm 5.1 10.0 mm
Treatment (g m-2) (g C m"2) (g N fm2) (g m=2) (g m"2)
Forest 304.1 (98.2) 142.4 (46.1) 4.4 (1.3) 26.4 (7.0) 10.2 (7.0)
P. caribaea 115.1 (27.1) 53.5(12.6) 1.6(0.3) 18.0(10.8) 3.0(3.0)
C. guianensis 235.6 (39.5) 105.8 (17.5) 2.4 (0.4) 18.8 (11.2) 19.0(11.8)
Leguminosae 277.8 (149.4) 133.9 (72.1) 4.2 (2.1) 29.2 (16.7) 0.0 (0.0)
E. paraensis 132.1 (21.8) 59.1 (9.8) 2.1 (0.3) 27.1 (5.1) 1.3 (1.3)

Table 5.3. Total root biomass (0 10 mm) and C and N contents of the 0 2 mm root diameter
class from mineral soil (0 20cm) under native forest and plantations at the Curua-Una
Reserve ( se). Estimates are from sampling in May and October 1995 (n = 3).
0 2 mm 0 2 mm 0 2 mm 2.1 5.0 mm 5.1-10.0 mm
Treatment (g m2gm) (g C m2) (g N m2) (g m-2) (g m-2)
Forest 387.5 (86.9) 181.3 (40.6) 5.7(1.3) 181.8 (18.1) 261.5 (22.1)
P. caribaea 101.6 (26.7) 47.1 (12.3) 1.4(0.3) 82.3 (32.2) 89.4 (39.2)
C. guianensis 393.7 (24.5) 176.0 (10.1) 4.1 (0.2) 129.5 (7.2) 120.9 (23.4)
Leguminosae 455.2 (40.6) 219.1 (19.4) 7.1 (0.7) 161.8 (55.4) 290.6 (131.6)
E. paraensis 521.2 (91.9) 232.8 (41.0) 8.5(1.4) 119.1 (11.0) 69.8 (7.4)








84

7-
S6- A
U 5- I
4- E

S2 y =.214 +.043 (x)
I 1- R =.84, p =.02
0 i i i i i l
20 40 60 80 100120140160

Annual Fine Aboveground Litterfall N Inputs (kg N ha )




Figure 5.3. The relationship between root C allocation (0 10 mm diameters) and
aboveground litterfall N inputs among plantations and terra firme forest at Curua-Una.


Estimates of microbial biomass-C and -N from surface soils at Curua-Una were lowest

under E. paraensis and the forest and highest under C. guianensis (Table 5.4). An analysis

of variance of treatment means did not detect a significant treatment effect (p = .30) for

biomass-C or -N.


Table 5.4. Microbial biomass for mineral soil (0 20 cm) under
native forest and plantations (n = 3) at the Curua-Una Reserve
( se).
Treatment ug C g'- soil ug N g-' soil
Forest 227.84 (9.4) 33.63 (1.3)
P. caribaea 236.11 (14.0) 34.85 (2.0)
C. guianensis 266.58 (25.6) 39.35 (3.7)
Leguminosae 265.15 (15.7) 39.14 (2.3)
E. paraensis 226.06 (15.8) 33.37 (2.3)








85

Relationships between CO2 Evolution Rates and Soil. Root. and Microbial Parameters

Mean monthly values for surface soil temperatures and moistures were not related to mean

monthly values for CO2 evolution rates from September 1994 to October 1995 (R2 = .05 and

.03, respectively). Annual estimates of soil C release, root biomass-C (0 10 mm) from the

surface root mat and surface soils, and microbial biomass-C (from surface soils) were derived

to estimate annual C pools and fluxes from surface soils at Curua-Una (Table 5.5). Total

root biomass-C and microbial biomass-C were not related to annual soil C release (R2 = .51

and .12, p = .28 and .64, respectively). Annual soil C release was inversely related to surface

soil texture (% clay + silt) while soil C release was related to forest floor mass at Curua-Una

(Figure 5.4).


Table 5.5. Total annual C release by microbial and root respiration, total root C pools (0 10
mm diameters) in surface organic layers and mineral soil (0 20 cm), and soil microbial C
pools (0 20 cm) under native forest and plantations at the Curua-Una Reserve.
Estimates of soil and forest floor C release were based on sampling from Nov. 1994 to Oct.
1995.
C release Root C Microbial biomass-C
Treatment (t ha" yr-') (t ha'1) (t ha'1)

Forest 8.38 5.48 0.419

P. caribaea 11.82 1.26 0.434

C. guianensis 10.79 4.10 0.490

Leguminosae 10.34 5.84 0.487
E. paraensis 3.88 0.415







86

15
14 A. y = 54.86-.482 (x)
13 R2 =.89, p =.05
12-
311 -
S 10-
C/ 9-
F
8-
S7-
06-
S5 I I
85 90 95 100
clay + silt (%)
15
14 -
1 B. y = 3.70 +.724 (x)
13- R2=.81, p=.09
12 -
o(
o 10
9-

S 8-
S7-
6-
u 5 I I I i

6 7 8 9 10 11 12

Forest Floor Mass (t ha- )


Figure 5.4. Relationship between (A) soil and forest floor C release and soil texture and (B)
forest floor mass among the forest and plantations at the Curua-Una Forest Reserve.








87

Discussion

CO2 Efflux from the Forest Floor

Total forest C exchange is dominated by C release from forest soils, and in the Amazon,

80 to 85% of total forest C fluxes from terra firme forests originates from the forest floor and

soils (Fan et al. 1990, Jarvis et al. 1996). CO2 efflux measured with more precise methods

such as dynamic chambers or static open chambers and gas chromatography ranged between

0.026 and 0.063 t C ha'' d-' from soils of lowland terra firme forests in Brazil (Table 5.6).

These values exceed my estimate of average daily soil CO2 efflux from the forest at Curua-

Una (0.022 t C ha"' yr'). As previously mentioned, it is likely that the static chambers using

a soda lime absorbent underestimated CO2 efflux at Curua-Una when evolution rates

exceeded 300 mg m"2 hr during the wet season of 1995. Nevertheless, the comparisons at

Curua-Una are valid because they gave an indication of differences in CO2 efflux among the

treatments, and these estimates are also useful indices of differences in root and heterotrophic

respiration rates and the seasonality of CO2 efflux under the forest and the plantations.

In contrast to the large differences in dry and wet season CO2 efflux at Curua-Una,

Trumbore et al. (1995) found little seasonal variation in terra firme forest soil CO2 efflux in

Paragominas (700 km east of Curua-Una). These authors indicated that CO2 production

occurring at soil depths to 8 meters buffered slower surface soil respiration in the dry season.

In addition, Nepstadt et al. (1994) reported that 70 to 80% of fine root biomass in forest soils

of Paragominas was found in the surface 1 meter, and Davidson and Trumbore (1995)

determined that 70 to 80% of soil CO2 production occurred at this same depth. Trumbore








88

Table 5.6. Forest floor and soil C release from Brazilian terra firme forests and Curui-Una
plantations. Average daily rates of C release were derived from average annual estimates at
Curua-Una using a soda lime static chamber. Daily rates from the other sites were estimated
using infrared analysis of samples from dynamic chambers or gas chromatography of samples
drawn from static chambers.
C Release
Location (t ha'' d-') Time of Sample Author

Curua-Una

Forest 0.022 annual average this study

P. caribaea 0.032 annual average this study

C. guianensis 0.029 annual average this study

Leguminosae 0.028 annual average this study

Para 0.063 annual average Trumbore et al.
1995

Amazonas 0.053 April May Fan et al. 1990

Amazonas 0.042 July Wofsky et al. 1988

Amazonas 0.041 July August Goreau and de
Mello 1985
Amazonas 0.026 December Keller 1986

Amazonas 0.031 March Keller 1986


et al. (1995) also estimated that 50 to 67% of total forest soil CO2 efflux in this region was

attributable to root respiration.

In their review of forest soil C dynamics, Raich and Nadelhoffer (1989) reported that total

soil respiration attributable to belowground sources (root respiration and turnover) ranged

from 70 to 80%. Using previously reported estimates of aboveground litterfall C inputs in

their calculations, Keller et al. (1986) and Wofsky et al. (1988) determined that root

respiration and decomposition contributed between 54 to 81 % of total soil C release in








89

Brazilian terra firme forests. At Curua-Una, aboveground fine litterfall C inputs during the

one year study ranged from 4.31 to 5.07 t ha"' (Chapter 4) and soil C release ranged from 8.4

to 11.8 t ha"' (Table 5.7). If I assume that the plantations and the forest were at steady state

and leaching losses of inorganic and dissolved organic carbon were minimal, then soil C

release from belowground sources (root + heterotrophic respiration) contributed at least 43

to 60% of total soil C release at Curua-Una (Table 5.7).


Table 5.7. Forest floor and soil C release attributed to root respiration and
decomposition at the Curua-Una Forest Reserve.
Aboveground C Annual Soil C % Soil C Release
Inputs Release from Belowground
Treatment (t ha'1 yr"') (t ha-' yr-') C Sources
Forest 4.78 8.38 43
P. caribaea 5.07 11.82 57
C. guianensis 4.31 10.79 60
Leguminosae 4.99 10.34 52


If fine root respiration and decomposition (turnover) contribute the majority of C to total

soil C release, why did soils under P. caribaea consistently have higher CO2 effiux than the

forest control (Figure 5.2)? Although the forest had higher mean root biomass (0 10 mm

diameter) in the surface root mat and in surface soils, these differences were not statistically

significant. Possibly, fine root turnover and respiration were higher under P. caribaea,

resulting in larger CO2 efflux from belowground sources.

Increased C allocation to belowground biomass and elevated root:shoot ratios are

theoretically an efficient manner for plants to obtain nutrients in nutrient-poor soils (Bloom








90

et al. 1985). For tropical forests, Vitousek and Sanford (1986) reported that fertile soils (i.e.,

Alfisols) supported forest stands with smaller fine root biomass and lower root:shoot ratios

compared to stands growing on infertile soils (i.e., Oxisols, Ultisols, and Spodosols). In

contrast, Nadelhoffer et al. (1985) reported that annual belowground C allocation in nine

temperate forests did not decrease as soil N availability increased, supporting Chapin's (1980)

hypothesis that plants occupying fertile sites have elevated fine-root turnover rates compared

to plants adapted to infertile sites. Therefore, higher estimates of fine root biomass in

resource-poor soils compared to resource-rich soils do not account for biomass that has been

annually shed and decomposed in the fertile environments (Tilman 1988). In this study, P.

caribaea potentially maintained higher rates of soil CO2 production by shedding larger

quantities of fine roots throughout the year studied, and by concurrently maintaining rapid

belowground turnover.

Raich and Nadelhoffer (1989) reported that total C allocation to roots increased with

higher quantities of litterfall; at Curua-Una, aboveground fine litter inputs and forest floor

mass were largest under P. caribaea (Table 4.2, 5.7). Although soil 0 horizons have lower

CO2 concentrations than surface soils because they are porous (Fernandez et al. 1993), the

deep, recalcitrant litter layer under the pine may have supported larger microbial biomass,

which resulted in continually high CO2 efflux as the microbes attempted to mineralize the litter

substrate. In support of this theory, CO2 release was related to forest floor mass at Curua-

Una (Figure 5.4).

The temperature and moisture contents of the forest floor and surface soils were also

potential factors in the difference in CO2 evolution rates between the forest and the pine








91

monoculture. Temperature and moisture are the two of the most important factors

controlling fluxes of C from litter and soil organic matter in forest ecosystems (Zak et al.

1993), and average annual temperatures of the surface soils under the pine were almost one

degree higher those under the forest control (Table 5.1). Although surface soil moisture

contents were higher under the forest, perhaps the increased temperatures under the pines

caused a corresponding increase in microbial activity in the forest floor and surface soils.

Finally, the static chamber technique using a soda lime absorbent was inadequate for

measurement of CO2 effluxes from termite mounds, stump mounds, ant nests, or large

decomposed logs because the inverted bucket could not be properly sealed in these locations.

These potentially are areas of high CO2 production (Zimmerman et al. 1982), and since the

forest had a higher number of these sites, I would have missed these fluxes and

underestimated C release under the forest compared to the plantations.

Surface soil particle size also influenced rates of soil CO2 evolution at CuruA-Una (Figure

5.3). Fine earth fractions are generally believed to protect soil organic matter from microbial

mineralization, particularly in Oxisols containing iron and aluminum oxides, which bind with

the carboxyl groups of soil organic matter in ligand exchange reactions (Oades et al. 1989).

In support of this theory, Motavalli et al. (1994) reported that clay contents were negatively

correlated with CO2-C release from Amazonian forest soils, and Lepsch et al. (1994) found

that soil organic C was positively related to clay + silt fractions in southern Brazil. Sorenson

(1981) and Van Veen et al. (1985) also indicated that soil organic matter and microbial

biomass-C were more stable in soils with high clay contents because they formed stable

complexes with the clay particles.








92

Microbial Biomass

Early reports of low soil microbial biomass-C in tropical soils were attributed to rapid

turnover and high predation (Theng et al. 1989). More recent estimates of biomass-C and

-N from soils in Costa Rica and Brazil contradict these earlier reports (Table 5.8). In fact,

microbial biomass in some tropical soils exceeds that of temperate forest soils which have

biomass-C values ranging between 289 and 1900 ug g 1 soil (Vance et al. 1987a, b).


Table 5.8. Microbial biomass-C from surface soils in lowland tropical forests and plantations
at CuruA-Una.
Location Soil order Depth (cm) ug C g-' soil ug N g-' soil Author

Curua-Una Oxisol 0 20 227 266 33 39 this study
La Selva, Inceptisol 0 15 700 2000 Henrot and
Costa Rica Robertson 1994
Amazonas, Oxisol 0 12 357 699 37 73 Motavalli et al.
Brazil 1994, 1995
Oxisol 0 5 1287 Luizao et al.
1992
6 20 765 Luizao et al.
1992
India Ultisol 0- 10 405 677 41 71 Srivastava 1992

Ultisol 0 10 487 744 51 -88 Singh et al. 1989


Surface soils under all the treatments at Curua-Una supported lower microbial biomass-C

than other tropical sites (Table 5.8). I attribute the low values at Curua-Una to the depth and

season of the surface soil sampling. Soil microbial biomass decreases with depth, and I may

have diluted the biomass-C pool by analyzing such a large portion (0 20 cm) of the surface

soils. Microbial biomass-C at Curud-Una during June was 0.4 to 0.5 % of total soil C (50

to 60 g kg-' soil), and this is a lower percentage of total surface soil C than soils near Manaus




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PLANT-INDUCED CHANGES OF SOIL CARBON AND NITROGEN DYNAMICS IN LOWLAND AMAZONIA, BRAZIL By CHARLES KENNETH SMITH A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1996

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ACKNOWLEDGMENTS I gratefully thank my major advisor, Dr. Henry Gholz, for his helpful guidance, editing, and efforts in procuring financial support for this study. I would also like to acknowledge the contributions of the other members of my supervisory committee, Drs. P.K. Nair, John J. Ewel, Nick Comerford, and Willie Harris, for their constructive criticism and advice during the development of my proposal and the writing of this dissertation. Drs. Nick Comerford and Willie Harris also provided laboratory space for some of the soil analyses presented here. In addition to my committee members, Dr. Ken Clark gave me useful insights on conducting field research in a remote location, and his assistance with my laboratory analyses was deeply appreciated. Financial support for this study was received through a competitive grant from the USDA National Research Initiative Program (Forest, Crop, and Rangeland Program), the USD A International Institute of Tropical Forestry (Puerto Rico), the Tropical Conservation and Development Program (University of Florida), and a fellowship from the National Security Exchange Program. The School of Forest Resources and Conservation also provided a research assistantship through Dr. Henry Gholz. Special thanks are extended to Professor Francisco de Assis Oliveira, Ms. Anadilza Baima, Ms. Dagma Costa, and Ms. Nailza Pedroso de Souza of the Faculdade de Ciencias Agrarias do Para for their assistance in Belem and at the Curua-Una Forest Reserve in Brazil. ii

PAGE 3

The Faculdade de Ciencias Agrarias do Para also provided valuable logistical support with customs and housing in Brazil. In addition, the Centro de Tecnologia Madeireira of the Superintendencia do Desenvolvimento da Amazonia provided housing and logistical support in Santarem and at the Curua-Una Forest Reserve. In particular, Dona Fatima Meckdece, Mr. Sebastiao Castro de Almeida, Mr. Antonio de Souza Pereira, and Mr. Jose Nildo Morais da Rocha were invaluable to the success of this study. Also, thanks to the people of Barreirinha who invited me into their homes and made living in a remote site much more enjoyable. In conclusion, I have to give another big thank you to my wife, Deborah McGrath, for her endless patience, support, good cooking, and assistance with my fieldwork. I would also like to acknowledge my remaining grandparents, Mrs. Betty Sullivant and Mrs. Jackie Smith, who have always supported my efforts despite all my travels and travails. Finally, I thank my parents, Charles and Alice Smith, for all those camping, fishing, and skiing trips that inspired my choice of a career. iii

PAGE 4

TABLE OF CONTENTS pa g e ACKNOWLEDGMENTS ii ABSTRACT vii CHAPTERS 1 INTRODUCTION 1 Literature Review 3 Conclusions and the Primary Objectives and Hypotheses of this Study 18 2 STUDY SITE DESCRIPTION AND PLANTATION SELECTION 21 Introduction 21 Climate and Forest Composition 23 Soils 26 Tree Plot Establishment 27 Plantation Selection 29 Summary 32 3 SOIL HOMOGENEITY 33 Introduction 33 Materials and Methods 35 Results and Discussion 36 Conclusions 46 4 LITTERFALL AND NITROGEN-USE EFFICIENCY OF PLANTATIONS AND PRIMARY FOREST IN THE BRAZILIAN AMAZON 48 Introduction 48 Materials and Methods 50 iv

PAGE 5

Results 53 Discussion 61 Conclusions 71 5 THE RELATIONSHIP BETWEEN FINE ROOT AND MICROBIAL BIOMASS, ABOVEGROUND LITTERFALL, AND C0 2 EVOLUTION UNDER TREE MONOCULTURES AND TERRA FIRME FOREST IN THE BRAZILIAN AMAZON 73 Introduction 73 Materials and Methods 76 Results 79 Discussion 87 Conclusions 95 6 FINE LITTER CHEMISTRY, DECAY, AND NITROGEN DYNAMICS UNDER PLANTATIONS AND PRIMARY FOREST IN LOWLAND AMAZONIA 97 Introduction 97 Materials and Methods 99 Results 104 Discussion 115 Conclusions 125 7 SOIL NITROGEN DYNAMICS AND PLANT TO SOIL FEEDBACK MECHANISMS UNDER MONOCULTURES AND PRIMARY FOREST IN LOWLAND AMAZONIA 126 Introduction 126 Materials and Methods 128 Results 133 Discussion 142 Conclusions 161 8 A FRAMEWORK OF PLANT-INDUCED CHANGES OF SOIL CARBON AND NITROGEN DYNAMICS IN A LOWLAND TROPICAL ENVIRONMENT AND CONCLUSIONS 162 Plant to Soil Feedback Mechanisms A Framework 162 Conclusions 169 v

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LIST OF REFERENCES 171 BIOGRAPHICAL SKETCH 198 vi

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PLANT-INDUCED CHANGES OF SOIL CARBON AND NITROGEN DYNAMICS IN LOWLAND AMAZONIA, BRAZIL By CHARLES KENNETH SMITH December 1996 Chairperson: Dr. Henry L. Gholz Major Department: School of Forest Resources and Conservation The principal objective that guided this study was to determine if plantations consisting of tree species with contrasting phenologies, resource requirements, and biochemical characteristics had significantly changed carbon (C) and nitrogen (N) dynamics in a lowland tropical ecosystem when planted on an initially uniform soil. The replicated, monospecific plots consisted of Pinus caribaea var. hondurensis, Euxylophora paracusis Huber, Carapa guianensis Aubl., and a Leguminosae combination (Dalbergia nigra Allemao ex Benth., Parkia multijuga Bent., Dinizia excelsa Ducke). The experimental plots were established in the Curua-Una Forest Reserve in the east-central Amazon Basin, and replicated plots in adjacent, undisturbed native forest were used as a control. Soils under the experimental units were determined to be homogeneous based on mineralogical, textural, and vii

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chemical analyses of surface and subsurface soils. During the year examined (September 1994 to October 1995), the plantations and the native forest control produced widely varying quantities of aboveground litterfall (8 to 10.3 t ha' 1 yr" 1 ), had wide differences in aboveground N inputs from litterfall (43 to 1 15 kg ha" 1 yr" 1 ), and also had significant differences in forest floor mass (8 to 1 1 1 ha' 1 ) and turnover (0.78 to 1 .2 years). In addition, annual litter N inputs were related to root mat and mineral soil fine root C allocation (R 2 = .84). Leaf litter decay differed significantly among the treatments, but no single chemical constituent was found to be a good predictor of leaf litter decay despite large differences in initial leaf litter chemistries. There were large differences among the treatments for annual net N mineralization rates (195 to 328 kg ha' 1 yr' 1 ), and annual net N mineralization rates were related to N allocation to fine roots (R 2 = .96) and total fine litterfall N inputs (R 2 = .81). These results suggest that at Curua-Una, plant-to-soil feedbacks existed because the treatments with the highest N concentrations in foliage, litterfall, and N concentrations in fine roots supported the highest rates of soil N transformations. viii

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CHAPTER 1 INTRODUCTION Vegetation is a primary factor in soil genesis (Jenny 1980) and tree species have been shown to alter soil chemical and physical properties in several temperate forest ecosystems (e.g., Mergen and Malcolm 1955, Challinor 1968, Alban 1969). A trend frequently reported in earlier work was that conifers acidified surface soils and reduced soil fertility compared to hardwood species (Handley 1954). Stone (1975) dismissed the results of many studies which found deleterious effects of trees on soil properties because they lacked plot replication and had confounding effects of land-use history (e.g., intensive agriculture). In forest ecosystems, the largest pools of ecosystem carbon (C) and nitrogen (N) are found in the forest floor and mineral soil (Cole and Rapp 1981, Edwards and Grubb 1982, Jordan 1985), and changes in aboveand belowground quantity and quality of plant litter should alter rates of litter and soil C and N transformations. Work conducted in replicated experimental units has demonstrated that groups of plants with different ecological characteristics, such as leaf and root litter quantity and quality, can influence soil C and N dynamics (Binkley 1994). Feedback loops between soil and vegetation in a system may develop if plants control soil N supply rates by varying the quantity and quality of aboveand belowground litter inputs (Wedin and Tilman 1990). 1

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2 Plant-induced soil changes are important to understand because of their effects on ecosystem nutrient dynamics and competition between plant species, and the implications for vegetation management. For example, if a successional plant community or plant species occupying a site affects soil N availability, then this plant-driven influence on N supply could control competition and succession (Ewel 1986, Vitousek and Walker 1989). High withinstand nutrient-use efficiency is characterized by high dry mass: nutrient ratios in plant litter (Vitousek 1982) and often results in efficient within-stand cycling of nutrients that limit plant production (Vitousek et al. 1982). Plant communities growing on infertile sites generally have higher nutrient-use efficiencies than those found on fertile sites (Chapin 1980, Vitousek 1982), and many lowland tropical forests circulate high quantities of N and have low withinstand nitrogen-use efficiency or NUE (Vitousek 1984, Vitousek and Sanford 1986). If soil nutrient mineralization rates are controlled by litter decomposition rates (McClaugherty et al. 1985), litter quality (Gower and Son 1992), or any other factor directly linked to the specific character of the vegetation occupying a site, then plant-soil-plant feedback loops may stabilize forest species composition over time (Pastor et al. 1984). As Binkley (1994) summarized in a review of plant induced soil changes, questions related to soil degradation or improvement are not useful unless they address the specific mechanisms behind observed changes in soil processes or properties. Thus, the objectives of this introduction are to 1) review previous work about plant-induced soil changes, 2) briefly discuss the mechanisms behind these changes, and 3) outline the objectives and principal hypotheses of this study.

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3 Literature Review Plant Effects on Soil Moisture, Temperature, and Structure Plants can alter soil moisture and temperature through differences in canopy leaf area that attenuates incoming shortwave and outgoing longwave radiation (Binkley 1994). For example, soil temperatures under deciduous trees in seasonally dry tropical forests might be expected to rise during the dry season, which generally is the time of heaviest litterfall (Murphy and Lugo 1995), although this temperature rise could also result from an increased number of warm and cloudless days. On the other hand, the buildup of litter layers under individual species insulates the mineral soil from temperature and moisture fluctuations (Lai et al. 1980), and these off-setting factors may explain the ambiguity of past results. For example, in a study conducted on afforested sand dunes in California, Amundson and Tremback (1989) found no differences in mineral soil temperatures among stands of Eucalyptus globulus Labill., Pinus radiata D. Don, and Quercus agrifola Nee. despite large differences in O horizon thicknesses under each stand. In contrast, Qashu and Zinke (1964) reported lower mean annual soil temperatures at a 60 cm depth under 13 -year-old monospecific stands of scrub oak {Quercus dumosa Nutt.) compared to Coulter pine (Pinus coulterii B. Don) in southern California. Tree species affect soil moisture primarily through canopy rainfall interception and transpiration which are influenced by the specific conductance of leaves, leaf area, and phenology (Kaufrnann 1985). In general, evergreen tree species that maintain high leaf areas throughout the year transpire more water annually per unit land area than species that remain leafless for extended periods. In modeling canopy water interception and transpiration in

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4 monospecific stands with identical basal areas, Kaufmann (1985) determined that the annual transpiration of aspen forests, {Populus tremuloides Michx ), which is deciduous, was 50% of that from forests of lodgepole pine (Pinus contorta Dougl. Ex Loud), which is evergreen, thus resulting in higher soil moisture under the aspen. Gower and Son (1992) reported significantly higher mineral soil moisture content under red oak (Quercus rubra L.) compared to four conifer species in a replicated common garden experiment in Wisconsin. In addition, Nihlgard (1971) found soil water content was higher under beech (Fagus silvatica L.) compared to adjacent stands of Norway spruce {Picea abies (L.) Karst.). Plant-induced changes in soil structure may result from differences in plant rooting densities and depths combined with the effects of plant litter quantity and quality on the size and activity of soil macrofaunal communities. Soil fauna influence soil bulk densities and particle size through their tunneling and nest building activities, and they frequently use root channels as conduits through the soil profile (Lavelle et al. 1992). In Florida, crayfish tunneling and their burrows under pine plantations were found to influence surface lateral water flow and soil mixing (Stone 1993). Plant litter chemical composition, especially nitrogen and lignin contents, is also known to influence soil faunal activity (Tian et al. 1993). In one of the few studies to examine how tree species influence soil structure indirectly through effects on soil faunal communities, Graham and Wood (1991) indicated that soils under 41 -year-old Coulter pine lacked earthworms while the A horizons under similarly aged scrub oak and ceanothus {Ceanothus crassifolia Ion.) were composed of earthworm casts and contained higher clay contents than soils under pine. These authors also stated that

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5 earthworms were moving clay-sized particles to the surface soils, while earthworm depauperate soils under pine were experiencing illuvial accumulation of clay in the B horizon. Plant Effects on Soil Faunal and Microbial Communities Early research comparing soil communities in "mor" soils (consisting of an acidic litter layer with little mixing of forest floor and mineral soil) and "mull" soils (less acid forest floor well mixed into A horizon) led Rommell (1935) to conclude that mor soils were dominated by acid-producing fungi and mull soils by "neutrophilous" bacteria. These microbial communities and their by-products were thought to regulate forest floor characteristics (e.g., pH), thus mor and mull forming tree species could directly influence microbial populations. Broadfoot and Pierre (1939) were among the first to relate variations in leaf chemistry to decomposition rates and differences in microbial populations colonizing the forest floor under different tree species. Considering more current work, Theodorou (1984) found that the surface soils under Pinus radiata had higher fungal but significantly lower total microbial biomass-N than adjacent eucalypt woodlands. Keinzler (1986) sampled soils under the same stands studied by Perala and Alban (1982) in Minnesota and determined that surface soils under aspen had more invertebrates, ten times more bacteria, and twice as much fungal growth than surface soils under pine and spruce. Under similar climatic regimes, plant litter quantity and quality are also thought to regulate soil macrofaunal activity (Cuendet 1984, Tian et al. 1993). Gast (1937) reviewed previous research from the Harvard Forest and the observed preferences of earthworms for aspen, white ash {Fraxinus americana L.), and basswood {Tilia glabra Vent.) leaves over red oak

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6 leaves. In particular, Gast (1937) commented on the importance of the mixing of organic material and mineral soil by earthworms, particularly with leaves from tree species with higher mineral nutrient contents. This division of organic material (comminution) and mixing with mineral soil and stimulates soil microbial activity by increasing the surface area of leaf particles (Swift et al. 1979, Luxton 1982). Thus, tree species with more palatable leaf litter can increase soil macrofaunal populations, which influence microbial activity and leaf decomposition. In support of this theory, Zou (1993) found that Paraserianthes falcataria (L.) Nielson plantations in Hawaii (where earthworms are not indigenous) had significantly higher earthworm densities than adjacent Eucalyptus saligna Sm. plantations, and he attributed this finding to the higher litter quality of Paraserianthes. Also in Hawaii, Aplet (1990) determined that earthworm biomass was increased by the presence of high N litter under the colonizing, N 2 -fixing shrub Myrica faya. Plant Effects on Soil pH Much of the early research on plant-induced soil changes focused on the forest floor and mineral soil pH, with the prevalent view that conifers both acidified and degraded soils compared to hardwoods (Ovington 1953). In a review of early European work, Miles (1985) observed that the establishment of conifers and ericaceous shrubs led to the formation of mor soils while hardwoods developed mull soil. Handley (1954) summarized the thinking on plant-induced soil changes during the first half of the century by stating that when climate, geology, and topography were held constant, a change in vegetation will bring a change in soil properties, with conifers generally causing a decline in soil pH.

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In contrast to Ovington (1953) and Handley's (1954) earlier work, Rennie (1955) expressed the view that there was no clear evidence to indicate that conifers caused rapid changes in soil pH over short time periods. Later, Stone (1975) pointed out that the spatial heterogeneity of soil properties and inadequate sampling in non-replicated plots led some researchers to wrongfully attribute short-term changes in soil chemical properties to tree influences. In support of Stone's argument, Tamm and Hallbacken (1986) sampled for soil pH under Norway spruce (Picea abies (L.) Karst ), beech (Fagus silvatica), and oak (Quercus sessiliflora Salisb.) stands in Sweden in 1927 and 1982, and they found no differences among species at either time period, although all the stands experienced a decline in pH overtime. In addition, Erickson and Rosen (1994) found no significant differences in surface soil pH among replicated blocks of four conifers after 36 years. More recent work has documented changes in soil pH attributable to plant influences, but the generalization that conifers always acidify surface soils has not withstood the test of time. For example, Alban (1982) determined that four tree species altered the top 30 cm of a soil profile in Minnesota, and a hardwood species (aspen), accumulated more exchangeable acidity compared to two pine species. Binkley and Richter (1987) attributed Alban's finding to aspen's higher base cation uptake and concurrent release of FT from roots in the upper mineral soil. In addition, France et al. (1989) found no significant differences in pH or exchangeable acidity among two conifers and two hardwoods in the top 1 5 cm of soils beneath replicated, 27-year-old plantations in Ontario, Canada. In North Carolina, Richter et al. (1994) reported an increase in KCl-exchangeable acidity over 28 years in the surface soils of a loblolly pine {Pinus taeda L.) forest. Finally, Binkley and Valentine (1991) found surface soil pH under

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8 replicated, 50-year-old plots of green ash (Fraxinus pennsylvanica Marsh.) were higher than eastern white pine (Pinus strobus L.) and Norway spruce. In the Pacific Northwest of the United States, there are well established effects of N-fixing tree species such as red alder (Alnus rubra Bong.) on soil pH. These derive from NH + 4 mineralized from N-enriched litter and the subsequent leaching from surface soils of NO" 3 with a base cation, which leaves behind H + to displace cations on soil exchange sites (Van Miegroet and Cole 1988). Nitrification processes in soils under red alder in Washington represented an internal H + source ten times greater than input by precipitation (Stuanes et al. 1992). In addition, decreases in soil pH under red alder are attributed to the greater acid strength of soil organic matter found under alder stands (Binkley and Sollins 1990). In the tropics, Sanchez et al. (1985) indicated that plantations of Gmelina arborea Roxb. and Pinus caribaea had surface pH that varied as much as 1 unit in Jari, Brazil. In Costa Rica, surface soil pH declined under three types of successional plant communities and a monoculture 5 years after clearing and burning, and soil pH values under all treatments were observed returning close to pre-burn levels (Ewel et al. 199 1). In the dry tropics, Mailly and Margolis (1992) found a 0.8 unit decline in soil pH (7.3 to 6.5) under 34-year-old Causuarina equisetifolia Forst.& Forst. plantations in Senegal. Individual trees within a stand have also been shown to affect soil pH in the temperate zone. Riha et al. (1986) indicated that soil pH variability between individual trees in monospecific stands of red pine {Pinus resinosa Ait ), Norway spruce, and sugar maple (Acer saccharum Marsh.) was greater than that encountered between pure stands of the three species. Boettcher and Kalisz (1990) found lower levels of surface soil pH under crowns of

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Liriodendron tulipiferaL. compared to soils under Tsuga canadensis (L). Carr. in Kentucky. In contrast, Zinke and Crocker (1962) sampled soils under a giant sequoia (Sequoiadendron giganteum (Lindl.) Buchholz) estimated to be 1500to 3000-years-old and found pH did not vary with distance from the stem. In drier habitats that do not support continuous tree cover, individual perennial plants such as bluebunch wheatgrass (Pseudoroegneria spicata (pursh) A Love) and sagebrush {Artemisia tridentata (Rydb.) Beetle) greatly influence soil pH, which was shown to vary by as much as 1.3 units in 0.5 meters (Jackson and Caldwell 1993). In addition, Charley and West (1975) reported a species influence in surface soil pH under Artemisia tridentata and Atriplex confertifolia (Torr.& Frem.) in the Great Basin, and isolated trees in tropical savannas in West Africa have been shown to increase soil pH under their canopies (Mordelet etal. 1993). The possible mechanisms of plant-induced changes in soil pH have been summarized by Binkley et al. (1989) and Binkley and Richter (1987). These include the following: 1 Tree canopy effects on forest floor depositional rates of acidic atmospheric chemicals. 2. Humified organic matter from plant species increase the quantity of weak acids stored in the soil, and these weak acids increase FT concentrations in the soil solution. 3 Increasing the acid saturation of soil exchange complex or decreasing base saturation allows more FT to move into solution. 4. Contributing soil humus that is strongly acidic that will donate more FT to the soil solution.

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10 5. Biological nitrogen fixation which leads to increased N in litterfall, increased nitrification, and increased nitrate leaching with a base cation. 6. Plant uptake of cations with a concurrent release of H + to the soil solution. Plant Effects on Soil Cations Tree species can significantly alter levels of extractable base cations in surface soils, for example, Binkley and Valentine (1991) observed higher concentrations of K + Ca 2+ Mg 2+ at a 0-15 cm depth under 50-year-old, replicated plots of green ash (Fraxinus pennsylvanica Marsh) and white pine (Pinus strobus L.) compared to Norway spruce. Son and Gower (1992) also reported a wide range of extractable cations among tree species in surface soils of 28-year-old plots. In contrast, France et al. (1989) found no significant differences for extractable cations among four tree species (two hardwoods and two conifers) in 27-year-old replicated plots in Canada. At Jari, Brazil, Russell (1983) found 8.5-year-old plantations of Gmelina arborea increased Ca 2+ concentrations in surface soils to a level similar to adjacent native forest. In addition, Pinus caribaea Mor. lowered base concentrations 9.5 years after establishment relative to Gmelina and native forest. Belsky et al. (1989) sampled soils under isolated trees in tropical savanna and detected higher total K + and Ca 2+ under trees compared to adjacent grasslands. These authors stated that higher concentrations under tree canopies resulted from nutrient uptake from surrounding soils and deposition of nutrients in litter and in dung by animals that rest and feed near the tree.

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11 Plant Effects on Soil Phosphorus Over time, tree species have been clearly shown to affect various phosphorus (P) pools. Polglase et al. (1992) demonstrated an age affect with labile inorganic P under Eucalyptus regnans F. Muell. which declined from 34 ug g" 1 soil at stand establishment to 2.3 wg g" 1 soil at 16 years. Gholz et al. (1985) observed a decrease in double-acid-extractable P over time in replicated stands of Pinus elliottii Engelm.(2 to 35 years old), with the most rapid change occurring the first 18 years (to a 1 meter depth). In replicated plantations in Hawaii, Zou et al. (1995) found labile organic P concentrations and soil acid phosphatase activity were higher in Parasericmthes falcataria (L.) Nielson plantations compared to Eucalyptus saligna Sm. In paired plantations and secondary forests in Puerto Rico, Lugo (1992) reported higher total P concentrations (1 meter depth) under Pinus caribaea compared to secondary forest. In other paired stands, secondary forest had higher total P than two mahogany (Swietenia macrophylla Jacq.) stands. In Brazil, Russell (1983) observed higher total P contents (kg ha" 1 ) under Gmelina plantations (8.5 years old) compared to Pinus caribaea (9.5 years old) or native forest. Isichei and Muoghalu (1992) determined that available phosphorus (Bray extraction) was higher under tree canopies than grassland in northern Nigerian savannas. Tergas and Popenoe (1971) found that pure stands of Heliconia sp. and Gynerium sp. accumulated four times more extractable P in surface soils as mixed fallows of the same age in P deficient Inceptisols of Guatemala. Mechanisms behind plant-induced changes in soil phosphorus are not only linked to subsoil mining of plant available P and the return of this P through litter (Comerford et al. 1984), but

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12 also can be attributed to plant associations with fungi and bacteria (Binkley and Richter 1987). Plant roots, bacteria, and fungi can produce many low-molecular weight organic acids (Fox and Comerford 1990) including phosphatases which aid in the mineralization of organic P (Fox and Comerford 1992), and plant species have been shown to influence soil acid phosphatase activity (Zou et al. 1995). Plant Effects on Carbon and Nitrogen Dynamics Plant-induced changes in soil C and N dynamics have recently received much attention, with studies conducted in homogeneous forest patches or natural stands, semi-arid grasslands, and in common garden experiments. It is convenient to study C and N dynamics jointly because they are covalently bonded in organic matter (McGill and Cole 1981), and thus they are linked together in soil organic matter dynamics. There are also no mineral sources of C and N from forest soils. Much of the interest in plant-induced changes in C and N dynamics arose as a result of the concept of feedback loops between plant and soil because plants occupying infertile sites were observed to return less N to soil in litterfall while species on fertile sites shed litter with higher N concentrations, thus maintaining higher soil N concentrations (Hobbie 1992). In addition, work conducted in semi-arid grasslands and savannas identified plant-induced "islands" of high nutrient concentrations under isolated trees and shrubs (Charley and West 1975, Tiedemann and Klemmedson 1973). Nevertheless, Gower and Son (1992) and Binkley (1994) signaled that many previous studies confused site effects with species effects, and they recommended that the best way to identify plant influences on soil properties was to establish a common garden consisting of plants with different characteristics on a homogeneous soil.

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13 The Return of N in Litterfall and NUE If positive N feedback loops exist in nature, plant species with high N concentrations in litter (low nitrogen-use efficiency or NUE) would influence site fertility by increasing the soil N supply (Vitousek 1982). Theoretically, this feedback loop reinforces the dominance of species that are well adapted to productive sites (Tilman 1988). Traits typical of such species include higher tissue N concentrations and high N allocation to aboveground plant material (Vitousek 1982). Another type of feedback loop would include vegetation that maintained low N concentrations in litter (high NUE) combined with high root biomass to outcompete potential site successors for limited N supply (Wedin and Tilman 1990). Experimental evidence demonstrating the correlation between site quality and litter NUE include Birk and Vitousek' s (1986) study of loblolly stands growing on sites with varying N availabilities. These authors determined that within-stand NUE decreased in sites with high N availability, and that changes in NUE were attributed to a decrease in nutrient absorbing efficiency in low N sites. In contrast, Gower and Son (1992) found that Norway spruce, a species frequently claimed to be a "site degrader" in Europe, had a lower NUE than red oak and white pine, two species that are typically found on moderately fertile and fertile sites (Harlow et al. 1979). The quantity and quality of root litter and mycorrhizal turnover directly affects soil N supply rates, yet belowground litter dynamics are poorly understood and have been estimated in few ecosystems (Vogt et al. 1986). Most studies of fine root production and turnover have been conducted in litter layers and surface soils (Attiwill and Adams 1993), and few have related root dynamics to nutrient availability (Nadelhoffer et al. 1985). In the tropics, fine

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14 roots in a lowland Amazonian forest at San Carlos, Venezuela had 25% turnover per month in surface soils, and fine root N inputs in this terra firme forest were estimated to be approximately six times higher than aboveground fine litter N inputs (Vitousek and Sanford 1986). Forest Floor Mass and Turnover The early studies of plant-induced soil changes focused on the pH and mass of the forest floor, the two factors that were believed to control the formation of mull and mor soil types (Handley 1954). In contrast to the early dogma that forest floor mass under conifers always exceeded that under hardwoods, Perala and Alban (1982) found no differences in forest floor mass between 40-year-old hardwood and conifer stands in Minnesota. Gower and Son (1992) did find that total forest floor biomass under red oak was much lower (8.7 t ha" 1 ) than forest floor mass under four conifers (24 43 t ha" 1 ) in 28-year-old plots in Wisconsin. In the tropics, some pine plantations have been shown to develop high forest floor mass compared to other species planted in monocultures or adjacent natural forest. For example, Lugo et al. (1990) estimated forest floor litter mass to be 27 t ha" 1 under a 26-year-old Pinus caribaea plantation in Puerto Rico, and total forest floor N contents (kg ha" 1 ) under Pinus caribaea were higher than total forest floor N contents under nine other monocultures. In addition, Cuevas et al. (1991) reported significantly higher standing stocks of total litter under 1 1-year-old Pinus caribaea (10.5 t ha" 1 ) compared to adjacent secondary forest (5.0 t ha" 1 ). The turnover rate of forest floor mass has been estimated for forests around the world by first assuming steady state conditions and then by dividing annual aboveground fine litterfall by forest floor mass. This turnover ratio (KJ for lowland tropical forests is generally greater

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15 than one because the decomposition of forest floor litter is faster than inputs from aboveground litterfall. Lowland tropical forests have K L values ranging from 1.0 to 3.3 (Anderson and Swift 1983), and one lowland Amazon forest was reported on the lower end of this range at 1.1 (Klinge 1973). Plant Effects on C Release and Microbial Biomass Annual plant species can affect microbial composition and population size (Chanway et al. 1991), and individual plants in agricultural fields have been shown to alter mycorrhizal populations (McGonigle and Fitter 1990). In an experiment with four perennial grass species, Bever (1994) reported that negative feedback effects existed between grass species and their native soil communities. The growth and survival of grasses in their own soil inocula were lower than for species grown in "exotic" inocula. Thus, the potential long-term influences of a plant species on microbial biomass and their activity are great. Plant-induced changes in leaf area will impact temperature and moisture regimes under forest canopies which directly affect soil microbial populations and their activities (Singh and Gupta 1977, Gupta and Singh 1981). Also, plant-induced changes in release of soil C and in microbial biomass may result from differences in fine-root biomass and turnover, and variations in aboveand belowground litter quantity and quality. Soil respiration rates are known to vary widely across natural forests and plantation types (Raich and NadelhofFer 1989, Raich and Schlesinger 1992), and soil C release attributed to plant root respiration ranges from 35 to 90 % of total soil C release (Edwards and Harris 1977, Ewel et al. 1987, Bowden et al. 1993, Nakane et al. 1995, Thierron and Laudelout 1996).

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16 Changes in plant life forms or plant species that affect soil C availability may also influence soil N cycling. Soil carbon availability has been found to be an important control on N cycling rates in soil, and C release measurements may be an index of soil N cycling (Hart et al. 1994). In mineral soil, no relation between microbial respiration and net N mineralization has been reported (Johnson and Edwards 1979, Johnson et al. 1980), but gross N mineralization and immobilization are known to be correlated with microbial respiration in incubated samples (Schimel 1986, Hart et al. 1994). Plant Effects on Litter Quantity, Quality, and Decomposition Plant species have evolved with a wide range of leaf chemistries, and rates of decomposition and mineralization of nutrients from plant litter have been shown to be affected by initial N, lignin, holocellulose, and polyphenol concentrations. In British Columbia, soil organic matter (SOM) formation is affected by different plant associations (deMontigny et al. 1993), and the availability of forest floor N is influenced by tree species (Prescott et al. 1993). Simulations of interactions between plant tissue chemistry, herbivores, and litter decomposition have also shown that plant tissue chemistry influences ecosystem C and N dynamics (Pastor and Naiman 1992). Leaf litter lignin/N (L/N) ratios of several temperate zone species have been shown to be good predictors of decay rates (Melillo et al. 1982), and initial leaf chemistries can be used to predict the length of time required to convert fresh litter into soil organic matter (Aber et al. 1990). In contrast, Edmonds (1980) found that litter C/N ratios were better predictors of leaf decay rates than initial lignin concentrations. In microcosm tests, Taylor et al. (1989) found C/N ratios were better predictors of mass loss than L/N ratios for leaf material

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17 containing low initial lignin concentrations. Despite wide differences in initial carbon fractions and C/N ratios, long-term decomposition studies have found that the chemical constituents of leaf litter converge to similar values over time (Berg et al. 1984, Aber et al. 1990). Other C fractions in leaf litter may regulate decomposition, and polyphenolic contents have been found to control N release rates from tropical plant material with high N concentrations (Palm and Sanchez 1990, 1991). Constantinides and Fownes (1994) incubated litter from 12 species with a wide range of initial leaf chemistries and determined that N loss was best predicted by initial N and polyphenol concentrations. In Pinus muricata (D. Don) stands in California, polyphenol concentrations in litter inhibited N mineralization from dissolved organic nitrogen encased in tannin complexes (Northup et al. 1995). Plant Effects on N Mineralization Rates In replicated common gardens, four of Jenny's (1980) five soil forming factors (i.e., climate, vegetation, relief, parent material, time) can be held constant with only vegetation varying across experimental units. These experiments give a more precise estimate of plantinduced affects on soil N mineralization, although pre-establishment soil homogeneity is rarely quantified. In a 28-year-old common garden experiment in Wisconsin, Gower and Son (1992) found significant differences between five species for mineral soil nitrate and ammonium concentrations, annual net N mineralization, and annual nitrification. Binkley and Valentine (1991) reported significantly higher net N mineralization rates under eastern white pine compared to green ash in another common garden trial in Connecticut. In a number of studies, leaf litter chemistries have been correlated with net N mineralization rates. In mixed species stands in Wisconsin, McClaugherty et al. (1985)

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18 reported that N mineralization rates were correlated with decomposition rates of leaf litter originating from the stands' dominant tree species. Prescott and Preston (1994) stated there was a connection between litter and nutrient supply in cedar-hemlock forests, and they attributed low N in the forest floor under western red cedar {Thuja plicata Donn) to its recalcitrant litter. In addition, Gower and Son (1992) indicated that initial leaf litter L/N ratios for five species were related to annual net N mineralization in surface soils in Wisconsin, and Stump and Binkley (1993) determined that leaf litter L/N ratios were better predictors of net N mineralization than initial leaf N concentrations alone. Conclusions and the Primary Objectives and Hypotheses of this Study Researchers have identified plant-induced soil changes in homogeneous forest patches, natural stands, savannas, grasslands, and in replicated common gardens. In a review of the influence of tree species on soil properties, Binkley (1994) stated that initial leaf chemistry, SOM dynamics in mineral soil, and soil biota activity were the key variables when considering how species influence soil nutrient availability. Most previous studies were conducted in temperate environments, and little is know about plant-induced changes in Oxisols, although this soil order is one of the most widespread in the world, covering approximately 8 .9 % of Earth's land area, mostly in the tropics (Brady 1990). Previous work has demonstrated that physical and chemical soil properties change after short-term exposure to monocultures of tree species with varying leaf litter quantities and qualities. Soil moisture, temperature, physical structure, microbial and faunal communities, nutrient status, and mineralization rates are influenced by plant species composition, and plant-soil-plant feedback loops may influence forest ecosystem dynamics. Plant-induced soil

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19 changes are important to understand because of their effects on nutrient dynamics and plant succession, and their implications for forest management. In this context, the overall objective guiding this study is to determine if four tree monocultures consisting of species with contrasting phenologies, resource requirements, and biochemical characteristics have significantly changed carbon and nitrogen dynamics in a lowland tropical ecosystem when planted on an initially uniform soil. The study used replicated plots of four monocultures and undisturbed native forest as a control, and the four monocultures consist of Pinus caribaea var. hondurensis, Euxylophora paraensis Huber, Carapa guianensis Aubl., and a Leguminosae combination (Dalbergia nigra Allemao ex Benth., Parkia multijuga Bent. Dinizia excelsa Ducke). All the plots were established in the Curua-Una Forest Reserve in the east-central Amazon Basin. The specific objectives of this study are the following: 1. To determine the effects of above-ground fine litter quality and quantity on litter storage and turnover rates, decomposition, soil respiration, soil carbon storage, N mineralization, and nitrification under the monocultures and undisturbed native forest, and 2. To determine the factors controlling the processes of soil organic matter formation, composition, and degradation under each monoculture and undisturbed native forest. The hypotheses that relate to these objectives are the following: 1. Surface soils under monocultures with low leaf litter quality (high L/N ratios) will have higher carbon and lower N contents than under monocultures with higher leaf litter quality. 2. Leaf litter with lower L/N ratios will decompose faster resulting in higher net N mineralization rates in the forest floor and surface soil.

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20 3. Higher nitrogen-use efficiency will be associated with lower net N mineralization rates in surface soils. 4. C0 2 evolution, inorganic N pools, and net N mineralization will be highly seasonal in the natural forest control and the four monocultures. 5. Monospecific stands with higher NUE will have less soil total N and inorganic pools relative to the control and other treatments with higher NUE, thus creating a feedback loop from plant to soil to plant. This dissertation is divided into eight chapters, including this introduction. The second chapter describes the study area, its climate, and the history of the experimental plots. The third chapter examines the assumption of initial soil homogeneity under the monocultures and native forest. The fourth chapter discusses differences in litterfall N contents, litterfall/standing crop quotients, and NUE among the plantations. The fifth chapter examines differences among the plantations and forest for soil C release, fine root biomass, and microbial biomass, along with their interrelationships. The sixth chapter explores differences in leaf litter chemistry among the plantations and the forest, and the controls on first-year mass loss and long-term litter decomposition. The seventh chapter describes patterns of N mineralization among the plantations and the forest, including an attempt to explain the mechanisms behind observed differences in N transformations among the treatments. The final chapter is a synthesis of how tree species with different ecological characteristics can influence soil properties in lowland Amazonia, and I will present a framework of plant-soil interactions at Curua-Una.

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CHAPTER 2 STUDY SITE DESCRIPTION AND PLANTATION SELECTION Introduction The world's largest contiguous tropical moist forest is located in the Amazon Basin of South America, and it covers 5.5 million km 2 that extend through nine countries (Browder 1988). Approximately 66 percent of lowland Amazonia (below 1000 m.a.s.l.) is located in Brazil (Eden 1990), and 49 percent of Brazil's portion of Amazonia is covered by dense tropical forest, primarily in the northern states of Para, Amazonas, Amapa, Roraima, and Maranhao (Browder 1988). In the 1950's, foresters working for the Food and Agricultural Organization (FAO) and the Brazilian government postulated that relatively few tree species would be utilized to supply Brazil's future needs for timber. They therefore established plantation trials of potentially commercial native and exotic tree species from 1959 to 1975 at the Curua-Una Experimental Station (Dubois 1971, Palmer 1977), on a site selected as representative of the terra firme (upland, non-flooded) forests that cover over 80% of the Amazon Basin (IBGE 1990, Unlet al. 1981). The field studies presented here were carried out at the Curua-Una Experimental Station in eastern Para, at the confluence of the Amazon and Curua-Una Rivers (2S, 54 W), 110 km east of Santarem (Figure 2. 1). Curua-Una is a 72,000 ha forest reserve, and it is the 21

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Figure 2.1. Location of the Curua-Una Reserve.

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23 oldest forest research station in the Amazon Basin. The life zone in this region is classified as tropical moist forest (Holdridge 1978) and the native forest has both deciduous and evergreen broadleaf tree species in the overstory. Most terra firme forest at the reserve is on the planalto, which is a plateau that rises to 180 m.a.s.l. approximately 6 km from the CuruaUna River. Climate and Forest Composition Average yearly rainfall at the station is 1900 mm, most of which falls from December to July, and the average daily temperature is 26C (SUDAM 1979a, b). During the course of this study, which took place from September 1994 to October 1995, most of the monthly rainfall totals fell within 3 standard deviations of ten year monthly averages (1983 to 1993) from Belterra, the nearest agriculture station (150 km) that records monthly rainfall (Figure 2.2). Rainfall during April and May in 1995 was higher than the 10-year average for these months at Belterra, but a review of long-term weather data from this region indicated that unusually high rainfall totals have been recorded during these months in past years. Average daily temperatures during the 1994-1995 field season remained between 26 and 28 C, and the warmest temperatures were recorded during the dry season months from August to November (Figure 2.3). At Curua-Una, inventories of trees over 25 cm diameter breast height (DBH) in 1 ha plots in terra firme forest on the planalto estimated timber volumes of 157 to 170 m 3 ha" 1 (Heindsjik 1958, Pitt 1961, Glerum and Smit 1965). Data from these inventories indicated that there were 33 to 37 trees over 25 cm diameter per ha and 103 to 140 tree species per ha.

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24 Figure 2.2. Average monthly rainfall from 1994 to 1995 at the Curua-Una Reserve compared to 10-year averages ( 3 sd) from 1983 to 1993 from the Belterra Agricultural Station.

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25 38 36 34 32 S 28 H 26 24 22 20 Maximum Minimum Average \ \ / w — n i i i i i i i 1 1 1 r NDJFMAMJ J A S O Month Figure 2.3. Mean daily maximum and minimum temperatures from 1994 to 1995 at the Curua-Una Forest Reserve.

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26 The families most frequently represented in these inventories on the Curua-Una planalto were the Leguminosae, Sapotaceae, Lecythidaceae, and Lauraceae (SUDAM 1971). Soils Sombroek (1966) described the planalto soils at Curua-Una as a 15 to 20 m thick, uniform, yellow, heavy textured, kaolinitic, sedimentary clay with no stratification. He classified this soil as an Oxisol and named this soil type "Belterra clay", which is the predominant soil type on the planalto stretching from the Tapajos to the Xingu River. IBGE (1990) estimated that this soil type covers 29% of the Brazil's Amazonian lowlands. The origin of this kaolinitic clay is widely disputed with several authors claiming it is of lacustrine origin from the late Pliocene early Pliestocene (Sombroek 1966, Putzer 1984, Klammer 1984). Irion (1984) believes the Oxisol was formed in situ because the clays contain quartz sand which would not appear together with the fine clays in lake deposited sediments. Soil surveys and analyses were carried out on the planalto soils at Curua-Una by Day (1961) and Cate (1960) before the removal of approximately 10 hectares of primary forest in 1958. Day and Cate sampled soils in the center of the tree plots utilized in this research project, and also from forest adjacent to the research plots. Although the number of samples analyzed is unknown, their results indicated that these soils had a high clay fraction and low extractable phosphorus (Table 2. 1). X-ray diffraction of the samples detected a high quantity of kaolinite and low levels of gibbsite in surface soils and at 1 meter at this site (Cate 1960, Sombroek 1966). Cate (1960) proposed that resilication of surface soils was occurring in this region because a high silica clay (kaolinite) is found above the more silica depleted gibbsite. This theory has been recently

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27 Table 2.1. Analyses of soils by Cate (1960) and Day (1961) prior to tree plot establishment at Curua-Una Forest Reserve. pH Total P Bulk Density Depth (cm) C (%) N (%) (H 2 0) (mg kg 1 ) clay (%) (g cm" 3 ) 0 30cm 3.56 0.26 4.4 3 73 0.96 90 130cm 0.56 0.04 4.9 3 84 1.06 revived by Lucas et al. (1993) who proposed that the soil mineral composition of these ecosystems is maintained by the cycling of silica by vegetation. Tree Plot Establishment The 12 experimental tree plots used in this study were planted between 1959 and 1961 and in 1973, and these plots were selected from the approximately sixty species trials that were established on the planalto (Figure 2.4). The objective of these plantings at the reserve was to identify which native or exotic species grew well in plantations in this environment (Dubois 1971). In addition to the species trials, several experiments with selective cutting and natural regeneration were carried out in km 2 blocks located near the species trials (Figure 2.4). The plots that were established as species trials were cleared of natural vegetation, burned, and planted with seedlings raised in a nursery at the research station. All native species have their provenance from native forest at Curua-Una and Pirtus caribaea seed came from Belize (Dubois 1971). At the time of planting the plots received "one-half tablespoon of phosphate" per seedling which had no effect on growth or survival (Pitt 1961). The only treatment each plot received since establishment was the clearing of understory growth (stems cut at base), and the Pinus caribaea plots were thinned (50% reduction of stems) in 1980.

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28

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29 The three native forest control plots (Figure 2.5) have not been disturbed by fire or cutting since 1955 when United Nations personnel first arrived at the site. Workers who have lived at the reserve since 1960 indicated that the terra firme forest did not show any signs of disturbance when they first arrived. The encampment of the reserve was located next to the Curua-Una River on a site that was once occupied by Native Americans, so, it is possible that the forest plots may have been disturbed by humans in the past. Indeed, charcoal was found during the course of this study in surface soils and at a 100 cm depth in the forest plots. Sombroek (1966) also reported finding charcoal in undisturbed forest at the reserve down to 150 cm, which he attributed to Indian fires that burned roots deep into the soil profile. Anthropogenic black earth or terra preta is widespread along the banks of the Curua-Una River which is 6 km from the research plots. Plantation Selection The plantations were chosen according to the following three criteria: 1) The presence of three plots (replicates) of a species or combinations of species that had similar foliar and leaf litter characteristics, 2) The replicates could not have been dominated by native, successional vegetation, and 3) As wide a range of phenologies, resource requirements, and biochemical characteristics were to be included Based on this screening, three replications each of Pimis caribaea, Euxylophora paraensis, Carapa guianensis, and the Leguminosae combination (one replicate each of Dalbergia nigra, Parkia multijuga, and Dinizia excelsa) were selected out of the sixty species that were originally planted at this site. Pimts caribaea was chosen because it is known to have high C/N ratios in leaf litter, high litterfall, high standing crop mass, and low N concentrations in litter (Lugo 1992). Carapa

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30 A N NF = Native Forest PC =' Pinus caribaea CG Carapa guianensis LEG = Leguminosae EP = Euxylophora paraensis Figure 2.5. Layout of the replicates used for this study at the Curua-Una Forest Reserve.

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guianensis is native to this region, had leaf litter quality close to that of the mixed native forest control, and had high survivorship in three plots. The Leguminosae treatment does not contain three plots of the same species, but was selected for study to include a treatment with high N concentrations in aboveground litterfall relative to the native forest control. Since there were no replicated plots of any known N-fixing species, I combined three separate unreplicated plots of species that had pinnately compound leaves and high N concentrations in foliage and leaf litter to form this treatment. The inclusion of the combined Leguminosae treatment does not allow me to attribute any effects on soils directly to a N-fixing species because none of theses are reported to fix atmospheric-N (Allen and Allen 1978). Including a treatment with high N inputs in leaf litterfall provided a good contrast with the native forest control and the other three plantations that had lower litter quality. The Euxylophora paraensis stand was younger than the other treatments (23 years compared with 36 years), but I included this species because the plantations were mature at 23-years-old, and I believed any soil effects would be evident after this amount of time. In addition, Euxylophora paraensis is native to this region and has leaf litter chemical characteristics intermediate between native forest and Pinus caribaea, thus its inclusion helped achieve my objective of including trees with a range of biochemical characteristics in leaf litter. The native forest control plots were located in a block of undisturbed forest adjacent to the plantations. These plots were established to test for differences between soils under undisturbed natural forest and the plantations at the initial phase of this project, and to

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32 contrast and compare seasonal C and N transformations between natural forest and the plantations over a one year period. Summary Replicated plots were selected from previously established plantations and seasonally dry, lowland Amazonian forest to test the hypothesis that trees with varied ecological characteristics will affect soil dynamics. The plantations were selected primarily to span a range of leaf litter characteristics and because they were replicated. Recently undisturbed native forest was selected as an experimental control to compare and contrast with the monocultures. The replicates were not originally established with a randomized experimental design, but the 15 plots (4 plantations + 1 control = 5 treatments x 3 replicated plots) will be analyzed as a completely randomized design with treatment effects fixed. All the experimental plots were within 1 .5 km from one another, received approximately 1 900 mm of precipitation annually, and were growing on a soil previously identified as an Oxisol.

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CHAPTER 3 SOIL HOMOGENEITY Introduction Previous studies examining plant-induced soil changes frequently encountered the confounding effects of site variability, soil heterogeneity, and land-use history (Stone 1975). For a study to accurately assess how plants influence soil properties, more precise estimates can be made if experimental material is planted on an initially homogeneous substrate ( Wedin and Tilman 1990). If soils are initially uniform then any differences in soil properties that arise over time among experimental treatments may be attributed to plant effects (Gower and Son 1992). Common garden tree experiments are used to examine the influence of tree species on soil properties because they offer researchers older, replicated plots of tree monocultures located in close proximity (Binkley 1994). In many previous, long-term retrospective studies examining changes in soil properties under tree monocultures, initial soil sampling was not carried out because the objective of most common garden experiments was to compare genotypic and phenotypic responses of tree species grown on a common site (Zobel and Talbert 1984) Initial soil homogeneity was frequently assumed based on indirect evidence such as the proximity and similar slope position of experimental units, block randomization, and similar soil textures (Gower and Son 1992 ). 33

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34 Although soil properties are spatially heterogeneous (Robertson et al. 1988), soils are classified into orders according to characteristics such as effective cation exchange capacity (ECEC), mineralogy, texture, and horizonation (Soil Survey Staff 1992). Previous soil sampling in the upland, terra firme forest at the Curua-Una Reserve indicated that the forest was growing on an Oxisol (Sombroek 1966), the predominate soil order in lowland Amazonia (IBGE 1990). These old and highly weathered soils are a good substrate on which to examine plant influences because they are deep, well aerated, and highly uniform. Compared to other soil orders, Oxisols cover the largest portion of the tropics (i.e., 22%, Van Wambeke 1992), and 9% of the earth's land area (Brady 1990), yet little is known of plant influences on their chemical and physical properties. Since the majority of aboveground detritus and a large proportion of belowground turnover occurs in surface soils, subsurface soils are less influenced by plants and may maintain physical and chemical characteristics despite changes in plant cover. Although lowland forests may affect soil mineralogy in the long-term (Lucas et al. 1993), short-term changes in vegetation are less likely to change mineralogy. The comparison of surface soil texture and mineralogy and subsoil chemical properties may be a reliable indicator of initial soil homogeneity under adjacent, replicated experimental units. Thus, the objectives of this chapter are to 1) compare and contrast physical and chemical soil properties between the forest control and each monoculture, and 2) determine if soils under the forest control and each treatment were initially uniform.

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35 Materials and Methods Soil Sampling and Statistical Analysis For the determination of soil physical and inorganic properties, four randomly located cores were taken at 0 to 20 cm and 95 to 105 cm depths in each of the 15 plots (4 monocultures + 1 control = 5 treatments x 3 replicated plots = 15 plots) and combined by plot and depth. All soils were passed through a 2 mm sieve, had fine roots removed by hand, were air dried, and then stored at approximately 4 C in sealed plastic bags until analysis. Soil particle size was analyzed in triplicate from pooled samples from each plot by the pipette method (< 2mm fraction) and volume weight determination for the fraction > 2 mm (Soil Survey Laboratory Staff 1992). Soil mineralogy was determined by x-ray diffraction of the fine earth fraction (<2 mm), using samples from each depth combined by treatment (i.e., 0 -20 cm and 100 cm). One sample per treatment was analyzed for mineralogy at each depth using one KCL and one magnesium saturated tile per treatment (Whittig and Alardice 1986). Effective cation exchange capacity (ECEC), pH, total C, N, P, and base cations were determined from four randomly located samples per plot that were combined and subsampled in triplicate at each depth (0 20 cm and 100 cm). ECEC was estimated as the sum of 1 N NH 4 OAc (pH 7) extractable base cations and 1 N KC1 extractable aluminum (Thomas 1982). Soil pH was determined in a 2:1 water suspension (McLean 1982) Total N and C were measured with a Carlo-Erba nitrogen autoanalyzer 1500, and extractable P, Al, and base cations were determined by Inductively Coupled Argon Plasma Spectroscopy (ICAP) after Mehlich I extraction.

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36 Significant differences for mean soil physical and chemical properties between the forest and the four plantations were determined by analyzing replicate means (n = 3) with a one-way analysis of variance with treatment (plantations + forest control) as a fixed effect. A completely randomized design (CRD) was used because the treatments were not randomly assigned to blocks when the original species trials were installed. Because the plantations were 23 to 36-years-old, I also could not randomly assign the treatments to the experimental units (replicates), but I chose to use a CRD because I assumed the experimental units were similar and any error attributable to differences among the replicates was small. In fact, precipitation, temperature, relief, slope, soil parent material, understory cleaning, and fertilizer applications were the same for all 12 of the monospecific plots. If soil texture, mineralogy, and subsoil chemical characteristics could be shown to be similar, then the any variation in surface soil properties among the treatments should be attributable to the species or group of species that was established upon it. Finally, significant differences between soil properties under the forest and each of the plantations were analyzed with Dunnett's t-test following the analysis of variance to identify which plantations had changed soil properties compared to undisturbed forest soils. Results and Discussion Soil Properties Only the percentage of clay and sand in soils under E. paraensis differed significantly from the soils under the forest; the physical properties of soils under the other plantations did not significantly differ from the forest soil (Table 3.1). Although the clay and sand contents of surface soils under E. paraensis were different than the forest control, the total fine earth

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37 fraction (clay + silt) of surface soils were similar (85 vs 95%). The mineralogy of surface soils and subsurface soils was similar under all the plots, and kaolinite was the predominate mineral at both depths (Figures 3.1-3.5). In addition, quartz was present in all the samples from surface soils and absent at 100 cm, which was evidence that the soils under the plantations and the forest had been influenced by similar pedogenic processes (Figures 3.13.5). These findings also indicated that the principal inorganic components of the soils under the plantations and the forest were similar at the time of plantation establishment. Table 3.1. Mean physical soil characteristics ( se) at the 0 20 cm depth, Curua-Una Forest Treatment clay (%) Soil property silt (%) sand (%) Mineralogy Forest 59.6(1.5) 35.9(1.4) 4.5 (0.5) Kaolinite P. caribaea 54.5 (6.6) 34.5 (3.7) 11.0(3.3) Kaolinite C. guianensis 51.5 (1.5) 39.8(2.1) 8.7(1.4) Kaolinite Leguminosae 57.2 (4.0) 36.5 (3.5) 6.3 (0.6) Kaolinite E. paraensis 40.3 (3.2)* 44.4 (2.7) 15.3 (2.1)* Kaolinite The ECEC of surface soils from the monocultures ranged from 1.11 to 1.73 cmol c kg" 1 soil, and there were no significant differences between the control and any of the monocultures (Table 3.2). Considering this soil's mineralogy, ECEC, depth, and moisture regime, this soil is a Typic Haplustox according to the USDA criteria (Soil Survey Staff 1992). These surface soils have a lower ECEC than several Amazonian Oxisols analyzed by Motavalli et al. (1994), which ranged from 2.93 to 4.00 cmol c kg" 1 soil. The lower levels of ECEC from soils at Curua-Una may result from deeper sampling (0 20 cm) compared to

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D: NATIVE FOREST(0-1 5CM) MG(GLY) (35kV. 20mA) File 23M MDI Scan 2-32/03/ 1/#1 001. Anode CU 38 750Kaolinite Kaolinite T 10 30 15 20 25 2-Theta Figure 3.1a. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under native forest at Curua-Una. ID: NATIVE FOREST 100CM MG(GLY) (35kV. 20mA) File 2AM MDI Scan 2-32/ 03/ 1/#1 001. Anode CU 750250Kaolinite 7.16 359 I Kaolinite Goethite 4.44 S ' 10 ' ' 15 ' ' 20 ' ' 25 3D 2-Theta Figure 3.1b. Mineralogy as detected by x-ray diffraction from subsurface soils (1 m) under native forest at Curua-Una.

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ID: PINE PLANTATION 0-1 5CM(MG(GLY) (35kV. 20mA) File: 25M MDI Scan: 2-32/ 03/ 001. Anode: CU 39 7505002502-Theto Figure 3.2a. Mineralogy as detected by x-ray diffraction from surface soils (0 20cm) under Pinus caribaea at Curua-Una. ID: PINE PLANTATION 100CM(Mg(GLY)) (35kV. 20mA) File: 26M.MDI Scan: 2-32/03/ 1/#1 001. Anode: CU 15 20 2-Theta Figure 3.2b. Mineralogy as detected by x-ray diffraction from subsurface soils (1 m) under Pinus caribaea at Curua-Una.

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ID: Ken Smith CG Mg-sat tile (35kV. 20mA) File: 0027.MDI Scon: 2-32/03/ 1 /#1 001 Anode: CU 7502-Theta Figure 3.3a. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under Carapa guianensis at Curua-Una. ID: Ken Smith CG Mg-sot tile (35kV. 20mA) File: 0025.MDI Scan: 2-32/ 03/ 001. Anode: CU 10002-Theta Figure 3.3b. Mineralogy as determined by x-ray diffraction of subsurface soils under Carapa guianensis at Curua-Una. (100 cm)

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ID: Ken Smith File: 001 9.MDI -20cm Mg-soltile (35kV. 20mA) Scan: 2-32/03/ 1/#1 001. Anode: CU 41 10007505002501 4 10 25 30 15 20 2-Theto Figure 3.4a. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under the Leguminosae plantations at Curua-Una. Figure 3.4b. Mineralogy as determined by x-ray diffraction of subsurface soils (100 cm) under the Leguminosae plantations at Curua-Una.

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ID: Ken Smith EP-20cm Mg-sattile (35kV. 20mA) File: 0023.MDI Scon: 2-32/03/ 001 Anode: CU 42 1000 1 3 8 500 2-Theta Figure 3.5a. Mineralogy as determined by x-ray diffraction of surface soils (0 20cm) under Euxylophora paraensis at Curua-Una. ID: Ken Smith EP-100cm Mg-sottile (35kV. 20mA) Re: 0021 MDI Scan: 2-32/03/ 1//1 001. Anode: CU 2-Theta Figure 3.5b. Mineralogy as determined by x-ray diffraction of subsurface soils (100 cm) under Euxylophora paraensis at Curua-Una.

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43 0) u +-> M i I PL, 'S CQ on f— 5 5' i= oo SI 00 M o oo H 2 -r„ — 00 o oo H oo CJ w _j r O 2 W 6 u 00 — \ V — < — / — \ NO o I — J* CN -* in — ' — ^ — */"> m 00 r-; ci ii rn fi cn — s rn Co Co ~" ri — ', r-i — < > — fi CN o o O NO ON ON d ON d CN in l /N NO K' V CN en — 1/1 V So 1 o o o o d d o d x in ON NO IT, ON 00 NO >* Tt -r •*r V — s CN O c d o d, — • o o in CN in o c u e i % 1 S CO s S 73 .91 CJ c/l | £ 0) CO 'o >i £ " CO CD I CD c o § "oo oo b oo uj 3 u r— 'oo o oo Z -r oo O 00 f— w u G c 00 1/1 CN O ON ON c Q 0 5 3 S: u ON d -3on' On CN -T — > ^ f O d d d d d On o in 00 NO >n / — \ — i V X ? ON m Co CN d o o d 00 o X in ri 5 NO -r -r 5 3 a, t3

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44 samples analyzed by Motavalli et al. (1994), which came from depths of 0 8 and 0 12 cm. Soil organic matter provides negative charge in highly weathered, variable charged soils (Gillman 1985), and the lower organic matter contents of the Curua-Una samples from 12 20 cm may have diluted the ECEC. The concentrations of Mehlich I extractable P in these surface soils ranged from 3.64 to 3.83 mg kg' 1 dry soil and these values are similar to those reported for tropical lowland forests in Sabah, Malaysia (Proctor et al. 1988) and montane forest in Puerto Rico (Cuevas et al. 1991). The concentrations of Mehlich I extractable P in surface soils at Curua-Una were much lower than estimates of acid extractable P reported from a wide range of tropical lowland forests in a review by Silver (1994). Mehlich I extractable P and base cations under the plantations did not significantly differ with the forest at either depth, and each decreased dramatically in concentration with depth (Tables 3.2, 3.3). Although these measures do not give an indication of what was available for plant uptake, they did indicate that the plantations had not influenced Mehlich I extractable P and base cation concentrations in the soil. Surface soil total N concentrations ranged from 2.68 to 4. 10 g kg" 1 dry soil, values similar to those from lowland forests in Costa Rica (Heaney and Proctor 1989) and Sarawak (Proctor et al. 1983). These values are also similar to a mean value of 3.2 g N kg" 1 soil reported for Oxisols in a review by Sanchez et al. (1982). Total N concentrations decreased with depth under all the plantations and the forest (Tables 3.2, 3.3), and surface soils under the forests had higher total N concentrations than all the plantations except for E. paraensis (Table 3.2). In contrast, there were no significant differences between the forest and the plantations for subsurface total N concentrations (Table 3 .3). If my assumption of initial soil homogeneity

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45 under the plantations and the forest was correct, these findings indicated that the plantations had influenced surface soil N dynamics. Although C and N are linked in forest soil biogeochemical cycling (McGill and Cole 198 1), surface soil total C concentrations under the forest were only significantly higher than under C. guianensis (Table 3.2). Surface soil total C concentrations ranged from 49.62 to 62.94 g kg" 1 dry soil (Table 3.2), and decreased greatly in subsoils (Table 3.3). The surface soil total C concentrations reported here are higher than the mean value of 38.00 g kg' 1 from surface soils (0-15 cm) of 19 Oxisols reviewed by Sanchez et al. (1982). Only subsurface total C concentrations under £ paraensis were significantly higher than under the forest (Table 3.3). The C/N ratios of surface soils at Curua-Una ranged from 15.33 (E. paraensis) to 18.60 (P. caribaea), and these are much higher than the mean Oxisol surface soil C/N ratio of 1 1 .87 reported by Sanchez at al. (1982). The subsurface C/N ratios ranged from 16.0 (legumes) to 18.9 (E paraensis), and these ratios are higher than those reported in a worldwide study of C and N storage by Post et al. (1985), who determined that tropical dry and moist forest soil profiles to a 1 m depth had C/N ratios of 13 to 15. Surface soil pH ranged from 4.48 to 4.98 (Table 3 .2) and decreased with depth under the plantations while increasing slightly under the forest (Table 3 .3). These data indicate that P. caribaea did not acidify surface soil pH, contrary to results reported from temperate zone sites that were reforested with conifers (Brand et al. 1986). Forest sites occupied by N-fixing species have decreased surface soil pH in temperate environments (Van Miegroet and Cole 1984), and this phenomenon has been attributed to H + production resulting from increased nitrification rates in combination with greater acid strength of soil organic matter under N-

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46 fixers (Binkley and Sollins 1990). At Curua-Una, the Leguminosae treatment did not acidify surface soils compared to the forest, and surface soil pH under the legumes was higher than in the forest control (Table 3.2). I did not note any nodulation on fine roots under the legumes, thus perhaps these species did not fix atmospheric-N or they fixed N only when they were younger. These findings suggest that soil nitrification rates were higher under the forest, which had a wide number of potential N-fixing legumes growing in it, or perhaps that the inputs of a wide range of litter types into the forest control maintained a higher acid strength of soil organic matter. Conclusions Several lines of evidence support the conclusion that initial soil physical and chemical properties under each monoculture were not significantly different than those observed under present day primary forest at the Curua-Una Reserve, and these include the following: 1 There were no significant differences in surface soil texture between plantations of P. caribaea, C. guianensis, and the Leguminosae combination and native forest. Only surface soils under E. paraensis differed from the forest control in their percentage of clay and sand. 2. The dominant mineral constituent under all the plantations and the forest control in both the surface soils (0 20 cm) and subsurface soils (100 cm) was kaolinite. 3. There were no significant differences between any of the monocultures and the forest for total C and N, or extractable P, and base cations at the 1 m depth. 4. All the research plots shared the same slope and altitudinal position on the plateau (slope was 0 to 1 %, altitude 1 80 m), and they were exposed to the same rainfall and temperature regimes.

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47 Although there were no significant differences between the forest and the plantations for total N at a 1 m depth, this variable differed significantly in surface soils. The physical properties between native forest and the plantations were similar and these soils had only been exposed to a change in vegetative cover. Therefore, this indicates that the plantations may have influenced N dynamics of these surface soils. The primary objective guiding the remainder of this work is to identify the mechanisms of these plant-induced soil changes, with an emphasis on the altered soil N dynamics.

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CHAPTER 4 LITTERFALL AND NITROGEN-USE EFFICIENCY OF PLANTATIONS AND PRIMARY FOREST IN THE BRAZILIAN AMAZON Introduction Aboveground fine litterfall and decomposition are critical processes for transferring nutrients from aboveground forest biomass to soils (Golley et al. 1975, Swift et al. 1979). In seasonally dry tropical forests (< 100 mm rainfall for four consecutive months), peak litterfall usually coincides with the dry season (Wright and Cornejo 1990). In the east-central Brazilian Amazon, which has a distinct dry season ranging from August to November (Klinge and Rodrigues 1968, Nepstad et al. 1994), peak litterfall occurs at the onset of the dry season in riverine and upland forests (Franken et al. 1979). Forest floor mass also increases during the dry season in this region (Klinge 1977). In forest ecosystems worldwide, foliar nitrogen (N) concentrations, fine litterfall N contents, and within-stand nutrient-use efficiency (NUE) have been used as indices of N availability and soil fertility (Van den Driessche 1974, Vitousek 1982). In a review of tropical forests receiving more than 1500 mm of precipitation annually, Vitousek and Sanford (1986) noted that Amazonian terra firme forests had foliar N concentrations intermediate to the other tropical forests examined, which indicated that N may not be limiting to forest growth in this region. Nitrogen return in litterfall of lowland tropical forests is higher than for 48

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49 temperate forests (Proctor 1984), and in lowland Amazonia, Vitousek (1984) suggested that N availability did not limit growth of terra firme forests growing on Oxisols because they had relatively low within-stand NUE (represented by the litterfall dry mass:nitrogen ratio). Differences in surface soil nutrient concentrations due to species or stand composition may be identified if climate, species composition, successional status, and soil type are controlled (Vitousek and Sanford 1986). If N concentrations in Oxisols supporting Amazonian terra firme forest do not limit aboveground litter production, forest conversion to tree plantations that consist of species with varied nutrient requirements and aboveground litter quantities and qualities might change N dynamics in these soils. After native forest removal, site preparation, and reforestation, the N status of these soils should recover to varying degrees based on the characteristics of the plants that dominate a particular site. For example, I demonstrated in Chapter 3 that total N concentrations in surface soils under plantations and terra firme forest have changed significantly over a 20 to 35 year period. I therefore hypothesize that mature plantations which are planted on an initially uniform soil will have varying litterfall N inputs, foliar N concentrations, and NUE, and that these three factors will be related to differences in surface soil N concentrations. Differences in litterfall N inputs, foliar N concentrations, and NUE among the treatments (the four plantations and the forest control) may also indicate that the plantations are influencing N availability in these surface soils. The specific objectives of this chapter are to 1) describe the physical environment of each plantation and the adjacent undisturbed forest, 2) determine the timing and quantity of aboveground litterfall, 3) determine the turnover quotients and N contents of aboveground

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50 litterfall, 4) estimate NUE and foliar N concentrations, and 5) relate foliar N concentrations, within-stand NUE, and litterfall N contents with total N concentrations in surface soils. Materials and Methods Plot Establishment an d Inventory Three of the plantations were established between 1959 and 1961, with one replicate of each installed each year, and the Euxlyophora paraensis plots were planted in 1 973 Despite its younger age, this monoculture was included because it was mature and its leaf litterfall had characteristics different from the other plantations. The forest control has not been disturbed recently, although charcoal was found at soil depths from 5 cm to 1 00 cm, indicating that it has been influenced by fire in the past. All the overstory trees in each replicate ^ 10 cm diameter at breast height (DBH) were measured for height and diameter, but the exterior row of trees in each replicate was not included in estimates of basal area because the vegetation surrounding each plot was variable. For each treatment, basal area (m 2 ha" 1 ) was estimated from trees with diameters at breast height (DBH) t 10 cm. Survival was calculated based on original stocking levels of 1600 trees ha* 1 (2.5 m spacing). Understory vegetation was sampled with five, randomly located .001 ha circular subplots located in each main plot. Vegetation Area Index Vegetation area index (VAI) was obtained with a LAI-2000 Plant Canopy Analyzer (LICOR Inc., Lincoln, Nebraska), which integrates readings of aboveand below-canopy diffuse sky radiation from five zenith angles. The LAI-2000 measures all light-blocking objects, thus VAI is a more appropriate term than leaf area index (Strachan and McGaughey 1996). In

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51 broadleaf canopies, trees are assumed to have randomly distributed hand horizontal foliage, and the LAI-2000 output approximates single-sided leaf area index (LAI). The LAI-2000 output is usually adjusted for conifer stands because needles are not arranged randomly (LICOR 1991). To obtain VAI in each plot, I took an initial reading in a road cut nearest to each plot, then entered the plot and took ten random measurements that were compared to each initial reading. I sampled each plot from 7-9 AM during one week in July and one week in October. I sampled at this hour because at these times of the year skies were uniformly clear, and the solar disc was not visible. I used the 270 view cap for all measurements, and the LAI-2000 device was pointed in the same direction for the initial reading and for all ten readings in each plot. VAI was estimated for each plot based on an average of the individual ten readings as compared to the initial reading (which I substituted for above-canopy conditions). Litterfall r Forest Floor Mass. and NIJE Aboveground fine litterfall was estimated by placing six 1 m 2 traps in each replicate of each treatment (3 replicatess x 6 traps = 18 traps per treatment). Traps were placed systematically to provide even coverage over each plot, always one row inside the boundary of each replicate. They were suspended 40 cm above the forest floor, and fine litter was collected in nylon screen with 1 mm mesh. Litter was collected every 14 to 20 days from October 1994 to October 1995. Fine litter was separated into leaves, fruits, cones, flowers, bark, and branches (< 2 cm diameter), and dried at 105C until a constant weight. During every collection period the trash fraction was partitioned into each of the previous categories according to what it most resembled, and insect parts and feces were discarded. Fine litter

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52 that was kept for nutrient analysis was dried to a constant weight at 60 C and refrigerated until analysis. Every four months, fine litterfall was separated into overstory and understory components to estimate the impact of non-overstory species. Monthly litterfall totals were calculated by determining the average daily litterfall rate for each collection period. To estimate forest floor mass, one 50 cm 2 quadrat was randomly placed on the forest floor of each plot every three months, and all fine forest floor material inside the quadrat to mineral soil was removed. Thus for 12 months, 4 quadrats x 3 plots = 12 quadrats analyzed per treatment. Forest floor material was separated into leaves, fruit plus flowers, and bark plus branches (< 2 cm diameter), and dried at 105 C until a constant weight. Nitrogen-use efficiency (NUE) is the ratio of the rate of dry matter production:rate of nitrogen taken up by a stand of trees (Hirose 1971, Grubb 1989). I did not estimate either of these rates, thus I calculated NUE using Vitousek's (1982, 1984) practical definition of NUE which is the ratio of the dry matternitrogen content of litterfall or the inverse of litter N concentrations. I also assumed that the plantations and the forest were at steady-state, aboveground net primary productivity was equal to litterfall, and that the N lost in litter was equal to N taken up by the stands. Nutrient and Statistical Analysis Fine litterfall material kept for nutrient analysis was ground through a 1 mm sieve, then finely ground with a ball grinder and subsampled for C and N concentrations in a Carlo Erba Nitrogen 1500 Analyzer in triplicate. Estimates of total fine litterfall and leaf litter N were derived from samples collected during September 1994, January 1995, and April 1995 To estimate total fine litterfall N, samples were bulked by plant fraction (leaves, fruit plus flower,

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53 bark plus branch) and treatment and analyzed in triplicate. Total fine litterfall N contents (kg ha' 1 ) were derived by multiplying litterfall N concentrations of each plant fraction by the total weight (t ha" 1 ) of each fraction that fell from during the study period. Total fine litterfall N concentrations were then estimated by dividing total litterfall N (kg ha" 1 ) by total litterfall (t ha' 1 ). Leaf litter samples from the three months were analyzed separately for N concentrations by month and treatment. Leaf litter N contents (kg ha' 1 ) were estimated by multiplying total leaf litterfall (t ha' 1 ) by average leaf litter N concentrations from leaf litter collected in September, January, and April. Foliar N concentrations were estimated from foliage removed from the mid-canopy of each plantation and the forest in September 1994. Foliage was shot out of the canopy with a shotgun, oven dried at 60C until a constant weight, and refrigerated until analysis. The age of the foliage collected was unknown. Significant differences among experimental treatment means (n = 3) for litterfall, forest floor mass, and leaf litter nitrogen concentrations were determined with Tukey's HSD multiple comparison following one-way analysis of variance using PROC GLM in SAS (SAS 1988) with treatment effect fixed. Relationships between litterfall, forest floor mass, nitrogen contents in litterfall and soil N concentrations were determined with PROC REG in SAS. Results Tree Density. Basal Area r and Species Diversity Stem density (> 10 cm DBH) ranged from a low of 463 per ha in the forest control to 1 095 per ha in the C. guianensis plots (Table 4. 1). Only one liana > 1 0 cm DBH was rooted in the forest plots, although smaller lianas were numerous and larger lianas rooted outside the

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54 £ 09 &g o CO o '3 Eg j U res C CO J3 u IS 2 a "5 R & 9 a S, 55 & i S -2 a a J C o [X, II > Z 'fi S o i X O em o o o S 1/0 1/1 ON 00 <-si On PI o o On O 1 Ov ^ 2 *? 00 "/I m m \6 i/> ON 1/1 _ rvo >/-> fS Q 00 lO o m „" rq o NO .3 BO u 2 s 'J < -a •Si X X X -3 o X X E o O rz X > u c I 1 5 o 5 *c5 % OQ I 3 u O > ri NC a rs rs 1/1 (-.] o ON dbh dbh E 0 0 10cm V V M u Speci Fami

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55 plots entered them through the canopy. Average basal area (trees £ 10 cm DBH) ranged from 17.9 m 2 ha" 1 (E. paraensis) to 46.1 m 2 ha (legumes), and average diameters were highest for P. caribaea (28.7 cm). The families most represented with trees > 10 cm DBH in the forest included the Leguminosae, Sapotaceae, Chrysobalanaceae, and Melastomataceae. Previous inventories in the reserve, which covered much larger areas, found the most widely represented taxonomic families were the Leguminosae, Sapotaceae, Lecythidaceae, and Lauraceae (SUDAM 1971). The pine overstory had the lowest stem density of all the plantations because it was thinned in 1980. Although the sample size used to estimate species richness under the plantations and the forest were identical, direct comparison of understory species richness was not possible because the plot sizes of the plantations and the forest ranged from 0.05 ha to 10 ha (Table 4. 1). Nevertheless, it is interesting to note that the pine understory supported more families than the forest, although the total number of tree seedlings and saplings in the pine plantation was much lower than in the forest. Vegetation Area Index In July, which is the peak litterfall season for the forest control and three of the plantations (Figure 4.1), VAI ranged from 2.1 (Leguminosae) to 3.75 (E. paraensis). Except for the Leguminosae treatment, VAI decreased for each treatment and the control in October, which is in the middle of this region's dry season. The LAI-2000 underestimates LAI in conifer canopies (Gower and Norman 1991), but I did not use a correction factor to estimate VAI because broadleaf understory vegetation also contributed to VAI in these plots. Trees in the replicates of P. caribaea were also widely spaced due to thinning, and I believed application of a correction factor would have

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200 160 cr 120 80 | -J 40 200 56 200 I I I I I I I I NDJ FMAM J J A SO Month II I II I I I I I ND J FMAM J J A SO Month 160 1 120 Verfai 80 40 NDJ FMAM J J A SO Month NDJ FMAM J J A SO Month 200 — •— Leaf Litter — — Bark + Branches — Fruit + Flowers ND J FMAM J J A SO Month Figure 4.1. Fine litterfall ( se) at the Curua-Una Forest Reserve from 1994 1995 under A) the native forest control, B) P. caribaea, C) C. guianensis, D) the Leguminosae, and E) E. paraensis.

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57 overestimated VAI. The legume plantations had the lowest VAI of all the treatments despite having the second highest stem density. Litterfall and Forest Floor Mass Total fine litterfall ranged from 8.09 1 ha" 1 yr" 1 under E. paraensis to 10.35 t ha" 1 yr' 1 under P. caribaea (Table 4.2). Mean total litterfall differed significantly among treatments (p< .05) primarily because of differences for total litterfall between E. paracusis and P. caribaea (Table 4.2). The percentage of total fine litterfall contributed by the understory and adjacent vegetation in the plantations ranged from 0. 1 (E. paraensis) to 6 % (P. caribaea), therefore the contribution of other species to litterfall inputs and forest floor mass was minimal in each plantation. Table 4.2. Total fine litterfall, forest floor mass, and mean residence times (inverse of litterfall standing crop quotient) for fine litter for native forest and four plantations at the Curua-Una Reserve ( se). Values followed by different letters are significant Total fine litter Standing crop 1/K L Treatment (t haV 1 ) (t ha" 1 ) (yr) Forest 9.76 (.58) ab 7.28 (1.0) b 0.77 P. caribaea 10.35 (.43) a 11.06 (1.2) a 1.06 C. guianensis 8.81 (.74) ab 10.26 (0.7) ab 1.16 Leguminosae 10.19 (1.3) ab 8.02 (1.3) ab 0.78 E. paraensis 8.09 (.63) b 7.77(1.0) ab 0.96 Total monthly litterfall for each category was similar for the plantations consisting of species native to the region, with peaks in leaf and branch litterfall in June and July (Figure 4. 1). Peak leaf fall for P. caribaea occurred during dry season, with heaviest branch fall in

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58 December (Figure 4.1b). The forest had the highest fruit and flower fall starting at the commencement of the rainy season (December) and lasting until March (Figure 4.1a). The plantations of species native to the region had similar peaks in fruit and flower fall, and the legumes had two distinct peaks when P. multijuga and D. excelsa flowered at different times. Pinus caribaea displayed steady fruit and flower production all year (Figure 4. lb). Total fine litterfall N contents ranged from 43 kg ha" 1 to 134 kg ha (Table 4.3). These values were not analyzed for significant differences because they were derived from pooled samples of fine litterfall over three months, and I did not obtain replicate means from which to estimate sample variance. Within-stand NUE was highest for P. caribaea and lowest for the legume plantation. Table 4.3. Total fine litterfall N concentrations and contents and NUE for native forest and the four plantations at the Curua-Una Reserve. Standard errors are not reported for litter because values are from one pooled sample (per treatment) from litter collected in September 1994, January 1995, and April 1995. Nitrogen-use efficiency (NUE) is the ratio of litterfall Treatment Total fine litterfall N(mgg-l) Total fine litterfall N (kg ha 1 ) NUE Forest 11.79 115.1 84.7 P. caribaea 4.70 43.1 240.1 C. guianensis 10.36 91.2 96.6 Leguminosae 13.19 134.4 75.8 E. paraensis 7.34 59.4 136.2 Forest floor mass ranged from 7.28 to 1 1 .06 t ha" 1 and mean residence times for fine litter ranged from 0.77 to 1.16 years (Table 4.2). Significant differences among treatments for

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59 mean forest floor mass were detected (p< .05) primarily because P. cahbaea had much higher forest floor mass than the forest (Table 4.2). The contribution of understory and adjacent vegetation to forest floor mass ranged from 0% under E. paraensis to 8.5% under P. cahbaea (data not shown). Total Leaf Litterfall. Leaf Litter Standing Crop, and Leaf Turnover Total leaf litterfall ranged from 6.45 t ha" 1 yr" 1 (E. paraensis) to 8.09 t ha" 1 yr" 1 (P. cahbaea), and there were no significant differences among treatments for total mean leaf litterfall (Table 4.4). Leaf litter standing crop varied widely and ranged from 3 .06 t ha" 1 (Leguminosae) to 8.19 1 ha" 1 (C. guianensis), and significant differences among means of leaf litter forest floor mass were detected at p <, .05 (Table 4.4). Leaf turnover quotients were highest for the legume plantations, and mean leaf residence time was shortest for the legumes (0.38 years) and the forest (0.5 years). Leaf material (including rachis) of C. guianensis had the longest turnover time (Table 4.4). Table 4.4. Leaf litterfall, leaf standing crop, and mean residence time for leaf litterfall for native forest and plantations at the Curua-Una Reserve ( se). Values followed by different letters Total leaf litter Leaf standing crop 1/K L Treatment (t haV 1 ) (t ha" 1 ) (yr) Forest 6.81 (.55) a 3.42 (.52) c 0.50 P. caribaea 8.09 (.37) a 7.85 (.99) ab 0.97 C. guianensis 7.46 (.71) a 8.19 (.48) a 1.09 Leguminosae 7.92 (1.3) a 3.06 (.62) c 0.38 E. paraensis 6.45 (.59) a 3.97 (.46) be 0.61

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60 Foliar and Leaf Litter N Concentrations As might be expected, the plantations of the Leguminosae had the highest foliar and leaf litter N concentrations while P. caribaea had the lowest N concentrations. Leaf litter from the legumes had the highest N concentrations at 15.6 mg g" 1 dry leaf matter, and P. caribaea had the lowest concentrations at 4.5 mg g" 1 dry leaf matter (Table 4.5). Nitrogen concentrations of live foliage were also highest in the Leguminosae plantations (18.8 mg g" 1 dry leaf matter) and lowest in P. caribaea (9.0 mg g" 1 dry leaf matter). As with leaf litter, foliar N concentrations in the legumes and the forest did not differ significantly, but foliar N concentrations for the other three plantations were significantly lower than both the forest and the legumes (Table 4.5). The N content of total leaf litterfall was highest under the legumes and lowest under P. caribaea (Table 4.5). The foliar N: leaf litter N ratio was highest for P. caribaea and lowest for the legume plantations. Table 4.5. Nitrogen contents and concentrations of leaf litterfall, and foliar nitrogen concentrations of leaf litter from native forest and four plantations at the Curua-Una Forest Reserve ( se). Treatment Foliar N (mg g"' ) Leaf Litter N (mg g' 1 ) N content (kg ha" 1 ) Foliar N :Litter N Forest 17.98 (0.7) a 13.97 (0.2) a 95.34 22.3 Pinus 9.00 (0.2) c 4.50 (0.2) c 36.40 50.0 Carapa 13.26 (0.2) b 10.87 (0.6) b 80.50 18.0 Leguminosae 18.82 (1.6) a 15.65 (0.4) a 123.94 16.8 Euxylophora 14.54(0. 1) b 8.60 (0.4) b 55.47 40.8

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61 Discussion Forest Tree Density, Basal Area, and Species Richness Previous studies in lowland Amazonian primary forest in Brazil have found tree densities (> 10 cm DBH) in natural forest ranging from 230 to 460 per ha, and these densities are much lower than those for terra firme forest at San Carlos in Venezuela (Table 4.6). Tree density in the undisturbed forest at Curua-Una is also in the lower range of the 414 957 trees per ha reported for tropical forests worldwide (Phillips and Gentry 1994). Basal area for the forest control was 26.2 m 2 ha" 1 which was similar to other Amazonian terra firme forests (Table 4.6). Larger scale inventories in Curua-Una region (100% inventories of 1 ha plots) have found a total of 103 140 tree species per ha in this region with 38 -42 species > 25 cm DBH per ha (Heindsjiik 1958, Pitt 1961, Glerum and Smit 1965). Tree species diversities of lowland Amazonian forests are difficult to compare because researchers have inventoried plots of variable sizes and used variable minimum diameters (Table 4.7). Species richness in the forest at Curua-Una was similar to values reported for lowland Amazonian forest near Manaus and in Roraima (Table 4.7). For example, Klinge et al. (1975) reported 50 tree species (stems > 15 cm DBH) and 20 families contributing to overstory diversity in a 0.2 ha plot near Manaus with the Leguminosae, Sapotaceae, Euphorbiaceae, Lecythidaceae, and Vochysiaceae the most widely represented families with stems over 15 cm DBH. Terra firme forest in the Peruvian Amazon is the most diverse in the world with 283 tree species represented in 580 stems per ha > 10 cm DBH (Gentry 1988). According to Gentry (1995), tropical dry forests have 50 to 70 tree species > 2.5 cm DBH per 0. 1 ha and moist semi-evergreen forests have 100 to 1 50 species > 2.5 cm DBH per 0. 1

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62 Table 4.6. Tree densities and basal areas (trees > 10 cm DBH) of lowland Amazonian forests. Location ana r oresi 1 ype Tree Density \n na j Basal Area ^111 Ila y T? f*ff*rf*n i**p DiaZlllun ic/Tu JlrmV t 1 nnii T In 0 v^urua-una 46^ 96 9 tnic ctuHv rvuf alllld 410 O Thnmn^nn pt al 1 007 i 1117111 u svjii wi ai i y s * Para 9^0 448 ZjU, 44o LMilLN CI ul. 1 7Jv Para S04 PirpQ pt nl 1 053 niii ci ui. l/jj Para 4£0 Zo.Z J)Z. 1 Pnmnhpll pf ] 0 Si A P /- t\ri in tOVVft 11 fYYiO rciuvioii itzrru jirfntz I all aJIlOIlO ^80 VJCIlLiy l/OO IVllbiidnd 847 npntrv/ 1 088 VJCIlLiy 1 700 Venezuelan ivrru jirrnv Con nr1r\c 670 786 97 8 z / 0 I TVil nnH Mnrnhv I 08 1 UIll allU iviuiuily 1 7ol San Carlos 748 23.1 Jordan and Uhl 1978 Columbian river terraces Caqueta 610 30.0 Duivenvoorden 1996 Suriname lowland forest Kabo 477 25.2 Jonkers and Schimdt 1984 ha. Lugo and Murphy (1986) found that tropical dry forest has 35 to 90 tree species per ha (^ 10cm DBH) and a basal area between 17 and 40 m 2 ha" 1 These authors also indicated that wet tropical forest has 50 to 200 tree species > 10 cm DBH and basal areas between 20 and 75 m 2 ha" 1 Although my figures for species richness in the forest were summed over three 0. 1 ha plots, tree species richness and basal area for terra firme forest at Curua-Una falls in between classifications for dry and moist lowland tropical forest.

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63 Table 4.7. Tree species diversity in plots of various sizes in lowland Amazonia. Location and Forest Type Minimum 111 o vi oior uiameier Plot Size (na) n OpcClCa Brazilian terra firme Curua-Una ;> 10 cm U.J 3 / una biuuy Curua-Una ^ 25 cm l ^8 4? JO HZ Pitt 1Q61 r ILL i/Ul Para ;> 10 cm i 1 1 o 1 OZ PnmnMI pf nl 1 086 l^allipUCll CL al. 1/OU Para ^ 10 cm j.j 1 7Q I 11 Ci CL ul. 17JJ Para 2. l\J Clll 1 X7 Rlark nl 1950 UlUCN CL ul. 1 V — V .rtJudZOndS ^ 10 cm I i Rlarkrtal 19Sft /MUaZUUuo Z 1 J CI 1 1 0 1 SO Klinpp et al 197S ^ 2.5 cm 0 1 U. 1 J L fipnfrv 1 078 vjciiiiy i y i o i\.oraima ^ 10 cm U.ZJ j j *f / i nompson ei ui. \yyL reruvian iciTa iirrnc I ailalllUHU ^ 10 cm i Zo_> vjciiLiy 1-700 IVliMlallu i> 10 cm I i L 1 J fWitrv 1 088 vjciiiiy 1700 V CllCZ^UCiail LC11U 1111UC Q on i nrlr\c kjclll Vul lUo III CTY\ 2 iu cm ft s uj" / y T rhl nnH Mnrnhv 1 08 1 win uiiu iviujpiiy i / o i San Carlos i: 10 cm 1 83 Uhland Murphy 1981 Columbian river terraces Caqueta ^ 10 cm 0.1 37 Duivenvoorden 1996 Suriname Kabo ^ 5 cm 1 108 Jonkers and Schimdt 1984 Vegetation Area Index My estimates of VAI for the forest control are lower than previously reported values of LAI in other Amazonian forests, perhaps because researchers have used different techniques to estimate LAI. For example, LAIs of 5.2 and 6.7 estimated from leaf dry weight/area

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64 relationships have been reported for terra firme forests in Venezuela (Jordan and Uhl 1978, Saldarriaga and Luxmoore 1991). Mc William et al. (1993) determined leaf area by similar methods for terra firme forest near Manaus and reported a LAI of 5.7. These authors sampled this forest during October, and the climate and temperature regime in this region is similar to that at Curua-Una. VAI of the forest stands at Curua-Una was estimated during clear days using the LICOR-2000, and it is possible that VAI was underestimated due to reflections of sunlight offleaf surfaces (LICOR 1991). These estimates for Amazonian forest leaf area are at the lower end of values reported worldwide for tropical forests, which have LAIs ranging from 6 to 17 (Leith 1975). Total Fine Litterfall and Seasonality of Litterfall Total fine litterfall (9.7 t ha' 1 yf 1 ) in the forest from October 1994 to October 1995 is higher than most values reported from Brazilian terra firme forest, which range from 6.9 to 9.9 t ha" 1 yf 1 (Table 4.8). Dry season intensity and the number of deciduous trees located in relatively small forest plots greatly influence total fine litterfall in this region; thus, variation in fine litterfall among sites is to be expected. Total fine litterfall in the forest control was greatest during the onset of the dry season (June August), so, if N concentrations in fine litterfall do not vary greatly over the year, aboveground N additions to the forest floor were also highest at this time. Seasonality of total fine litterfall has been widely reported in Brazilian terra firme forests, with peak litterfall in this forest type occurring at the onset of the dry season (Franken et al. 1979). Other neotropical lowland forests with distinct dry and wet seasons also have peak litterfall during the driest months of the year including semi-evergreen forest in Panama

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65 Table 4.8. Total fine litterfall and forest floor mass in lowland Amazonian forests. Total Fine Forest Floor Litterfall Mass Location and Forest Type (\ til" 1 1TT-^\ [i na yr ) (.tna ) V R L i\.eierence Kf ITllf Otl / /? yV/~1 T 1 V YVA SJ Drj/lllon lerru JlrlnH Curua-Una 9.7 1.2 1.34 This study Roraima 9.3 4.6 2.02 Scott etal. 1992 Roraima 6.9 3.1 2.22 Scott etal. 1992 Manaus 7.6 7.2 1.05 Klinge 1977 Manaus 7.3 Klinge and Rodrigues 1968 Manaus 7.9 Frankenetal. 1979 Para 9.1 Uhletal. 1988 Para 9.9 Klinge 1977 Para 8.0 Dantas and Phillipson 1989 (Wieder and Wright 1995), and deciduous forest in dry forest in Chamela, Mexico (MartinezYrizar and Sarukhan 1990) and Costa Rica (Borchert 1994). Total fine litterfall under P. caribaea slowed only during February (Figure 4. lb), and was consistently high compared to the other plantations throughout the year. Pinus caribaea planted in plantations produces more litterfall than similarly aged plantations of broadleaf tree species (Table 4.9) and secondary forests in Puerto Rico (Cuevas et al. 1991, Lugo 1992). Previous studies have found that forest leaf litter contributes approximately 60 to 75% of total fine litterfall (Bray and Gorham 1964, Proctor 1984). Leaf litter in the forest fell within this range at 69.5% of total fine litterfall (Figure 4.1a). The 13.9% contribution of reproductive parts in the forest control is a higher percentage than previously reported values

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66 Table 4.9. Total fine litterfall, forest floor mass, and litterfall: standing crop quotients (K L ) for )lantations at Curua-Una and other tropical sites. Location and Forest Type Age Total Fine Litterfall (t ha'yr"') Forest Floor Mass (tha 1 ) K L Reference Curua-Una P. caribaea 36 10.35 11.06 0.93 this study C. guianensis 36 8.81 10.26 0.85 this study Leguminosae 36 10.19 8.02 1.16 this study E. paraensis 23 8.09 111 1.04 this study Puerto Rico P. caribaea 7 7.5 9.1 0.82 Lugo 1992 P. caribaea 12 12.1 10.5 1.15 Cuevas et al. 1991 P. caribaea 21.5 14.5 18.8 0.77 Lugo 1992 P. caribaea 26 27.2 Lugo et al. 1990 Swietenia macrophylla 17 12 13.2 0.90 Lugo 1992 S. macrophylla 26 12.5 Lugo et al. 1990 S. macrophylla 49 14.1 9.76 1.44 Lugo 1992 Nigeria P cnrihflpfl J L Lit 1/ W t U 10 1 V/ 1 0 7 nguiijoui dnu Bada 1979 Costa Rica Stryphnodendron microstachyum 5 11.7 Montagnini et al. 1993 Hyeronima alchorneoides 5 8.2 Vochysia guatemalensis 5 12.6 Vochysia ferruginea 5 9.5

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67 in Amazonian terra firme forests. Reproductive parts in other lowland forests in Brazil have contributed 2.5 % (Klinge and Rodrigues 1968) to 13 % (Scott et al. 1992) of total fine litterfall. The year studied may have been a mast year in the forest; other lowland tropical forests have occasional years of exceptionally high fruit production (Stocker et al. 1995). Forest Floor Mass and Turnover Quotients The forest floor mass in the forest (7.28 t ha" 1 ) is almost identical to Klinge's (1973) standing crop estimate in a terra firme forest near Manaus, but much smaller than the estimate from Uhl et al. (1988) in a terra firme forest in Paragominas, Para (Table 4.8). My estimate for forest floor mass is also much higher than estimates for terra firme forest in Roraima (Table 4.8). Other seasonally dry neotropical lowland forest have similar estimates of forest floor mass, including a semi-evergreen forest in Panama (7.8 t ha" 1 Wieder and Wright 1995), and dry forests in Indian Church, Belize (7.2 t ha" 1 Lambert et al. 1980) and Chamela, Mexico (7.6 t ha" 1 MartinezYrizar 1995). Forest floor mass for P. caribaea and C. guianemis are at the high end of world estimates for forest floor mass in tropical and temperate forest ecosystems (Anderson and Swift 1983) Firms caribaea planted in plantations in Puerto Rico and Nigeria have similarly high standing crops of litter (Table 4.9). Pinus caribaea at Curua-Una is not as productive as similarly aged stands in Puerto Rico, possibly due to lower stand densities and slower growth at Curua-Una. Montagnini et al. (1993) indicated that broadleaf species planted in plantations in Costa Rica had statistically different forest floor mass after only 4.5 years after establishment, and total fine litterfall in these young plantations was equal to litterfall in the 36-year-old plots at Curua-Una (Table 4.9).

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68 Fine litter turnover quotients (K L ) in Curua-Una terra firme forest were slower than for a forest in Roraima, possibly due to Roraima's longer wet season (Table 4.8). Fine litter turnover quotients in the forest and legume plantations were similar to values reported from other lowland forests in Sarawak, New Guinea, and Malaysia which ranged from 1.0 to 1.7 (Anderson and Swift 1983). Seasonally dry lowland forest in Panama has a similar fine litter turnover quotient, ranging from 1.5 to 1.7 (Weider and Wright 1995). Fine litter in the monospecific plots of P. caribaea, E. paraensis, and C. guianensis had slow turnover rates which were similar to values reported for P. caribaea plantations in Puerto Rico (Table 4.9). Turnover quotients below one are infrequent in tropical lowland forests because fine litter decomposition is accelerated by warm temperatures and moist conditions (Anderson and Swift 1983). These low turnover quotients indicated that aboveground fine litter, particularly leaf and branch litter, in these plantations was relatively recalcitrant to decomposition. Litterfall N Inputs and Their Relationship with Soil N Total fine litterfall N inputs and leaf litter N concentrations from the forest were similar to those previously reported for terra firme forest in Brazil and Venezuela (Table 4.10). These values for aboveground litterfall N inputs and leaf litter N concentrations are intermediate compared to other tropical forests on a global scale (Vitousek and Sanford 1986). Total fine litterfall N inputs and leaf litter N concentrations under the legume plantations at Curua-Una fell within the range of values for terra firme forest, but the other plantations had values below that of undisturbed forest (Table 4. 10).

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69 Table 4.10. Total fine litterfall N contents and leaf litter N concentrations for Amazonian Location and Forest Type Total Fine LUlUl Jll IN (kgNha'yr 1 ) T oof I tttar XT Lxai Lriiter in (mg g"') Reference Rra7ilian tprrn firtnp lji n/.iimn it/ i u J*f #c 1 1 s 1 7 Q7 1 j.y 1 mis stuuy rvui uiiiiu 1 1 8 1 1 I j oCOll el 31. lyyZ Rnrnima 1 5 OLULL CI al. l77i Manaus 106 15 Klinge and ivuui iKUwa i7Uo ivioilaUd 1 8 1 o LUl/.uO 1V5V Manaus 109 14 Luizao 1989 Para 157 17 Klinge 1977 Para 115 Dantas and Phillincnn 1 Q8£ rlllllipbUll IVoD Venezuelan terra flrtne San Carlos 123.8 16.3 f^iipvn^ flnH V/UvV(U tlMU Medina 1986 Curua-Una plantation P. caribaea 43.1 4.5 this study C. guianensis 91.2 10.87 this study Leguminosae 134.4 15.65 this study E. paraensis 59.4 8.6 this study Puerto Rico plantation P. caribaea (6-yr) 14 3 Lugo 1992 P. caribaea (20.5-yr) 86 4.5 Lugo 1992

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70 Aboveground Litterfall and Total N in Surface Soils Total litterfall N contents, leaf litter N contents, within-stand NUE, and foliar N concentrations were not significantly related to total surface soil N concentrations under the forest and the plantations at Curua-Una. Total leaf litterfall (t ha" 1 yr" 1 ) was inversely related to total surface soil N (R 2 = .90, p < .01), with the lowest values for total soil N found in stands with higher leaf litterfall (Figure 4.2). Total soil N is composed of organicand inorganic-N, thus the significance of this finding is limited because it does not indicate if variations in leaf litter inputs have impacted soil N transformation rates or N availability. Also, belowground litter contributes greatly to total soil N, but I have not quantified these rates. In the next chapters, I will present data that examines seasonal net N mineralization rates, and I will attempt to determine the relationships between litterfall and N dynamics in the forest floor and mineral soil. o O to O CO 5 2 5 1 Y= 8.64 -.724 R 2 = .90, p = .02 6 T 8 9 10 Total Leaf Litterfall (t ha' 1 yr 1 ) Figure 4.2. The relationship between leaf litterfall and total N in surface soils under the forest (F) and plantations (P = Pinus, C = Carapa, E = Euxylophora, L = legumes) at the CuruaUna Reserve.

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71 Conclusions The plantations and forest represented a wide range of physical environments with basal areas, VAIs, tree densities, and tree species diversities varying widely. The forest is similar to other Brazilian terra firme forests with respect to litterfall N contents, the number of trees per hectare, and foliar N concentrations. Low within-stand NUE in the forest suggests that this lowland terra firme forest was not severely limited by nitrogen availability. The functioning of the plantations differed in varying degrees from the forest that they replaced, and at the time of this study these mature plantations were producing widely varying quantities of litterfall, had wide differences in aboveground N inputs from litterfall, and also had differences in forest floor mass and turnover. Within-stand NUE was lowest for the legume plantations and the forest, and highest for the P. caribaea plantation; NUE was not correlated with total N in surface soils. Other indices of site fertility such as foliar N concentrations and total litterfall N contents were also not related to total surface soil N among the treatments. The only significant relationship between aboveground litter production and total soil N was derived with total leaf litterfall (R 2 = .90), indicating that greater leaf litter production was related to smaller amounts of total N in surface soils. Relating litterfall and foliar characteristics to total soil N does not give an indication of the rates of N turnover in these surface soils and the forest floor, or how N transformation rates and N availability have been altered by the plantations. In addition, observations of unconfined litter turnover has shown that aboveground litterfall was decomposing at different rates under the forest and plantations, and that total fine litter and leaf litter N concentrations

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were not related to turnover. Therefore, other leaf chemical components must be controlling decomposition, and the influence of initial litter chemistry on leaf and needle decomposition rates should be determined under the plantations and the forest control. The differences observed among the stands for litterfall and aboveground N inputs demonstrated that the species comprising these plantations have different genotypic and phenotypic responses to this lowland Amazonian site. Although differences in surface soil total N concentrations signified that the plantations altered N dynamics in this environment, further evidence is needed to justify the claim that these plantations induced changes in soil chemical properties. Several questions that should lead to identifying the existence of plantinduced soil changes include the following: 1) Are there differences in fine root and microbial biomass among the treatments, and are they related to aboveground litterfall inputs?, 2) How have these observed differences in aboveground C and N inputs, forest floor mass, and litter turnover rates influenced forest floor and mineral soil C and N transformation rates?, and 3) How does initial leaf litter chemistry influence soil organic matter formation under each treatment? These questions will guide the work presented in the following chapters.

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CHAPTER 5 THE RELATIONSHIP BETWEEN FINE ROOT AND MICROBIAL BIOMASS, ABOVEGROUND LITTERFALL, AND C0 2 EVOLUTION UNDER TREE MONOCULTURES AND TERRA FIRME FOREST IN THE BRAZILIAN AMAZON Introduction Plants species with different nutritional requirements, phenologies, and litter chemical characteristics may induce changes in soil chemical properties and processes over time (Gower and Son 1992). A primary mechanism regulating plant-induced soil changes is the feedback effect that litter quality and quantity have on forest floor and soil nutrient mineralization and subsequent cycling (Binkley 1994). Characteristics of plants that dominate infertile environments (e.g., low N availability) include relatively high carbon (C) allocation to roots, low tissue N concentrations, low litter quality (i.e., high C/N ratios), and slow litter turnover (Chapin 1980, Tilman 1988). These plant characteristics may induce plant to soil feedbacks because they create conditions of low soil N availability and aid in the exclusion of plants that require higher soil N concentrations for establishment and growth (Tilman and Wedin 1990). In contrast, high tissue N concentrations, high quality litter (i.e., low C/N ratios), and rapid litter turnover characterize plants that dominate fertile environments (Vitousek 1982). The rapid cycling of soil N through high quality litter and root turnover is considered a positive feedback mechanism for species requiring high soil N availability to 73

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maintain N-rich tissues and to support the higher net primary productivity (NPP) that is characteristic of more fertile sites. Plant species may also influence soil nutrient cycling by altering belowground turnover through their impacts on microbial populations and activities. For example, plant-induced changes in leaf area will impact temperature and moisture regimes under forest canopies, thereby directly affecting the soil microbial biomass (Singh and Gupta 1977, Gupta and Singh 1981). Annual plant species influence microbial composition and population size by altering root exudates and through infection specificity between microbes and plant species (Chanway et al. 1991). Individual plants in agricultural fields have been shown to alter mycorrhizal populations through preferential mycorrhizal infection of grasses and forbs (McGonigle and Fitter 1990). In an experiment with four perennial grass species, Bever (1994) reported that negative feedback effects existed between grass species and their native soil communities, and survival of grasses in their own soil inocula was lower than for species grown in "exotic" inocula. Van Veen et al. (1989) indicated that microbial activity was also related to how a plant or group of plants impacted nutrient availability through root uptake, and these changes in soil nutrient status directly influenced microbially mediated processes. If plant species or groups of similar species influence root biomass and turnover in conjunction with microbial biomass and microbial activity, then the influence of a plant species on soil respiration and C storage potentially are great. The primary sources of C release from the forest floor and soils are microbially mediated decomposition of aboveand belowground litterfall and root respiration (Raich and Nadelhoffer 1989). Soil C release attributed to plant root respiration ranges from 35 to 90 % of total soil C release (Edwards and Harris 1977,

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75 Ewel et al. 1987, Bowden et al. 1993, Nakane et al. 1995, Thierron and Laudelout 1996), and root respiration plus heterotrophic respiration of C from dead roots generally contributes approximately 75% to total soil respiration in forest soils (Raich and Nadelhoffer 1989). Soil C0 2 evolution rates are temperature dependent, particularly in climates that are seasonally cold (Lloyd and Taylor 1989), and the highest soil evolution rates recorded were from moist, lowland tropical forests that had a warm climate, high annual precipitation, and large annual litter inputs (Raich and Schlesinger 1992). Soil C0 2 evolution rates contribute greatly to ecosystem C fluxes, and in terra firme forests in Brazil, the largest flux of ecosystem C release comes from the forest floor and soil (Fan et al. 1990, Jarvis et al. 1996). Experimental evidence suggests that groups of plants or plant species with similar characteristics influence soil processes through feedback effects involving both aboveand belowground litter quality and turnover. I decided further evaluate the hypothesized plant-tosoil feedback effects by examining differences in fine root biomass, microbial biomass, and soil C release among plantations and native forest in a lowland tropical environment. The primary hypothesis of this work was that the treatments that developed the smallest fine root and microbial biomass in mineral soil would also have had the slowest annual soil C release. The objectives were the following: 1) To determine if monocultures have altered soil C release compared to the forest and to identify seasonal patterns of soil C release among the treatments, 2) To determine if soil C release was related to litterfall inputs, forest floor mass, fine root biomass, or microbial biomass, and 3) To determine if fine root C allocation in the surface soil was related to litterfall inputs.

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76 Materials and Methods Soil Temperature. Moisture Contents, and CO Evolution The tree plots and soils previously described in Chapters 2, 3, and 4 were utilized for this portion of the study except that soil temperatures and C0 2 evolution rates were not estimated under E. paraensis because of logistical constraints. Surface soil temperatures (2 cm depth) were estimated monthly under the forest control, P. caribaea, the Leguminosae treatment, and C. guianensis by randomly placing two dial thermometers in each replicate once a month during the C0 2 evolution experiment. Soil moisture contents were estimated gravimetrically by collecting four randomly located samples per plot per month (0 20 cm), combining two samples, and drying the two combined samples per plot at 105 C until a constant weight. Forest floor and soil C0 2 evolution rates were estimated with the static chamber soda lime technique (Anderson and Ingram 1993) and following the recommendations of Raich and Nadelhoffer (1989). This technique underestimates C0 2 efflux, particularly when evolution rates are above 300 mg m" 2 hr" 1 (Cropper et al. 1985, Ewel et al. 1987, Nay et al. 1994) However, soil C0 2 emission rates determined with static chambers using a soda lime absorbent were correlated with emission rates in dynamic chambers in Massachusetts over a range of 150 370 mg m' 2 hr" 1 (Raich et al. 1990). The soda lime technique was chosen to estimate C0 2 evolution rates at Curua-Una because it was easy to operate at the remote field site, and allowed me to make at least a qualitative comparison of evolution rates among the plantations and the forest. C0 2 evolution rates were estimated monthly from September 1994 to October 1995, and each month, three buckets were randomly located in each plot of the forest, P. caribaea, C.

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77 guianensis, and the legume treatment. Each bucket was 23 cm diameter at the open end and had a chamber space of 6835 cm 3 Buckets were placed 2 cm into mineral soil 72 hours before estimating C0 2 evolution rates, and seedlings or other live vegetation were removed from the sampled area before the buckets were installed. After 72 hours passed, 40 g of 6-12 mesh soda lime were placed into aluminum cans 8 cm in diameter and 5 cm deep, and the cans were placed under the inverted buckets for 24 hours. The soda lime was dried immediately for 12 hours at 105C, and C0 2 evolution rates were estimated following correction for water uptake (Anderson and Ingram 1993). Soil C0 2 evolution was not estimated in October 1994. Root and Microbial Bi omass and Statistical Analysis Root biomass (0-10 mm diameter) of the surface root mat was estimated by random sampling during May (the height of the rainy season) and October (height of the dry season). I sampled three randomly located points in each plot by inverting a bucket 23 cm in diameter and cutting around the bucket edge. I then cut the roots entering the surface soil and removed the sample from the surface organic layer. The sample was washed with deionized water successively through a 2 mm and .425 mm soil sieve, debris was removed by hand, and roots were hand sorted into three diameter classes (0 -2 mm, 2. 1 5.0 mm, 5 .1 10 mm). All roots (dead and living) were oven-dried to a constant weight at 60 C and weighed. Nitrogen and carbon concentrations were determined on the finest fraction (0-2 mm diameter) with a Carlo Erba 1500 Nitrogen Analyzer after grinding the tissue. Root biomass and nutrient contents from surface soils (0 20 cm) were also estimated during May and October. During each of these months, I sampled six randomly chosen locations in each plot with a soil auger. Samples were washed with deionized water through

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78 a 2 mm and .425 mm soil sieve, and hand sorted into the three diameter classes. All roots (0 10 mm diameters) were oven-dried to a constant weight at 60 C and weighed. Nitrogen and carbon concentrations of the finest root fraction (0-2 mm diameter) were determined by combining the 6 samples from each plot. Microbial biomass-C and -N were estimated from surface soil samples taken during June 1995. Four randomly located soil cores (0 20 cm) from each plot were combined and one 50 g sample per replicate was kept for analysis. Soils were then refrigerated at field moisture contents for seven days until analysis. Estimates of microbial biomass-C were made following the ninhydrin-reactive nitrogen technique of Amato and Ladd (1988). Microbial biomass-N was calculated by multiplying ninhydrin-reactive N released from fumigated soils by 3.1 as recommended by Amato and Ladd (1988). The experiment was analyzed as a completely randomized design with treatment and month effects fixed. Each plantation and the control (forest) had three replicates, thus replicate means (n=3) of soil moisture, soil temperature, and C0 2 efflux were analyzed with a repeated measures ANOVA over the thirteen months sampled (September 1994 to October 1995). Estimates of C0 2 evolution rates were log transformed prior to analysis to homogenize variance. Seasonal means (dry season vs. wet season) of root biomass were also log transformed prior to analysis with a repeated measures ANOVA. Microbial biomass was only estimated once during the year, thus this experiment was analyzed as a one-way ANOVA with treatment effect fixed. Relationships between C0 2 efflux and the other variables were determined with PROC REG in SAS (SAS 1988).

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Results Surface Soil Temperat ures. Moisture Contents, and CO -, Evolution Mean monthly surface soil temperatures (2.5 cm) ranged between 25 and 28 C throughout the year (Figure 5.1) and were a full degree higher under P. caribaea compared to C. guianensis (Table 5.1). Repeated measures ANOVA of mean monthly surface soil temperatures detected significant differences among the treatments (p = .0001). Mean monthly soil moisture contents (gravimetric) peaked during the height of the rainy season in April, May, and June, and surface soils under the forest maintained the highest moisture contents throughout the year (Figure 5.1). Mean annual surface soil moisture contents were lowest under E. paraensis and highest under the forest (Table 5.1). Repeated measures ANOVA of mean monthly soil moisture contents detected significant differences among the treatments (p = .01). Pinus caribaea maintained the highest forest floor and soil C0 2 evolution rates throughout the year, while the forest consistently maintained the lowest rates (Figure 5.2). Mean annual soil C0 2 evolution rates under the forest were much lower than under the plantations (Table 5.1). Repeated measures ANOVA of log transformed estimates of monthly soil C0 2 evolution rates detected significant differences among the treatments (p = .003). Root and Microbial Biomass Estimates of mean fine root biomass (0-10 mm) from the surface root mat were lowest under P. caribaea and highest under the forest (Table 5.2). The forest and the legume plantations allocated the highest C and N contents to the 0 2 mm diameter class in the root mat (Table 5.2). Repeated measures ANOVA of log transformed data detected no treatment

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80 i i i i i i i i i i i r SNDJFMAMJ J A S O Month 30 29 28 82 ratu 27 \ CD emp 26 25 i — • — Forest — — Pinus Carapa — -— Legumes Euxylophora 24 i r i i i i i SNDJFMAMJJASO Month Figure 5.1. Mean monthly soil temperature and moisture ( 1 sd) from September 1994 to October 1995 at the Curua-Una Forest Reserve. Soil parameters were not estimated during October 1994.

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81 CM CM o o I T'l I I I I I I I I SNDJ FMAMJ J ASO Month i i i i i SNDJ FMAMJ J ASO Month o 800 600 400 200 0 'I I I I I I I M I I I SNDJ FMAMJ J ASO I I I I SNDJ FMAMJ J ASO Month Month Figure 5.2. Mean monthly forest floor and soil C0 2 evolution rates ( se) from September 1994 to October 1995 under A. The forest control, B. P. caribaea, C. C. guianensis, and D. the Leguminosae at the Curua-Una Forest Reserve. Soil C0 2 evolution was not estimated during October 1994.

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82 Table 5.1. Average monthly soil temperatures ( C) soil moisture (gravimetric, 2.5 cm), and C0 2 evolution rates (n = 13) from mineral soil under native forest and plantations at the Curua-Una Reserve ( se). Treatment Soil temperature ( C) Soil moisture (%) C0 2 efflux (mg m" 2 hr" 1 ) Forest 25.6(0.12) 31.4(0.8) 350.0 (17.5) P. caribaea 26.5 (0.24) 29.2 (0.8) 495.9 (20.5) C. guianensis 25.2 (0.32) 29.9 (0.6) 450.9(17.8) Leguminosae 26.3 (0.16) 29.3 (0.6) 430.2 (17.8) E. paraensis 28.9 (0.6) effect (p = .63), and no season effect (p = .59) for total root biomass (0 10 mm) in this surface layer despite large differences among treatment means. In the surface organic layer, there were also no treatment effects (p = .61) detected for mean biomass in the fine root class (0-2 mm diameter) following the repeated measures analysis of log transformed data. Estimates of mean root biomass (0-10 mm) from surface soils (0 20 cm) were lowest under P. caribaea and highest under the legume plantations (Table 5.3). In the finest root class (0-2 mm), E. paraensis had the largest biomass and C and N contents, while P. caribaea had the smallest biomass and allocated the least C and N in surface soils of all the treatments (Table 5.3). Repeated measures analysis of log transformed data did not detect a significant treatment effect ( p = .07) for total root biomass (0-10 mm), but a significant treatment effect was detected for the 0 2 mm root class (p = .002). In addition, no significant season effect was detected among the treatments for the 0 -2 mm diameter class (p = .37). Total root C allocation (0-10 mm diameter) in the surface organic layer and surface soils was related to aboveground litter N inputs (Figure 5.3).

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83 •2 o .a .a 2 § 60 ao c — M o u 0 E a '-5 *i o o in E £ o ^ w-i g CN E E cn £0 00 o o o Cl r--' v ' w IN o O o CI O on o — 00 I N f— i I— J o o l/"l ro 00 (N no 00 00 On' (N 1 — 1 CN CN C CI cn d o ri o vq (N (N' CN SO CN ri 00 rOn' in 00 On CN ci m ci On' IT) O CI in s ts m On' 00 00 On' On CN CO CN 00 ^ m in CN O CI rCI ts CN u. O PL, I I 03 O C 3 if) s ft) I E CO '-5 o o 03 c E E T3 C OS 2 ^ u ^ V II o "5 £ .2 13 * *-> 7 c -a Arte 73 *j § £ o? >S „ § u oi e ^ E 3 c o E 2 p§ • 2; -*— X) — cd so E o — t5 J: £u •*-• C — 1 o = 00 O 0 0 On NO (18.1) (32.2) (7.2) (55.4) (11.0) 00 Cl 00 00 CN 00 On r) nO On' — CI Cl CN r~0 O 0 rin *t 00' Cl O 0 O On' Cl O 00 00
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84 7 20 40 60 80 100120140160 Annual Fine Aboveground Litterfall N Inputs (kg N ha ^ ) Figure 5.3. The relationship between root C allocation (0-10 mm diameters) and aboveground litterfall N inputs among plantations and terra firme forest at Curua-Una. Estimates of microbial biomass-C and -N from surface soils at Curua-Una were lowest under E. paraensis and the forest and highest under C. guianensis (Table 5.4). An analysis of variance of treatment means did not detect a significant treatment effect (p = .30) for biomass-C or -N. Table 5.4. Microbial biomass for mineral soil (0 20 cm) under native forest and plantations (n = 3) at the Curua-Una Reserve Treatment wg C g" 1 soil ?/g N g" 1 soil Forest 227.84 (9.4) 33.63 (1.3) P. caribaea 236.11 (14.0) 34.85 (2.0) C. guianensis 266.58 (25.6) 39.35 (3.7) Leguminosae 265.15 (15.7) 39.14(2.3) E. paraensis 226.06 (15.8) 33.37 (2.3)

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85 Relationshi ps between C0 : Evolution Rates and Soil. Root, and Microbial Parameters Mean monthly values for surface soil temperatures and moistures were not related to mean monthly values for C0 2 evolution rates from September 1994 to October 1995 (R 2 = .05 and .03, respectively). Annual estimates of soil C release, root biomass-C (0-10 mm) from the surface root mat and surface soils, and microbial biomass-C (from surface soils) were derived to estimate annual C pools and fluxes from surface soils at Curua-Una (Table 5.5). Total root biomass-C and microbial biomass-C were not related to annual soil C release (R 2 = .5 1 and .12, p = .28 and .64, respectively). Annual soil C release was inversely related to surface soil texture (% clay + silt) while soil C release was related to forest floor mass at Curua-Una (Figure 5.4). Table 5.5. Total annual C release by microbial and root respiration, total root C pools (0-10 mm diameters) in surface organic layers and mineral soil (0 20 cm), and soil microbial C pools (0 20 cm) under native forest and plantations at the Curua-Una Reserve. Estimates of soil and forest floor C release were based on sampling from Nov. 1994 to Oct. 1995. C release Root C Microbial biomass-C Treatment (t haV) (t ha 1 ) (t ha 1 ) Forest 8.38 5.48 0.419 P. caribaea 11.82 1.26 0.434 C. guianensis 10.79 4.10 0.490 Leguminosae 10.34 5.84 0.487 E. paraensis 3.88 0.415

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86 co
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87 Discussion C_Q 2 Efflux from the Forest Floor Total forest C exchange is dominated by C release from forest soils, and in the Amazon, 80 to 85% of total forest C fluxes from terra firme forests originates from the forest floor and soils (Fan et al. 1990, Jarvis et al. 1996). C0 2 efflux measured with more precise methods such as dynamic chambers or static open chambers and gas chromatography ranged between 0.026 and 0.063 t C ha" 1 d" 1 from soils of lowland terra firme forests in Brazil (Table 5.6). These values exceed my estimate of average daily soil C0 2 efflux from the forest at CuruaUna (0.022 1 C ha" 1 yr' 1 ). As previously mentioned, it is likely that the static chambers using a soda lime absorbent underestimated C0 2 efflux at Curua-Una when evolution rates exceeded 300 mg m" 2 hr ~ l during the wet season of 1995. Nevertheless, the comparisons at Curua-Una are valid because they gave an indication of differences in C0 2 efflux among the treatments, and these estimates are also useful indices of differences in root and heterotrophic respiration rates and the seasonality of C0 2 efflux under the forest and the plantations. In contrast to the large differences in dry and wet season C0 2 efflux at Curua-Una, Trumbore et al. (1995) found little seasonal variation in terra firme forest soil C0 2 efflux in Paragominas (700 km east of Curua-Una). These authors indicated that C0 2 production occurring at soil depths to 8 meters buffered slower surface soil respiration in the dry season. In addition, Nepstadt et al. (1994) reported that 70 to 80% of fine root biomass in forest soils of Paragominas was found in the surface 1 meter, and Davidson and Trumbore (1995) determined that 70 to 80% of soil C0 2 production occurred at this same depth. Trumbore

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88 Table 5.6. Forest floor and soil C release from Brazilian terra firme forests and Curua-Una plantations. Average daily rates of C release were derived from average annual estimates at Curua-Una using a soda lime static chamber. Daily rates from the other sites were estimated using infrared analysis of samples from dynamic chambers or gas chromatography of samples drawn from static chambers. Location C Release (t ha 1 d" 1 ) Time of Sample Author Curua-Una Forest 0.022 annual average this study P. caribaea 0.032 annual average this study C. guianensis A Alfl annual average this study Leguminosae (J.Uzo annual average this study Para 0.063 annual average Trumbore et al. 1995 Amazonas 0.053 April May Fanetal. 1990 Amazonas 0.042 July Wofskyetal. 1988 Amazonas 0.041 July August Goreau and de Mello 1985 Amazonas 0.026 December Keller 1986 Amazonas 0.031 March Keller 1986 et al. (1995) also estimated that 50 to 67% of total forest soil C0 2 efflux in this region was attributable to root respiration. In their review of forest soil C dynamics, Raich and Nadelhoffer (1989) reported that total soil respiration attributable to belowground sources (root respiration and turnover) ranged from 70 to 80%. Using previously reported estimates of aboveground litterfall C inputs in their calculations, Keller et al. (1986) and Wofsky et al. (1988) determined that root respiration and decomposition contributed between 54 to 8 1 % of total soil C release in

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89 Brazilian terra firme forests. At Curua-Una, aboveground fine litterfall C inputs during the one year study ranged from 4.3 1 to 5.07 1 ha" 1 (Chapter 4) and soil C release ranged from 8.4 to 11.8 1 ha' 1 (Table 5.7). If I assume that the plantations and the forest were at steady state and leaching losses of inorganic and dissolved organic carbon were minimal, then soil C release from belowground sources (root + heterotrophic respiration) contributed at least 43 to 60% of total soil C release at Curua-Una (Table 5.7). Table 5.7. Forest floor and soil C release attributed to root respiration and decomposition at the Curua-Una Forest Reserve. Treatment Aboveground C Inputs (t ha 1 yr 1 ) Annual Soil C Release (t ha 1 yr 1 ) % Soil C Release from Belowground C Sources Forest 4.78 8.38 43 P. caribaea 5.07 11.82 57 C. guianensis 4.31 10.79 60 Leguminosae 4.99 10.34 52 If fine root respiration and decomposition (turnover) contribute the majority of C to total soil C release, why did soils under P. caribaea consistently have higher C0 2 efflux than the forest control (Figure 5.2)? Although the forest had higher mean root biomass (0 10 mm diameter) in the surface root mat and in surface soils, these differences were not statistically significant. Possibly, fine root turnover and respiration were higher under P. caribaea, resulting in larger C0 2 efflux from belowground sources. Increased C allocation to belowground biomass and elevated root: shoot ratios are theoretically an efficient manner for plants to obtain nutrients in nutrient-poor soils (Bloom

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90 et al. 1985). For tropical forests, Vitousek and Sanford (1986) reported that fertile soils (i.e., Alfisols) supported forest stands with smaller fine root biomass and lower root: shoot ratios compared to stands growing on infertile soils (i.e., Oxisols, Ultisols, and Spodosols). In contrast, Nadelhoffer et al. (1985) reported that annual belowground C allocation in nine temperate forests did not decrease as soil N availability increased, supporting Chapin's (1980) hypothesis that plants occupying fertile sites have elevated fine-root turnover rates compared to plants adapted to infertile sites. Therefore, higher estimates of fine root biomass in resource-poor soils compared to resource-rich soils do not account for biomass that has been annually shed and decomposed in the fertile environments (Tilman 1988). In this study, P. caribaea potentially maintained higher rates of soil C0 2 production by shedding larger quantities of fine roots throughout the year studied, and by concurrently maintaining rapid belowground turnover. Raich and Nadelhoffer (1989) reported that total C allocation to roots increased with higher quantities of litterfall; at Curua-Una, aboveground fine litter inputs and forest floor mass were largest under P. caribaea (Table 4.2, 5.7). Although soil O horizons have lower C0 2 concentrations than surface soils because they are porous (Fernandez et al. 1993), the deep, recalcitrant litter layer under the pine may have supported larger microbial biomass, which resulted in continually high C0 2 efflux as the microbes attempted to mineralize the litter substrate. In support of this theory, C0 2 release was related to forest floor mass at CuruaUna (Figure 5 .4). The temperature and moisture contents of the forest floor and surface soils were also potential factors in the difference in C0 2 evolution rates between the forest and the pine

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91 monoculture. Temperature and moisture are the two of the most important factors controlling fluxes of C from litter and soil organic matter in forest ecosystems (Zak et al. 1993), and average annual temperatures of the surface soils under the pine were almost one degree higher those under the forest control (Table 5.1). Although surface soil moisture contents were higher under the forest, perhaps the increased temperatures under the pines caused a corresponding increase in microbial activity in the forest floor and surface soils. Finally, the static chamber technique using a soda lime absorbent was inadequate for measurement of C0 2 effluxes from termite mounds, stump mounds, ant nests, or large decomposed logs because the inverted bucket could not be properly sealed in these locations These potentially are areas of high C0 2 production (Zimmerman et al. 1982), and since the forest had a higher number of these sites, I would have missed these fluxes and underestimated C release under the forest compared to the plantations. Surface soil particle size also influenced rates of soil C0 2 evolution at Curua-Una (Figure 5.3). Fine earth fractions are generally believed to protect soil organic matter from microbial mineralization, particularly in Oxisols containing iron and aluminum oxides, which bind with the carboxyl groups of soil organic matter in ligand exchange reactions (Oades et al. 1989). In support of this theory, Motavalli et al. (1994) reported that clay contents were negatively correlated with C0 2 -C release from Amazonian forest soils, and Lepsch et al. (1994) found that soil organic C was positively related to clay + silt fractions in southern Brazil. Sorenson (1981) and Van Veen et al. (1985) also indicated that soil organic matter and microbial biomass-C were more stable in soils with high clay contents because they formed stable complexes with the clay particles.

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92 Microbial Biomass Early reports of low soil microbial biomass-C in tropical soils were attributed to rapid turnover and high predation (Theng et al. 1989). More recent estimates of biomass-C and -N from soils in Costa Rica and Brazil contradict these earlier reports (Table 5.8). In fact, microbial biomass in some tropical soils exceeds that of temperate forest soils which have biomass-C values ranging between 289 and 1900 ug g _1 soil (Vance et al. 1987a, b). Table 5.8. Microbial biomass-C from surface soils in lowland tropical forests and plantations at Curua-Una. Location Soil order Depth (cm) ug C g"' soil ug N g' 1 soil Author Curua-Una Oxisol 0-20 227 266 33-39 this study La Selva, Costa Rica Inceptisol 0-15 700 2000 Henrot and Robertson 1994 Amazonas, Brazil Oxisol 012 357 699 37-73 Motavalli et al. 1994, 1995 Oxisol 0-5 6-20 1287 765 Luizao et al. 1992 Luizao et al. 1992 India Ultisol 010 405 677 41 -71 Srivastava 1992 Ultisol 010 487 744 51-88 Singh etal. 1989 Surface soils under all the treatments at Curua-Una supported lower microbial biomass-C than other tropical sites (Table 5.8). I attribute the low values at Curua-Una to the depth and season of the surface soil sampling. Soil microbial biomass decreases with depth, and I may have diluted the biomass-C pool by analyzing such a large portion (0 20 cm) of the surface soils. Microbial biomass-C at Curua-Una during June was 0.4 to 0.5 % of total soil C (50 to 60 g kg" 1 soil), and this is a lower percentage of total surface soil C than soils near Manaus

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93 (3.5 to 5.3%, Luizao et al. 1992), throughout the Amazon (0.5 to 2.7 %, Motavalli et al. 1994), and in temperate forest (1.8 to 2.9 %, Vance et al. 1987). Also, I sampled soils in June during the height of the rainy season, and although these soils are very well drained, they could have experienced microsite waterlogging which reduced microbial biomass-C during that time of the year. Root Biomass and N Content Nepstadt et al. (1991) reported that Oxisols supporting terra firme forests in Brazil's eastern Amazon contained 73% of total forest root mass (0 10 m soil depth) in the upper 20 cm of the soil horizon (including the root mat). They also determined that 42% of total root length was found to this same depth. It is also important to note that at this same site in Paragominas, approximately 23% of total root length and 16% of total root biomass was found below a 1 m depth, indicating that previous studies examining total root biomass in the surface 1 m may have missed significant portions of total root biomass in these systems (Nepstadt et al. 1991). This deep rooting is reported by Nepstadt et al. (1994) to be common in other regions of the Amazon, and these authors hypothesized that deep roots in these seasonally dry forests have an important role in maintaining dry season water and nutrient supplies to these semi-evergreen lowland forests. Compared to two other terra firme forests in lowland Amazonia, root biomass in the surface organic layer and surface soils in the forest at Curua-Una is most similar to terra firme forest near Manaus, which has a similar climate (Table 5 .9). The terra fume forest in Venezuela had substantially larger fine root biomass in the surface organic layer which is perched on top of a sandy horizon 10 40 cm deep (Stark and Spratt 1977). Although the

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94 Table 5.9. Root biomass in Amazonian terra firme forests. Location depth sampled (cm) diameter (mm) dry weight (t ha 1 ) author Curua-Una root mat 0-2 3.04 this study 2.1 10 0.36 mineral soil 0-2 3.87 2.1 10 4.43 Arna7nrui
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95 Amazon, but was significantly larger than mean fine root biomass under P. caribaea in mineral soil in both periods sampled (p = .04 and .001, respectively). These differences in fine root biomass (0-2 mm diameters) may reflect P. caribaea' s ability to capture necessary resources with smaller belowground C allocation. Alternatively, P. caribaea may have allocated large quantities of C to belowground biomass and maintained high root turnover (but I did not assess these). At Curua-Una, P. caribaea had the smallest belowground C and N fractions allocated to forest floor and surface soil roots, maintained the largest amounts of C in forest floor mass, and had the lowest inputs of N in fine litter (Table 4.2, 4.4). How did this buildup of recalcitrant litter, low inputs of N from litter, and low N allocation to root biomass affect soil N availability? In addition, allocation of root C (t ha" 1 yr' 1 ) was related to litter N inputs in the plantations and the forest (Figure 5.3), suggesting aboveground N inputs were linked with root dynamics at Curua-Una. Conclusions In this chapter, I have attempted to demonstrate that forest floor and soil C dynamics at Curua-Una have been altered by the presence of plantations. The most significant findings of this work include the following: 1) There were significant differences in surface soil moisture, temperature, C0 2 evolution, and surface soil fine root biomass among the plantations and forest, indicating that the change in vegetation has influenced soil properties and soil C storage,

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96 2) There was a negative relationship between surface soil particle size (% clay and silt) and C0 2 evolution rates, perhaps indicating that fine soil particles protected soil organic matter at this site, 3) Forest floor mass was positively related with annual soil C release, while root biomass and surface soil microbial biomass were not related to forest floor mass, 4) Annual forest floor and soil C release from belowground C sources were estimated to contribute more than 50% to total annual C release, 5) Litter N inputs were positively related to forest floor and mineral soil fine root C allocation at this site.

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CHAPTER 6 FINE LITTER CHEMISTRY, DECAY, AND NITROGEN DYNAMICS UNDER PLANTATIONS AND PRIMARY FOREST IN LOWLAND AMAZONIA Introduction Leaves account for most fine litterfall in forests (Bray and Gorham 1964, Proctor 1984), and decomposition and nutrient mineralization of those leaves are key components of ecosystem nutrient cycling. Tree species have evolved a wide range of leaf chemistries, and decay rates and mineralization of nutrients from tree litter are regulated by litter chemistry in addition to temperature and moisture (Waksman and Starkey 1931, Jenny et al. 1949, Swift et al. 1979). Leaf litter has two phases of decomposition (Berg 1986, Melillo et al. 1989), and the first phase of decay follows an negative exponential model until approximately 20 % of original mass remains (Olsen 1963, Aber et al. 1990). The second phase of mass loss approaches an asymptotic value as the relative proportion of recalcitrant material in leaf litter increases with time (Weider and Lang 1982, Berg et al. 1984). Lignin and holocellulose fractions of leaf litter with varied initial chemistries converge over time (McClaugherty et al. 1985, McClaugherty and Berg 1987), suggesting that the long-term degradation products from forest leaf litter may be similar despite large differences in initial leaf chemistries (Mellilo et al. 1989). Proximate carbon (C) fractions (e.g., lignin, cellulose, and polyphenols) or initial nitrogen (N) and phosphorus (P) concentrations regulate phase 1 decomposition of leaf litter in many 97

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98 forests (Horner et al. 1988, Taylor et al. 1989, Vitousek et al. 1994). In some forest systems, total C:N ratios (Edmonds 1980) were better predictors of leaf litter decay than initial nutrient concentrations. In California, initial leaf litter C:P ratios were more important determinants of mass loss than C:N ratios or initial nutrient concentrations (Schlesinger and Hasey 198 1). Carbomnutrient ratios are not always the best predictors of mass loss because C fractions of leaf litter vary in their susceptibility to leaching and attack by microorganisms (Swift et al. 1979). Proximate C fractions in leaf litter are generally placed into three categories including extractives (fatty acids, sugars, and phenolics), holocellulose, and lignins (Ryan et al. 1990), and these fractions are known to decay differentially (Minderman 1968, Berg et al. 1982). No single C fraction or C fraction: nutrient ratio has been found to regulate long-term leaf litter decomposition across a range of leaf litter qualities and regions, and studies have variously shown that initial leaf litter lignin:N ratios (Mellilo et al. 1982), cellulose:lignin:N ratios (Entry and Backman 1995), lignin concentrations (Fogel and Cromack 1977), and polyphenolic concentrations (Palm and Sanchez 1990) were good predictors of decay rates. In microcosms with controlled temperature and moisture levels, leaf litter decay is primarily related to its chemical properties, although the physical structure of leaf litter may also play an important role (Taylor et al. 1989, Cortez et al. 1996). When a common leaf litter is placed at field sites with different climates, mean annual temperature and moisture are the dominant rate-regulating factors. For example, in Hawaii, substrates of consistent quality had apparent temperature Q 10 values ranging from 4 to 1 1 (Vitousek et al. 1 994). At a larger geographical scale that encompasses several climatic regimes, actual evapotranspiration (AET) has been shown to be the dominant regulating factor of leaf litter decay (Meentemeyer

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99 1978, Johansson et al. 1995). Across this same regional scale, leaf litter lignin concentrations may play a larger role in regulating decay in the later stages (i.e., phase II) of decomposition (Johansson et al. 1995). In this chapter, I examine the influence of initial C fractions and N concentrations on leaf litter decay under the plantations and the forest at the Curua-Una Forest Reserve. The primary objectives of this study were to: 1) identify the C fractions that controlled leaf litter decomposition under plantations and undisturbed forest in a lowland tropical environment, 2) determine if there were significant differences for mass loss among the leaf litter that had widely different initial C fractions and N concentrations, 3) determine what, if any, initial leaf litter chemical constituent regulated leaf litter N dynamics during one-year field incubations, and 4) determine if leaf litter decomposition changed significantly when litter was placed outside its stand of origin. Materials and Methods Field Methods Decomposition Study Fresh leaf litter (1 to 5 days) was collected during the first week of September 1994 from litter traps installed in the plantations and the forest. This collection occurred during the season of peak fine litterfall (Chapter 4), and also corresponded with the annual dry season which continued until November. Only leaf decomposition was examined because approximately 70% of fine litterfall in most forest systems consists of leaf fall (Bray and Gorham 1964, Proctor 1984). Live leaf samples were mixed with abscised foliage from the Leguminosae treatment because I was unable to collect sufficient quantities of leaf litter fall in these three plantations. Approximately 25% of the samples placed in the Leguminosae

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100 bags were from live leaf material. All the samples were air-dried, and approximately 15 g were placed in 15 cm x 25 cm nylon bags with a 1 mm 2 mesh. Pinus caribaea samples were placed in bags of 12 cm X 28 cm. Oven-dry mass for samples in each bag was derived by oven-drying randomly chosen subsamples (100 g) to a constant weight at 60 C to obtain a correction factor. These initial oven-dried samples were kept for chemical analysis. The litterbags were placed in the field September 21, 1994, and each replicate received 24 bags of litter (from the plot of origin) divided into 2 groups of twelve. Each group of twelve was randomly placed on the forest floor in a lm 2 subplot, and each month, one bag from each subplot was removed, oven-dried, weighed, and then the two bags from each plot were bulked for chemical analysis. The last samples were removed during September 1995. Mixed Site Study A separate study was conducted with litter collected during February 1995 to examine if leaf litter placed outside of its stand of origin would decompose at rates comparable to those under its native stand. Leaf litter collected from litter traps under the forest, P. caribaea, and the Leguminosae were air-dried, and approximately 5 g were placed in 1 5 cm x 25 cm nylon bags with a 1 mm 2 mesh. These three treatments were chosen because the represented leaf litter with a high L/N ratio (P. caribaea), leaf litter with high lignin and high N concentrations (the forest), and leaf litter with a low L/N ratio (Leguminosae). Leaf litter from the three Leguminosae plots were mixed in equal proportions for this part of the study. Subsamples (100 g) from the initial leaf litter were oven-dried at 60 C to a constant weight to obtain a correction factor. In each plot of the forest, P. caribaea, and the Leguminosae, I placed 1 8 leaf litter bags (six each from P. caribaea, the forest, and the Leguminosae) in a 3 m x 3m

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101 subplot during March 1995. Six bags of each treatment were removed every 3 months for 1 year (until March 1996). Samples removed from the field were oven-dried at 60 C to a constant weight, and these samples were analyzed only for mass loss. Laboratory Methods Initial and one-year-old litter samples from the decomposition study (not the mixed site study) were analyzed for total C, N, lignin, cellulose, hemicellulose, and polyphenol concentrations. Each month, the two bags removed from each replicate were oven-dried at 60C to a constant weight, weighed, and then combined. Each combined sample was ground to pass a 1 mm mesh in a Wiley mill. Subsamples to be analyzed for total C and N concentrations were finely ground with a ball grinder, and submitted for analysis in a Carlo Erba Nitrogen Analyzer 1500. Lignin and cellulose concentrations were determined on oven-dried samples ground to pass a 1 mm mesh following the acid detergent fiber (ADF) methodology of Van Soest (1963) and Goering and Van Soest (1970). Hemicellulose concentrations were determined by estimating total cell wall constituents (hemicellulose + lignin + cellulose) by the neutral detergent fiber (NDF) technique (Golding, Carter, and Moore 1985), and then subtracting the ash-free ADF from the ash-free NDF. Polyphenol concentrations were determined following the methods of King and Heath (1967) and Allen et al. (1974) and are reported as tannic acid equivalents. The compounds extracted with this procedure include hydrolyzable and condensed tannins and non-tannin polyphenolics (Anderson and Ingram 1993).

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102 Statistical Methods Leaf Chemistry Initial leaf chemistry was analyzed as a one-way analysis of variance with treatment effects fixed. Replicate means of leaf litter chemistry were used in this analysis (n = 3), and Dunnett's t-test was used to identify leaf chemistries that were significantly different from the forest. Absolute values of N and C fractions were used for statistical analysis of samples that were left in the field for one year. To obtain absolute values, the concentrations of the chemical constituents were multiplied by the fraction of the original litter mass remaining and expressed as a percent of the original N or C fraction content. The leaf chemistries of yearone samples were analyzed as a one-way analysis of variance using replicate means (n = 3), and Dunnett's t-test was used after analysis of variance to identify leaf chemistries that were statistically different than the forest. Mass Loss Mass loss from leaf litter of the five treatments was analyzed using the standard exponential decay function y = e" 1 (Olsen 1963), where y is the fraction of initial mass remaining, t is time, and k is the decay constant. For this analysis, I followed Aber et al. (1990) where k = the exponential decay constant fit to time series data and K = the litterspecific exponential model, which is an estimate of the time required to reach the end of phase 1 decomposition (20% mass remaining). This estimate of K is derived by fitting the exponential model separately to data for each litter type. Significant differences among the treatment's decay constants (k) were analyzed with a one-way analysis of variance with treatment effects fixed. The k values were normalized by log transformation of the inverse of k prior to analysis (Little and Hills 1978). I used PROC REG (SAS 1988) to identify

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103 relationships between the exponential decay constant (k) for each treatment's leaf litter and initial and year-one leaf chemical constituents. For the mixed site study which utilized litter from only three treatments, significant differences among each treatment's k were analyzed with a nested analysis of variance (n = 3). Replicate (plot) and leaf litter effects were analyzed according to a randomized block design. However, replicate effects were nested within location (treatment), so the effects of the replicates were modeled with location effects and replicate within location effects. Differences among location effects (treatment were litter was placed) were assessed by using variability among replicates within locations as the error term. Again, prior to analysis, k values were normalized by log transformations. Changes in Nitrogen and Carbon Fractions I used Aber and Melillo's (1980, 1982) inverse-linear relationship to correlate the percentage of original mass remaining with relative N concentrations of leaf litter to predict N concentrations of litter at the end of phase 1 decomposition (i.e., 20 % mass remaining). This relationship has been shown to be a good predictor of long-term N dynamics using shortterm decay data in temperate forest ecosystems (Aber et al. 1990). Nitrogen concentrations of the initial oven-dried leaf litter and material removed monthly from October 1 994 to September 1995 were used to derive the inverse-linear predictions. Indices of the fractions of holocellulose, lignin, and polyphenol concentrations in leaf litter were estimated initially and after the field incubations to compare changes in C fractions of litter placed in decomposition bags. The lignocellulose quotient, or HLQ (McClaugherty and Berg 1987), was estimated as the fraction of holocellulose (hemicellulose + cellulose) in

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104 lignocellulose (lignin + holocellulose). I also derived a polyphenol quotient as (polyphenol concentration / (lignin + holocellulose + polyphenol) to examine changes in the relative proportion of polyphenolics over time. Significant differences among these ratios were detected with a one-way ANOVA of arcsin transformed data with treatment effects fixed Results Initial Leaf Chemistry Initial leaf litter N concentrations (Table 6. 1) of litter collected in September 1994 ranged from 0.4 % (P. caribaea) to 1.6 % (Leguminosae). Analysis of variance detected significant differences among treatments for N concentration (p = .001), and N concentrations of leaf litter of P. caribaea and E. paraensis were significantly lower than the forest (Table 6.1). There were also significant differences among initial C concentrations (p = .0001), and leaf litter of E. paraensis had significantly lower C concentrations than the forest (Table 6. 1). Lignin concentrations, which ranged from 19.0 % (Leguminosae) to 45.9% (forest), were higher than holocellulose concentrations for leaf litter from the forest, P. caribaea, and C. guianensis (Table 6. 1). There were significant differences in leaf litter lignin concentrations (p = .002), but not for holocellulose concentrations (p = .10). Leaf litter polyphenol concentrations were lowest for E. paraensis (2.63%) and highest in the Leguminosae (4.86%). There were no significant differences in leaf litter polyphenol concentrations (p = .17). Year-one Leaf Chemistry Nitrogen concentrations (absolute values) in leaf litter incubated for one year were lowest in P. caribaea and highest in C. guianensis (Table 6.2). There were significant differences

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105 Table 6.1. Initial leaf chemistry from leaf litter used in the decomposition bag experiment (n = 3) at the Curua-Una Reserve ( se). Values followed by (*) were significantly different than the forest (p < .05). Leaf Litter Characteristic Treatment N (%) C (%) Lignin (%) Holocell. (%) Poly. (%) Forest 1.40 (.08) 49.2 (.28) 45.9(3.0) 21.9(1.7) 2.6 (.12) P. caribaea 0.44 (.04)* 50.1 (.19) 37.8 (4.5) 30.9 (0.2) 3.6(50) C. guianensis 1.30 (.04) 47.3 (.11) 35.1 (4.0) 30.1 (1.5) 3.8 (.60) Leguminosae 1.63 (.34) 47.5 (.95) 19.0(5.0)* 26.0 (4.8) 4.8 (1.1) E. paraensis 0.70(.02) 43.3 (.40)* 21.4(1.0)* 23.9(0.6) 2.6 (.58) among the litter types for N and C concentrations (p = .004 and .000 1 respectively), and N concentrations of one-year-old P. caribaea leaf litter was significantly lower than forest leaf litter. Carbon concentrations of leaf litter from P. caribaea and C. guianensis were significantly higher than the forest, while the Leguminosae leaf litter had significantly lower C concentrations than the forest (Table 6.2). Lignin concentrations were higher than holocellulose concentrations for all the litter types examined after one year in the field (Table 6.2), and there were significant differences among treatments for both lignin and holocellulose concentrations (p = .02 and .0001, respectively). One-year-old leaf litter under/ 3 caribaea and E. paraensis had significantly higher holocellulose concentrations than one-year-old leaf litter under the forest (Table 6.2). Polyphenol concentrations in one-year-old leaf litter were much lower than the initial values, and ranged from 0.05% to 0.40% (Table 6.2) There were no significant differences among the treatments for polyphenol concentrations of one-year-old leaf litter (p = .09).

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106 Table 6.2. Leaf litter chemistry after one year field incubations from the decomposition bag experiment (n = 3) at the Curua-Una Reserve ( se). Absolute values based on original leaf mass. Values followed by (*) significantly different than the forest (p < .05). Leaf Characteristic Treatment N (%) C (%) Lignin (%) Holocell. (%) Poly. (%) Forest 1.18 (.14) 26.9 (.31) 34.6 (2.7) 13.5 (1.9) 0.22 (.02) P. caribaea 0.51 (.01)* 35.1 (.22)* 35.8 (0.8) 25.5 (1.6) 0.40 (.03) C. guianensis 1.27 (.17) 29.9 (.39)* 38.0(6.5) 19.8 (1.1) 0.21 (.05) Leguminosae 0.84 (.08) 18.5 (.29)* 23.8 (2.5) 8.9 (0.4) 0.36 (.18) E. paraensis 0.84 (.02) 25.9 ( 28) 25.9(1.6) 21.5 (1.5)* 0.05 (.02) Mass Loss and Nitrogen Dynamics Pinus caribaea leaf Utter lost the least mass, and Leguminosae the most, from September 1994 to October 1995 (Figure 6.1). The k values ranged from 0.39 (P. caribaea) to 1.13 (Leguminosae), and the exponential mathematical model accounted for 90 to 97 % of the variation in the data (Table 6.3). Analysis of variance of each treatment's k detected statistically significant differences among the treatments (p = .005), and the forest k was significantly different than that of the Leguminosae treatment (p <05). In the mixed site study in which I utilized leaf litter from only three treatments, all three litter types had similar mass remaining after one year in the field (Figure 6.2). The k values ranged from 0.46 (forest litter placed under the Leguminosae) to 0.68 (Leguminosae litter placed in the forest), and the exponential mathematical model accounted for 72 to 96% of the variation in the data (Table 6.4). Pinus caribaea maintained the most consistent k values across all the sites (k = 0. 54-0.56), and the Leguminosae litter maintained the largest k value under each site (Table 6.4). The ANOVA of k values from the mixed site study detected no

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^9 o> CD s CD .O) 110 100 90 80 70 60 50 40 30 20 10 107 Forest P. caribaea C. guianensis Leguminosae E. paraensis i i i i i i i i i i i i i SONDJFMAMJJAS Month Figure 6.1 Mass loss from leaf litter placed in fine mesh bags under the plantations and forest at the Curua-Una Forest Reserve from September 1994 to September 1995. significant interaction between leaf type and location (p = .98), and leaf decomposition rates were not significantly different among locations (p = .66). The effects of litter bag location were statistically significant (p = .03), and the fastest decomposition occurred under the forest. The litter-specific exponential model (K) predicted that the time required to reach 20 % mass loss (i.e., the end of phase I decomposition) ranged from 1.33 years (Leguminosae) to

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108 u C u 9 c t/l c 5s o > .S oo .1 I E C O E o (N 03 C o o g c C a 2 "S 8 4) S3 1_ o a. — 1 T3 § 8 | a c o -a c u 5 u 0) o 35 i s V o C u O BJ a. i> c r c o O DE o o cn CN 00 00 o fi CN to to CN Ov cn rn CN f On iv~i Os Ov o ON t OV o vO 00 Ov CO rto o •~n K U U ed o c g •v. &) Forest i ft u E rs oo u —1 2 a 5,

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^ 100 ON Di .£ e q: CO CO CD 80 60 40 20 109 Mar July Nov Month Mar ^ 100 ON CD CD E CD q: CO CO en 80 60 40 20 0 Mar July Nov Month Mar ^ 100 ON .£ cd 6 CD q: CO CO CD 80 60 40 20 Mar July Nov Month Mar Figure 6.2. Mass loss of leaf litter (mixed site study) from P. caribaea, the Leguminosae, and mixed litter from the forest placed in A. the forest, B. P. caribaea, and C. the Leguminosae. Litter bags were incubated in the field from March 1995 to March 1996.

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110 Table 6.4. Exponential decay constants (k), coefficients of determination, p values, and predicted time to 20% mass remaining (K) derived from the litter-specific exponential model for leaf litter decomposition from the mixed site study at the Curua-Una Forest Reserve. Litter was field incubated from March 1995 to March 1996. Location and Litter Type k R 2 P K Forest control Forest .522 .72 .18 2.83 P. caribaea .567 .96 .02 2.78 Leguminosae .684 .79 .12 2.14 P. caribaea Forest .558 .88 .07 2.81 P. caribaea .559 .96 .02 2.74 Leguminosae .670 .81 .09 2.20 Leguminosae Forest .461 .81 .07 3.28 P. caribaea .548 .96 .02 2.88 Leguminosae .597 .85 .09 2.54 4.04 years (P. caribaea) for the decomposition study (Table 6.3). For the mixed site study that utilized leaf litter from only three treatments, K constants ranged from 2. 14 years for Leguminosae leaf litter placed in the forest to 3 .28 years for forest litter placed under the Leguminosae (Table 6.4). Considering the decomposition study that examined decay under all five treatments, leaf litter in the legume plantations had the most depletion of their initial N content, and leaf litter forest and C. guianensis also experienced N depletion (Figure 6.3). The two treatments with the lowest initial leaf litter N concentrations, P. caribaea and E. paraensis had net N

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§> 100 (0 E 20 0 Foresf P. caribaea C. guianensis Leguminosae E. paraensis \ 30 60 90 T 180 270 360 Days Figure 6.3 Net N accumulation and depletion of leaf litter from September 1994 to September 1995 at the Curua-Una Forest Reserve.

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112 accumulation over the year long incubation (Figure 6.3). The total N change in leaf litter during the incubations (difference in absolute N concentrations initially to year one) ranged from an accumulation of 1 .47 g N kg" 1 dry litter (E. paraensis) to a depletion of 7.9 g N kg" 1 dry litter (Leguminosae). The slopes of the inverse-linear curves derived from one year of leaf litter N dynamics at Curua-Una (Table 6.3) were significantly different than zero (p < .06), and predicted values of absolute N concentrations at the end of phase-one decomposition ranged from 0.29% (P. caribaea) to 0.56% (C. guianensis). Lignocellulose and Polyphenol Quotients Initial lignocellulose quotients (HLQ) of the leaf litter (Table 6.5) ranged from 0.32 (forest) to 0.58 (Leguminosae). Holocellulose was a more significant component of the proximate C fraction of the Leguminosae leaf litter compared to the other treatments. HLQ decreased after one year in the field, and ranged from 0.27 (Leguminosae) to 0.45 (E. paraensis). HLQ diminished 53% in leaf litter from the Leguminosae and only 8% in the P. caribaea litter after one year. There were significant differences among treatments for arcsin transformed of initial HLQ (p=.0006) and HLQ after one year incubations (p=01). The HLQ of leaf litter from the plantations was significantly different than the forest initially, but only E. paraensis had a significantly higher HLQ compared to the forest after the incubations (Table 6.5). Initial polyphenol quotients (Table 6.5) were much lower than HLQ because polyphenol concentrations were a much smaller fraction of the total litter C content than lignin or holocellulose. Initial polyphenol quotients ranged from 0.037 (forest) to 0.108 (Leguminosae). Year-one polyphenol quotients decreased for all the litter types, with the

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113 Table 6.5. Ligno-cellulose quotients (HLQ) and polyphenol quotients of initial and year one leaf chemistry from the decomposition experiment at the Curua-Una Forest Reserve Ligno-cellulose quotient Polvphenol quotient Treatment Initial Year-one Initial Year-one Forest 0.324 (.03) 0.282 (.04) 0.037 (.00) 0.004 (.00) P. caribaea 0.453 (.02)* 0.415 (.02) 0.050 (.00) 0.006 (.00) C. guianensis 0.464 (.02)* 0.347 (.05) 0.057 (01) 0.002 (.00) Leguminosae 0.583 (.03)* 0.275 (.03) 0.108 (.04) 0.010 (.00) E. paraensis 0.528 (.01)* 0.454 (.02)* 0.054 ( 01) 0.001 (.00) largest decrease found in leaf litter of E. paraensis (98%). Initial polyphenol quotients were not significantly different among the treatments (p = .21), and year-one polyphenol quotients were significantly different at a p value of .08. Relationship between Decay Constants. Leaf Litter Chemistry, and N Loss The k values derived from the decomposition experiment were not related to initial leaf litter chemical constituents or indices, for example, there were no significant relationships between k and C/N ratios, L/N ratios, polyphenol/N ratios, lignin+holocellulose/N ratios, initial N concentrations or proximate C fractions. Nitrogen accumulation or depletion from leaf litter in the decomposition bags was not related to litter chemistry or initial quotients and indices. Annual N accumulation or depletion from fresh litter was related to C and holocellulose loss during the one year incubations (Figure 6.4).

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114 1 I I I I I I I I I 0 2 4 6 8 10 12 14 16 18 Holocellulose Depleted (g kg ) from Leaf Litter after One Year Incubation Figure 6.4. Relationships between N accumulation and depletion with A. C loss and B. holocellulose loss from leaf litter incubated for one year at the Curua-Una Forest Reserve

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115 Discussion Mass Loss The decay rates for the non-legume leaf litter at Curua-Una was slower than rates reported from similar decomposition studies in lowland tropical sites (Anderson and Swift 1983). In fact, they were similar to rates recorded from temperate zone studies (Anderson and Swift 1983, Aber et al. 1990), montane tropical forests (Tanner 198 1), and lowland tropical forests growing on infertile soils. For example, three species from the Bana forest of San Carlos, Venezuela, which is dominated by tree species with sclerophyllous leaves, had decay rates similar to the forest control and non-legume plantations at Curua-Una (Cuevas and Medina 1988). Rates of mass loss of the leaf litter from the forest control were slower than for confined leaf litter from the Dipterocarp and Heath forests of Sarawak (Anderson et al. 1983), lowland forest of Cameroon (Songewe et al. 1995), and terra firme forest of San Carlos, Venezuela (Cuevas and Medina 1988). At Curua-Una, estimates of unconfined leaf litter turnover (years to 95% mass loss), derived from litterfall: standing crop ratios (K L ), were higher than turnover times (K = time to 20% mass remaining) derived from the litter-specific exponential model (Table 6.6). Theoretically, litter that is comminuted by soil fauna is easier for microorganisms to decompose because division of litter creates a greater surface area to volume ratio for microbes to attack (Swift et al. 1979). In lowland tropical ecosystems, litterbag mesh size has been shown to be correlated with decomposition rates, with larger mesh sizes related to faster were greater than weight losses in fine mesh decay of high quality leaf litter in Nigeria (Tian et al. 1992). However, in Sarawak, Anderson et al. (1983) reported weight losses of leaves

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116 Table 6.6. Time predicted to 20% mass remaining with the leaf litter specific exponential model (AT) derived from the decomposition study and Olson's (1963) 95% mass loss (3/K, ) based on leaf fall:standin g crop ratios.. Treatment K (years) 3/K L (years) Forest 2.68 1.50 P. caribaea 4.04 2.91 C. guianensis 3.23 3.29 Leguminosae 1.33 1.16 E. paraensis 2.92 1.85 in coarse meshed decomposition bags (10 20 mm) ed bags (0.4 mm) in two forest types (alluvial and limestone) while decay in fine mesh bags exceeded that in coarse mesh bags in two other forest types (Dipterocarp and Heath). In temperate zone studies, litter bag mesh size has had no clear effect on mass loss of leaf litter in some systems (McClaugherty et al. 1985, Berg et al. 1996), but fine mesh sizes are reported to slow fine litter decomposition in others (Coleman and Crossley 1996). The effects of mesh size oon litter decomposition rates may be dependent on the activity of the forest floor and soil fauna and the quality of fresh litter (Anderson et al. 1983). Soil faunal biomass was not estimated at Curua-Una, but visual inspection of the litter layer and surface soils revealed that faunal activity was increased during the wet season. Therefore, unconfined litter at Curua-Una may be decomposed faster than in bags due to comminution by soil fauna. The estimates of K and 3/K L were almost identical for C. guianensis because its long woody rachises were resistant to decay. These rachises comprised a large portion of the litter standing crop, therefore, they were resistant to decay inside and outside the decomposition bags. The inclusion of leaf petioles and rachises in the decomposition bags of all the

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117 treatments slowed the estimates of mass loss for these samples, which may explain why decay at Curua-Una was slower than at the terra firme site at San Carlos. For example, Cuevas and Medina (1988) did not include petioles in their leaf bags, and their estimates of decay rates may have slowed if their samples had included this recalcitrant portion of the leaf. Mass LossThe Mixed Site Study The A: values and time to 20% mass remaining (K) of leaf litter from the mixed site study at Curua-Una were different than those estimated from the decomposition study (Tables 6.3, 6.4). Leaves used for the mixed site study were collected during February, which is the season when leaf litterfall was the lowest at Curua-Una (Figure 4.1). Potentially, the leaf litter quality is different at this time of year, especially in the forest, and microbial activity would also be influenced by the increased moisture during this season. In support of this argument, Lowman (1988) reported that leaf litter decay in Australian rainforests was significantly impacted by the initiation of decay or the timing of leaf litter placement on the forest floor. Also, fresh leaves were not used from the Leguminosae for the mixed bag study, and this leaf litter could have been of lower quality than the leaf material used for the decomposition study, which consisted of 25% fresh leaves. In addition, less leaf mass was placed in the fine mesh bags, i.e., 5 g vs 15 g, and potentially this kept the confined leaves less compacted in the bags. Thus, direct comparison of the decay rates from these two studies is not possible because substrate quality and quantity were not homogeneous for the two studies. The significant location effect detected for decay rates in the mixed site study indicated that the treatments at Curua-Una influenced the physical or biological microenvironments

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118 under their canopies, and that these factors regulated leaf litter decomposition rates. The decay constants (k) for all three leaf litter types that were placed under the forest were significantly higher than those under the Leguminosae (Table 6.4), suggesting that the high tree species diversity in the forest may support more diverse microbial communities able to degrade a wide variety of leaf litter types. In the forest, litterfall was contributed from 57 tree species per ha (Table 4. 1), which likely included a wide variety of leaf litter qualities. Blair et al. (1990) reported that mixed species litter assemblages that had wider variation in leaf litter quality or resource heterogeneity supported microbial communities that decomposed a wider range of leaf litter types more rapidly. McClaugherty et al. (1985) reported that a variety of litter placed under aspen stands was attacked by a white rot fungus, and this caused the more rapid decay of leaf litter. In that study, litter decay was affected by the environment in which the litter was placed, suggesting that stands of tree species with different ecological characteristics may support different populations of microorganisms. At Curua-Una, the forest may also be influencing decay rates because of increased inputs of labile C from throughfall and stemflow. Inputs of chemical elements in forest throughfall have been reported to be large enough to affect net mineral accumulation in the forest litter layer (Laskowski et al. 1995). Although throughfall was not analyzed in this study, high concentrations of dissolved organic carbon from the forest canopy may have provided a C source for heterotrophic microorganisms in that site. This C source could have provided the microbial community under the forest with the energy to accelerate decomposition (Ibrahima et al. 1995).

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119 Decomposition Study Changes in Carbon Fractions In other decomposition studies, holocellulose concentrations of leaf litter from temperate and tropical tree species usually have exceeded lignin concentrations (Berg et al. 1984, Cuevas and Medina 1988, Aber et al. 1990). At Curua-Una, leaf litter from the Leguminosae and E. paraensis had higher holocellulose concentrations compared to lignin, but lignin concentrations of leaf litter from the forest, P. caribaea, and C. guianensis exceeded holocellulose concentrations (Table 6.1). Similarly high lignin concentrations have been previously reported, particularly in dry environments or for tree species growing in soils with low nutrient availablity. For example, Cuevas and Medina (1988) reported higher lignin concentrations in two tree species of the Low Bana Forest of San Carlos, Venezuela. In addition, abscised leaves from Ceanothus megacarpus from southern California had lignin concentrations 47% higher than holocellulose concentrations (Schlesinger and Hasey 1981), and fresh leaf litter from a chronosequence of Pinus elliottii (3-35-years-old) had consistently higher lignin concentrations compared to holocellulose concentrations (Ghloz et al. 1985). In addition, Berg et al. (1996) and Berg and Ekbohm (1993) reported similarly high lignin concentrations (35-51%) for conifers and beech in Sweden. There are three possible explanations for the the high lignin concentrations in leaf litter from the forest at Curua-Una. The first is that leaf litter for the decomposition study was collected during the middle of dry season, and perhaps I collected proportionately more sclerophyllous leaves from tree species that dropped their leaves during the height of the dry season. Secondly, I included rachises and petioles in the leaf decomposition bags and in the analysis of chemical constituents, and they probably increased the lignin concentrations of the

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120 leaf litter. For example, C. guianensis has a compound leaf with a long woody rachis (20 cm) which is much more recalcitrant than the leaves attached to it. Other tree species in the forest also had similarly long, woody rachises. Third, during the initial leaf litter collection, I chose leaf litter that had minimal insect damage, and I may have inadvertently selected the most recalcitrant leaf types if the most palatable leaf litter had more surface area missing due to foraging by insects. The decrease in HLQ (Table 6.5) indicated that holocellulose concentrations in leaf litter decreased faster than lignin concentrations, which was likely due to the degradation of the isolated, non-encased holocellulose present in the leaf cell walls that is easily decomposed by microorganisms (Alexander 1977). The increase in absolute lignin concentrations relative to holocellulose in C. guianensis, the legumes, and E. paraensis (Table 6.2) also may have occurred because recalcitrant lignin-like products were formed from the recondensation of degraded cell wall materials as decomposition proceeded (Berg and Theander 1984). Mellilo et al. (1989) indicated that the major factor limiting decomposition at the later stages of leaf decay was lignin degradation, because the cellulose remaining in later stages of decomposition was lignin-encrusted. These authors also suggested that at this stage of decomposition (i.e, end of phase I to phase II), all litter chemistries, irrespective of their initial chemical constituents, are reduced to their least common denominator, lignin and ligninassociated products. As previously discussed, Berg et al. (1984) and McClaugherty and Berg (1987) used an index of the relative amounts of holocellulose and lignin in decomposing leaf litter and they designated this index as HLQ or the ratio of holocellulose to lignocellulose. Because

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121 holocelluolse is in the numerator of this ratio, the HLQ decreases over time as the nonencrusted holocellulose in leaf litter is oxidized by microorganisms. The HLQ reaches an asymptotic value during the later stages of decomposition, usually in the range of 0.4 to 0.5 for leaf litter of temperate zone tree species (McClaugherty and Berg 1987). This roughly corresponds with the ligno-cellulose index (LCI) of 0.5 that Aber et al. (1990) reported for leaf litter from nine temperate tree species at the end of phase I decomposition. At CuruaUna, the year-one HLQ's decreased from the initial values as expected, but these ratios were moving towards the 0.2 to 0.3 range which would correspond with the expected LCI of 0.7 0.8 from Melillo et al. (1989) in the later stages of decomposition. Again, despite large differences among the treatments for initial HLQ, these values converged over the one year field incubation, suggesting that leaf litter chemistries do degrade to a common endpoint over time. The initial polyphenol concentrations of leaf litter at Curua-Una were similar to values reported for leaf litter from a wide range of tree species and litter qualities in previous studies (Table 6.7). The polyphenol quotients of leaf litter declined dramatically after one year incubations (Table 6.5), indicating that polyphenols rapidly degraded or leached from litter in this environment compared to other C fractions. Previous studies have reported rapid losses of polyphenolics (Anderson 1973, Schlesinger and Hasey 1981) which may be leached from leaf litter or, as Suberkropp et al. (1976) suggested, may become complexed with protein during the initial stages of decomposition and isolated in the lignin fraction of the leaf material. Schlesinger and Hasey (1981) also proposed that this formation of polyphenolprotein complexes explained the increase in leaf litter lignin fractions over time.

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122 Table 6.7. Polyphenol concentrations of leaf litter from tropical sites. Location and Species Polyphenol (%) Author Curua-Una Forest and plantations 2.66-4.86 This study Sarawak mixed forest 1.5-2.28 Anderson et al. 1983 Peru, Leguminosae 0.69-3.61 Palm and Sanchez 1991 Hawaii, non-legumes 2.12 7.94 Constantinides and Fownes 1994 Hawaii, Leguminosae 1.90-20.99 Constantinides and Fownes 1994 Nitrogen Dynamics Lignin/N ratios were not correlated with k values at Curua-Una, but an interesting trend occurred during the year-long incubation. Year-one L/N ratios of the forest leaf litter and P. caribaea leaf litter were lower than initial ratios, which indicated that these treatments lost a higher absolute quantity of lignin compared to nitrogen. In contrast, the L/N ratios of leaf litter from C. guianensis and the Leguminosae rose over one year, indicating an absolute loss of more N than lignin from these treatments. McClaugherty and Berg ( 1 987) reported that, in general, leaf litter with lignin concentrations over 30% experience a net decrease in absolute lignin concentrations in the early stages of decompositon, while litter with low initial lignin concentrations have absolute increases of lignin in the early stages of decompositon. These net increases in absolute lignin concentrations are possibly due to the reformation of lignin-like compounds, which may explain lignin increases in leaf litter of C. guianensis and the Leguminosae. The leaf litter of E. paraensis had a L/N ratio that was identical at the two

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123 time periods, indicating an absolute increase in both N and lignin concentrations after the year-long incubation. Both leaf litter cellulose/N and polyphenol/N ratios declined more than lignin/N ratios after one year incubations, suggesting that holocellulose and polyphenols were easily leached and degraded compared to lignin in this environment. Also, at later stages of decomposition, leaf litter degradation products form stable ligno-nitrogeneous compounds which may render leaf litter N less available to microbial community (Berg and Theander 1 984). McClaugherty and Berg (1987) reported that N concentrations increased during the later stages of decomposition when HLQ was constant, and they hypothesized that N concentrations were an important rate regulating factor at this stage of decomposition. At Curua-Una, polyphenolic concentrations were not related to N release or accumulation in the decomposition study, but they are known to be important regulators of N dynamics in other systems. Polyphenolics are known to control N release from litter of leguminous species, but this effect was thought to be short-term and most important when fresh litter was applied as a mulch in agroforestry systems (Fox et al. 1990, Palm and Sanchez 1991). Polyphenolics may control the short-term release of N in leaf litter because they are thought to bind to organic-N and render it unavailable for microbial consumption. For example, the hot-methanol soluble fraction of polyphenols have immobilized labile-N fractions from fresh tree leaves (Oglesby and Fownes 1992, Constantinides and Fownes 1994). Polyphenolics in leaf litter also affect long-term N dynamics in forest ecosystems. For example, polyphenol concentrations in leaf litter controlled N mineralization rates from dissolved organic nitrogen (DON) encased in tannin complexes in Pinus mnricata in California (Northup et al. 1995).

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124 Despite the wide differences in initial N concentrations, absolute leaf litter N concentrations predicted from the inverse-linear curves converged to similar values at 20% mass remaining (Table 6.3). The relative N concentrations at 20% mass remaining were within the range reported from long-term study of leaf litter from temperate forest species in Massachusetts and Wisconsin (Aber et al. 1990). I obtained similar results for the C fractions, again indicating that litter chemical properties converged to common chemical components in later stages of decomposition (see Melillo et al. 1989, Rustad 1994). Nitrogen depeleted or accumulated from leaf litter incubated for one year was related to their A: values at Curua-Una, but this relationship was highly dependent on the inclusion of the Leguminosae litter (Figure 6.5). Nitrogen dynamics of leaf litter in the decomposition bags were not just determined by leaf and microbial N dynamics, but by N inputs of throughfall and stemflow, and root and mycorrhizal penetration. Also, as previously mentioned, the physical 1.2 1.1 g 1.0 c 0.9 £ 0.8 o O 0.7 2 0.6 H Q 0.5 0.4 0.3 y = .498 + .0096 (x) R 2 = .90 p = .009 20-10 0 10 20 30 40 50 60 70 N Accumulated or Depleted from Leaf Litter (kg ha' 1 yf 1 ) Figure 6.5 The relationship between the decay constant (k) and N dynamics from litterbags after one year field incubations at the Curua-Una Forest Reserve.

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125 and biological environment under each treatment at Curua-Una impacted leaf litter decay rates. Blair (1988) suggested that microbial incorporation of N in throughfall and fungal translocation of N were important factors in leaf litter N dynamics in North Carolina, and are possible reasons why N was accumulated during decay of leaf litter in this region. Conclusions This study examined rate regulating factors of leaf litter decay by incubating leaf litter with a wide range of initial chemistries in a lowland tropical environment. The findings of this study included the following: 1) No single initial chemical constituent or ratio was a good predictor of leaf litter decay, 2) Chemical properties of leaf litter converged towards similar chemical characteristics after one-year incubations, and 3) The forest, which had high tree species diversity, influenced the physical and biological characteristics of the site in ways that increased rates of leaf litter decay of a common substrate.

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CHAPTER 7 SOIL NITROGEN DYNAMICS AND PLANT TO SOIL FEEDBACK MECHANISMS UNDER MONOCULTURES AND PRIMARY FOREST IN LOWLAND AMAZONIA Introduction The largest storages of forest ecosystem carbon (C) and nitrogen (N) are found in the forest floor and mineral soil (Cole and Rapp 1981, Edwards and Grubb 1982, Jordan 1985), and changes in aboveand belowground litter quantity and quality could have significant impacts on rates of detrital C and N transformations in these pools. Previous work has demonstrated that plants with different ecological characteristics, such as leaf and root litter quantity and quality, can influence soil C and N dynamics (Binkley 1994). Feedback loops between soil and vegetation in a system may develop if plants with similar biochemical characteristics and phenologies control soil N supply rates by varying the quantity and quality of aboveand belowground litter inputs (Wedin and Tilman 1990). Plant-induced soil changes are important because of their effects on ecosystem nutrient dynamics and competition between plant species, and the implications for vegetation management. For example, if a successional plant community or plant species occupying a site affects soil N availability, then this plant-driven influence on N supply could control competition and succession (Ewel 1986, Vitousek and Walker 1989). If soil nutrient mineralization rates are controlled by litter decomposition rates (McClaugherty et al. 1985), litter quality (Gower and Son 1992), fine root biomass (Aber et al. 1985, Nadelhoffer et al. 126

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127 1985, Binkley et al. 1986) or any other factor directly linked to the specific character of the vegetation occupying a site, then plant-soil-plant feedback loops may influence forest species composition over time (Pastor et al. 1984). Potentially, four of Jenny's (1980) five soil forming factors (i.e., climate, vegetation, relief, parent material, time) can be held constant in replicated common gardens, with only vegetation varying across experimental units. These experiments give a more precise estimate of plant-induced affects on soil N mineralization, although pre-establishment soil homogeneity is rarely quantified. For example, in a 28-year-old common garden experiment in Wisconsin, Gower and Son (1992) found significant differences beneath five tree species in mineral soil nitrate and ammonium pools, annual net N mineralization, and annual net nitrification. They also indicated that initial leaf litter lignin/N (L/N) ratios were related to annual net N mineralization in surface soils. In another common garden trial in Connecticut, Binkley and Valentine (1991) reported significantly higher net N mineralization rates for four months under eastern white pine (Pinus strobus L.) compared to green ash (Fraxinus americana L). Theoretically, plants occupying infertile sites (e.g., low N availability) invest more C in belowground biomass to increase their ability to scavenge available nutrients (Tilman 1988), and they also maintain lower nutrient concentrations in plant tissue and litterfall (Chapin 1980). In contrast, plants adapted to highly fertile sites (e.g., high N availability) invest more C into shoot growth and photosynthetic tissues to better compete for light or crown position (Tilmann 1988). Plants growing in fertile sites also maintain higher nutrient concentrations in live tissues and litter (Vitousek 1982), which may result in positive plant-soil feedbacks to N cycling in these systems.

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128 Previous work has shown that the conversion of terra firme forest in the Curua-Una Forest Reserve, Brazil to plantations has significantly altered total surface soil N concentrations, fine litterfall N contents, leaf decomposition, and soil C release. I hypothesized that conversion of this forest type to monocultures has altered soil N transformation rates and pools sizes, and that monocultural plots with low leaf litter quality (high L/N ratios) had slower net N mineralization rates. In addition, I predicted that plantations with low N concentrations in leaf litterfall would have smaller inorganic-N pools than the forest or plantations that senesced litter with high N concentrations. The objectives of this chapter are to evaluate these hypotheses by determining the following under each plantation and the forest: 1) monthly and average annual inorganic-N pools for one year, 2) monthly and annual net N mineralization and nitrification rates, 3) inorganicN fluxes in the surface organic layer, and 4) the mechanisms that caused observed differences in N transformations and pools among an undisturbed forest and plantations. Materials and Methods Nitrogen Mineralization Monthly net N mineralization and nitrification was estimated in situ using a revised version of Distefano and Gholz's (1986) methodology. In situ soil incubations to estimate net N mineralization have been widely used in a variety of forest ecosystems and have gained acceptance because they allow researchers to approximate net N transformations under field conditions (Binkley and Hart 1989). Although the assumption is made that in situ incubations do not highly alter mineralization rates, in situ methods may modify natural soil conditions in several ways (Adams et al. 1989), including the following: 1) cutting of fine roots stops

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129 plant uptake of inorganic-N, 2) changes in soil bulk density, 3) alteration of soil water flow, and 4) cessation of N and labile organic carbon inputs from litter sources, roots, and associated mycorrhizae. Adams et al. (1989) demonstrated that shorter incubation periods of 1 to 2 weeks reduced these containment artifacts of in situ incubations compared to incubations of 4 to 8 weeks. I used incubation periods averaging 2 1 days at the Curua-Una Reserve. In each plot, 4 polyvinylchloride (PVC) tubes 24 cm long and 5 cm in diameter (inner) were filled with soil removed from four random locations (0 20 cm depth) with a Dutch soil auger. At each location, this soil was mixed, the roots removed by hand, and soil was repacked into the PVC tube. The PVC tube was then capped at the bottom end with one plastic ring (5 cm outer diameter) filled with 5 g of anion exchange resin (IONAC NA-38, OH' form) and one ring filled with 5 g of cation exchange resin (DOWEX 50W-X8, H + form). The resin was encapsulated in fine-meshed mosquito netting that was glued on the ring edge. These rings were placed at the bottom of the tube to capture nitrate-N + nitrite-N (NO' 3 -N + NO ~ 2 -N) and ammonium-N (NH + 4 -N) that was leached from the soil as water moved through the tube. The top of the tube was capped with one ring filled with mixed bed resin (anion + cation exchange resin, Rexyn 101 H-OH) to remove incoming inorganic-N from rainfall, throughfall, and stemflow. Tubes were then placed in the soil where the original soil core had been removed, and the surface organic layer was replaced on top of the tube so that surface soil temperature and moisture conditions were mimicked. For each plantation and the forest, 4 tubes were randomly placed in each of the three replicate plots; thus each month, 4 tubes x 3 plots x 5 treatments = 60 tubes that were processed. Time-zero soil

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130 samples (20 g fresh weight) were taken from the same soil sample that was placed in the tube, and 10 g of this soil (fresh weight) were extracted with 100 ml of 2M KC1 for NH>N and NO" 3 -N analyses. Soil moisture was determined gravimetrically (10 g soil) from the same samples. The tubes were left in the ground for approximately 2 1 days, removed, and 1 0 g of soil plus the anion and cation resins were extracted separately for inorganic-N with 100 ml of 2M KC1. The top ring of mixed-bed resin was not analyzed but cleaned for use in the following month's incubation. One drop of 5% mercuric chloride was added to the 20 ml of solution saved from each extraction to stop microbial growth because samples could not be refrigerated immediately after processing. NH + 4 -N was determined colorimetrically with the indophenol blue technique (Keeney and Nelson 1982 ), and NO" 2 -N + NQ 3 -N was determined colorimetrically by the copperized cadmium reduction method (Keeney and Nelson 1982). Seperate analyses of NO" 2 -N concentrations were not attempted, thus all NO" 3 -N concentrations reported here include NO" 2 -N concentrations. Net mineralization for each tube was calculated as [time final NH + 4 -N + NO" 3 -N (soil) time zero NH + 4 -N +NO" 3 -N (soil)] + (NH + 4 -N and NO" 3 -N) extracted from the anion and cation exchange resins. Inorganic-N concentrations extracted from the ion exchange resins were adjusted to wg g" 1 dry soil d" 1 by dividing total concentrations extracted from the resins by the amount of dry soil in each tube and by the number of days the tubes were incubated. To obtain the amount of dry soil in each tube, I weighed them in the field and corrected their weight for moisture content. Net nitrification was estimated as the difference between time

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131 final and time zero NO" 3 -N concentrations in soils + NO" 3 -N concentrations on the anion exchange resin. Nitrogen Fluxes in the Surface Organic Layer Fluxes in the surface organic layer (root mat) were estimated once in the rainy season (May) and once in the dry season (October) by placing 5 g (dry weight) of anion and cation exchange resin in nylon bags. In each plot, five stations consisting of one anion and one cation bag were placed along a transect running through the center of each plot. Thus in May and October, there were 5 stations x 3 replicates x 5 treatments = 75 total estimates of inorganic-N fluxes. The bags were left in the root mat for 8 days, because termites and ants damaged them if left longer. Anion and cation bags were extracted seperately with 100 ml of 2M KC1, and 20 ml of the extractant were kept for analysis. Again, one drop of 5% mercuric chloride was added to preserve samples until refrigeration. NFT 4 -N and NO" 3 -N concentrations were determined as previously described for the soil incubation tubes. Daily nitrogen fluxes were estimated by dividing total NH + 4 -N or NO" 3 -N concentrations from the resin extracts by the number of days they were left in the field (8) Statistical Analysis A repeated measures analysis of variance (ANOVA) in PROC GLM (SAS 1 988) was used to test for significant differences among treatments, months, and month by treatment interactions for all N transformation rates, pools, and fluxes. Each class variable was analyzed as a fixed effect. I treated each replication (3) as an experimental unit, and means from each replication per month were used for analysis. Thus, for each treatment month combination, n = 3. The repeated measures ANOVA was used to detect significant differences for yearly

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132 averages among treatments and months. This ANOVA used 4, 11, and 40 degrees of freedom for the class variables of treatment, month, and treatment x month interaction. The error variance used to calculate the F statistic for the month and treatment by month interaction effects had 1 10 degrees of freedom. The monthly inorganic-N pools were log transformed, and the N transformation rates were transformed with signed square root transformations prior to analysis of variance. Signed square root transformations were used for N transformation rates because the transformed data included negative values when net immobilization occurred. Transformations were necessary because plots of residual vs. predicted values for all N transformation rates and pool sizes following analysis of variance displayed unequal variance. The Greenhouse-Geisser F statistic was used to estimate P values for these analyses because the assumption of compound symmetry was violated. Linear regression was used to relate average monthly estimates of net N mineralization rates under all the treatments (5 treatments x 12 months = 60 estimates) with mean monthly nitrification and ammonification rates and inorganic-N pool sizes to determine if one of these parameters was related to net N mineralization. Annual net N mineralization and nitrification rates were estimated by summing average monthly rates for each treatment, and these annual rates were regressed against estimates of inorganic-N pools, fine litterfall characteristics, soil fine root biomass, microbial biomass, and soil C release under each plantation and the forest. This was an effort to identify processes that were related to N transformation rates in initially uniform soils that have been dominated by tree species characterized by a wide range of litter qualities and quantities.

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133 Results Monthly Inorganic N Pools in Surface Soils Monthly NH + 4 -N pools were consistently higher than monthly NO" 3 -N pools throughout the year for all the treatments (Figures 7.1a-e). A large increase in NH + 4 -N pool size occurred during April under each of the treatments, while NO" 3 -N pools stayed at consistent levels throughout the year, except under P. caribaea. The NO" 3 -N pool size exceeded NH + 4 N pools only under the forest control during the dry season and under P. caribaea throughout the year. Yearly treatment averages of inorganic-N pool sizes and ammonification rates were significantly different during the year examined (Table 7. 1). Average monthly NH + 4 -N and N0 3 -N pools were highest under the forest, and the Leguminosae and P. caribaea treatments maintained the second highest NH + 4 -N and NO" 3 -N pool sizes, respectively (Table 7.2). The large differences between monthly NH 4 -N pools under the forest and P. caribaea contributed most of the variation detected among treatments in the repeated measures ANOVA. Large differences in monthly NO" 3 -N pool size between the forest and C. guianensis and E. paraensis contributed to the variation detected among the annual means.

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NDJFMAMJ J ASO Month Figure 7.1. Mean monthly inorganic-N pool sizes under A. terra firme forest, B. Pinus caribaea, C. Carapa guianensis, D. Leguminosae, and E. Euxylophora paraemis.

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Table 7.1. Probability values from the repeated measures ANOVA of yearly averages for N transformation rates and pool sizes. All data were transformed prior to analyses. Variable Source of Variation treatment month month x treatment Net N Min. Rate .35 .0001 .52 Net N Nitr. Rate .11 .0004 .65 Net N Amm. Rate .02 .0001 .05 NH + 4 -N Pool .002 .0001 .02 NOyN Pool .006 .0001 .45 Table 7.2. Yearly averages of the ammonium-N and nitrate-N pool sizes in mineral soil (0 20 cm) under native forest and four monocultures at the Curua-Una Reserve ( se). Treatment Ammonium Pool (mg NH + 4 -N kg 1 soil) Nitrate Pool (mg NO>N kg 1 soil) Forest 7.66 (.99) 4.24 (.40) P. caribaea 4.83 (.60) 4.14 (.55) C. guianensis 6.85 (.68) 0.57 (.12) Leguminosae 7.51 (.72) 1.36 (.16) E. paraensis 5.78 (.81) 0.47 (.08)

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136 Net N Mineralization Rates Net monthly N mineralization rates were highest under all the treatments during July and August (Figure 7.2), which is the end of the rainy season in this region. Small peaks of net nitrification occurred during January under all the treatments, during the start of the rainy season. Net N immobilzation occurred under all the treatments during February, which is the season of the heaviest fruit and flower production. Although there were large differences among the treatments in annual net N mineralization and nitrification (Table 7.3), these transformation rates were not significantly different because of high variance. There was a significant month effect for all N transformation rates and inorganic-N pool sizes (Table 7 1). In general, net monthly nitrification rates exceeded net N mineralization rates, and net NH + 4 -N immobilization occurred in mineral soils during every month except March and August (Figure 7.2). During the months when net ammonification rates exceeded zero, net N mineralization exceeded net nitrification (e.g., March and August). Average daily N mineralization rates for the 1994 to 1995 season ranged from 0.29 to 0.49 ug g" 1 soil, and average daily nitrification rates exceeded 100% of net N mineralization for all the treatments (Table 7.4). In addition, net daily NH + 4 -N immobilization occurred in mineral soils under each of the treatments (Table 7.4). Annual net N mineralization rates in mineral soil (0 20 cm) ranged from 195 kg ha" 1 yr" 1 to 328 kg ha" 1 yr" 1 and annual net nitrification rates for each treatment were greater than annual net mineralization rates under all the treatments (Table 7.3).

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3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 i m i I i i i I I I NDJFMAMJ J A SO Month cm) 3.0 -i 2.5 o 2.0 l 1.5 1.0 0.5 1 0.0 :> -0.5 §> -1.0 3.0 -i cm) 2.5 o CNJ 2.0 1.5 1.0 r0.5 i 2: 0.0 -0.5 -1.0 3.0 -t o r-\ — 1 2.5 C\l i 2.0 1 .0 t— i 1.0 so/7 0 5T — 1 0.0 -0.5 Co -1.0 N D J F MAM J J A SO Month 137 o o CD I I I I I I NDJFMAMJ J ASO Month 3.0 2.5 1.0 0.0 -1.0 I I I I I I NDJFMAMJ J ASO Month Net N Min. Rate Net NO' 3 -N Rate Net Nhf 4 -N Rate i i i i i i i i i i i ND J FMAMJ J ASO Month Figure 7.2. Net N transformation rates in mineral soil under A. terra firme forest, B. P. caribaea, C. C. guianensis, D. the Leguminosae, E. E. paraensis from November 1994 to October 1995 determined from in situ incubations of 21 days at the Curua-Una Reserve.

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138 3 ON On C O m C ON c On 00 — tl io > 6 O >> o c o •c f V cn On O ON* 00 no' rin in NO m rvq NO m no' >ri m NO o> o CN ON IT) r-> oo' wi On' CN On 00 m rs in cn c E u t/J CO ha o § &) 9 6. CO in o c E tg 3 | 8 On cu 0 s \n '-5 OJ E cu > o u CO > CO 2 -h o SJ E £ O CU GO O CD CO 0 c 'O CO 1 E CO 3 g u C CD E .E 3 c/) CD C O U to C CD i*5 CO > CD G C CO £ .23 CO C CD C O u CO CD "O i— c/5 cC CO •c .2 +- cO E C So 0 o \a a 2 "ft £ c 1 S co 2 .2 h n C 7 CO j>> O n c c£ CO CO C CO U Z CD -> > CD • — c CO CD cO 3 B .n On t' ~ i_ r. ^ O •g c 2 o CO "O E 8 .a cd +- +- ^ f3 CD u > CO V 3 — G< T C o — •a o cO N v CD Z .£ oo CD — ^ V o NO N? o o o O oc ON oo oo ON NO en (N m m o o o o o 00 00 ON~ o o o 00 O CN o> " O m NO NO o o o o O o m o m o NO o o o NO ON o ro r*i in o o o o O o C/3 CD U o PL, 8 3 1 0 CO o c 1 00 CD -J S<3 -v. I 8, i*3

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139 Nitrogen Fluxes in the Surface Organic Layer The NH + 4 -N fluxes in the root mats under all the treatments were highest during May, at the height of the rainy season (Table 7.5). A significant treatment effect was detected during May (p = .04) but not during October. The mean NH + 4 -N flux in soils under E. paraensis was signifcantly lower than the forest during this time. The was a significant month effect for NH + 4 -N fluxes (p=.008) with no significant month by treatment interaction (p= 90). The NO" 3 -N fluxes in the root mat were considerably lower than NH + 4 -N fluxes during the wet and dry season, and no significant treatment effects were detected for NOyN fluxes for each month examined (i.e., May vs. October). Overall treatment and month effects were detected for NO" 3 -N fluxes (p = .03. and .0003, respectively), with no treatment by month interactions (p=.41). Relationships between N Transformation Rates. N Pools. Litterfall, and Soil C Release Monthly averages of mean daily net N mineralization rates in surface soils under all the treatments were related to monthly averages of mean daily nitrification rates (Figure 7.3). Linear relationships were found between monthly averages of daily net N mineralization rates and monthly averages of daily ammonification rates ( R 2 = .22, p = .0001), but monthly averages of NH>N pool sizes (R 2 = 10, p = .01) and NO" 3 -N pool sizes (R 2 .003, p =.66) were not related to monthly averages of net N mineralization rates. Estimates of annual net N mineralization were highly related to estimates of fine root biomass, C release, litterfall parameters, and soil inorganic-N pools (Table 7.6). Monthly averages of net N mineralization rates were not correlated with mean monthly C0 2 evolution rates (data not shown), but annual net N mineralization was related to annual soil C release

PAGE 148

140 . -< 5? 2 o g c C to 03 O CO e Q uo > 03 c o 03 s ON fo o O o d d d d — — ^-^ — in o O o o d o o oi O no NO OO — O (N I— cs 3 I

PAGE 149

141 2 y = -0.81 + 0.950(x) F?=71,p = .001 1 O) 0 -1 i 0 1 i 2 ug NO'Ng-i soil d~ 1 (020 cm) Figure 7.3. The relationship between net N mineralization and net nitrification under all the treatments at the Curua-Una Reserve. Each data point is a corresponding monthly mean for net N mineralization and nitrfication (n = 60).

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142 Table 7.6. The relationships between net annual N mineralization and root, soil, and litterfall parameters in the forest floor surface soils under plantations and primary forest at the CuruaJna Reserve. All regressions significant at p < .05. Parameter Intercept olope 1)2 K Fine Roots (0-2 mm) T% . XT /x 1_ -1 \ Root mat N (t ha ) 134.0 /lO/l 1 4z41 u.% Root mat biomass (t ha ), 0 2 Izo.Z 04.22 A AA u.yu Root mat + surface sou biomass (0 1 0 mm) 120.6 1 C C A 15.54 0.75 O "1 J T"' a T~"1 T"> a' Soil and Forest Floor Respiration C Release (t ha' 1 yr" 1 ) 699.7 -41.3 0.93 Surface Soil Chemical Properties Average Monthly NH 4 + -N Pools (mg kg" 1 soil) -22.63 42.53 0.88 Litterfall Total fine litterfall (kg ha" 1 yr' 1 ) 140.4 1.29 0.81 Leaf litter N concentration (g kg" 1 ) 136.2 1 1.07 0.80 (Table 7.6). Net annual N mineralization was related to mean leaf litter N inputs (kg ha" 1 yr' 1 ) (R^.70, p .07), and net annual N mineralization was not related to mean L/N ratios of leaf litter (R 2 =.38, p=.27). Discussion Inorganic-N Fluxes in the Surface Organic Layer Nepstad et al. (1991) estimated that 42 % of the total root length and 74 % of the total root biomass in terra firme forests in the eastern Amazon were located in the upper 20 cm of mineral soil. In other Amazonian terra firme forests, surface organic layers contained approximately 30% of the total fine root biomass to a 1 meter depth (Klinge 1973, Stark and Spratt 1977). St. John (1983) demonstrated that the fine roots above the mineral soil

PAGE 151

143 encountered organic matter deposited on the forest floor at random, then branched profusely after contact to increase root length and surface absorbing area at these nutrient rich sites. This high allocation of tree carbon to the surface root mat is beneficial because this close association of fine roots and fresh organic matter results in an efficient recovery of nutrients from aboveground litterfall (Went and Stark 1968, Stark and Jordan 1978). The fluxes of NH + 4 -N in the root mat at Curua-Una were much higher than NO" 3 -N fluxes in the wet and dry season under all the treatments (Table 7.5). Although I did not estimate inorganic-N pools in the root mat, the higher NH + 4 -N fluxes indicated that NH\,-N was in solution and availabile for plant uptake at higher concentrations than NO>N. Vitousek and Matson (1988) found NH + 4 -N pools 47 times larger than NO" 3 -N pools in the root mat of a terra firme forest near Manaus. They also determined that only 38% of net N mineralization was due to net nitrification in the root mat while net nitrification contributed over 1 00% to net N mineralization in surface soils. They also found that net N mineralization was 3 .6 times higher in the root mat as compared to the mineral soil. Montagnini and Buschbacher ( 1 989) determined that a root mat under Venezuelan terra firme forest had a net N mineralization rate thirteen times higher than that of the mineral soil during the wet season. In addition, root mat NH + 4 -N pools were three times higher than N0 3 -N pools, and root mat inorganic-N pools were much larger than inorganic-N pools in the mineral soil. Surface Soil Inorganic N Pools Average monthly inorganic-N pools in surface soils under terra firme forest at Curua-Una were larger than most values reported at other lowland Amazonian sites, but within the range of values from other lowland tropical sites (Table 7.7). The range of values for inorganic-N

PAGE 152

144 pools from theplantations at Curua-Una were similar to those found under much younger monocultures (4-years-old) by La Selva, Costa Rica (Table 7.7). Except for Luizao et al. (1992), the values reported from the other Amazonian sites were not based on year-round sampling, so they may not reflect the seasonal variation that I observed in the surface soil inorganic-N pools. In addition, different extraction techniques and sample preparation may confound comparisons of inorganic-N pool sizes at these sites (Kenney and Nelson 1982, Hart and Binkley 1984). What caused the differences in inorganic-N pool sizes among theplantations and native forest at Curua-Una? Average annual fine litterfall N contents, leaf litter L/N ratios, and leaf litter N concentrations were related to average annual NH + 4 -N pools in surface soils at this site, suggesting that fine litterfall quantity and quality may be regulating NH + 4 -N pool sizes in these Oxisols (Figure 7.4). Litterfall N content and leaf quality parameters were not related to NO" 3 -N pool sizes at Curua-Una (Figure 7.4), thus other mechanisms such as competition between plant roots and nitrifying bacteria for NH + 4 -N may be more influential in controlling these pool sizes. I did not estimate fine root turnover at Curua-Una, but belowground litter dynamics should greatly influence mineral soil inorganic-N pool sizes through the varying quantities and quality of root litter and turnover (Vogt et al. 1986). The forest had larger average fine root biomass (0-2 mm) in the root mat and surface soils, but these differences were not statistically significant (Table 5.2). Average annual NH + 4 -N pools was related to fine root biomass of the root mat, but fine root biomass in the mineral soil was not related to NFT 4 -N

PAGE 153

145 o '5. o CJ c ib 'oo GO 00 2 B O o o TJ C CO 4) H -- c/j o /CO N '5 CO OJ _o o u CJ Tf Tj— r-' iri — ON GO ON o CO 3 CQ c CO C c o <5j c u CO c D -CO 1 3 o CO CO C o i CO CO c o % CO E CO c o p4 CO 'E o -a c o s , E CU c ca CD CD C CD > NO ON ON 00 o ON On o o -C o o c c ir. CO Der oc (/) 1988 ON pu pu T3 "3 CO CO C nini CO cO cu UIU >. o u S 00 -o 00 rrs ous o s, CO -*-> c 3 CO nta Ma > Ma Mo C/3 IE Mo SO so On ^ ^ m N-r o 2 NO ON o co _o co u c co cy3 CO o B JO o -J CO o CO -*— I CO o U T O O — in oo On o CO B va va V CI va co B D i va Sel Sel Sel Sel rua Sel La La La La CO *— B Cu La

PAGE 154

9.0 '8 i + 1.5 0.0 — p y3.82 + 0.03 (x) R 2 = .92, p = .009 9.0 7.5 6.0 2 O) S. 4.5 > 3.0 O = 1.5 0.0 y = 2.55 + -.004 (x) R 2 = .008, p=.88 146 I i +I 50 100 150 50 100 150 Litterfall N Content (kg ha' 1 yf 1 ) 9.0 7.5 6.0 0.0 F E y = 7.82 + .034 (x) # = .67^ = .08 Litterfall N Content (kg N ha' 1 yf 1 ) 9.0 20 40 60 80 100 y = .59 + .041 (x) R 2 = .39, p = .25 Lignin:N (gg~ 1 ) 20 40 60 80 100 -1 Ugnin:N(gg ) 9.0 -i 7.5 6.0 I 4.5 ^ i + 3.0 1.5 0.0 y = 3.97 + 2.32 (x) R 2 = .94, p = .005 T en 5 O 9.0 7.5 6.0 4.5 3.0 1.5 0.0 y = 2.97 + -.74 (x) R 2 = .03,p = .75 0.0 2.0 0.0 0.5 1.0 1.5 Leaf Litter N (%) 2.0 0.5 1.0 1.5 Leaf Litter N (%) Figure 7.4. The relationships between average annual inorganic-N pool sizes and litterfall N contents and leaf litter quality from the forest and the plantations at the Curua-Una Forest Reserve.

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pool sizes (Figure 7.5). NO" 3 -N pool sizes were not related to surface soil fine root biomass, and no relationship existed between NO" 3 -N pools and fine root biomass in the root mat (Figure 7.5). Binkley et al. (1986) and Wedin and Tilman (1990) reported that fine root biomass in surface soils under forest and grasses, respectively, were negatively correlated with NO" 3 -N concentrations. These authors hypothesized that high root biomass in mineral soil potentially would create more root surface area for inorganic-N uptake, therefore, concentrations of the mobile NO" 3 -N may be influenced by total fine root biomass. This relationship held in the mineral soil under all the treatments at Curua-Una, but I did not estimate total root length. Total fine root biomass is less related to nutrient uptake capacity than total fine root length (Newman 1966), therefore, total fine root biomass is not a good indicator of the total surface area available for inorganic-N absorption. The seasonality of inorganic-N pools at Curua-Una was reflected in September and October when NO" 3 -N pools under all the treatments increased slightly, and N0 3 -N pools under the forest and P. caribaea exceeded NH + 4 -N pool sizes (Figure 7.2). These two months were the driest of the year, and surface soil NO" 3 -N losses due to leaching were low. These low leaching losses were reflected in the very small quantities of NO" 3 -N extracted from the anion exchange resins at the base of the incubation cores (data not shown). Also, during the course of the dry season, microsite denitrification potentially was at a minimum because rainfall was infrequent and these soils are very well drained. Compared to the other treatments, surface soils under P. caribaea maintained the smallest NH + 4 -N pools throughout the year, but one of the largest nitrate-N pools. A possible explanation is that surface soils under P. caribaea had smaller fine root biomass, and this

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148 9.0 7.5 I ^ 6.0 £ 4.5 %3.0 o.o y = 3.59 + .07 fxj R 2 = .95, p = .004 O) 3 o 700 750 200 250 300 350 2 1 0 y= 1. 83 + 0014 (x) R 2 = .004, p = .91 100 150 200 250 300 350 Root Mat Biomass (g m' 2 ) Root Mat Biomass (g m ) 9.0 ^ 7.5 &6.0 £4.5 X3.O X S> 1.5 0.0 y = 4.88 + .0044 (x) R 2 = .34, p = .29 9.0 -. 1 7.5 Oj 6.0 4.5 1 00 3.0 b 1.5 0.0 y = 5.25 + .008 (x) R 2 = .50, p = .18 c E 200 400 600 200 400 600 Soil Biomass (g m' 2 ) Soil Biomass (g m ) Figure 7.5. The relationships between average annual inorganic-N pool sizes and fine root biomass (0-2 mm diameter) in the root mat and surface soils (0 20 cm depth) under the forest control and monocultural plots at the Curua-Una Reserve.

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149 resulted in less competition between plant roots and autotrophic nitrifiers for uptake of available NH + 4 -N. The maintenance of relatively small NO' 3 -N pools under E. paraensis and C. guianensis, despite high net nitrification rates throughout the year (Figures 7.1, 7.2), possibly indicates that either higher rates of uptake or leaching losses of NO* 3 -N occurred in mineral soil, or that these species supported microbial populations that immobilized larger quantities of NO' 3 -N throughout the year. There were no significant differences detected among treatments for net nitrification rates, thus net NO" 3 -N production in soils under these two monocultures was as rapid as under the other treatments. Also, effective cation exchage capacity (ECEC) among the treatments was similar, therefore anion exchange capacity and leaching losses among treatments should not have varied greatly. Microbial biomass-N did not differ significantly among treatments during the wet season (Table 5.4), but biomass estimates did not indicate the size of inorganic-N fluxes through these pools. The most plausible explanation is that these two monocultures take up greater quantities of NO" 3 -N. Why did NH + 4 -N concentrations under all the treatments show such a distinct peak in April? This peak corresponded with the termination of cone,fruit, and flower production under all the stands, except for P. caribaea which dropped reproductive parts continually throughout the year. I believe that internal plant demands for production of reproductive parts decreased at this time. Also, the heaviest precipitation of the year fell during April, and microsite waterlogging may have inhibited the aerobic nitrifying bacteria in the mineral soil. In addition, microbial heterotrophs may have gained a competitive advantage over autotrophic nitrifiers at this time because of increased inputs of labile C from throughfall and stemflow.

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150 The decrease in NH + 4 -N pool sizes in all the stands during the dry season of 1995 may be a result of a reduction in labile C sources and microbial heterotrophic activity. Hart et al. (1994) indicated that when labile C sources diminish, microbial heterotrophs compete less efficiently with autotrophic nitrifiers for available NH + 4 -N in forest soils. As rainfall decreased in September and October, labile C leached from fine litterfall and the surface organic layer might have diminished to levels that inhibited microbial heterotroph activity. The maintenance of leaf area and the thick surface organic layers kept surface soil moistures at 20 25% throughout the dry season, which was probably high enough to maintain autotrophic nitrifier activity. With lower NH + 4 -N inputs in solution from heterotrophic N mineralization of fresh organic matter, these nitrifiers may have diminshed levels of available NH + 4 -N in surface soils, despite the slower rates of plant uptake. Net N Mineralization and Nitirification Rates Net N transformations in lowland tropical forests are the highest of all terrestrial ecosystems, and the highest surface soil net N mineralization rates yet reported (822 kg ha" 1 yr" 1 ) were recorded from lowland rainforest at La Selva, Costa Rica (Vitousek and Denslow 1986). Although annual net N mineralization in surface soils under the treatments at CuruaUna (195 328 kg N ha" 1 yr" 1 ) were not as high as those of La Selva, they were considerably higher than rates estimated in temperate forest soils (i.e., 5 100 kg ha" 1 yr" 1 Binkley and Hart 1989, Attiwill and Adams 1994). Because incubation periods and techniques vary from study to study, I grouped previously reported values of net N mineralization rates by ranges of average daily rates (Table 7.8). These ranges were estimated from field and laboratory incubations of varied time periods, and

PAGE 159

151 they were obtained from surface soils during different times of the year. The range of average daily net N mineralization rates from terra firme forest at Curua-Una falls within the range of previously reported mineralization rates in the Amazon (Table 7.8). The higher rates from La Selva may be caused by continuously distributed rainfall and the maintenance of microbial activity in surface soils throughout the year. Aboveand belowground litterfall are less seasonal than at Curua-Una, and this continual supply of litter may stimulate microbial activity and high N fluxes through microbial biomass at La Selva (citation). Net N immobilization occurred in surface soils at Curua-Una during the month of February (Figure 7.2), and I believe this was due to reduced belowand aboveground litter inputs at this time of the year. Fruit production was highest during February and March (Figure 4. 1), and plant demand for inorganic-N is likely to be very high to support the development of fruits and to maintain shoot and root growth. Rainfall was relatively high during February (200 mm), and tree growth and microbial activity were also probably high. The microbial biomass may have immobilized available inorganic-N in mineral soils because fresh inputs of organic-N from litter sources decreased during this time and because of competition from tree roots for available N. Methodological Considerations Net nitrification contributed over 100% to net N mineralization almost every month under the forest and plantations at Curua-Una (Figure 7.2). Net ammonification (organic-N to NH + 4 -N) occurred rarely, with positive rates occurring in March and April (Figure 7.2). Large increases of the NO" 3 -N pools in the incubations in short time periods were likely due to rapid increases in soil nitrifier populations (Sabey at al. 1959, Matson and Vitousek 1981,

PAGE 160

152 I § 1 | •si fi .8 O D 1 3 '55 •h C o 03 0 +3 ll "O to C V 2 g w £ £ ft 5 2 8 J 1 5 0 oo 1 a S 9 '1 | g I CO CJ 'ft o 09 U o c e < 00 O c o & •C £ Z •a .j .S ^ c on w a o c u .£3 2 £ C o 1 5 o T3 C eg u a. >, H -^j on U i— o ON On 3 OO H 03 3 C C 03 3 C c o3 >> GO NO da d 'oo NO NO oi) m 3 n — O o on u £ 1 ed c_> 'a. o -a c a o I— 1 CQ of C D t 5 3 u i 1— CQ i u O ON On • cd CCj w OJ 1/2 b 03 O o c o 00 03 oo & T3 O O o F3 S DQ etf "S o ~o G o 00 00 ON a o i -a c o3 id, u OO 3 O C o ed oj O 2 B 1 i i l 3 5 00 ON 1 '9 'n ed u u u. CQ CQ DQ C/5 oo" oo' o3 03 o3 C c c O o o N 3 03 NO U On JG GO ON U ON o3 -O o J= o 00 nc 3 in 03 DQ GO C U T3 C GO On Q 00 pu 03 -o ON 03 c "2 "3 03 c nin '2 *- o DO CD u so 03 ITS e oj C On 3 O u +-< B Mo 00 On Ma > Rol Mo c o o3 NO ed 3 u c u > in _o 03 u B O on o3 OJ m U o3 U o3 o U of _> GJ OD 03 o3 3 G C 03 o 03 CN r4 03 m 03 -J 03 _>
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153 Van Miegroet et al. 1990). This increase in NOyN pools inside the incubation tubes may also be caused by the exclusion of allelopathic compounds exuded from fine roots (Rice 1984, Vitousek and Matson 1985) or the accumulation of inorganic-N due to the absence of root and mycorrhizal uptake of NH + 4 -N or NOyN (Pastor et al. 1984). The ratio of net nitrification to net N mineralization is an index of the activity of autotrophic nitrifiers in the forest floor and soils (Robertson 1982, Harris and Riha 1991), and this ratio exceed 1 in mineral soils under the forest and plantations at Curua-Una. This indicates that the initial nitrifier population size in these field incubations was large throughout the year. Sizes of nitrifier populations and nitrification rates are limited by NH + 4 -N availability (Robertson 1984), but at Curua-Una mean monthly NH + 4 -N pool sizes under all treatments were not significantly related to mean monthly net nitrification rates (R 2 = .02, p = .24). Pastor et al. (1984) found that inorganic phosphorus (P) supply had regulated nitrification in mineral soils in Wisconsin, and possibly available P may have also have been limiting nitrifier activity in soils at Curua-Una. Phosphorus availability in Oxisols is usually very low (Van Wambeke 1992), and the soils at Curua-Una likely had low concentrations of P available for plant and microbial uptake. The high nitrification rates in the field incubations may have also resulted from artifacts of the incubation technique that favored the autotrophic bacterial populations in the soil. In general, nitrifying bacteria are thought to be weak competitors with microbial heterotrophs for available NH + 4 -N in soils (Jones and Richards 1977). DeBoer et al. (1996) has also demonstrated that the activity of soil nitrifier populations may be limited by ammonifying bacteria. I believe that inside the incubation tubes, autotrophic nitrifiers dominated microbial

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154 heterotrophs because of the loss of a C supply to heterotrophs from fine roots and mycorrhizae (Hart et al. 1994). The fine roots inside the tubes were removed prior to incubation, and the incubation tubes were capped with mixed-bed resins which trapped labile C and mineral N before they entered the incubation environment. Although I could not have removed severed hyphae and very small root tips, these C sources would have been mineralized rapidly in the warm and moist conditions inside the tubes. During the incubation period, I believe microbial heterotroph demand for organic-N declined with decreasing C availability, and the autotrophic nitrifiers were free to convert available NH + 4 -N to NO" 3 -N. Also, the exclusion of plant roots eliminated uptake of NH + 4 -N and competition for mineral sources of N between plant roots (with their associated mycorrhizae) and the autotrophic bacteria. Thus, the field incubation technique may have modified the soil environment in ways that favored autotrophic nitrifiers, which also had initially high populations. A complete understanding of soil N transformations at Curua-Una was difficult to assess completely with measures of net N mineralization and nitrification, which estimate N dynamics through the differences in pool sizes over time. The use of 15 N isotope dilutions has demonstrated that net rates estimate but a small fraction of gross rates of N mineralization because they do not quantify microbial immobilization of inorganic-N (Davidson et al. 1991, Davidson et al. 1992, Hart et al. 1994). Therefore, if microbial populations at Curua-Una were immobilizing large amounts of inorganic-N during the incubations, I would have underestimated N transformation rates because I did not quantify immobilized inorganic-N. Finally, I have not considered dissolved organic nitrogen (DON), which is important in many forest floor nutrient fluxes (Quails et al. 1991, Northup et al. 1995). In particular,

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155 conifer species are associated with ectomycorrhizae that are exuding enzymes capable of desorbing organic forms of N associated with tannic acids. These protein tannin complexes contain N that is unavailable for many plant life forms and are viewed as a feedback mechanism that ensures adequate N availability for conifers growing on infertile sites (Northup et al. 1995). Nevertheless, the role of DON remains unclear in tropical ecosystems, and may be an important but undiscovered factor in N availability and cycling in these environments. Nitrogen Pulses in Surface Soils A large pulse of net N mineralization was detected in surface soils under the forest and plantations at the onset of the dry season, corresponding to the time of peak aboveground litterfall (Figure 7.2). Pulses of nutrient mineralization in seasonally dry tropical forests have also been observed during drying-and wetting cycles (Lodge et al. 1994), and precipitation at the beginning of the wet season has been shown to initiate decompostion of litter deposited during the dry season (Cornejo et al. 1994). In addition, Weider and Wright (1995) demonstrated that moisture seasonality controlled fine litter decomposition in a seasonally dry lowland forest in Panama, possibly leading to a pulse of mineralized nutrients from the litter layer at the start of the wet season. Although research in seasonally dry tropical forests has shown that nutrient mineralization pulses occur in surface soils at the beginning of the wet season (Singh et al. 1989, Raghubanshi et al. 1990), data from Curua-Una indicate that sporadic rainfall at the end of the wet season (July August), combined with the onset of peak litterfall (June July), induced pulses of N mineralization in surface soils under all the treatments. Dry-wet cycles

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156 are common at this time of the year in the east-central Amazon, as the Inter-Tropical Convergence Zone moves farther north and high pressure begins to dominate. Precipitation events become more infrequent and convectional storms drop heavy rainfall once every seven to ten days at the end of the wet season. At Curua-Una, alternate cycles of wetting and drying of the forest floor and mineral soil during the onset of the dry season and the beginning of the wet season resulted in the microbial mediated cycles of nutrient release and immobilization previously reported in other seasonally dry tropical forests (Jaramillo and Sanford 1995). Similar results have been reported from soils of Amazonian terra firme forest near Manaus, where a pulse of net soil N mineralization was recorded during laboratory incubations from soils collected at the start of the dry season (Luizao et al. 1992). Luizao et al. (1992) hypothesized that this pulse was a result of rewetting incubated soil in the laboratory. Other studies in seasonally dry tropical forests (in India and Mexico) have also demonstrated large net N mineralization in surface soils after rewetting soils collected during the dry season (Singh et al. 1989, Garcia-Mendez et al. 1991, Davidson et al. 1993). This N flush in rewetted surface soils incubated in laboratories has been attributed to microbial lysis upon wetting (Marumoto et al. 1977, Kieft et al. 1987, Cabrera 1993), diffusion of inorganicN to microbes (Davidson et al. 1993), increased available organic substances desorbed from soil surfaces (Seneviratne and Wild 1985), and increased exposure of organic surfaces during dry periods (Birch 1959). Soils were incubated in situ at Curua-Una, and the mechanisms controlling the pulse of N mineralization was likely due to the rewetting of the surface soils and surface litter

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157 following rain events. Fine root proliferation in tropical dry forest in Mexico was cued by increases in soil moisture during the wet season (Kavanagh and Kellman 1 992), and fine root mortality increases greatly where tropical forests experience extended dry seasons (Singh and Singh 1981, Srivastava et al. 1986, Kummerow et al. 1990). At Curua-Una, I believe that as rainfall became more infrequent in July, fine roots in the root mat and surface soils began to die, and this belowground litter input added to the large inputs from aboveground sources. Although the surface litter layer dried out in between rainfall events, surface soil moisture contents were maintained only a few percent lower than peak wet season levels (Table 5 .1), and microbial activity in mineral soil probably was maintained at higher levels than in the surface organic layer. In addition, infrequent heavy rainfall might have leached organic and inorganic substrates from the fresh litter into the mineral soil. This was accentuated because the surface root mat had been drying and had less surface area involved in nutrient absorbtion at this time of year. This flush of nutrients from the surface layer entered the mineral soil where microbes had been actively processing the increased litter input from fine roots, and this resulted in high rates of net N mineralization. Concurrently, there may have been less mineral N uptake by plants at this time of the year, especially in the drier surface organic layer. Less plant uptake of available N combined with higher ammonification and nitrification rates resulted in the net N mineralization pulses in these soils. Evidence for Plant-Soil Feedback Mechanisms Previous work in common garden experiments, homogeneous forest patches, and replicated plots of grass species have identified litter quality and belowground N dynamics as mechanisms for plant-induced soil changes (Pastor et al. 1984, Wedin and Tilman 1990,

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158 Gower and Son 1992). Nitrogen dynamics were examined in these experiments because N limits plant growth in many environments, and also because plants species maintain varied N concentrations in belowground and aboveground litter inputs to the soil. This has enabled researchers to compare plant species that have efficient and inefficient use of N, and to quantify their impacts on soil properties. At Curua-Una, the close relationship between annual net N mineralization and root, soil, and litterfall parameters suggests that plant to soil feedback mechanisms were operating under the plantations. Annual net N mineralization was highly related to fine root N contents in the root mat and fine root biomass in surface soils (Table 7.6), and these findings were similar to previous reports from temperate ecosystems. For example, Nadelhoffer et al. (1985) determined that N allocation to roots increased with net N mineralization in a temperate forest in Wisconsin. Wedin and Tilman (1990) also found that belowground N allocation for five grass species in Minnesota was strongly related to net N mineralization rates in mineral soils. These findings indicated that plant species growing in soils with higher N mineralization and N turnover rates may allocate more N to fine roots, which in turn are shed as high quality belowground litter. At Curua-Una, the stands with the largest pools of fine root N in the surface organic layer potentially shed the highest quantities of N in belowground litter, which would have contributed to higher rates of N turnover in mineral soils. Annual net N mineralization was also related to total litterfall N inputs and leaf litter N concentrations (Table 7.6). If high litterfall quality and fine root biomass are associated with increased net N transformation rates at Curua-Una, then plant to soil feedbacks may have been operating at this site. When planted on the uniform soils at Curua-Una, a tree species

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159 that was genetically adapted to poorer sites such as P. caribaea deposited high quantities of low N litterfall on the forest floor. Much of this litterfall with a high CM ratio remained undecomposed on the forest floor while the microbial biomass was immobilizing N to process it. Pinus caribaea also maintained lower fine root biomass and allocated less N to its fine roots, which potentially indicated that there was less fine root N turnover under this species. Considering that net N transformations were also the slowest under P. caribaea, it appeared that N dynamics were slowed in soils occupied by pine. In contrast, the Leguminosae plots had higher aboveground litterfall N inputs, allocated higher quantities to fine roots, and maintained much higher N transformation rates than P. caribaea despite being planted on similar soils adjacent to the pine. If I assume that these stands at Curua-Una were at steady state, and that the ratio of the dry mass of litterfall :N content of litterfall is an index of within-stand nitrogen-use efficiency (NUE, Vitousek 1982), then was NUE at Curua-Una related to available N? Nitrogen-use efficiency of litter production ranged from 76 (Leguminosae) to 240 (P. caribaea), and net N mineralization was inversely related to NUE (Figure 7.6). The inverse relationship between net N mineralization and NUE at Curua-Una indicated that the less efficient use of N in the forest and the legume plantations resulted in more rapid N transformation rates in mineral soil compared to the pine, which had a much more efficient N-use as reflected in the low N concentrations in aboveground litter. These patterns of plant to soil feedbacks at Curua-Una were similar to those reported for a variety forest ecosystems in North America (Vitousek et al. 1982). These authors postulated that tree species that dominate infertile sites were selected to have efficient

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480 420 \ 360 !g 300 9> ^ 240 .c ^ 180 •fc 12060 0 160 Y = 514.83.36(x) + 0.008(x 2 ) R 2 = .81 ,p<.05 0 40 80 —\ 1 1 r 120 160 200 240 NUE Figure 7.6. The relationship between net N mineralization and NUE of the plantations and forest at the Curua-Una Forest Reserve. nutrient-use and to maintain low concentrations in live plant parts These tree species shed litterfall with low nutrient concentrations, which reduces soil nutrient availability, and feedbacks between the plant and soil maintain low nutrient availabilities indefinitely. At Curua-Una, tree species or groups of species with varied characteristics and resource requirements changed soil N transformation rates in directions that were predictable according to N concentrations in aboveground litter and fine root biomass in this lowland tropical site.

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161 Conclusions In summary, year-long observations of N transformations and pool sizes confirmed that plantations consisting of species with a range of phenologies, growth requirements, and chemical characteristics altered soil N dynamics in a lowland tropical environment after establishment on similar soils. The most significant findings of this study include the following: 1) Inorganic-N pools in mineral soils under the monocultures and native forest displayed a treatment effect over the year studied, but net N mineralization rates were not significantly different despite large differences in annual net N mineralization, 2) Net nitrification:N mineralization ratios exceeded one under all the treatments, possibly due to high initial populations of nitrifiers or artifacts of in situ field incubations, 3) Large flushes of net N mineralization in surface soils occurred at the end of the wet season, corresponding with peak aboveground litterfall, and possibly belowground root turnover, 4) There were significant relationships between annual net N mineralization rates and N allocation of fine roots, N inputs in total fine litterfall and leaf litterfall, N concentrations in live leaves, and soil C release, and 5) The plantations at Curua-Una impacted N availability and this change was related to within-stand NUE.

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CHAPTER 8 A FRAMEWORK OF PLANT-INDUCED CHANGES OF SOIL CARBON AND NITROGEN DYNAMICS IN A LOWLAND TROPICAL ENVIRONMENT AND CONCLUSIONS Plant to Soil Feedback MechanismsA Framework A general framework of how species or groups of trees with specific characteristics alter forest C and N dynamics is presented in Figure 8. 1 The forest floor and soil pools each have several internal components that are directly and indirectly altered by the type of tree that dominates a site. The boundary for this system is the plantation or a group of trees within a forest that have similar phenologies, chemistries, and growth requirements. The major pools in this framework are the tree component, the forest floor, the surface soil, and subsoils. Carbon and N fluxes are designated by arrows and indicate movement of organic and inorganic-C and N through the system. When soils and climate are homogeneous, pool sizes and fluxes between and outside of the pools are regulated by the characteristics of the dominant tree species and its interaction with the biotic and abiotic factors operating in the environment. Tree Component The major inputs to the tree pool are C (C0 2 ) and N (N) fixation, and the uptake of inorganic and sometimes organic-N from the forest floor and the two soil pools. The tree 162

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163 ira u co X C a) C Z> O kO) i o c CD o ^ = 8 S g | if I E 1 | c CO "5 .9 I 2 E eg co 8 c o g o CO c 2 ho c o 'S3 CO CO N c co 0 o '•c = CO 2 a. CO c CO CD ;= CD _i 2 _i CO c CO o co CD o >. E •o c co CO *— o p m cd CO E to 'c CO CD o o o t— OJ CO o 0) CD o CO r 3 c g 1 o to c CO o C CO g Q) 1 § O) N "C C := CO 2 2 CL O CD xCB C CO CD ~ CD T g co •£[ f 1 5 1 z i s cm" o O o CD CO E "O CO I co 3 C CO t1 c\i O (A -Q 3 CO CD c o CO CD z s lo 6 U c o 3 3 § p. J" Hi O X 3 '£ 2 0 JS a. o — Q. O <4_ 1 3 CO O §1 0 <= S 1 oo u, C £ 1 o — 1 1 — c. 5 jg

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164 component determines the quantities of C and N that are fixed in to this pool because these inputs are regulated by how well each species is adapted to the environment in which it is growing, and in the case of N-fixation, if tree roots are associated with N-fixing bacteria. The primary outputs (loss of C and N) from this pool result from herbivory and frugivory, aboveand belowground litterfall, throughfall and stemflow of organic and inorganic-C and -N, root exudates, and plant respiration. Net primary production will be directly influenced by the tree species that occupy a site because they will grow at different rates according to how well they are adapted to a particular site. In addition, losses of C and N from the tree pool will be directly influenced by the quality and quantity of leaves and fruit available for consumption by insects and frugivores. Also, species composition will influence throughfall and stemflow through the canopy and leaf structure and the susceptibility of leaves to leaching or uptake of C and N compounds. Indirect outputs from the tree component that may be altered by species composition include the quantities of light and rainfall that reach the forest floor and soil surface, and the quantities of C and N that are deposited on the forest floor as insect frass or animal dung. The form and depth of the tree canopy will also influence how much light and water penetrates to the forest floor which will impact forest floor temperatures and understory plant composition. Labile C and N return in insect frass can be a substantial input to the forest floor and surface soils (Grace 1986, Lovett and Ruesnik 1995), and insects are known to attack some tree species preferentially over others (Lowman 1 992, Lowman and Heatwole 1992). Species composition, richness, and leaf area index are important factors affecting

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165 herbivory in the tropics (Brown and Ewel 1987), and plantations are potentially susceptible to high losses of foliage. Forest Floo r Component This pool includes soil fauna, free living microorganisms, animals, aboveand belowground litter, and live roots with their associated mycorrhizae or bacteria, and root exudates. The inputs to the forest floor were previously listed as the outputs from the tree pool. Although litter will eventually decay into soil organic matter (SOM) with similar chemical characteristics, the rates of decay and leaching losses in this system are dependent on the litter quality derived from the tree pool. Higher quality litter inputs (high N concentrations) will increase rates of N outputs to the soil pools or to plant uptake, and concurrently, higher Utter quality may stimulate forest floor soil faunal and microbial activity. Outputs from this pool include organic and inorganic-N and -C that is mineralized and leached from decaying litter or uptaken by tree roots. Also, C and N in leaves which are physically moved to the subsoil by ants or termites may be substantial. Finally, C0 2 and NO x that is respired into the atmosphere through plant and microbial respiration are also sources of N and C losses, and for C in particular, a major ecosystem loss Surface Soil Component This pool includes the soil, tree roots and their associated mycorrhizae or bacteria, SOM, soil fauna, free living microorganisms, burrowing animals (mammals and reptiles), root litter, and root exudates. This surface soil pool receives C and N inputs from the forest floor and the subsoil pool. As previously discussed, the inputs from the forest floor are primarly products of litter decay or leached materials, and these two components are directly linked

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166 to the tree species that occupy the site. Inputs of C and N from the subsoil include C0 2 produced by deep roots and NOx from denitrification. Outputs from the surface soil pool includes C0 2 from root and microbial respiration, NOx from microbial respiration, NH + 3 -N volatilization, and leached organic and inorganic molecules. Subsoil Component The subsoil pool consists of the soil below a 1 meter depth, tree roots and their associated mycorrhizae, soil fauna, and root litter. The C and N inputs to the subsoil pool come from exudates of tree roots and their mycorrhizae, root turnover, C0 2 respired from roots, leached inorganic molecules, and forest floor leaf litter in ant and termite colonies. Outputs from this pool include subsoil fine particle materials (moved by termites, ants, and cicads) to the forest floor, surface soils, and to trees (nests), the loss of ions to ground water, uptake of ions by deep roots, and C0 2 respired from deep roots and microorganisms. Examining the Framework P.caribaea vs. the Leguminosae As previously discussed, this study suggests that plant to soil feedback mechanisms existed in this lowland tropical environment. In particular, fluxes of forest floor NH + 4 -N, pools of surface soil NH + 4 -N, and fine root biomass-C in the forest floor and surface soil were highly related to NUE. This suggests that high quality litter supplied the forest floor and surface soils with a substrate from which N was readily leached or mineralized This resulted in a higher allocation of fine root biomass, which may be an adaptation of species in response to high soil N availability. This in turn results in a feedback from soil to plant as the higher N availability and higher root biomass lead to increased NH + 4 -N uptake, which energetically is more efficiently converted into amino acids than NO" 3 -N. This increased uptake of NH + 4 -N

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167 is required by plants with low NUE because they abscise leaves with higher N concentrations, and therefore, require more N to maintain their relatively N-rich foliage. Although all the inputs and components of the framework (Figure 8.1) were not examined during the course of this study, I did investigate many of the components and some of the fluxes. By comparing two treatments with widely different NUE, 1 can illustrate how species or groups of similar species influence the forest floor and surface soil processes (Figure 8 .2). The P. caribaea and Leguminosae treatments had widely different N concentrations in live foliage and fine litterfall, therefore they should illustrate how soils may be impacted by shortterm exposure to trees with litterfall of different qualities (Figure 8 .2) Nitrogen inputs from litterfall under the Leguminosae were approximately three times greater than under P. caribaea, and the higher quality litter of the legumes had a mean residence time of 0.78 years compared to 1.06 years under P. caribaea. The Leguminosae invested more C to fine root biomass in the forest floor and surface soils, a growth strategy that reflects an investment in nutrient absorbing surface area for increased N uptake to support the N-rich foliage Mean total soil N was higher, and surface soil C:N ratios lower, under the Leguminosae compared to P. caribaea. In addition, N transformation rates were higher under the Leguminosae, and average surface soil NH + 4 -N concentrations were also much higher under the Leguminosae than under P. caribaea (Figure 8 .2). The Leguminosae had higher tissue concentrations of N, which resulted in higher quality aboveand belowground litter. This higher quality litter decayed faster than the low quality pine litter, and this resulted in larger NH + 4 -N pools and mineralization rates under the Leguminosae. This increased N availability resulted in a larger root biomass distributed in

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168 1 co m 2 I s g §5 rt O) § ^ P CO CO I i c c o o 5 ^ CL CL c c Z O o a 8 s s fl. lo t-: c\j o S 5 ioo ^ 2 o n! q q co o _i o CO $ c o o u. 8 e o LL oo c 2 o "55 CL CL 2 2 o o O) O) c c C C CO CO CO CO 3 3 CO CO E E 2 2 8 8 DC DC O 0. § S3 toi S>8S J co ^ T II H II 1 1 LL if i I c o oo CO CD CO c o oo CO 0 CO 2? Q CM O O o Q. CO O) CO o CNJ 8 CO y— CO a ^2 § io co lo CM II II (0 CO t fJL05 O) C || || C Ol O . . O P o o) o) '-5 — • Q. v v to 8 | Z O o 1 3 1 1 i o o E F F Z Z < E a o £ o3 'S ~ a Si 'o
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169 Conclusions The potential mechanisms for plant-induced soil changes are numerous, and experimental evidence indicates that tree species or groups of tree species do alter soil properties. These changes are inevitable, especially since organisms as large and long-lived as trees shed or respire a substantial part of the C and nutrients that they acquire each year. Shedding of biomass directly affects the activities of a multitude of other organisms that are dependent on the trees for sustenance. This study determined that the most evident feedbacks at Curua-Una were related to N concentrations and NUE, which were associated with higher NFT 4 -N pools in the forest floor and surface soils. Species with higher NUE also maintained larger fine root biomass. Although fine root biomass is not always related to total root C allocation, this study suggests that N in soils under the native forest and the Leguminosae is cycled more rapidly and in higher quantities than the other treatments. Finally, in response to this N enrichment, theses stands developed larger fine root biomass, perhaps as a strategy to take up larger quantities of available soil N for tissue maintenance and production. This study may also have implications for the low intensity management of plantations in the Amazon. The plantations at Curua-Una are evidence that plantations may survive in this environment, but commercial species developed with bad form due to attack by shoot borers and other insects. For example, C. guianensis had high survival but their crowns were damaged by shoot borers at a young age, and they had bad stem form which reduced their commercial value. Perhaps planting blocks of trees in mixtures of twenty to thirty valued species would reduce damage by herbivores.

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170 There was a high regeneration of native species under the P. caribaea, which might be an effective tool for reforestation of degraded sites. Although P. caribaea at Curua-Una was not as productive as other tropical pine plantations, this species had high survival and could be used to re-establish vegetative cover on degraded sites. Pinus caribaea has also been successfully grown for paper and pulp products in the Amazon at Jari, but at high costs for fertilization and fire protection (Fearnside 1988, Palmer 1991). Considering the implications for long-term production of forest products on the same site, the response of soil nutrient status to biomass removals in other Amazonian lowland sites indicates that long-term rotations or fertilization would be necessary to let soils recover from losses of nutrients due to harvesting and removal of biomass (Russell 1983). Currently in Brazil, large tracts of forest remain uncut, and the emphasis will continue to be placed on the harvesting of commercial species in primary forest with little thought for management until the forest stocks are depleted. Logging operations in the vicinity of Curua-Una are currently unaware of the plantations and selective cutting trials that were implemented at the Forest Reserve (Ros-Tonen 1993), and the results of these long-term studies will probably continued to be ignored until forest operations are forced to produce future wood products from the same locations.

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LIST OF REFERENCES Aber, J.D. and J.M. Melillo. 1980. Litter decomposition: measuring relative contributions of organic matter and nitrogen to forest soils. Can. J. Bot. 58:416-421 Aber, J.D. and J.M. Melillo. 1982. Nitrogen immobilization in decaying hardwood leaf litter as a function of initial nitrogen and lignin content. Can. J. Bot. 60:2263-2269. Aber, J.D., J.M. Melillo, and C.A. McClaugherty. 1990. Predicting long-term patterns of mass loss, nitrogen dynamics, and soil organic matter formation from initial fine chemistry in temperate forest ecosystems. Can. J. Bot. 68:2201-2208. Aber, J.D., J.M. Melillo, K.J. Nadelhoffer, C.A. McClaugherty, and J. Pastor. 1985. Fine root turnover in forest ecosystems in relation to quantity and form of nitrogen availability: a comparison of two methods. Oecol. 66:3 1 7-32 1 Adams, M.A., P.J. Polglase, P.M. Attiwill, and C.J. Weston. 1989. In situ studies of nitrogen mineralization and uptake in forest soils: Some comments on methodology. Soil Biol. Biochem. 21:423-429. Alban, D.H. 1969. The influence of western hemlock and western redcedar on soil properties. Soil Sci. Soc. Am. Proc. 33:453-457. Alban, D.H. 1982. Effects of nutrient accumulation by aspen, spruce, and pine on soil properties. Soil Sci. Soc. Am. J. 46:853-861. Alexander, M. 1977. Introduction to Soil Microbiology Second Edition. John Wiley and Sons, New York. 467 pp. Allen, O.N. and E.K. Allen. 1978. The Leguminosae. A Source Book of Characteristics, Uses, and Nodulation University of Wisconsin Press, Madison. 812 pp. Allen, S.E., H.M. Grimshaw, J. A. Parkinson, and C. Quarmby. 1974 Chemical Analysis of Ecological Materials Blackwell Sci. Pub., Oxford. 565 pp. Amato, M. and J.N. Ladd. 1988. Assay for microbial biomass based on ninhydrin-reactive nitrogen in extracts of fumigated soils. Soil Biol. Biochem. 20: 107-1 14. 171

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173 Berg, B. and O. Theander. 1984. Dynamics of some nitrogen fractions in decomposing Scots pine needle litter. Pedobiol. 27: 261-267. Bever, J.B. 1994. Feedback between plants and their soil communities in an old field community. Ecol. 75:1965-1977. Binkley, D. 1994. The influence of tree species on forest soils: processes and patterns. In: Proceedings of the Trees and Soil Workshop, Lincoln University 28 February 2 March 1994. D. J. Mead and I.S. Cornforth Eds. Agr. Soc. New Zealand Spec. Pub. No. 10. Lincoln University Press, Canterbury, pp 1 33. Binkley, D., J. Aber, J. Pastor, and K. Nadelhoffer. 1986. Nitrogen availability in some Wisconsin forests: comparisons of resin bags and on-site incubations. Biol. Fertil. Soils. 2:7782. Binkley, D. and S C. Hart. 1989. The components of nitrogen availability assessments in forest soils. Adv. Soil Sci. 10:57-1 12. Binkley, D. and D. Richter. 1987. Nutrient cycles and FT budgets of forest ecosystems. Adv. Ecol. Res. 16:1-51. Binkley, D. and P. Sollins. 1990. Factors determining differences in soil pH in adjacent conifer and alder-conifer stands. Soil Sci. Soc. Am. J. 54:1427-1433. Binkley, D. and D.Valentine. 1991. Fifty-year biogeochemical effects of green ash, white pine, and Norway spruce in a replicated experiment. For. Ecol. Mgmt. 40: 13-25. Binkley, D., D. Valentine, C. Wells, and U. Valentine. 1989. An empirical analysis of the factors contributing to 20-year decrease in soil pH in an old-field plantation of loblolly pine Biogeochem. 7:39-54. Birch, H.F. 1959. Further observations on humus decomposition and nitrification. Plant and Soil. 9:262-286. Birk, E.M. and P.M. Vitousek. 1986. Nitrogen availability and nitrogen use efficiency in loblolly pine stands. Ecol. 67:69-79. Black, G.A., T. Dobzhansky, and C. Pavan. 1950. Some attempts to estimate species diversity and population density of trees in Amazonian forests. Bot. Gaz. 1 1 1 :4 13-425 Blair, J.M. 1988. Nitrogen, sulfur, and phosphorus dynamics in decomposing deciduous leaf litter in the Southern Appalachians. Soil Biol. Biochem. 20:693-701.

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BIOGRAPHICAL SKETCH Ken Smith was born on September 16, 1961, in Oklahoma City, Oklahoma. He attended Thomas B. Doherty High School in Colorado Springs, Colorado, graduating there in 1979. He began his college education at Allen County Junior College in Iola, Kansas, which he attended on a baseball scholarship. After receiving his Associate of Arts in 1981, he began his professional training in forestry at Colorado State University in 1982. He received his Bachelor of Science in Forestry in May 1986 and immediately began service as a Peace Corps Forestry Volunteer in Guinea, West Africa. After returning from Africa, he entered the University of Florida in the fall of 1990 to pursue a Master of Science in Forestry, specializing in forest tree improvement. He received his M.S. in 1992 and started his Ph.D. program in Forest Ecology during the fall of 1992 at the School of Forest Resources and Conservation, University of Florida. 198

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. inry L. ProfessdV'bf Forest, Resources and Conservation I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. P.K.R. Nair Professor of Forest Resources and Conservation I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. — "fohn J. Ewel Professor of Botany I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Nicholas B. Comerford Professor of Soil and Water Science

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Willie G. Harris Professor of Soil and Water Science This dissertation was submitted to the Graduate Faculty of the School of Forest Resources and Conservation in the College of Agriculture and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. December, 1996 )irector, School Conservation Forest Resources and Dean, Graduate School