Group Title: Movement and transformations of urea-N in three forest soils of the Southeastern Coastal Plain /
Title: Movement and transformations of urea-N in three forest soils of the Southeastern Coastal Plain
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Title: Movement and transformations of urea-N in three forest soils of the Southeastern Coastal Plain
Physical Description: xii, 145 leaves : ill. ; 28 cm.
Language: English
Creator: Sarigumba, Terencio I., 1939-
Publication Date: 1974
Copyright Date: 1974
 Subjects
Subject: Forest soils -- Fertilization   ( lcsh )
Nitrogen fertilizers   ( lcsh )
Soil Science thesis Ph. D
Dissertations, Academic -- Soil Science -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (Ph. D.)--University of Florida, 1974.
Bibliography: Includes bibliographical references (leaves 136-144).
General Note: Typescript.
General Note: Vita.
Statement of Responsibility: by Terencio I. Sarigumba.
 Record Information
Bibliographic ID: UF00098179
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000407872
oclc - 38038424
notis - ACF4195

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MOVEMENT AND TRANSFORMATIONS OF UREA-N
IN THREE FOREST SOILS OF THE SOUTHEASTERN
COASTAL PLAIN











By

TERENCIO I. SARIGUMBA

















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








UNIVERSITY OF FLORIDA

1974













ACKNOWLEDGMENTS


I sincerely thank Dr. William L. Pritchett and Dr. Wayne H. Smith

for acting as Cairman and Co-chairman, respectively, of the Super-

visory Committee, for helping develop the problem, for their construct-

ive criticisms during the preparation of this dissertation, and for

their overall guidance and support throughout the study. I also thank

Luther C. Hammond and Dr. Robert S. Mansell for their valuable suggest-

ions on the soil physical aspects of the problem. Special thanks are

due to Dr. Jonn A. Cornell for his guidance in the statistical analysis

of the data.

I heartily extend appreciation to the following: (1) the Cooperative

Research in Forest Fertilization (CRIFF) program for financial support,

(2) Mrs. Mary C. McLeod for her valuable help in tne aborarory,

(.) Dr. Herman L. Breland, Mrs. Helen A. Brasfield, a-d Miss ".,na V. del

Mindo for the mineral analysis, (4) Dr. Russell Baliara, Mr. Roulin L.

Voss, and Mr. James B. Emmons for their help and brotherly com' non-

ip, (5) Mr. James W. Gooding for helping faciii:a.e comput-e naiysis

o Ene data and his friendship, and (6) Mrs. Paulit3 R. Puzo.n '*r

t'.,pi tnis dissertation.

.the lii,,t of my life, Nattie, I owe a world for her ,ni rsiand-

i,g, iin ,tiJaki, n crifice during moments of crisis, nelp ii f :,

- ll-e tion and preparation of the experimental materials and laboratory

Nork, and patience in typing the draft of this dissertation. A special

appreciation to Edzel and Glenn for being a constant source of

"inspirational distractions."















TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ................................. ............ ii

LIST OF TABLES ............................................... vi

LIST OF FIGURES .............................................. x

ABSTRACT ..................................................... xi

INTRODUCTION ....................................... ......... 1

LITERATURE REVIEW ............................................ 4

The Use of N in Forest Fertilization .................... 4
Urea in Forest Fertilization ............................ 5

Immobilization ..................................... 8
Volatilization ..................................... 10
Denitrification .................................... 13
Leaching ........................................... 16
Injury .to Plants ................................... 21
Summary ............................................ 23

Overall Fates and Reactions of Urea ..................... 24

Clay Adsorption .................................... 24
Plant Uptake ....................................... 27
Ureolysis .......................................... 28
Consequences of Ureolysis .......................... 30

Volatilization ................................ 30
Cation Exchange ............................... 30
pH R ise ....................................... 31
Other Consequences ............................ 31

MATERIALS AND METHODS ............................... ........ 33

Soil Materials .......................................... 33
Experiment 1. Incubation ................................ 35
Experiment 2A. Lysimeter Pots with Two-year-old
Slash Pine ......................................... 39
Experiment 2B. Lysimeter Pots with One-year-old
Slash Pine ........................................ 43
Experiment 3. Greenhouse Soil Columns ................... 45
Experiment 4. Laboratory Soil Columns ................... 48
Analysis of Leachates ................................... 49








TABLE OF CONTENTS (continued)

Page

Soil Extraction and Analysis .......................... 53
Tissue Analysis ....................................... 54

RESULTS .................................................... 55

Experiment 1. Incubation .............................. 55
Experiment 2A. Lysimeter Pots with TwO-year-old
Slash Pine ....................................... 67
Experiment 2B. Lysimeter Pots with One-year-old
Slash Pine ....................................... 78
Experiment 3. Greenhouse Soil Columns ................. 86
Experiment 4. Laboratory Soil Columns ................. 94

DISCUSSION ................................................. 100

Ureolysis ............................................. 100
Ammonification ........................................ 103
Nitrification ......................................... 104
Volatilization ........................................ 109
Nutrient Leaching ..................................... 113

Losses of N ...................................... 113
Losses of Other Nutrients ........................ 118

Mineralization ........................................ 119
Nutrient Retention .................................... 121

Retention of N ................................... 121
Retention of Other Nutrients ..................... 122

Change in pH .......................................... 123
Slash Pine Growth Response ............................ 124

SUMMARY AND CONCLUSION ..................................... 127

APPENDIX ................................................... 131

LITERATURE CITED ................ .......................... 136

BIOGRAPHICAL SKETCH ........................................ 145














LIST OF TABLES


Page

1. Selected chemical and physical properties of three
forest soils of the Southeastern Coastal Plain ........... 34

2. Experimental designs ..................................... 37

3. Rainfall over the experimental site during the period
April to December, 1973 .......................... ..... .. 50

4. Leachate concentrations of nitrate and chloride .......... 51

5. The effects of urea fertilization under 100 cm of soil
water tension on the population of ureolytic micro-
organism in the soil after one week of incubation ........ 56

6. Statistical analysis of parameters measured in
Experiment 1: Incubation for forty-two days of a Leon
soil treated with three levels of urea-N under four
soil moisture regimes .................................... 57

A. Significance of the effects of soil moisture
regimes, rates of urea-N, and incubation period
on extractable urea-N, NH4-N, and NO3-N and
so il pH .............................................. 57

B. The effects of soil moisture regimes and rates
of urea-N on extractable urea-N and NH4 and
so il pH .............................................. 57

C. The effects of soil moisture regimes on
extractable urea-N, NH4-N, and N03-N and soil
pH ....................... ............................ 58

D. The effects of urea-N rates on extractable urea,
NH4-N, and NO3-N, and soil pH ........................ 58

E. The effects of varying lengths of incubation
period on extractable urea-N, NH4-N, and N03-N
and soil pH .......................................... 59

7. Statistical analysis of parameters measured in Experi-
ment 2A: Lysimeter pots with two-year-old slash pine ..... 68








LIST OF TABLES (continued)


Page

A. Significance of the effects of soil moisture regimes
and fertilizer treatment on slash pine growth and
soil parameters .......... ...... .................. 68

B. The effects of soil moisture regimes and N fer-
tilizer treatments on Ca uptake, K leaching,
residual P, and leachate pH ......................... 69

C. The effects of soil moisture regimes on slash
pine growth response, nutrient uptake, leaching,
and retention, and leachate pH ...................... 69

D. The effects of N fertilizer treatments on slash
pine growth response, nutrient uptake, leaching,
and retention, soluble salts, and leachate pH ....... 70

8. Leaching losses of soluble salts during a period of
nine months from the lysimeter pots with two-year
old slash pine fertilized with urea and ammonium
sulfate under two soil moisture regimes ................. 73

9. Relationships among growth response, nutrient
uptake, and leaching losses of nutrients and soluble
salts in Experiment 2A: Lysimeter pots with two-year old
slash pine .............................................. 75

10. Distribution of N in the lysimeter system fertili-
zed with urea and ammonium sulfate under two soil
moisture regimes ........................................ 76

11. Distribution of N in the lysimeter system fertili-
zed with urea and ammonium sulfate under two soil
moisture regimes ........................................ 77

12. Statistical analysis of parameters measured in Experi-
ment 2B: Lysimeter pots with one-year old seedlings ..... 80

A. Significance of the effects of shading and N
fertilizer treatments on slash pine growth and
soil parameters ................................... 80

B. The effects of shading on dry weight production,
K uptake, Ca leaching and leachate pH ............... 81

C. The effects of N fertilizer treatments on diameter
growth, nutrient uptake and leaching,
and leachate pH ..................................... 81

13. Distribution of N in the lysimeter system fertilized
with urea and ammonium sulfate under direct (exposed)
and indirect (shaded) sunlight .......................... 84








LIST OF TABLES (continued)


Page

14. Distribution of N in the lysimeter system fertilized
with urea and ammonium sulfate under direct (exposed)
and indirect (shaded) sunlight ........................... 85

15. Statistical analysis of parameters measured in
Experiment 3: Greenhouse soil columns .................... 87

A. Significance of the effects of soil moisture
regimes, urea fertilization, soil types, and soil
depths on soil parameters ............................ 87

B. The effects of soil moisture regimes on volatilization
and leaching losses of N ............................. 87

C. The effects of soil types and soil moisture regimes
on leaching losses of NO3-N .......................... 88

D. The effects of soil types and urea fertilization on
non-extractable and KC1-extractable N and soil pH .... 88

E. The effects of soil moisture regimes on N
volatilization and leaching .......................... 89

F. The effects of urea fertilization on N volati-
lization and leaching, non-extractable N, KCl-
extractable N, and soil pH ........................... 89

G. The effects of soil types on N volatilization,
non-extractable N, and KCl-extractable N ............. 90

16. Statistical analysis of parameters measured in
Experiment 4: Laboratory soil columns .................... 95

A. Significance of the effects of soil types, PMA,
sampling time, and soil depth on leachate and soil
pH and KCl-extractable and leachable N ............... 95

B. The effects of soil types and PMA treatment on
N leaching losses and leachate and soil pH ........... 95

C. The effects of soil types and sampling time on
leaching losses of urea-N and KCl-extractable N ...... 96

D. The effects of soil type and depths on KCl-extract-
able N ............................................... 96

E. The effects of soil types on leachable and
KCl-extractable N and leachate and soil pH ........... 97








LIST OF TABLES (continued)


Page

F. The effects of PMA treatment on N leaching losses
and leachate and soil pH ............................ 97

G. The effects of sampling time leachable and
KCI-extractable N and soil pH ....................... 98

H. The effects of soil depths on KC1-extractable
NH -N and NO -N ..................................... 98

17. Total amount of leachates collected from the lysimeter
pots with two-year-old slash pine fertilized with urea
and ammonium sulfate under two soil moisture regimes .... 115

18. Standard errors of the mean of parameters measured
in Experiments 1 to 4 ................................... 132

A. Experiment 1. Incubation ............................ 132

B. Experiment 2A. Lysimeter pots with two-year-old
slash pine .......................................... 132

C. Experiment 2B. Lysimeter pots with one-year-old
slash pine .......................................... 133

D. Experiment 3. Greenhouse soil columns ............... 134

(1) Parameters involving volatile and
leaching losses of N ............................ 134

(2) Parameters involving non-extractable and
KCl-extractable N and soil pH ................... 134

E. Experiment 4. Laboratory soil columns ............... 135

(1) Parameters involving N leaching losses
and leachate pH ................................. 135

(2) Parameters involving KC1-extractable N
and final soil pH ............................... 135


Nlx













LIST OF FIGURES


Page

1. The fates of urea ....................................... 25

2. Moisture retention curves of three forest soils
of the Southeastern Coastal Plain ....................... 36

3. A. Two-year-old slash pine grown in four-gallon
glazed ceramic lysimeter pots filled
with Leon soil ....................................... 40

B. One-year-old slash pine grown in round-bottomed,
three-gallon glazed ceramic lysimeter pots
filled with Leon soil ................................ 41

4. Variations in soil matric suction at the surface
and bottom of pots with and without seedlings ........... 42

5. Schematic diagram of the lysimeter systems employed
in Experiments 2A and 2B ................................ 44

6. Device for trapping ammonia volatilized from urea
applied to the soil columns in the greenhouse ........... 47

7. The relationship between ammonium-N values (ppm)
determined by steam distillation and ammonia
selective electrode methods ............................. 52

8. The effects of soil moisture regimes, urea-N levels, and
incubation period on ureolysis and ammonification ....... 63

9. The effects of soil moisture regimes, urea-N levels,
and incubation period on nitrification .................. 65

10. The effects of soil moisture regimes, urea-N levels,
and incubation period on soil pH ........................ 66

11. Accumulative volatilization losses of ammonia from
urea applied to three soil types under two soil
moisture regimes ........................................ 92

12. The effects of urea and ammonium sulfate fertilization
of potted slash pine under two soil moisture regimes
on leachate pH and leaching losses of nitrate-N ......... 107
/'








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



MOVEMENT AND TRANSFORMATIONS OF UREA-N
IN THREE FOREST SOILS OF THE SOUTHEASTERN
COASTAL PLAIN



By

Terencio I. Sarigumba
December, 1974

Chairman: Dr. W. L. Pritchett
Major Department: Soil Science

A series of lysimeter, greenhouse, and laboratory experiments

were conducted to determine the influence of soil moisture, soil

organic matter and clay content, and rates of urea application on

the transformation and movement of urea in forest soils. It was

hoped that the results of these tests would help characterize the

transformation pathways of urea-N applied to Southeastern Coastal

Plain forest soils possessing various characteristics.

High rates of ureolysis and ammonification were observed when

soil moisture conditions were maintained within the range of 0 to

170 cm of soil water tension. When the soil matric suction was

allowed to reach 345 to 1000 cm of water, about 8 to 50 % of the added

urea-N remained in its unhydrolyzed form at the end of 42 days.

About 8 to 18 % of the added urea-N was lost due to volatilization

of ammonia when the soil matric suction was allowed to reach 500

cm of water. Ammonification was observed to be the predominant

conversion pathway of urea-N. Nitrification was generally low under

all conditions tested. Urea fertilization invariably increased

soil pH.








Rates of ureolysis and ammonification were faster in Plummer fs

with 20 % organic matter than in Leon fs or Bladen sl with 0.92

and 2.75 % organic matter, respectively. Phenylmercuric acetate

was more effective as a urease inhibitor in the Leon fs than in

the Plummer fs.

Urea fertilization caused a decrease in the non-extractable N

component of the soil, indicating an increased rate of mineralization

of the organic N fractions. The amounts of non-extractable N decreased

from 93 to 65 %, 98 to 77 %, and 97 to 92 % of the total initial N

content in the Leon fs, Bladen sl, and Plummer fs, respectively. The

degree of decomposition of the soil organic matter components appeared

to be the main factor affecting the urea fertilization-N mineralization

relationships.

Urea appeared to be a better fertilizer than ammonium sulfate for

two-year-old slash pine, particularly under high soil moisture conditions.

Favorable growth response to urea fertilization was probably due to a

temporary rise in soil pH which may have favored better nutrient

availability, increased metabolic efficiency resulting from an increased

supply of ammonium ions, the possible uptake of unhydrolyzed urea and

the products of ureolysis such as ammonia and carbonates, and minimized

leaching losses of nutrients. Poor growth response to ammonium sulfate

was probably due to high concentrations of soluble salts and high leaching

losses of nutrients. An accounting of the N distribution in the lysimeter

experiment showed that urea and ammonium sulfate fertilization resulted

in slash pine uptake of 64 and 49 %, leaching losses of 17 and 51 %,

and apparent gaseous losses of 19 and 0 % of the added N, respectively.














INTRODUCTION

Steadily increasing demand for wood products and ever decreasing

land areas available for expanded commercial timber production have

led to the use of forest fertilization as one of the intensive forest

management tools to increase wood production per unit land area.

Although forest fertilization has been practiced in some countries for

over 70 years (Tamm, 1968), it only has been during the last decade

that forest fertilization has gained recognition as an effective

silvicultural method to increase tree growth.

In the Southeastern United States, results from field experiments

undertaken by the Cooperative Research in Forest Fertilization (CRIFF)

program have indicated that about 2 million hectares (ha) of 12- to 16-

year-old pine plantation, 310,000 ha of pines planted yearly, and

another 310,000 ha of existing plantation that will annually reach an

age of 12 to 16 years are responsive to both nitrogen (N) and phos-

phorus (P) fertilization (CRIFF ANNUAL REPORT, 1973). Considering

positive indications that a large percentage of these potentially

responsive areas will be fertilized, it is reasonable to expect

that about 60,000 to 80,000 tons of N will be used annually in the

future.

There are over 2000 grades of fertilizer containing N (Hauck,

1968) but only a few formulations have been commonly utilized as N

sources, particularly in forest situations. One of these N carriers

is urea. This fertilizer has been widely used in stimulating the








growth of established and newly planted forest stands in many coun-

tries. Results have been so encouraging that forest fertilization

operations in Sweden (Tamm, 1968), Japan (Mitsui, 1967), Canada

(Roberge et al., 1970), the Pacific Northwestern USA (Norris and Moore,

1971), and the Southeastern USA (CRIFF ANNUAL REPORT, 1973) have

included urea as an N source. These, among other developments such as

those associated with agronomic practices, have greatly stimulated

increased production of urea at such levels that justify the claim

that the future of the fertilizer N supply market belongs to urea

(Harre and Mahan, 1973).

Several favorable attributes have led to the wide acceptance of

urea as a forest fertilizer. These are (1) high N concentration (45 %

to 46 %), (2) low leachability after conversion to ammonium (NH4),

(3) capacity to temporarily raise the pH of acidic soils, (4) low

potential for ground water contamination with nitrates, (5) non-combus-

tible or explosive, (6) low corrosive properties that lend to easy and

low-cost handling, and (7) blends well with other fertilizer materials.

However, there are some problems that tend to negate the effective

and efficient use of urea in forest ecosystems. After urea is added

to forest soil-plant systems, microbial and humus immobilization,

volatilization, denitrification, leaching, and injury to seedlings may

cause problems.

Particular forest situations determine whether to use urea for a

given forest ecosystem. Thus, it is necessary to characterize and

understand the transformation pathways the fertilizer undergoes under

the influence of the prevailing forest site factors. The primary

concern of this study was the movement and transformations of urea-N





3


in forest soils. The objectives were to (1) study the movement and

transformations of urea as affected by soil type, soil moisture

regimes, and levels of N application, and (2) determine the effects

of urea and ammonium sulfate [(NH4)2S04] fertilization on growth

response of potted slash pine, soil pH, and residual soil N.














LITERATURE REVIEW

The Use of N in Forest Fertilization

Comprehensive reviews on worldwide forest fertilization were com-

piled by White and Leaf (1957), Stoeckeler and Arneman (1960), and Baule

and Fricker (1970). Progress and potential of forest fertilization in

North America have been reviewed by Gessel (1968), Beaton and Tisdale

(1969), Roberge et al. (1970), and Weetman and Hill (1973). The poten-

tial for forest fertilization in the Southeastern Coastal Plain of USA

was recently reviewed by Pritchett and Smith (1970).

Experimental results indicating favorable growth response of

different tree species to N fertilization are well documented (Gessel

and Walker, 1956; Baule and Fricker, 1970; Weetman and Hill, 1973).

Despite the widespread deficiencies of N in most soils, N fertiliza-

tion of forests have not always produced positive responses. Several

experiments with N fertilization of planted pine seedlings resulted in

poor growth response (Hughes and Jackson, 1962; Pritchett and Llewellyn,

1966; Pritchett, 1972). Several factors have been considered respon-

sible for such poor response: high solubility and rapid leaching of

most N fertilizer, high salt index, stimulation of weeds which compete

with the tree seedlings for nutrients or water, suppression of

mychorrhizal development at high N levels, improper fertilizer place-

ment, and poor timing of fertilizer application (Austin and Strand,

1960; Pritchett and Robertson, 1960; Bengtson and Voigt, 1962; Richards

and Wilson, 1963).








The above factors suggest that the solution to the effective use

of N fertilizers may be in choosing the proper application of the

correct N source. Different tree species have different silvical

characteristics and soil types widely vary in their physical and

chemical properties. These factors plus climatic conditions and

biological influences affect the rate of N conversion, utilization, and

losses. However, the problem of choosing the proper N source may be

simplified by the fact that coniferous trees generally respond to

ammoniacal sources and that in acid forest soil, nitrification is

generally slow (Smith, et al., 1971).

Urea in Forest Fertilization

Urea is the N fertilizer most commonly used to stimulate growth

of established stands in several countries. In Sweden, economical

returns from forest fertilization trials with urea led to large-

scale application of urea, particularly by the Swedish Cellulose

Company (Hagner, 1966; Tamm, 1968). Favorable results of experiments

conducted and compiled by the Ministry of Agriculture and Forestry

in Japan have resulted in this country becoming one of the leading

manufacturers and users of urea (Mitsui, 1967). Stagnating black

spruce stands in the boreal forests of Canada showed significant

growth stimulation from a combined treatment of thinning and urea

fertilization (Roberge et al., 1970). The authors attributed the

favorable growth response to the mobilization of N contained in the

thick humus accumulated on the forest floor. In the Pacific North-

western USA, major commercial coniferous timber types have responded

to N fertilization, regardless of source (Norris and Moore, 1971;

Cochran, 1973). But because of its high N content and adaptability







to aerial application, urea has been the main source of N for operation-

al fertilization in this region. Urea is also the recommended N source

for the operational fertilization of N responsive sites in the South-

eastern Coastal Plain of the USA (CRIFF Annual Report, 1973).

There are several reasons behind the wide acceptance of urea as a

forest fertilizer. Urea is the most concentrated (45 % to 46 % N) dry

N source available and this makes it superior to other sources in

terms of handling cost per unit N weight. This is especially signifi-

cant in forest fertilization operations which usually cover vast area

of land with considerable tonnages of fertilizer materials.

Although highly soluble in water, urea is more resistant to

leaching than NH4NO3 (Garn, 1973). Leaching of unhydrolyzed urea is

believed to be retarded by certain mechanisms associated with the clay

and organic matter contents of the soil (Chin and Kroontje, 1962;

Broadbent and Lewis, 1964; Mitsui, 1967; Ogner, 1972). Under suitable

conditions following addition of urea to acid forest soils, ammonifi-

cation is the predominant transformation pathway. This is particularly

significant in the acid flatwoods sites of the Southeastern USA where

slash pine (Pinus elliottii Engelm. var. elliottii) is one of the

important native species that grow well with NH4 as the predominant N

form (Smith, 1972; personal communication). Since nitrification in

this region is in most cases negligible following urea fertilization

of acid forest soils (Pritchett and Smith, 1970; Smith et al., 1971)

the potential for polluting ground water with nitrates is minimal.

Upon ureolysis, urea causes soil pH to rise (Kresge and Satchell,

1960; Fiedler, 1971; Beaton, 1973). In the microsites immediately

surrounding the fertilizer granules, pH may reach 11.0 (Fiskell, 1972;








personal communication). Although this pH rise is temporary, it may

be beneficial to acid forest soils in terms of enhanced availability

of nutrients (Beaton, 1973).

By virtue of Its non-combustible, non-explosive, and less

hygroscopic (when uncontaminated) properties, urea appears to be a

fertilizer material suitable for long-term storage, safe in transport

via low-cost water transportation, and possessing excellent blending

ability with other fertilizer materials,

The extensive use of urea, however, is somewhat hindered by

highly variable and often unsatisfactory growth response of fertilized

forest trees. Fiedler (1971) showed that in several cases, calcium

nitrate [Ca(NO3)2) and ammonium sulfate [(NH4)2S04] were 5 to 60 %

better than urea in terms of tree growth response. For example, in

Quebec, Canada dry matter production of pine and spruce for two growing

seasons was about 50 % less with urea fertilization than with Ca(NO3)2

or (NH4)2S04 applications. Diameter growth of a 50-year old spruce

stand in East Germany was increased by 30 % with Ca(NO3)2 but only by

13 % with urea. In Sweden, average growth response of coniferous

stands to urea has been reported to be nearly 35 % lower than the

growth response to NH4NO3 (Nommik, 1973b).

Urea may also be less effective than other N sources on other

crops. For example, Power (1974) reviewed the effects of urea ferti-

lization on grassland productivity and found that at application rates

higher than 90 kg/ha, NH4NO3 is 8 % to 12 % better than urea. A green-

house study on the fertilization of orchardgrass in Washington (Klock

et al., 1971) found that (NH4)2SO4 elicited greater dry weight response

than urea. The authors attributed the difference in response to sulfur









(S) deficiency of the plants. There may be some influence due to soil

pH which ranged from 5.2 to 6.1.

The reported superiority of Ca(N03)2 to urea could be partly due

to the effect of Ca on acidic forest soils. However, other processes

are certainly associated with the superiority of NH4N03 and (NH4)2504

to urea. These processes may include immobilization, volatilization,

denitrification, leaching, and injury to plants.

Immobilization

Several reports indicate that substantial amounts of urea-N added

to forest soils are immobilized by microbial cells and humus. After

reviewing the work of Huser (1969), Fiedler (1971) reported that 10 to

20 % of added urea may be used in the construction of microbial tissues

and up to 20 % in the formation of stable humus compounds. Overrein

(1970) reported higher values of immobilized N obtained after in-

cubating urea-treated raw humus. After 60 days of incubation at 12 C,

71 % of the urea-N added at the rate of 50 ppm was in the non-

mineralized form; about 8 % of which was KCl-extractable. At a

higher incubation temperature (20 C), maximum immobilization (75 %)

occurred with the 100 ppm treatment and only 4 % was KCl-extractable.

The relationship between urea-N application rate and temperature vis-

a-vis maximum immobilization indicates microbial action.

Although microbial immobilization may be high, this N form may

become available subsequent to the death and decomposition of microbial

tissues. However, the organic N forms may be held in stable humus

complexes and rendered unavailable for plant uptake over sustained

periods. Furthermore, the re-mineralized N may be gradually incorporated

in the non-extractable fraction of humus N (Overrein, 1970, 1972b).

The above process of immobilization probably occurs following the








hydrolysis of urea. There are reports that indicate immobilization of

urea in its molecular form. Chin and Kroontje (1962) postulated that

the formation of relatively stable urea-organic matter complexes re-

sulted from physical or chemical adsorption and was primarily related

to soil organic matter content. Subsequent investigations appeared to

confirm this hypothesis. Broadbent and Lewis (1964) obtained data from

a series of chromatographic elution that indicated the ability of urea

to form salts with carboxylic groups of soil organic acids. Using

mercuric chloride (HgCl) to inhibit urease activity, Mitsui, (1967)

found that over a concentration range of 0,01 to 1,00 M the adsorption

of urea by volcanic ash soil was linear while that of (NH4)2S04 peaked

at 0.10 M. It was also found that volcanic ash and peat soils had

higher capacities to adsorb urea (46 and 42 mg of urea-N per 100 g

soil, respectively) than coarser soils or soils with lower organic

matter content. Based on these data and some spectrographic evidence,

Mitsui (1967) postulated that the main mechanisms of urea adsorption by

soils is the formation of hydrogen (H) bonds with the carboxylic

(-COOH), carbonyl (-C=O), and phenolic (C6H6-OH) groups of soil organic

acids.

Ogner (1972) sutdied the effects of urea fertilization on raw

humus collected under spruce and Scots pine stands in Norway, Through

proximate analysis method of N extraction and organic matter fraction-

atlon, he obtained quantitative and spectrographic evidence of urea-N

incorporation in the humus matrix, Reactions with -COOH, -C=0, and

C6H60H groups were the mechanisms considered responsible for such

incorporation. Furthermore, Ogner cited the possibility of re-synthesis

of hydrolyzed urea-N upon reaction with the polymeric groups of soil








organic compounds. Hubert (1974, personal communication) stated that

urea-N incorporation by humic compounds is a result of the fact that N

is a structural part of the non-hydrolyzable heterocyclic organic

compounds that form through polymerization.

It would appear that the main mechanism of humus immobilization of

urea is associated with its ability to self polymerize (Mitsui, 1967)

or polymerize with the decomposition products of organic matter (Ogner,

1972). Said (1972) reported that soil adsorption of unhydrolyzed urea

was not related to organic matter content of the soil.- It may be

pointed out, however, that the soils used in the latter study had low

organic matter contents (0.41 to 2.22 %). It is possible that within

the usual range of organic matter contents of soils, there is a critical

organic matter percentage at which reactions postulated by Mitsui

(1967) and Ogner (1972) would start to operate. Salt formation with

soil organic acids (Broadbent and Lewis, 1964) may contribute to the

retention of unhydrolyzed urea by soils or may be a key step towards

polymerization reactions. Unless urea becomes part of the unhydrolyzable

heterocyclic organic compounds of the soil or is associated with clay

adsorption mechanisms, salt formation appears to be a short-term

mechanism of urea retention in the soil.

Volatilization

When urea is added to the soil, the volatilization pathway may

predominate, especially when Its application Is made Improperly or con-

ditions conducive to volatilization occur. Kresge and Satchell (1960),

Allison (1964), Gasser (1964), Bengtson (1970), Fiedler (1971), and

Watkins et al. (1972) reviewed the volatilization losses of ammonia

(NH3) from urea added to agricultural soils. Losses as high as 50 %








of the added N were reported. In general, high volatilization losses

were found to be associated with the following: (1) surface application

to bare soils, (2) calcareous soils or soils made so by liming,

(3) soils of low cation capacity (CEC), (4) soils of low buffering

capacity, (5) low soil moisture conditions (losses are especially

high during the drying cycle following the wetting of the soil),

(6) high soil temperature, (7) high wind speed, and (8) high urease

activity. Results from studies with agricultural soils cannot neces-

sarily be used to predict volatilization losses from forest soils.

Gaseous losses of ammonia (NH3) from acid forest soils are usually

considered low (Beaton, 1973). This appears to be borne out by the

results of several investigators. Overrein (1969) fertilized highly

acidic podzolic forest soil in Norway with urea-N at rates of 100, 250,

500, and 1000 kg/ha. Maximum volatilization loss was only 3.5 % of the

added N over a period of 48 days. Application of 672 kg/ha of urea-N

in similar podzolic soils in Newfoundland resulted in negligible gaseous

losses of NH3 and a rise in pH of only 0.4 to 0.5 unit (Bhure, 1970).

On a strongly acid fine sand under a natural slash pine stand in

Florida, Volk (1970) found volatilization losses of 4 % in 7 days

following surface application of 100 kg/ha of urea-N. Volk also found

losses of 2 % from an area that had been control-burned 5 weeks before

fertilization and 5 % from an area bared of loose debris.

On the other hand, other studies indicated much greater losses of

gaseous NH3. Fiedler (1971) reported that volatile losses resulting

from the fertilization with urea of a sandy soil under a jack pine

stand amounted to 15 % of the 112 kg/ha of the added N. He also

reported that losses increased with increasing rates of application.








In laboratory studies in which forest soils obtained from Douglas-fir

stands were held at different moisture and pH levels, Baker (1970)

detected up to 21 % of volatile losses from 448 kg/ha of urea-N

added. In Baker's study, soil pH increased to about 8 regardless of

the initial value, and volatile losses were decreased by lowering the

temperature from 13 C to 5 C and the soil moisture content to below

10 %. It was also observed that bringing the soil moisture content to

150 to 300 % delayed gaseous losses of NH3,

Watkins et al. (1972) studied the effects of air flow rates,

temperature, and pH on volatilization of NH3 from urea applied to

forest soils taken as undisturbed cores from Douglas-fir, western

hemlock, and sitka spruce stands in the Pacific Northwest. Their results

showed that at an air flow rate equivalent to 10 m/hr and temperature

of 19 C, NH3losses ranged from 6 to 30 % of the 224 kg/ha of urea-N

surface applied to bare mineral soils and 27 to 46 % for the same

application rates to forest floor surfaces.

Nommik (1973a) assessed NH3 losses from N15-labelled urea applied

to vertically isolated microplots (24.5cm in diameter and 15 cm deep)

under a 90-year old Scots pine stand. Evolved NH3 was not directly

monitored but indirectly determined from the difference between the N

added and the N recovered in the soil profile. Results showed that

after 13 days of exposure, gaseous NH3 losses were 25 % from small

pollet-size urea and 12 % from Labletted (2.06 g/tablet) urea. For

a 31-day exposure, unrecovered N was 27 % from the pellet-size and 15 %

from the tabletted urea.

The differences in the magnitude of volatilization losses

reported by various investigators may reflect the various factors








affecting volatilization. However, experimental procedures also may

be of importance. In certain studies, NH3 evolved is monitored by

allowing a sweep of air to pass across the soil surface under air-tight

enclosure. This method favors removal of NH3 by mass flow which may not

necessarily occur under field conditions. Other methods rely on dif-

fusion of gaseous NH3 into acidified sorbers. This system may develop

positive pressure relative to the soil air pressure such that volatili-

zation may be unrealistically retarded. The most reliable method of

assessing volatilization losses appears to be the one employed by

Nommik (1973a) which determines evolved NH3 from the difference between

N added and N recovered in the soil profile. This method, however,

requires labelled material and plant uptake accounting.

To minimize NH3 losses through volatilization, Allison (1964) and

Volk (1970) recommended that urea fertilizer should be mixed with the

soil. In forestry practices, however, it is not economical nor

practical to apply and incorporate fertilizer in such vast areas involving

considerable tonnages of fertilizer in stands often inaccessible to

ground equipment. Furthermore, when fertilization is done in young

stands where tree roots are already extensive, mechanically mixing the

fertilizer with the soil may damage the roots making them subject to

Fomes annosus infection. Thus, alternative means of minimizing NH3

losses and improving the efficiency of urea as a fertilizer are needed.

Characterizing the soil and environmental conditions that favor the

transformation pathways of urea to the extent that N availability is

optimized may provide such alternatives.

Denitrification

Denitrification is an N transformation pathway which returns








mostly molecular and nitrous oxide forms of N (N2 and N20, respectively)

to the air. Sites may suffer N deficits if this process predominates

in the N cycle. Alexander (1961) stated that except in fields populated

with legumes, a condition that often leads to N accretion, N deficits

are the rule.

Several examples of soil N depletion brought about by denitrifi-

cation and the conditions responsible for such losses were given by

Bremner and Shaw (1958), Alexander (1961), Carter and Allison (1961),

Allison (1963), Meek and Mackienzie (1965), and Borishova and

Zertsalov (1966). Denitrification losses ranging from 6 to 73 % of the

added N have been reported. In general, denitrification requires a

(1) good supply of decomposable organic compounds to serve as proton

donors and energy source, (2) high nitrite (NO2) or nitrate (NO3)

levels to serve as terminal proton acceptors, (3) poor drainage in

which microbial oxygen demand exceeds supply, (4) high acidity which

regulates the supply of protons, and (5) high temperature (within the

range of optimum biological activity).

In certain cases, denitrification may occur following the addition

of urea to forest soils. Upon complete ureolysis, subsequent reactions

may lead to the production of hydroxyl (OH) ions and nitrification.

In the first case, high pH may develop, especially at the microsites

immediately surrounding the urea granules. Such processes affect

nitrification by forcing the oxidation of NH4 to terminate at the NO2

form. This could happen because the microorganisms (Nitrosomonas)

responsible for the production of NO2 are less inhibited by high pH

than those (Nitrobacter) responsible for NO3 formation (Alexander,

1961; Fiedler, 1971). Martin et al. (1943) stated that pH 7.7 is the

threshold value beyond which NO2 could no longer be converted to NO3,








In this situation, decomposing organic matter (enhanced by urea

fertilization may protonate the accumulating NO2 (more easily than

the NO3 form) to produce water and N2 or N20. Accumulating NO2 may,

however, diffuse out of the highly alkaline pocket and be further

oxidized to N03 (Olson el al., 1971).

The soil pH optimum for nitrification is 8.5, but it can occur

over a pH range of 5.5 to 10.0 (Tisdale and Nelson, 1966). Nitrifica-

tion is known to take place in a few agricultural soils at pH as low as

3.8 but in forest soils, it may practically cease below pH 5 (Beaton,

1973). Because forest soils have existed for a long time at such low

pH levels that nitrifying organisms are nearly absent or rendered

ineffective (Pritchett and Smith, 1970; Smith et al., 1971), nitrifica-

tion in untreated acid forest soils is generally considered to be

of minor importance.

Cases of considerable nitrification, however, have resulted from

urea fertilization of forest soils, especially at high rates of N. For

instance, Roberge and Knowles (1966) found that over an incubation

period of 42 days, NO3 production in highly acidic humus (pH 3.3 to 4.7)

treated with 3,500 ppm of urea-N went as high as 15 % of the added N.

Overrein (1971) recovered 50 % of the added N as NO3 over 40 months

following the fertilization of forest soils (in lysimeter systems) with

1,000 kg/ha of urea-N. In the Southeastern USA, Smith et al. (1971)

obtained a maximum nitrification rate of 25 % during 24 weeks of

incubation of acid sandy soils treated with 90 to 180 kg/ha of urea-N.

It must be pointed out that the above data were obtained from in-

cubation studies or soil profiles enclosed in lysimeters. Both systems

may operate under conditions not found in nature. Despite the claim

of Overrein (1969) that processes involving accumulation and evolution








of N20 and N2 are not operative in relatively well aerated and highly

acidic forest soils, the above data would indicate the possibility that

urea-fertilized forest soils may produce considerable NO3 which may

be enzymatically or chemically reduced to N20 or N2,

Leaching

The ability of forest soils to restirct nutrient movement has

long been recognized (Joffe, 1933; Hardy, 1936; Ovington, 1960),

However, the intensified use of N in forest fertilization during the

last decade and the rising concern on environmental quality have led

to the recognition of N leaching losses as a problem of sufficient

importance to justify extensive research.

There are three general methods that have been used in assessing

leaching losses from the soil: lysimetry (filled-in or monolith), stream

sampling from gaged watersheds, and suction cup sampling,

Lysimeter studies are advantageous in that they often incur less

expense, generate quick results, facilitate replications (Hornbeck and

Pierce, 1973), and allow more complicated experimental designs, However,

results from lysimeter studies can not be readily applied to natural

conditions. Use of disturbed soil profiles, development of unnatural

water regimes, and consequently abnormal nutrient movements in lysimeter

soil make interpretation of results difficult. Use of tension-plates

at lysimeter bottoms may reduce some of these shortcomings (Cole, 1958),

but variations in leachate volumes through tension-plate lysimeters

(Cochran, 1970) may make quantification of leached nutrients difficult.

Furthermore, Nutter and Ike (1970) pointed out that lysimeters can only

sample discrete sections of the hydrologic continuum from stream to

ridgetop. Despite the shortcomings of lysimeters, the method is still








a very useful approach to studying nutrient movements, transforamtions,

and budgets.

Sampling streams draining from gaged watersheds can overcome some

of the above shortcomings. The approach incurs less problems with

interpretation of results because it integrates all the variables that

influence nutrient leaching (Nelson, 1970). Furthermore, the use of

gages facilitate quantification of data. The main disadvantages of the

gaged watershed method are that the system is not flexible to repli-

cations and complicated experimental designs and it involves considerable

expense and elaborate organization of personnel (Hornbeck and Pierce,

1973).

The use of suction cup samplers is a relatively new system of

collecting soil solution. Quantification is not possible with this

system, which is its main disadvantage. But it can facilitate immediate

detection of nutrient concentration changes at or near the sites of

chemical reactions or biological activities, which may not be accounted

for by the gaged watershed method. There seems to be no one perfect

system of assessing leaching losses of nutrients but the above may be

combined to obtain maximum results.

Broadbent et al. (1958) determined the comparative leachability of

urea and other NH4-bearing fertilizers through 22-cm laboratory columns.

They found that urea moved more readily than the other fertilizer mater-

ials. More recent investigations tend to contradict these results. Using

lysimeters equipped with tension-plates about 90 cm below the surface,

Cole and Gessel (1965) found very small leaching losses after applications

of urea or (NH4)2504 to a Douglas fir stand in the Pacific Northwest.

Fiedler (1971) reviewed the study of Huser (1971) and found that








urea was less leachable than NH4NO3 and that losses were greater from

coarse than fine textured soils. A lysimeter study carried out by

Overrein (1969) on a podzolic forest soil in Norway showed that leaching

losses of N from lysimeters fertilized with urea, ammonium chloride (NH4CI),

and potassium nitrate (KNO3) were 15, 100, and 300 times greater than the

losses from unfertilized plots. Nommik and Popovic (1971), using later-

ally isolated microplots in a Scots pine stand in Sweden, showed that

urea had much slower mobility than either Ca(NO3)2 or (NHq)2SO4. A

lysimeter study in Canada indicated leaching losses of less than 1 kg/ha

of urea-N applied to a black spruce stand (Roberge et al., 1971).

The differences in the findings of Broadbent et al. (1958) and the

other investigators may be resolved in terms of leaching depth and the

time leaching occurred. Broadbent et al. (1958) monitored their leachates

to a depth of 22 cm, 1 to 24 hours after application. Cole and Gessel

(1965), Overrein (1969), and Nommik and Popovic (1971) determined

leaching to depths of 90, 45, and 40 cm, respectively, and their

leaching times varied from weeks to months. The difference in space

and time may have allowed sufficient opportunity for urea to react with

the soil or the humus fraction, which consequently minimized leaching.

Tamm and Wiklander (1971) investigated the effects of N fertiliz-

ation on ground water and streams draining Swedish coniferous forests.

Their results showed that stream water concentrations of NO -N and

NH -N were 8.6 ppm and 5.9 ppm, respectively. They concluded that

fertilizing forests with urea would pose no danger of polluting the

ground water or streams provided the fertilizer is not directly spread

over water courses. In another study, Friburg (1971) applied 112 kg/ha

of urea-N around the perimeter of a forest lake in Sweden. In the first

summer after treatment, N concentration in the lake went up to-0.6 to








0.8 ppm and went down to 0.2 to 0.5 ppm in the second summer,

Moore (1970) reported that fertilization of Douglas-fir in

southwestern Oregon with 224 kg/ha of urea-N caused stream concentra-

tions in urea, NH4, and N03 to increase only slightly above background

values. All forms of N had returned to pre-treatment levels by the

fourth week after application. Aerial urea fertilization of two

forested creeks in Washington caused urea concentration to increase on

the day of application; NO3 levels increased to a maximum of 1.32 ppm

one week after fertilization and was back to pre-treatment levels

after 4 months (McCall, 1970).

Klock (1971) studied the effects of urea fertilization of burned

watersheds in north central Washington. At 60 days after fertilization,

1.37 kg of urea-N and 2,90 kg of NO3-N were estimated to have been

carried by streamflow from the watershed receiving 27,500 kg of urea-N.

The maximum concentrations detected were 0.6 ppm and 0.2 ppm for urea

and NO3-N, respectively.

Aerial application of 115 kg/ha of urea-N to a young hardwood

forest in West Virginia (Aubertin et al., 1973) yielded results some-

what different from the above. During the first growing and dormant

seasons after fertilization, 17.4 % of the added N was carried in the

stream discharge as N03-N. Maximum stream concentration of NO3-N

reached as high as 19.8 ppm, which was generated by a high storm flow.

A study on the effects of fertilizing young slash pine plantations

with different forms fo N on groundwater quality was undertaken by

the CRIFF program at the University of Florida, Gainesville (CRIFF

unpublished data). Peak N concentrations in the water samples were

detected 20 days after fertilization, as shown below:









N treatment Leachate concentrations
Rate NH -N NO -N
Source (kg/ha) (ppm) (pQm)

Control 0 0.30 1.40

Urea 90 4.40 0.30

Ureaform 90 1.20 2.60

NaNO 90 0.60 50.00

(NH4)2SO4 90 11.90 1.40

NH4NO3 90 4.65 2.00


Concentration of NO -N in samples of collected from sites treated

with NH NO3 peaked again at 14.2 ppm 37 days after fertilization,

while those collected from urea-treated sites peaked at 6.2 ppm 50

days after fertilization. The above data indicate that, in terms of

total NH -N plus NO -N movement, urea is the least leachable of the

N sources tested.

Concern has been commonly expressed on N leaching losses as

NO -N. This concern undoubtedly stems from the fact that most soils

have a low anion exchange capacity making nitrates easily leached,

unless denitrification or biological uptake takes place. Leaching

losses of N as NH4-N may also occur when the soil exchange sites are

temporarily overloaded or when the soil is inherently low in CEC

Thomas, 1970). Leaching of NH4-N may be further enhanced if, after

applying NH4-bearing or NH4-converting fertilizers, negligible nitri-

fication occurs, plant or microbial uptake is slow, and soil water

percolation is rapid. This was substantiated by the findings of

Aubertin et al. (1973) which indicated that urea fertilization

increased the NH -N concentration of stream water for 5 weeks before








it returned to its original level. The data shown in the above table

indicate that NH4-N is the main form of N involved in leaching losses of

urea.

Urea may be leached in its unhydrolyzed form. The solubility of

urea in water, its chemical or physical neutrality, and its low disso-

ciation constant (1.5 x 10-14 at 25 C) are attributes that make this

fertilizer susceptible to leaching in its unhydrolyzed form. Klock (1971),

Norris and Moore (1971), Aubertin et al. (1973), and Tiedemann (1973)

detected unhydrolyzed urea-N in stream water samples. However, they

claimed that most of the amounts detected were attributable to wildlife

excretion. It would appear that unhydrolyzed urea-N is not an impor-

tant form of leaching losses of N, especially if precipitation occurs

after fertilization to effect rapid infiltration and complete ureolysis,

Leading of unhydrolyzed urea may also be retarded by mechanisms

associated with the soil organic matter and clay contents (Chin and

Kroontje, 1962).

Injury to Plants

Urea has a salt index of 1.62 (Tisdale and Nelson, 1966), It

may be harmful to plants by the formation of biuret contaminants,

production of free NH3, and by accumulations of NO2.

Biuret contaminants, formed from 2 molecules of urea during its

synthesis and processing especially at high temperature, have been

formed in urea solutions heated to more than 50 C (Gasser, 1964),

Biuret toxicity has been mostly observed on germinating seeds and

leaves receiving foliar spray of urea solution, Several cases of

biuret toxicity were reviewed by Gasser (1964). However, it appears

that this problem has diminished in importance because present manu-








facturing technology is capable of minimizing biuret formation in the

factory (Philipps, 1973).

The free NH3 that is produced during or after ureolysis may be

harmful if absorbed by the roots in toxic amounts. Damage to agri-

cultural crops by NH3 injury has been fairly common as indicated by

the reviews of Gasser (1964) and Viets (1965). Mesa (1974) recently

observed that tomato seeds were prevented from germinating, or

germinating seedlings failed to survive, due to urea application to

an alkaline soil. Soil pH rises subsequent to ureolysis may be such

that activities of Nitrobacters are inhibited. This would result in

NO2 accumulation and toxicity. Court et al. (1962) described cases of

crop damage due to N02 accumulation, Passioura and Wetselaar (1972)

studied the effects of N fertilizer salts on wheat roots. They found

that (NH4)2S04 restricted root growth via increase in osmotic potential

of the soil solution. This effect lasted for 4 weeks. On the other

hand, urea more severely inhibited root growth by generating high NO2

concentrations (maximum of 100 ppm) and the toxic effects persisted

for 8 weeks. Accumulation of NO2 was likely in that experiment be-

cause of high initial soil pH (6.7 to 7.8).

Urea also had been observed to cause injury to nursery grown

slash pine seedlings which were transplanted to an acid flatwoods

soil (pH 4.3) in the Southeastern USA (Smith et al., 1971). Seedling

survivals in pots fertilized with urea were lower than those ferti-

lized with NH4N03 and ureaform, and survival decreased with increasing

N applications rates. Lowest survival resulted from urea-N application

of 448 kg/ha. Ammonia toxicity generated during or after ureolysis

was probably responsible for such low survival.








Potential damage owing to urea fertilization is most important

in forest nursery operations where seedlings are more susceptible to

either salt injury or toxicity due to accumulation of free NH3 and NO2.

Cases of fir and pine seedlings being damaged by soluble N fertilizers,

one of which is urea, were reviewed by Smith et al. (1971).

SUMMARY

From the foregoing reviews of research on the transformations of

urea in soils, a number of problems are brought into focus. These

problems involving the effective use of urea appear to be influenced

by the following conditions:

1. If water is added too soon after application, there may be a

flushing of the fertilizer out of the root zone.

2. If watering is delayed, conditions may favor high volatiliza-

tion rate.

3. If adequate water is added at the proper time, microbial and

humus immobilization may prevail.

4. If organic matter is high, volatilization or immobilization

may result.

5. If organic matter is low, volatilization and/or leaching may

prevail.

6. If temperature is high, rapid rate of volatilization is very

likely to prevail; if temperature is low, ureolytic activity

may not be sufficient for a rapid rate of conversion of the

fertilizer into its ionic forms.

To avoid the above kind of dilemma, It is important that the

transformation pathways and the complete spectrum of reactions conse-

quent to the application of urea to forest soils be thoroughly understood.








Overall Fates and Reactions of Urea

The cycling of N in forest ecosystems is an important nutritional

and environmental factor. Urea plays an important role in this cycling

because of the varied transformation pathways it undergoes in the soil-

plant system. The role of urea in the overall N cycling may be com-

plicated by the fact that even in its unhydrolyzed form, it may get

immobilized, leached, adsorbed on clays, absorbed by roots, and hydro-

lyzed (Fig. 1).

Clay Adsorption

Said (1972) found that soil adsorption of unhydrolyzed urea was

highly correlated with CEC and clay content. It was also demonstrated

by Mitsui (1967) that urea moved much slower in soils with high clay

content than coarser textured soils. Gasser (1964) found that urea

was adsorbed by sodium (Na), magnesium (Mg), calcium (Ca), barium (Ba),

and aluminum (Al), and montmorillonites. Furthermore, Pearson et al.

(1962) observed that when urea was applied to soils, Ca and Mg move

downwards less than when (NH4)2S04 was applied.

Clay adsorption of urea may involve H-bonding, polyvalent cations,

ligand exchange, water bridges, and soil organic compounds. McLean

and Peterson (1965) and Mitsui (1967) postulated that urea may form

H-bonds with some radicals associated with clay colloids as shown

below:

SiOH OH -------------0

0 -C= -----H C H
C-OH \NX

OH H H
1H20



























































Fig. 1. The fates of urea
u,








The amide groups may also react with cations in the same manner as

amines because the former are readily adsorbed in the cationic forms

by exchange for inorganic ions (Greenland, 1965).

Polyvalent cations act as bridges between clay and organic com-

pounds (Greenland, 1965). It has been pointed out earlier that urea

may be a polymeric component of certain humus structures. If such

organic compounds linked to clay edges via polyvalent cation bridges

carry urea molecules, immobilization of urea more stable than humus

immobilization would occur. Stability of this type of reaction may be

maintained because proteins, which predominate in organic matter,

attached to clay edges or lattices are resistant to microbial attack

(Greenland, 1965).

Soil organic compounds are adsorbed on clays, particularly

kaolinite, through ligand exchange (Greenland, 1971). In this type of

adsorption, the organic anion (R-COO-, for example) penetrates the

coordination shell of an iron (Fe) or aluminum (Al) atom in the

hydroxide surface, thus affecting incorporation of the anion with the

surface hydroxyl layer. Urea incorporation in this manner may be

achieved by virtue of its slightly electronegative property which can

stimulate the production of organic anions or by being a structural

part of organic anions.

Clay adsorption of uncharged organic compounds is weak (Greenland,

1956). But uncharged polymers are adsorbed strongly and in large

amounts (Emerson, 1963). Furthermore, Emerson (1963) also reported

that once uncharged polymers are adsorbed on clays, they are extremely

difficult to desorb. It would appear, therefore, that polymerized

urea compounds (Mitsui, 1967) and polymeric urea-humus structures may








adsorb on clay with considerable stability. From a plant nutrition

standpoint, direct clay adsorption of urea is more desirable than humus

complexation or adsorption through ligand bridges because in the former

urea may be made available through CEC reactions,

Plant Uptake

Studies on absorption, movement, and decomposition of urea in

plants suggest that the compound is absorbed both by roots and leaves

and moves intact until hydrolyzed at the points of growth Gasser (1964),

Although beneficial effects of urea on plant growth through foliar up-

take are well documented (Gasser, 1964; Tisdale and Nelson, 1966;

Pritchett and Eberhardt, 1972), less information on direct root

uptake is available. Most detections of absorption of unhydrolyzed

urea were made on cereals, particularly wheat and rice (Mitsui, 1967),

Because of the rapid hydrolysis to NH4-N in most soils, Tisdale and

Nelson (1966) believed that direct root absorption of unhydrolyzed

urea is a minor occurence, Several reports indicated that directly

absorbed unhydrolyzed urea would be hydrolyzed inside the plant

and that the NH3 produced would be utilized in the synthesis of

proteins (Mitsui, 1967). However, the same source indicated that urea

could be absorbed by plants with no urease and the urea incorporated In

protein compounds without recourse to the NH3 form. The postulated

mechanism of urea assimilation in this manner is the reverse reactions

of the ornithine cycle.

Direct uptake of unhydrolyzed urea may be beneficial because

(1) it can supply the plant with N even without the benefit of hydroly-

sis, (2) it can supply plants with carbon without requiring some

photosynthetic steps, (3) urea may be incorporated with the protein








compounds or amino acids with fewer physiological steps, (4) danger of

damage due to NH or NO2 toxicity may be averted, (5) it may provide

the primary means of N uptake from urea immobilized in humus structures

or adsorbed on clays, and (6) it can provide plants with N supply under

conditions unfavorable for rapid ureolysis. Durzan (1973) studied the

metabolic pathway of 14C-urea in 15-day-old spruce seedlings and found

that urea provided carbon (C) for the synthesis of allantoic acid and

carbamyl phosphate. He also found that urea served as a cofactor in

the synthesis of pyruvic acid and, through urease, provided C for the

synthesis of numerous sugars and organic acids.

Ureolysis

Ureolysis is the conversion of molecular urea to ammonium car-

bonate [(NH4)2CO3] through the action of H20 and urease. A series of

chemical reactions are involved in this process. Some of the reactions

were given by Sumner (1951).

HS-CH2 HS-CH2
I I I
H2N-C-NH2 + H2N-CH-RH---- H2N-C-NH-CH-R + NH [1]

(urea) (HS-urease) (urea-urease) (ammonia)

In Eq. [11, one amide (-NH2) is removed, picks up a proton from sulf-

hydryl urease (HS-urease), and forms NH3. Blakely et al. (1969) and

Sundaram and Laidler (1970) showed that carbamic acid (NH2COOH)

is the first product of ureolysis. In the presence of H20, the urea-

urease complex is hydrolyzed. This results in the release of the

HS-urease as it regains its proton from H20 and the formation of

NH2COOH. This process is shown in Eq. [2].

HS-CH
P 1 2
H2N- -NH-CH-R + H20 ------HS-urease + H2N-C-OH [2]
carbamicc acid)








Sumner (1951) did not show the reactions necessary to carry out

the conversion of urea to (NH4)2CO3. It is probable that the following

reactions represent the necessary intermediate steps according to Hart

and Schuetz (1972) and Manziek (1974, personal communication):


H2N-C-OH + NH H2N-i-O NH4 [3]

The NH3 formed in Eq. [1] provides the electronegative force to

remove a proton from NH2COOH with the consequent formation of NH4+ and

carbamate (NH2cOO-) ions which then neutralize each other through ionic

bonding. The remaining -NH2 of the original urea molecule may then be

deaminated as in Eq. [1] and, by undergoing the same reactions as in

Eqs. [2] and [3], (NH4)2CO3 is formed.

Eq. [3] is a reversible reaction and indirectly influences .the

fate of the NH3 produced in Eq. [1]. If, instead of reacting with

NH3, NH2COOH reacts- with some other nucleophiles, NH3 may be lost

through volatilization unless it finds another proton source in the

soil solution system. However, since these two reactants share common

source and site of reaction, proximity would increase the likelihood of

the reaction in Eq. [3]. Furthermore, NH3 is an efficient nucleophile

(Hart and Schuetz, 1972). The rate of Eq. [3] reactions would then

be dependent on the rate of NH3 and NH2COOH production on the one hand

and (NH4)2CO3 accumulation on the other.

Sumner (1951) reported two kinds of urease: HS-urease and

hydroxyl urease (HO-urease). These two enzyme species differ from each

other only in terms of reaction sites as Eqs. (1] and [4] would show.

HO-CH2

H2N- -NH2 + H2N-CH-R> H2N-C-O-urease + NH3 [4]

(HO-urease) (urea-urease)








As in Eq. [2], hydrolysis of the urea-urease complex would result in

HO-urease regaining a proton from H20 and the formation of NH2COOH.

Consequences of Ureolysis

Ammonium carbonate is highly unstable in the soil (Tisdale and

Nelson, 1966). The formation of this compound is essentially the

crossroad from which more varied transformation pathways of urea-N

would emanate.

Volatilization. The formation of NH3 during deamination of urea

(Eq. [1]) is not the likely source of gaseous loss because of the

readily available proton source in NH2COOH. The main mechanism of NH3

volatilization appears to be the following:

(NH4)2CO 2NH4 + CO- 3 2NH + H2CO3 [5]
S3 3 (Carbonic acid)

The two moles of NH3 are potentially volatilizeable, unless protons

are available. Likely sources of protons are free acids and organic

acids. The following reactions may prevent NH3 from being lost through

volatilization:

NH + H+--- NH4+ [5a]

RCOOH + NH3 )RCOO- + NH RCOO-NH4 [5b]

The formation of H2CO3 (Eq. [5]) depends upon the carbon dioxide (CO2)

equilibrium status between the soil and the atmosphere. Reactions

involved in this process will not be covered in this paper. Excellent

coverage of this subject was given by Hassett (1972).

Cation Exchange. Olson et al. (1971) gave the following reactions

to show how NH3 may be involved in cation exchange reactions:

]-H + NH3 ]--- + NH4+ ]-NH [6a]

]OH + NHN +- + NH rT0-NH4+ [6b]
3 4 1 ---








]-OH2 + NH -- ]-OH + NH4 [6c]

Reactions with bicarbonates (HCO%) have two alternatives: the

formation of H2CO3 (Eq. [7c]) or participation in cation exchange

reactions. An exchange reaction with potassium (k) is shown below.

]-K + HCO + NH4+ ]-NH4 + KHCO3 [6d]

pH Rise. During the first few days following urea addition to the

soil, a rise in pH is usually observed. This rise is controlled by the

reactions that are driven by the products of ureolysis. Although OH

ions are not explicitly formed in Eqs. [Sa] through [6c], these reac-

tions are indirectly alkaline-producing since protons are utilized in

the conversion of NH3 to NH4+. Direct production of OH ions influencing

pH rise is shown below,
2-
CO- + 2H20 H2CO + 20H [7a]

CO2- + H20 -- HCO_ + OH [7b]

HCO3 + H20 H2CO3 + OH [7c]

NH3 + H20 + NH4 + OH [7d]

Only one mole of OH may be produced (Eq. [7b]) when the HCO ion is

involved in reactions similar to Eq. [6d]. In Eq. [7d], leftward

direction would seem to predominate because OH is more electronegative

than NH3 (Hart and Schuetz, 1972). However, if sinks for NH+ are

available at the moment of its formation (biological uptake or cation

exchange reaction, for example), OH" may be liberated to cause a pH

rise. In general, high CEC, high free acid concentration, and abundant

organic acids would tend to hold down volatile losses of NH3 and buffer

pH rise.

Other Consequences. The two moles of NH4 that are produced for

every mole of urea hydrolyzed may react with the concomitantly produced








CO3. One or both moles may be nitrified, denitrified, leached,

absorbed by roots, immobilized by microorganisms or the soil humus, or

fixed by clays. Nitrification is the oxidation of NH4 through microbial

action (Eq. [8a]). On the other hand, denitrification is the evolution

of N2, N20, NO, NO2 arising from the enzymatic reduction of N03 (Eq.

[8b]). The following reactions were given by Olson et al. (1971):
NH4 Nitrosomonas NO2 Nitrobacter NO3 [Sa]

NO3 enzymatic ) NO- NO ----N 0--N [8b]
3 reduction 2 2 2

As mentioned earlier, high pH may inhibit the action of Nitrobacter

more than that of Nitrosomonas such that NO2 may accumulate and

increase the potential of denitrification. The enzymatic reduction of

NO3 occurs under anaerobic conditions where NO3 is used as a terminal

proton acceptor in the decomposition of organic matter by facultative

anaerobes, or when NO3 is utilized as oxygen (02) source by autotrophic

bacteria for the oxidation of inorganic compounds in the soil

(Alexander, 1961; Olson et al., 1972).

Clay fixation of N may occur when, in the presence of K, the clay

crystal lattice collapses and traps NH4 (Olson et al., 1971). This

process can be reversed or prevented in the presence of Ca and Mg.














MATERIALS AND METHODS

Soil Materials

Three soils (Leon fs, Bladen sl, and Plummer fs), typical of

forested sites of the Southeastern Coastal Plain, were used in this

study. Samples were taken from the A horizons (15 cm deep) in control

plots in a series of CRIFF experiments (Pritchett and Smith, 1970).

The Leon series is a member of the sandy, siliceous, thermic

family of Aeric Haplaquods. These sandy soils have thin, very dark

gray Al and light gray A2 horizons that total less than 76 cm thick,

and dark humus B horizons. Samples were taken from a somewhat poorly

drained site in Lake Butler, Florida. This site is one of those found

to be highly responsive to N (especially if applied with P) fertiliza-

tion (Pritchett and Smith, 1970).

The Bladen series is a member of the clayey, mixed, thermic family

of Typic Ochraquults. These soils have loamy A horizons, gray, fine-

textured, Bt horizons containing mottles of yellowish-brown, strong

brown and shades of red, and a seasonally high water table. Samples

were taken from a control plot of a field experiment which is located

in a poorly drained site in Glynn County, Georgia. This site was not

as N responsive as the Leon site, but this soil was selected because of

its high clay content (Table 1) which influences urea-N mobility.
1/
The Plummer series is a Grossarenic Ochraquult- member of a loamy

siliceous, thermic family. Plummer soils typically have thin, gray to



1/ This is a provisional subgroup.

















Table 1. Selected chemical and physical properties of three forest
soils of the Southeastern Coastal Plain


a b Total
Soil type pHa N OMc sandd Silt Clay CEC
------------------ ----------------- meq/lOOg

Leon 4.3 0.026 0.92 88.0 10.0 2.0 7.7
Bladen 4.2 0.081 2.75 52.0 30.0 18.0 12.6
Plummer 3.5 0.385. 20.00 64.0 25.0 11.0 21.5
Plummer + De 3.5 0.710 25.00 -

Soil pH using 1:1 soil-water suspension. Unless otherwise specified,
all soil pH data reported in these studies are determined in the
same manner.

bTotal N by salicylic acid modification of the macro-Kjeldahl method.

COrganic matter content by potassium dichromate oxidation method.

From the CRIFF Uniform Fertilizer Experiments Installation Report.

eSamples consisting of loose debris that did not pass through the
2-mm sieve.








black Al horizons underlain with thick, gray to white sandy A2

horizons; these horizons overlie a strongly gleyed, loamy argillic

horizon. Samples were taken from a control plot of a field experi-

ment located in a poorly drained site near palatka, Florida. The

CRIFF Progress Report (Unpublished data, UF, Gainesville, FL, 1971)

indicated that 22 kg N/ha elicited a negative height growth

response while 90 kg N/ha effected greater height growth than the

unfertilized plots. This soil was selected for this study because

of its high organic matter content (Table 1) which has.some influence

on urea-N transformations.

With these soils comparisons of the movement and transformation

of urea-N in soils with low organic matter and clay contents (Leon

fs), high clay content (Bladen), and high organic matter content

(Plummer fsl) were possible. Chemical and physical properties of

the soils are given in Table 1,

Experiment 1. Incubation

Fifty-gram samples of oven-dry Leon soil were each weighed into

screw-cap glass jars. Three levels of urea-N (0, 118, and 236 ppm)

were added and thoroughly mixed with the soil. Four soil moisture

regimes defined by a soil moisture retention curve (Fig. 2) were

maintained. Thus, on an oven-dry weight basis, 25,0, 5.4, 2.5, and

1.8 % of water was needed to effect soil moisture regimes having

soil matric suction of 0, 100, 345, and 1000 cm of water,

respectively. Each treatment was replicated twice on each soil

moisture regime (see Experimental design, Table 2A). The caps of

the glass jars were replaced with perforated aluminum foil which

















* Plummer fs

0 Bladen sl

a Leon fs -


1I1 I I I
100 200 300 400 500 600 70
Matric suction (cm of water)

Fig. 2. Moisture retention curves of three forest soils of the
Southeastern Coastal Plain.


60



x
Ab)
C
0
u

L


0 4o







Table 2. Experimental designs

A. Experiment 1. Incubation


Urea-N Soil moisture regimes (cm of water)
treatment 0 100 345 1000
-------------- N added, ppm ----------------
1 0 0 0 0
2 118 118 118 118
3 236 236 236 236

Note: Number of replications = 2; number of samplings = 5.

B. Experiment 2A. Lysimeter pots with two-year-old slash pine

Soil moisture regimes
Treatment (cm of water)
symbola 170 500
---- N added, kg/ha ----

0 0 0
U1 224 224
Sl 224 224
U2 448 448
S2 448 448

aTreatment symbols: 0 = no N added; UI = N added as urea;
S1 = N added as ammonium sulfate; U2 = 2 x UI; S2 = 2 x S1.
Note: Number of replications = 4.

C. Experiment 28. Lysimeter pots with one-year-old slash pine

Treatment Shaded Exposed
symbols side side
------ N added, kg/ha ------

0 0 0
Ul 224 224
51 224 224
U2 448 448
52 448 448

aSee footnote a, Table 2B.

Shaded side did not receive direct sunlight while the exposed
side received direct sunlight during most of the day.

Note: Number of replications = 2.








Table 2. continued.

D. Experiment 3. Greenhouse soil columns


Soil Soil moisture regimes (cm of water)
type 170 500
-------------kg N/ha ------------------

Leon 0 448 0 448
Bladen 0 448 0 448
Plummer 0 448 0 448
Plummer + Da 0 448 0 448


a
Soil columns overlain with Plummer debris that
through the 2-mm sieve.

Note: Number of replications = 2.

E. Experiment 4. Laboratory soil columns


did not pass


Days after fertilization


Soil


type 2 5
--------- PMAa added, ppm -------------

Leon 0 50 0 50
Bladen 0 50 0 50
Plummer 0 50 0 50
Plummer + D 0 50 0 50

aPMA = phenylmercuric acetate.

bSee footnote a, Table 2D.

Note: Urea-N was added at a uniform rate of 100 ppm.
Number of replications = 2.








was fitted to each jar with a rubber band. The samples were incu-

bated at 28 C. Moisture conditions were maintained for each regime

by watering to the original moisture levels twice a week through

weight adjustment. Subsamples were collected 2 days, 1 week, 2 weeks,

4 weeks, and 6 weeks after fertilization and extracted for pH

determination, and urea-N, NH4-N, and N03-N analysis. At the end

of 1 week, soil extractions from samples under soil moisture regime

of 100 cm of water were plated so that ureolytic mircoorganisms

could be quantified and some identified (Skerman, 1959).

Experiment 2A. Lysimeter Pots with Two-year-old Slash Pine

In May, 1972, two-year-old slash pine seedlings were planted

in 4-gallon glazed ceramic lysimeter pots (Fig. 3A) with Leon soil.

Since the pots were flat-bottomed, a soil moisture potential profile

was established to determine the possibility of waterlogging at

the pot bottoms (22 cm below the surface) as this would affect the

overall experimental design. Tensiometers were installed at two

depths (Icm and 21 cm below the soil surface) in the pots with and

without seedlings. Results indicated that no such waterlogging

occurred (Fig. 4).

In April, 1973, all pots were uniformly fertilized with P as

concentrated superphosphate (CSP), K as potassium chloride (KC1),

and fretted trace elements (FTE 503) at rates equivalent to 112, 90,

and 112 kg/ha, respectively.' Nitrogen from two sources, [(NH2)2CO

and (NH4)2S04], was added at three levels: 224, and 448 kg/ha.

The N treatments, replicated four times, were randomized over every

ten pots (Table 2B). The fertilizers were spread on the soil surface

and covered with an additional 2-cm layers of the same soil.















































Fig. 3A. Two-year-old slash pine grown in four-gallon glazed ceramic lysimeter pots filled with
Leon soil. -D













































Fig. 3B. One-year-old slash pine grown in round-bottomed, three-gallon glazed ceramic
lysimeter pot filled with Leon soil.








o at surface of pots with seedlings

at bottom of pots with seedlings
0C at surface of pots without seedlings
O at bottom of pots without seedlings




80-





6o-


60 -







0







I I I I 1 I I i
50 100 150 200 250 300 350 400
Time in hours

Fig. 4. Variations in soil matric suction at the surface and bottom of pots with and without seedlings.








Since the lysimeter pots were in the open, moisture input was regulated

by covering the pots, but not the seedlings, with cellulose acetate

sheets (Fig, SA). Two soil moisture regimes, 170 and 500 cm of water,

were maintained as indicated by tensiometers which were installed at

10 cm below the soil surface. Each time the soil moisture potential

limits were approached, distilled water was added at amounts such that

400 to 500 ml of leachate was produced. The leachates were stored in

plastic beakers or bottles and refrigerated. Two months after fertili-

zation, the cellulose acetate sheets were removed. When no rainfall

occurred for a sustained period, the soil moisture regimes were main-

tained by adding enough distilled water to bring the soil matric

suction at the pot bottom close to 0 cm of water tensiometerr readings

of 10 to 15 cm of water indicated such condition), Samples of leachates

produced due to excessive rainfall were stored in plastic bottles and

refrigerated. Height and diameter were measured at the time of

fertilization and once a month thereafter.

Experiment 2B. Lysimeter Pots with One-year-old Slash Pine

In June, 1972, slash pine seeds were sown in lysimeter pots

filled with Leon soil. When the seedlings were one year old, they

were transplanted to 3-gallon, round-bottomed, glazed ceramic lysimeter

pots attached to the sides of the lysimeter pit (Figs, 3B and 5B), One

bench did not receive direct sunlight (designated as Shaded Side)

while the other bench received direct sunlight during most of the day

(designated as Exposed Side).

In July, 1973, (two weeks after transplanting), all pots uniformly

received the same rates and sources of P, K, and micronutrients, the

same rates and sources of N, and the same randomization scheme as in










A. Experiment 2A


leachate collector


B. Experiment 2B


1
s


soil



3-gal. cerami


-year-old -
lash pine Air


--- Hg reservoir




ceramic porous cup



bench


tygon tubing



leachate collector


Fig. 5. Schematic diagram of the lysimeter systems employed in
Experiments 2A and 2B.








Experiment 2A. The N treatments were replicated two times over each

bench (Table 2C). Moisture input due to rain was not controlled.

When no rainfall occurred for a sustained period, distilled water was

added to prevent soil moisture potential from exceeding 170 cm of

water. Leachates were sampled, stored in plastic bottles and refri-

gerated. Height and diameter were measured at the time of fertiliza-

tion and once a month thereafter.

In January, 1974, after measuring their final height and diameter,

the seedlings in both experiments were harvested. Foliage, stems, and

roots of each seedlings were separated and dried in an air-circulated

oven at 70 C for 3 to 4 days. After obtaining the dry weights, the

samples were ground in a Wiley mill to pass a 20 mesh screen. The

soil in the pots was sampled for analysis of pH and residual N, P, K,

Ca, and Mg.

Experiment 3. Greenhouse Soil Columns

Plexiglass tubes (11.75 cm in inside diameter) were cut into 38-cm

sections. One end of each section was sealed with a cut-out plexiglass

panel through which a rubber stopper fitted with a glass tubing was

inserted to provide a leaching outlet. The plexiglass columns were

placed on two racks and divided into four sets. The first set (L) was

filled with Leon fs, the second set (B) with Bladen sl, the third set

(P) with Plummer fs, and the fourth set (PD) with Plummer fs overlain

with 4-cm layers of loose organic debris that did not pass the 2-mm sieve

(Table 1). Tensiometers were installed through the side of each

column at 10 cm below the soil surface. Slash pine seeds were sown in

a seedbox and one seedling was transplanted to each soil column. When

the seedlings were three months old, the columns were uniformly








fertilized with P, K, and micronutrients at the same rates and sources

as in Experiments 2A and 2B. Nitrogen as urea was added at rates

equivalent to 0 and 448 kg/ha. Two soil moisture regimes were maintained.

In one soil moisture regime (W1), distilled water was added two days

after fertilization at amounts sufficient to produce about 150 ml of

leachate which was stored in plastic beakers and refrigerated. Each

time the soil moisture potentials reached 170 cm of water, distilled

water was added to bring the 5oil matric suction at the bottom of the

soil column close to 0 cm of water tensiometerr readings of 30 to 35 cm

of water indicated such condition) when no leachate was desired.

Sufficient amounts of distilled water were instantaneously added to

produce about 150 ml of leachate whenever desired. In W1, there were

three leachings made. In the other soil moisture regime (W2), distilled

water was added one week after fertilization and once a week thereafter

or whenever the soil moisture potentials reached 500 cm of soil water

tension. Only one leaching was made in W2. The soil types were ran-

domized with two replications for each N level and soil moisture

regime (see experimental design, Table 2D).

For trapping the NH3 volatilized from the urea applied, a system

of acidified NH3 sorbers similar to the one employed by Nommik (1973a)

was used (Fig. 6). This consisted of two foam rubber discs fitted

tightly inside a polyvinyl chloride (PVC) pipe (12.7 cm in length and

4.3 cm in inside diameter). Before using, the discs were leached

alternately with I M H3P04 and I M KOH, and finally with deionized

water. The discs were then soaked in a solution containing 50 ml of

85 % H3PO4 and 40 ml of glycerol per liter. The excess solution in each

sorber was removed by squeezing. To check NH3 contamination of the
























































Fig. 6. Device for trapping ammonia volatilized from urea
applied to the soil columns in the greenhouse.








system, four sorbers were placed on unfertilized soil columns.

The sorbers were replaced 1, 3, 5, 9, 14, and 21 days after fer-

tilization. The discs removed from the used sorbers were successively

leached with five 10-ml portions of 1 N KCI containing 6 ppm of phenyl-

mercuric acetate (PMA). The leachates were stored in plastic beakers

and refrigerated until analyzed.

The soil columns were sampled at three depths with a soil auger.

The soil samples were placed in polyethylene bags and stored under

refrigeration.

Experiment 4. Laboratory Soil Columns

Four sets of PVC sections (12.7 cm in length and 4.3 cm in

inside diameter) were prepared. The first set (four sections per set)

was filled with Leon fs, the second with Bladen sl, the third set with

Plummer fs, and the fourth with loose debris of the Plummer fs that

did not pass the 2-mm sieve. The amount of soil placed in each section

was such that a 10-cm soil column was attained. Each set was divided

into two subsets. To one subset, PMA was added at the rate of 50 ppm

and throughly mixed with the soil. Experimental design is shown in

Table 2E.

Each soil column was placed on a glass funnel provided with a fil-

ter paper (Watman No. 42). The soil columns were then supersaturated

with deionized water and allowed to drain. When drainage stopped, a

solution of urea-N was added to each soil column at a uniform rate of

100 ppm. Foam rubber discs (prepared in the same manner as in

Experiment 3) were fitted tightly at the top of each column. Two days

after fertilization, the foam rubber discs were removed from half of

the soil columns and leached with 1 N KCI containing 6 ppm of PMA.








The leachates were stored in plastic beakers and refrigerated. Each

soil column of this set was leached with deionized water in sufficient

amounts to produce about 120 ml of leachates. When drainage stopped,

the soil columns were sampled into 5-cm sections, stored in polyethy-

lene bags, and refrigerated. The same procedure was repeated for the

remaining set of soil columns 5 days after fertilization.

Analysis of Leachates

For Experiments 2A and 2B, leachate pH was measured using a glass

electrode and potentiometer, Analysis of urea-N was done by a colori-

metric method (Douglas and Bremner, 1971). Determination of NH4-N

was made by steam distillation, NO3-N by Devarda's alloy treatment

followed by steam distillation, except for the months of June and July

when high rainfall frequencies and intensities (Table 3) resulted in

large amounts of leachate. Since determination of NH4-N and N03-N by

the above methods is relatively slow, NH4-N was analyzed using the

ammonia selective electrode (ASE) and N03-N was determined using the

nitrate specific ion electrode (NSIE) during these months. Compared

to the steam distillation method of NH4-N determination, accuracy of

the ASE is very high (Banwart et al., 1972; Thomas and Booth, 1973).

The NSIE allows significant interference of chloride (Cl") when the

concentration of the latter exceeds ten times that of NO3. Table 4

appears to indicate that such interference did not occur. To verify

the accuracy of ASE method, some electrode measurements were calibrated

against steam distillation (SD) measurements. For NH4-N concentrations

greater than 1.0 ppm, the ASE method was adequately comparable with

the SD method, as indicated by the high correlation coefficient (Fig. 7).

Subsamples of the leachates were sent to the Soil Science















Table 3. Rainfall over the experimental site during the
period April to December, 1973


Rainfall
Month Date (cm)

April 11 30 none


Rainfall
Month Date (cm)

July 30 2.0
31 0.5


1 30 0.0


August


September 1 3
4
15
25
28

October 6
21
31


22 1.5 November 29 0.5
27 0.2
28 0.9 December 8 4.6
29 1.9 16 7.6
20 2.3


May.

June









July



















Table 4. Leachate concentrations of nitrate and chloride

NO Cl NO C1
Sample # SDa Sib SI Sample # SD SI SI
-----------------------ppm-----------------------

1 5 10 44 21 403 450 7
2 34 36 115 22 465 330 11
3 0 2 32 23 83 69 8
4 0 7 57 24 466 400 10
5 63 120 2 25 86 135 4
6 74 125 3 26 236 235 4
7 156 165 10 27 16 22 2
8 82 60 10 28 270 245 2
9 182 185 10 29 131 120 3
10 221 180 27 30 18 26 0
11 301 330 10 31 531 560 17
12 66 49 7 32 0 8 3
13 260 280 38 33 80 68 4
14 168 105 15 34 34 36 115
15 363 260 5 35 6 12 20
16 18 22 15 36 9 16 18
17 27 31 46 37 0 1 20
18 2 8 16 38 13 18 24
19 22 27 29 39 0 2 20
20 0 1 17 40 5 10 12

aDetermined by Devarda's alloy treatment followed by steam distil-
lation.


bDetermined by specific ion electrode method.


















































Steam distillation (S)


Fig. 7. The relationship between ammonium-N values (ppm) determined
by steam distillation and ammonia selective electrode methods.








Department Analytical Research Laboratory where P was determined by the

ascorbic acid method (Watanabe and Olsen, 1965), K by flame emission

spectrophotometer, Ca and Mg by atomic absorption spectrophotometer,

and soluble salts by electrical conductivity method.

Leachates collected In Experiments 3 and 4 were analyzed for pH,

urea-N by colorimetric method (Douglas and Bremner, 1971) NH4-N by

steam distillation, and NO3-N by Devarda's alloy treatment followed by

steam distillation.

The leachings from the NH3 sorbers were analyzed for NH3-N by

making 10-ml aliquots alkaline with 5 ml of 10 N NaOH immediately

prior to steam distillation. The amount of N detected in this system

was considered NH3-N volatilization loss.

Soil Extraction and Analysis

Soil samples in Experiments 2A and 28 were dried and sieved through

a 2-mm mesh. Total N was determined by the salicylic modification of

the macro-Kjeldahl method. For pH and KCl-extractable N (NH4-N and

NO3-N) determinations, the following were perfromed: Ten-gram air-dry

soil samples were weighed into 100-ml plastic beakers. After adding

10 ml of deionized water, the soil suspension was shaken for 15 minutes

in a mechanical shaker. After the pH determination, 20 ml of extracting

solution (2 N KCI containing 6 ppm of PMA) was added and the suspension

was shaken in a mechanical shaker for 45 minutes. The soil suspension

was then poured into a leaching tube provided with a filter paper

(Watman No. 42) and followed by two 15-ml portions of the extracting

solution. The filtrates were transferred to plastic beakers marked at

60 ml, made to volume with the extracting solution, and stored in the

refrigerator. Soil subsamples were analyzed for residual P, K, Ca and








Mg after extracting the soil with 1 N NH40Ac (pH 4.8). Phosphorus was

determined by ammonium molybdate-stannous chloride colorimetric method

(Jackson, 1958), K, Ca, and Mg by the methods previously described.

Soil samples in Experiments 1, 3, and 4 were analyzed without

being dried. Moisture content of each sample was determined on an

oven-dry weight basis. In Experiment 3, total N analysis and soil

extraction for pH, urea-N, NH4-N, and N03-N determinations were done

as above. Since the soil samples were not dried, addition of deionized

water for pH determination was done such that the total.water content

would effect a 1:1 ratio by weight with the soil. Soil extracts in

Experiment 3 were analyzed for KC1-extractable organic N by micro-

Kjeldahl digestion method (Bremner, 1965). In the characterization

of the different N fractions in the soil, the difference between

total N as determined by macro-Kjeldahl method and the amount of

KC1-extractable N was considered non-extractable N. The difference

between total KCl-extractable N as determined by micro-Kjeldahl

digestion followed by steam distillation and the amount of N detected

as NH4-N and NO3-N was considered KCl-extractable organic N. Conceivably,

the KCl-extractable organic N may include some of the added urea-N

that may have remained unhydrolyzed.

Tissue Analysis

Total N was analyzed from 1.0 g of the ground tissue by the

salicylic modification of the macro-Kjeldahl method. For the analysis

of P, K, Ca, and Mg, 2-gram ground tissue samples were dry-ashed at

500 C in a muffle furnace and the ash was dissolved in 50 ml 0.1 N HC1.

Phosphorus was determined by the ascorbic acid method, K, Ca, and Mg

by the methods previously described.















RESULTS

Experiment 1. Incubation

Platings of the extracts from soil incubated for I week gave

quantitative results which indicated a positive response of ureolytic

microorganisms to urea treatment (Table 5). The stimulative effect of

urea was not limited to ureolytic microorganisms; the total microbial

population increased more than four-fold and seven-fold when 118 ppm

and 236 ppm urea-N, respectively, were added. While 118 ppm urea-N

effected a population increase of ureolytic microorganism at the same

rate (four-fold) as that of the total microbial population, 236 ppm

urea-N caused the population of ureolytic microorganism to increase about

ten-fold more than the total population.

The urease-agar used in the platings had an initial pH of 6.9 and

a yellowish color. Presence of ureolytic microorganisms was indicated

by a halo of changing color (from yellow to purple) of the culture

medium around the developing colony. This color change resulted from

a rise in pH as NH3-N was produced during or after ureolysis. Fungal

ureolytic colonies germinated earlier and grew much faster than the

bacterial colonies. Subcultures taken from the ureolytic colonies

revealed a rather wide spectrum of microbial species (See note, Table 5)

that were apparently capable of producing the enyzme urease.

The effects of soil moisture regimes, urea-N treatment, and length

of incubation period on the disappearance of unhydrolyzed urea-N, for-

mation of NH4-N and NO3-N, and soil pH are shown in Table 6.

55, ,


S. . I i .





















Table 5. The effects of ure,
of water tension on
microorganism in thr
incubation

N Added (ppm)a Total MOb/g soi

0 3.7 x 106
118 13.5 x 106
236 20.0 x 106


ization 00 cm
lulation ,olytic
after or -. of


I UMOC/g soil % UMO

0.1 x 106 3.0
0.4 x 1 6 3.0
1.0 x 106 5.0


aNitrogen added as urea.

bMO = microorganism.

cUMO = ureolytic microorganism.

Note: The microorganisms identified from the isolates
were:

Fungi: Pyronelaea, Penicillium, Fusarium, Tri-
choderma, Sartoria, Aspergillus, and
Sporotrichum;

Bacteria: gram positive cocci and rods and one
negative rod;

Yeast: gram positive.









Table 6. Statistical analysis of parameters measured In Experiment 1: Incubation for forty-two
days of a Leon soil treated with three levels of urea-N under four soil moisture regimes

A. Significance of the effects of soil moisture regimes, rates of urea-N, and incubation period
on extractable urea-N, NH -N, and NO -N and soil pH


Soil moisture


Urea-N h Incubation


Parameters regimes (W) rates (T)T period (P) W x T

Urea-N ** ** ** *
NH -N ** ** ** *
NO -N ** ** ** NS
Soil pH ** ** ** **


B. The effects of soil moisture regimes and rates of urea-N on extractable urea-N and
NH -N and soil pH

Soil moisture regimes (cm of water)
0 100 345 1000
Parameters 0d 118 236 0 118 236 0 118 236 0 118 236


10.4a 15.9a O.Oa 9.la
14.3a 15.9a 3.6b 17.4a
5.9a 6.2a 4.5c 6.2b


8.3a
22.4a
7.0a


O.Ob 15.la 16.8a
3.5b 14.Oa 17.4a
4.4c 5.8b 6.7a


O.Oc 20.Ob
3.1a 5.la
4.6b 5.lab


Urea-N (%)e
NH -N (%)
Soil pH


O.Obf
3.4b
4.3b


29.7a
8.5a
5.7a









Table 6. continued.


C. The effects of soil moisture regimes on extractable urea-N, NH,-N, and
NO -N and soil pH

Soil moisture regimes (cm of water)
Parameters 0 100 345 1000

Urea-N (%) 8.8bc 5.8c 10.7b 16.6a
NH -N (%) 1l.2a 14.5a 11.6a 5.6b
NO -N (%) 1.6bc 2.2a 1.3c 1.3i
So0l pH 5.5bc 5.9a 5.6ab 5.2-


D. The effects of urea-N rates on extractable urea-N, NH,-N, and NO,-N
and soil pH

Urea-N added (ppm)
Parameters 0 118 236

Urea-N (%) O.Oc 13.6b 17.9a
NH -N (%) 3.4c 12.7b 16.la
NO -N (%) 2.3a 1.8b 0.7c
So 1 pH 4.4c 5.8b 6.4a










Table 6. continued.

E. The effects of varying lengths of incubation period on extractable urea-N,
NH,-N, and NO -N and soil pH
3


Incubation period (days)
Parameters 2 7 14 28 42

Urea-N (%) 23.5a 14.6b 5.6c 4.7c 3.7c
NH -N (%) 2.6e 7.7d 17.3a 11.9c 14.1bc
NO -N (%) 1.6bc 1.7ab 1.2c 2.1a 1.3bc
Sohl pH 4.1c 5.Ob 6.2a 6.3a 6.1a


aSoil moisture regimes (cm of water): 0, 100, 345, and 1000.

bUrea-N levels (ppm): 0, 118, and 236.

CIncubation period in days.

See footnote b above.

eExtractable urea-N, NH -N, and NO -N are expressed as percent
of the total initial N.

Differences in letters across each parameter indicate significant differences
(at .05 level) between means as indicated by Duncan Multiple Range test.

Note: For measures of statistical precision, see Table 18.
= significant at .05 level; ** = significant at .01 level;
NS = non-significant.








by urea-N treatment within each soil moisture regime is shown in

Table 68. Under 0 cm of soil water tension, urea-N that remained

unhydrolyzed amounted to 10.4 and 15.9 % of the total soil N with

urea-N treatments of 118 and 236 ppm, respectively. When the soil

matric suction was maintained close to 100 cm water, the percentage of

urea-N that remained unhydrolyzed was no longer significant. Ammoni-

fication under this soil moisture regime was increased by urea-N add-

ition, although the effects of the two levels of urea-N (118 and 236

ppm) were not different. Levels of urea-N distinctively influenced

changes in soil pH, with pH rising from 4.5 where no urea-N was added

to 7.0 where 236 ppm were applied.

Maintaining soil moisture conditions close to 345 cm of soil

water tension elicited effects on ureolysis and ammonification that

were nearly identical with those observed under 0 cm of soil water

tension. The differences in soil pH with respect to N levels followed

the following order: 0 < 118 < 236 ppm urea-N.

When soil matric suction was maintained close to 1000 cm of water,

the amount of unhydrolyzed urea-N recovered from samples receiving 236

ppm urea-N was significantly greater than from those receiving 118 ppm

urea-N. Urea-N treatment did not increase ammonification under this

soil moisture regime. Addition of 236 ppm urea-N caused soil pH to rise

slightly more than either 0 or 118 ppm urea-N (Table 6B).

The effects of soil moisture regimes on ureolysis, ammonification,

nitrification, and soil pH change are shown in Table 6C. Maintaining

soil matric suction close to 100 cm of water seemed to provide the

best environment for rapid ureolysis. Under this soil moisture regime,

the overall average amount of urea-N that remained unhydrolyzed was least.








Ureolysis was affected by soil moisture levels as follows:

100 > 0 > 345 > 1000 cm of soil water tension. Maintaining soil matric

suction as high as 1000 cm of water, however, reduced ammonification.

Nitrification occurred at a rather low level of production, especially

under 0, 345, and 1000 cm of soil water tension. It was only under 100

cm of soil water tension that nitrification was significantly higher.

Overall changes in soil pH were slightly affected by soil moisture

regimes in the following order: 100 2 345 2 0 21000 cm of water.

The overall effects of urea-N treatments on ureolysis and ammoni-

fication were in the following order: 236 > 118 > 0 ppm urea-N

Table 6C). Addition of 118 ppm urea-N appear to slightly increase

nitrification while 236 ppm urea-N tended to reduce it. For instance,

the percentages shown in Table 60 represent NO3-N production of 0.30,

0.34, and 0.17 mg resulting from urea-N treatments of 0, 118, and 236

ppm, respectively. Changes in soil pH resulting from urea-N treatments

occurred in the following order: 236 >118 >0 ppm urea-N.

The amount of urea-N that remained unhydrolyzed was higher after 2

days than either of the subsequent incubation periods (Table 6D).

Residual urea decreased after 7 days of incubation and even further

after 14 days of incubation. The amount of urea-N recovered after the

longer incubation periods (28 and 42 days) was negligible. Ammonifica-

tion was minimum (NH4-N = 2.6 % of total N) during 2 days of incubation

but Increased to 7.7 % of total N after 7 days and reached a maximum

value of 17.3 % of the total N after 14 days. Ammonification decreased

during the period 14 to 28 days and remained unchanged 28 to 42 days

after fertilization. The amounts of NH4-N produced during the last

two incubation periods were greater than those produced during the

first week of incubation. Nitrification did not seem to fluctuate as








widely as ammonification. A maximum production of NO3-N (2.1 %),

which was higher than those detected at the end of 2, 14, and 42 days,

was attained by incubating the soil for 28 days. Soil pH changed from

4.3 at the beginning of the incubation period to 4.1, 5.0, and 6.2 after

2, 7, and 14 days, respectively, but remained fairly constant there-

after.

Under 100 cm of soil water tension, complete disappearance of

unhydrolyzed urea-N occurred in 7 and 14 days after adding 118 and 236

ppm urea-N, respectively (Fig. 8B). Ureolytic activity was reduced

when soil matric suction was maintained near 345 cm of water (Fig. 8C)

and least efficient ureolysis occurred when the soil was kept under

higher soil matric suction (Fig. 8D). For a sandy soil such as Leon,

1000 cm of soil water tension probably approaches the wilting point.

Under 0 cm of soil water tension, there seemed to be a quiescent period

(2 to 7 days) during which the ureolytic microorganisms apparently

slowed down in their activity (Fig. 8A). After 7 days of incubation,

the ureolytic rate increased again, which finally resulted in complete

disappearance of unhydrolyzed urea-N during 28 to 42 days of incubation.

The ammonification process also showed characteristic patterns.

Under 0 and 100 cm of soil water tension (Fig. 8A and 8B) peak produc-

tion of NH4 coincided with the complete hydrolysis of urea-N. However,

after its maximum production, NH4-N decreased and apparently reached

minimum levels at about 28 days after fertilization. The rate of decrease

was faster under 0 than 100 cm of soil water tension. During the period

28 to 42 days of incubation, ammonificatlon increased again with 0 cm of

soil water tension favoring a slightly higher rate of increase than 100 cm

of soil water tension. Although the average ammonification process under
h .











100 ---- urea-N unhydrolyzed 50
N -- NH -N
80- n a no N added 40

l\ 118 ppm urea-N added
60- \ o 236 ppm urea-N added" 30


S40 20








4--
20-- 10V> ^ )










60-- 30
'0------i---






4o 20


20 10






Fig. 8. The effects of soil moisture regimes, urea-N levels, and incubation period on ureolysis
and ammonification.








345 cm of soil water tension vary from those that occurred under 0 and

100 cm of soil water tension (Table 6C), the pattern was different

(Fig. 8C). Under this soil matric suction (345 cm of water), 118 ppm

urea-N effected a steady build-up of NH4-N during the whole experimental

period. On the other hand, NH4-N production resulting from the addition

of 236 ppm urea-N showed a decline after its peak production at about

the 14th day of incubation. Under 1000 cm of soil water tension, about

19 % of the total initial soil N was in the NH4-N form 14 days after

the addition of 236 ppm urea-N. However, after peak production at the

14th day, NH -N steadily declined until the end of the experimental

period. A slight build-up of NH4-N resulted from the addition of 118 ppm

urea-N under the same soil moisture condition.

Nitrification occurred at a much lower level than ammonification

(Fig. 9). Maximum nitrification generally occurred under 100 cm of

soil water tension (Table 6C). The highest N03-N concentration

(19.2 ppm, soil basis) was detected 28 days after fertilization with 118

ppm urea-N under 100 cm of soil water tension (Fig. 98), Nitrification

resulting from the addition of 236 ppm urea-N under 0 and 100 cm of

soil water tension fluctuated with the length of incubation period, On

the other hand, nitrification under 345 and 1000 cm of soil water tension

appeared to steadily decline with increasing length of incubation

period (Fig. 9C and D). Nitrate-N production in the unfertilized

samples fluctuated with the different incubation periods under 100 and

345, remained essentially steady under 1000, and steadily declined

under 0 cm of soil water tension.

Incubating the unfertilized samples for 2 days under 0 cm of soil

water tension caused soil pH to drop from 4.3 to 3.6 (Fig. 1OA). With









A. 0 cm of soil water tension B. 100 cm of soil water tension


A IA


II


C. 345 cm of soil water tension I


U

U,


D. 1000 cm of soil water tension


no N added
118 ppm urea-N
0 236 ppm urea-N


II 1 0 __


7 14 21 28 35 42 7 14 21 28 35 42
Incubation period (days)

Fig. 9. The effects of soil moisture regimes, urea-N levels, and incubation period on nitrification.


0 5.0



4.0


3.0


2.0
1.0
0








3.0


2.0


1.0













































Incubation period (days)

Fig. 10. The effects of soil moisture regimes, urea-N levels, and incubation period on soil pH.








longer incubation periods, pH increased up to 5,0. Under 100 cm of soil

water tension (Fig. 10B), the drop in soil pH was only 0.1 unit but its

rise occurred earlier (at the 28th day) and was greater (1.1 pH units).

The patterns of soil pH changes in the unfertilized samples under 345 and

1000 cm of soil water tension (Fig. 10C and D) were essentially identi-

cal except that maximum pH rise under the latter was higher (5.2) than

under the former (4.9).

Addition of urea-N generally did not cause the pH to rise until

after 2 days of incubation. After 7 days of incubation under 0 and

1000 cm of soil water tension (Fig. 10A and B), soil pH increased

slightly. Under 345 cm of soil water tension, additions of 236 ppm

urea-N caused soil pH to rise to 6.0, while 118 ppm urea-N did not seem

to cause any significant pH change (Fig. 10C). After 14 days of incubation,

pH of the fertilized samples reached peak values under 0, 345, and 1000 cm

of soil water tension. Maximum pH (8.1) was attained under 345 cm of soil

water tension 14 days after the addition of 236 ppm urea-N. Towards the

end of the experimental period, all pH values declined. Highest and

lowest final soil pH values were detected under 345 and 1000 cm of soil

water tension, respectively. In terms of individual pH profiles with

respect to soil moisture regimes, soil pH peaked the highest under 345

and lowest under 1000 cm of soil water tension.

Experiment 2A. Lysimeter Pots with Two-year-old Slash Pine

Statistical analysis of the data collected in Experiment 2A are

shown in Table 7A. Variables significantly affected by N treatments

within each soil moisture regimes are shown in Table 78. Under the

high soil moisture regime (170 cm of water), 224 and 448 kg N/ha as

(NH4)2SO4 and 488 kg N/Ha as (NH2)2CO effected higher Ca uptake than












Table 7. Statistical analysis of parameters measured in Experi-
ment 2A: Lysimeter pots with two-year-old slash pine


A. Significance of the effects of
fertilizer treatments on slash
soil parameters


Parameters


Soil moisture
regimes (W)a


soil moisture regimes and
pine growth response and


N fertilizer
treatments (T)b


WxT


Growth response

Height ** ** NS
Diameter ** ** NS
Dry weight ** ** NS

Nutrient uptake

N ** NS
P ** NS
K NS ** NS
Ca ** ** *
Mg NS ** NS

Leaching losses


NH -N
NO -N
P
K
Ca
Mg
Soluble salts

Leachate pHC

1
2
3


Residual Nutrients


N
KCl-ext. N


Ca
Mg

Final soil pH









Table 7. continued.

B. The effects of
on Ca uptake,


Para-
meters C

Ca up-
take (mg) 232be
K leach-
ing (mg) 202a
Residual
P (mg) 93a
Leachate pH
1 5.4h


i1 moisture regimes and
leaching, residual P, and

Soil moisture regime
170
UI Sl "7 S2


418ab

100a

91a

6.2a


486a 526a

135a 175a

74a 106a

3.9c 5.9a


531

131

76

3.7c


ertilizer treatments
achate pH

'cm of water)
500
I Ul Sl U2 S2


499a

13Rh

131,'

5.2a


290b 325ab 273b

-'ra 137b 308a

163a 86b

4.4b 4.6b 4.2b


C. The effects of soil moisture regimes on slash pine growth response,
nutrient uptake, leaching, and retention, and leachate pH


Parameters


Growth response

Height (cm)
Diameter (mm)
Dry weight (g)

Nutrient uptake (mg)

N
P
K
Ca
Mg

Leaching losses

NH4-N (%)d
K (mg)
Ca (mg)

Residual P (mg)


Soil moisture regimes
(cm of water)
170 500


15.5
6.8
180.0


Level of
significance
(F test)


8.4
4.7
124.0


Leachate pH
1 5.0 4.6







Table 7. continued.

D. The effects of N fertilizer treatments on slash pine growth res-
ponse, nutrient uptake, leaching, and retention, soluble salts,
and leachate pH

N fertilizer treatments


Parameters


0 Ul S1 U2 S2


Growth response
Height (cm)
Diameter (mm)
Dry weight (g)

Nutrient uptake (mg)
N
P
K
Ca
Mg

Leaching losses
NH -N (%)
NO -N (%)
P (mg)
K (mg)
Ca (mg)
Mg (mg)
Soluble salsts (g)


Leachate pH
1


Residual
P


nutrients (mg)
89bc
109a
3534a


8.0b 15.4a
2.3c 6.8ab
82c 182ab


316c
80b
175c
211b
67c


0.11d
0.22c
254a
205a
712cd
282a
3.6c


918ab
164a
310a
458a
161a


9.9b
5.7b
138b


715b
133a
218bc
388a
103bc


1.76d 10.70b
0.76bc 0.52bc
121b 212a
119b 216a
574de 949b
216b 274a
3.2c 7.4b


15.6a
7.7a
197a


1098a
162a
310a
439a
168a


5.90c
2.52a
Illb
156ab
483e
153c
3.6c


5.la 5.7a 4.1b 5.2a
4.5a 4.7a 3.8b 4.2ab
5.6a 5.4a 4.7b 5.0ab


11 lb
89ab
3610a


75c
65b
2451b


135a
88ab
2914ab


10.9b
6.0b
161ab


868ab
138a
312bc
403a
10Obc


22.60a
1.04b
261a
220a
1128a
260ab
10.la


4.0b
3.7b
4.4b


81c
68b
2453b


aSoil moisture regimes: 170


and 500 cm of water.


bN fertilizer treatments: 0 = no N added; Ul = 224 kg N/ha as urea!
Sl = 224 kg N/ha as ammonium sulfate; U2 = 2 x Ul; S2 = 2 x Sl.

CpH of leachates collected at different times after fertilization:
1 = 1 week, 2 = 8 weeks, 3 = 9 months.

dNH4-N and NO -N are expressed as percent of the total initial
N content of the soil.

eDifferences in letters across each parameter indicate significant
differences (at .05 level) between means as indicated by Duncan
Multiple Range test.








the controls (no N treatment). Addition of (NH2)2CO at the rate of 224

kg N/ha did not cause any response in terms of Ca uptake. Under the

low soil moisture, regime (500 cm of water), addition of (NH2)2CO at the

rate of 224 kg N/ha resulted in higher Ca uptake than the addition of

224 or 448 kg N/ha as (NH4)2S04 or where no N was added. The effect of

448 kg N/ha under the same soil moisture regime was not different from

the effects of the other treatments.

Leaching losses of K under the high soil moisture regime was not

affected by N treatments. Significant variation, however, occurred

under the low soil moisture regime where adding 224 or 448 kg N/ha as

(NH4)2S04 resulted in higher leaching losses of K than adding either

levels of N as (NH2)2CO. Without N additions the amount of K lost

through leaching was less than the amount lost following (NH4)2SO4

fertilization but it was not different from the losses resulting from

(NH2)2CO fertilization.

Leaching losses of soluble salts were higher resulting from the

addition of 224 or 448 kg N/ha as (NH4)2SO4 than as (NH2)2CO under the

high soil moisture regime. Under the same soil moisture regime, leaching

losses of soluble salts were higher from pots to which (NH4)2S04 had been

added than from the control pots or those that received (NH2)2CO,

The five N treatments did not differ in their effects on residual

P under the high soil moisture regime. Under the low soil moisture regime,

however, there was significantly higher soil retention of P with either

224 or 448 kg N/ha as (NH2)2CO than with 224 or 448 kg N/ha as (NH4)2S04

or without N.

Variations in leachate pH were more distinct during the early part of

the experiment. Under the high soil moisture regime, 224 and 448 kg N/ha

as urea caused leachate pH to rise to 5,9 and 6,2, respectively,








Under the low soil moisture regime, only 224 kg N/ha as urea brought

leachate pH higher than 5.0. On the other hand, addition of N as

(NH4)2S04 caused leachate pH to reach as low as 3.7, particularly

under the high soil moisture regime. Growth response (height,

diameter, and dry weight production) and nutrient uptake (N, P. K, Ca,

and Mg) were higher under the high than low soil moisture regime

(Table 7C). The high soil moisture regime effected higher leaching

loss of Ca while the low soil moisture regime favored higher leaching

losses of NH4-N and K. Leachate pH during the early part of the experi-

mental period was generally higher under the high than low soil mois-

ture regime.

Greater height growth response was observed on pots receiving

(NH2)2CO (Table 7D). Higher level of N (448 kg N/ha) as (NH2)2CO was

apparently needed to elicit greater diameter growth and dry weight

production). Higher nutrient (N, P, K, Ca, and Mg) uptake was also

observed on slash pine fertilized with (NH2)2CO.

Leaching losses of N generally increased with increasing level of

N application but (NHj)2S04 favored much higher leaching losses of N

than (NH2)2CO. Leaching losses of P, K, Ca, and Mg, on the other hand,

were generally lower from pots receiving (NH2)2CO than those receiving

(NH4)2SO4. The amount of soluble salts leached through pots fertilized

with (NH4)2S04 was more than twice the amount lost from pots fertilized

with (NH2)2CO.

The effect of (NH2)2CO fertilization on leachate pH generally did

not vary from the control. However, (NH )2SO4 fertilization caused a

significant drop in leachate pH, especially 1 to 2 weeks after ferti-

lization.

The data shown in Table 8 suggest that generally higher amounts of








Table 8. Leaching losses of soluble salts during a period of nine months from the lysimeter
pots with two-year old slash pine fertilized with urea and ammonium sulfate under
two soil moisture regimes


Days after Soil moisture regimes (cm of water)
fertili- 170 500
nation 0a Ul Sl U2 S2 0 Ul Sl U2 S2

----------------------------.------------- g ------------------------------------

7 0.51 0.31 2.23 0.53 2.89
12 0.40 0.51 1.42 0.71 2.30
24 0.39 0.41 1.23 0.61 1.77
58 0.50 0.62 0.56 0.90 0.78
64 0.30 0.31 0.34 0.27 0.55
65 1.60 1.51 3.76 1.27 5.64
67 0.91 0.70 1.29 1.01 1.47 0.15 0.15 0.40 0.11 0.56
73 0.61 0.25 0.38 0.21 0.30 0.68 0.62 1.33 0.64 2.01
113 0.10 0.10 0.16 0.08 0.08 0.13 0.14 0.31 0.06 0.33
134 0.05 0.03 0.05 0.02 0.06
146 0.17 0.14 0.31 0.16 0.22 0.22 0.20 0.27 0.20 0.30
241 0.07 0.13 0.18 0.13 0.18 0.10 0.11 0.22 0.12 0.18
251 0.12 0.18 0.20 0.13 0.17 0.16 0.12 0.08 0.12 0.33

Total 3.74c 3.28c 7.12a 4.15c 9.00a 3.43c 3.26c 7.60b 3.13c 11.12a


aSee footnote b, Table 7A.








soluble salts were detected in leachates collected from pots fertilized

with (NHQ)2SO4 and these amounts were particularly high during the

early part of the experiment. Soluble salts detected 65 days after

fertilization were about 3 times more in the leachates collected from pots

fertilized with (NH4)2SO4 than those fertilized with (NH2)2CO. It was

two weeks before two slash pine seedlings fertilized with 448 kg N/ha

as (NH4)2S04 started to show symptoms of salt injury; the two seedlings

eventually died. The data referred to above, however, could not be

compared because variable time of leachate collection under each soil

moisture regime made the data difficult to statistically analyze. Despite

the inclusion of leaching data on soluble salts that occurred during the

latter part of the experimental period, statistical analysis of the

total leaching losses still showed that within each soil moisture regime,

(NH4)2S04 fertilization resulted in significantly higher leaching losses

of soluble salts than either the control or (NH2)2CO fertilization.

Correlation analysis was performed to establish the relationship

between growth response and nutrient uptake and leaching losses of nutrients

and soluble salts (Table 9). Results indicate that growth response

(height, diameter, and dry weight production) were positively correlated

with uptake of N, P, K, Ca, and Mg but negatively correlated with leach-

ing losses of of P, K, Ca, and Mg. Leaching losses of N03-N were

positively correlated with diameter growth and dry weight production.

Leaching losses of NH4-N and soluble salts were not correlated with

growth response.

Characterization of the distribution of N in the lysimeter system

(Experiment 2A) is shown in Tables 10 and 11. Nitrogen that was measured

in the uptake, leaching, and soil retention distribution was expressed as












Table 9. Relationships among growth response, nutrient uptake, and leaching losses of nutrients and soluble
salts in Experiment 2A: Lysimeter pots with two-year-old slash pine

Dependent Nutrient uptakea Leaching losses
variables UN UP UK UC UM LNH LNO LP LK LC LM SS
---------------------------------- correlation coefficients- ---------------------------------

Height (cm) 0.61, 0.72,_ 0.59.~ 0.671,, 0.59*. -0.20 0.15.- -0.53*, -0.62, -0.12 -0.23, -0.25
Diameter (mm) 0.77,, 0.691, 0.601, 0.682, 0.68,, 0.17 0.41," -0.50,, -0.578 -0.08 -0.46 -0.01
Dry weight (g) 0.92' 0.85 0.84"" 0.92. 0.86"" 0.-08 0.33 -0.63 -0.64 -0.03 -0.37 -0.08

aNutrient uptake: UN, UP, UK, UC, and UM = uptake (mg) of N, P, K, Ca, and Mg, respectively.

bLeaching losses: LNH, LNO, LP, LK, LC, LM, and SS = leaching losses of NH4-N (% of total initial N), NO3-N
(% of total initial N), P (mg), K (mg), Ca (mg), Mg (mg), and soluble salts (g), respectively.

Significant correlation at .05 level.

*Significant correlation at .01 level.










Table 10. Distribution of N in the lysimeter system fertilized with urea and ammonium
sulfate under two soil moisture regimes

Soil moisture regimes (cm of water)
170 500
N dlstrlbutTon 0a Ul S1 U2 S2 0 UI SI U2 S2
------------------------ of total initial N------------------------

Uptake 6.8 15.2 14.4 17.0 16.1 7.1 17.3 10.7 15.2 9.2
Leaching 0.4 2.6 7.7 8.2 17.8 0.3 2.3 11.5 6.4 20.8
Residual 94.9 76.7 74.6 64.4 61.1 97.1 78.2 73.5 65.0 62.4

Unaccountedb -2.1 5.5 3.3 10.4 5.0 -4.5 2.2 4.3 13.4 7.6

aSee footnote b, Table 7A.

bPercent of original soil N plus added N (total initial N) unaccounted for after nine
months. Negative sign indicates a gain of N by the soil. See footnote b, Table II.

Note: Total rain input of N/pot: NH4-N = 14.0 mg; N03-N = 15.0 mg.









Table 11. Distribution of N in the lysimeter system fertilized with urea and ammonium
sulfate under two soil moisture regimes

Soil moisture regimes (cm of water)
170 500
N distribution Oa Ul Si U2 S2 0 Ul Sl U2 S2
-------------------------- % of added N---------------------

Uptake 74.8 71.0 50.4 47.6 85.2 52,8 45.2 27.3
Leaching losse 13.2 38.2 24.3 52.7 11.1 56.3 19.0 61.8
Gaseous losses 27.1 16.2 30.8 14.8 10.8 21.1 39.8 22.5
................---------------------------------------------------------------------.

Removalc -2.1 3.1 5.2 1.8 5.1 -4.5 1.4 3.6 13.4 7.6

aSee footnote b, Table 7.

bBased on the assumption that the item unaccounted for in Table 10 was due to gaseous
losses as NH3 volatilization and/or volatilization.

CApparent depletion of native N which was determined by expressing as percent of the
total initial N the amount of N taken up by the seedlings, leached, and apparently
volatilized and/or denitrified in excess of the added N.









percent of the total initial N content of the soil (Table 10). Subtract-

ing the sum of the three pathways from the total initial N and expressing

the difference as percent of the latter would result in an unaccounted

for item. Under the high soil moisture regime, such item varied in

the following order: U2 Ul > 52 > Sl > 0. Under the low soil

moisture regime, the order was U2 > 52 > SI > Ul > 0. Negative

values corresponding to the controls signify that the sum of the

three measured pathways exceeded the total initial N. In Table 11, uptake

and leaching losses of N and the item unaccounted for in Table 10 are

expressed as percent of the added N. By assuming that most of the N

unaccounted for was due to gaseous losses as NH3 volatilization and/or

denitrification, another item was calculated, the apparent depletion

of the native N. This was done by expressing as percent of the total

initial N, the amount of N taken up by the seedlings, leached, and

apparently volatilized and/or denitrified in excess of the added N.

Under this item, depletion of the native soil N resulting from N

fertilization followed the following order: S1 > S2 > U1 > U2 > 0

and U2 > 52 > Sl > U1 > 0 under high and low soil moisture regimes,

respectively. Again, this shows that N seemed to be gained by the

unfertilized pots. Calculation using the amount of rain recorded

(Table 3) and rain concentrations of NH4-N and NO3-N (see Note, Table 10)

would indicate that N input due to rain could not account for the N

gained by the unfertilized pots.

Experiment 2B. Lysimeter Pots with One-year-old Slash Pine

The lysimeter pots used in Experiment 2A were flat-bottomed and

were, therefore, restrictive to downward water movement. Furthermore,

the seedlings had been growing in the pots for 11 months before ferti-








lization treatments were added and an examination of a few pots showed

that the pot soil volume was well colonized by the seedlings root, thus

inhibiting downward water flow through transpiration and, perhaps, as

a physical barrier by the root mass. Experiment 2B was, therefore,

conducted to characterize nutient movement through a lysimeter system

that provided less restriction to downward water flow than the system

employed in Experiment 2A. Since the pots in experiment 2B were

placed at two sides of the lysimeter pit which received varying degrees

of shading, the latter was considered a variable in the data analysis.

Most of the variables measured in Experiment 2B were non-signi-

ficant with respect to degree of shading (Table 12A). Among the

growth response variables, only dry weight production was signifi-

cantly affected. Other parameters affected by shading were K uptake

and pH of leachate collected 6 months after fertilization. Leaching

losses of Ca and pH of leachates 6 days and 6 weeks after fertilization

were highly significantly affected by degree of shading..

Nitrogen fertilizer treatments significantly affected diameter

growth, uptake of N and Mg, pH of leachates collected 6 days after

fertilization, and final soil pH. On the other hand, addition of

either (NH2)2CO or (NH4)2S04 significantly affected K uptake, nutrient

leaching losses, and pH of leachates collected 6 months after fertiliza-

tion.

Dry weight production of seedlings grown on the shaded side of

the lysimeter pit was slightly higher than those grown on the exposed

side (Table 128). Correspondingly, K uptake was higher by the shaded

seedlings than by the exposed plants. On the other hand, leaching

losses of Ca, as well as pH of leachates collected at different times





80





Table 12. Statistical analysis of parameters measured in Experi-
ment 2B: Lysimeter pots with one-year-old slash pine

A. Significance of the effects of shading and N fertilizer treatments
on slash pine growth response and soil parameters


Parameters


Degree of
shadina (S)a


Fertilizer
treatments (N)b S x N


Growth response

Height NS
Diameter NS
Dry weight *

Nutrient uptake

N NS
P NS
K
Ca NS
Mg NS

Leaching losses

NH -Nc NS
NO -N NS
P NS
K NS
Ca **
Mg NS


Leachate pH-

1
2
3

Final soil pH


NS *








Table 12. continued

B. The effects of shading on dry weight
Ca leaching,and leachate pH


production, K uptake,


Level of
Parameters Shaded side Exposed side significance

Dry weight (g) 32.2 20.2 *

Nutrient uptake (mg)
K 109 62 *

Leaching losses (mg)
Ca 474 573 **

Leachate pH
1 5.3 6.4 **
2 4.6 5.5 **
3 4.3 4.6 *

.The effects of N fertilizer treatments on diameter growth,
nutrient uptake and leaching, and leachate pH

Fertilizer treatments
Parameters 0 Ul Sl U2 S2

Diameter (mm) 4.2b 8.0a 5.8ab 8.6a 8.2a

Nutrient uptake (mg)
N 103b 280ab 142ab 318a 164ab
K 49b 168a 58b 103ab 50b
Mg O1b 32a 11b 21ab 14ab

Leaching losses
NH -N (%) 0.5d 7.4c 15.1b 12.2bc 26.9a
NO-N (%) l.Oc 2.8b 1.6c 4.4a 1.5c
P (mg) 248a 177b 248a 164b 294a
K (mg) 310a 167b 328a 180b 290a
Ca (mg) 561a 352b 684a 360b 600a
Mg (mg) 414a 144c 152c 96c 208b

Leachate pH
I 6.3a 6.1a 5.8ab 6.4a 4.6b
3 5.2a 4.4b 4.1b 4.6ab 3.9b

aThe pots were placed at two sides of the lysimeter pit which
received varying degree of shading. One side (shaded side)
did not receive direct sunlight while the other side (exposed
side) received direct sunlight during most of the day.

bFertilizer treatment: 0 = no N added: Ul = 224 kg N/ha as
urea; Sl = 224 kg N/ha as ammonium sulfate; U2 2 x Ul;
S2 = 2 x Sl.





82


Table 12. continued

CNH4-N and NO -N were expressed as percent based on total
initial N content of the soil

dpH of leachates collected at different times after fertili-
zation: 1 = 6 days, 2 = 6 weeks, 3 = 6 months.








after fertilization, were significantly higher from the exposed pots

than the shaded pots.

There was no significant difference in diameter growth with

respect to source of N. However, the control (no N added) resulted in

less diameter growth than where N was added. Uptake of N and K was

greater from the (NH2)2CO than (NH4)2S04 source, but only the low rate

of (NH2)2CO significantly increased Mg uptake over the other treatments.

Addition of N as (NH4)2S04 caused higher leaching losses of

NH4-N while (NH2)2CO fertilization resulted in more leaching losses of

NO3-N, although nitrification, in general, was low (Table 12C), Leaching

losses of P, K, Ca, and Mg seemed to be minimized by (NH2)2CO fertilization.

The effect of N treatment on leachate pH varied distinctly only in

the leachates collected 6 days after fertilization. Addition of 224 and

488 kg N/ha as (NH2)2CO caused soil pH to reach 6.3 and 6.4, respectively

while 448 kg N/ha as (NH4)2S04 tended to maintain leachate pH at the

level before fertilization.

In Experiment 2A, the nutrient uptake was generally greater than

nutrient leaching losses while the reverse was true in Experiment 28

(Table 13 and 14). Residual N values were approximately identical in

both experiments. While the unfertilized pots in Experiment 2A

apparently gained N, all treatments in Experiment 2B resulted in net

losses of N that could not be accounted for (Table 13). Table 14

expresses the N pathways as percent of the added N. On the shaded

side, apparent gaseous losses were greater from (NH2)2CO than from

(NH4)2504 fertilization. On the exposed side, however, variation in

apparent gaseous losses followed the following pattern: U1 > S2 >

U2 > Sl. In terms of apparent N depletion, the potted soil that











Table 13. Distribution of N in the lysimeter system fertilized with urea and ammonium
sulfate under direct (exposed) and indirect (shaded) sunlight

Shaded side Exposed side
N distribution Ob Ul S1 U2 S2 0 Ul S1 U2 S2
-----------------------% of total initial N------------------------

Uptake 2.6 7.4 2.8 8.6 2.6 2.5 4.0 3.0 2.4 3.1
Leaching 1.4 10.7 16.7 15.3 27.8 1.6 13.5 16.5 20.7 29.0
Residual 93.7 80.0 80.1 66.0 68.2 90.6 76.2 77.5 70.4 58.4
............................------------------------------------------------------------

Unaccountedc 2.2 1.9 0.4 10.1 1.4 5.3 6.3 3.0 6.5 9.5

aSee footnote a, Table 12A.

bSee footnote b, Table 12A.

CSee footnote c, Table 14.

Note: Rain nutrient input/pot: NH4-N = 7.8 mg; N03-N = 8.5'mg; Total = 16.3 mg.











Table 14. Distribution of N in the lysimeter system fertilized with urea and ammonium
sulfate under direct (exposed) and indirect (shaded) sunlight

Shaded Sidea Exposed Side

N distribution 0b Ul Sl U2 S2 0 UI Si U2 S2
----------------------------% of added N-------------------------

Uptake 42.6 16.2 29.0 8.8 23.0 17.3 8.1 10.5
Leaching 61.6 95.5 51.7 94.2 77.7 93.6 69.9 97.8
--------------------------------------------------------------------------------------

Gaseous Lossesc 10.9 2.3 34.1 4.7 36.2 17.3 21.9 32.2
-------------------------------------------------------------------------------

Removald 6.3 2.6 2.6 4.4 2.2 9.4 6.4 5.2 -0.1 12.1

aSee footnote a, Table 12A.

bSee footnote b, Table 12A.

CThis is based on the assumption that the item unaccounted for in Table 18 was due to
gaseous losses as NH3 volatilization and/or denitrification.

dApparent depletion of native N which was determined by expressing in % of.the total
initial N the amount of N taken up by the seedlings, leached, and apparently vola-
tilized and/or denitrified in excess of the added N.








received no N apparently lost considerable amount of its native N.

Depletion of native N in the shaded side did not seem to distinctly

vary with respect to sources and levels of N added. In the exposed

side, however, practically no N was lost from the native N content

of the potted soil fertilized with 448 kg N/ha as (NH2)2CO. On the

other hand, addition of 448 kg N/ha as (NH4)2SO4 resulted in the

highest amount of native N depletion.

Experiment 3. Greenhouse Soil Columns

Statistical analysis of the variables measured in Experiment 3 are

shown in Table 15. Under the high soil moisture regime (170 cm of

water), addition of 488 kg N/ha as urea did not cause significant

NH3-N volatilization. Under the low soil moisture regime, however,

significant amount of NH3-N was volatilized from the added urea-N.

This pattern is better emphasized in Fig. 11. Significant amount of

unhydrolyzed urea-N and NH4-N leached through the fertilized columns

under the high soil moisture but not under the low soil moisture

regime.

The one variable affected by soil moisture regimes within each

soil type is the leaching loss of NO3-N (Table 15C). This variation

occurred only in one soil type, Bladen, where the high soil moisture

regime caused significantly higher loss of NO3-N.

The effects of urea-N additions within each soil type are shown

in Table 15D. Additions of urea at a rate equivalent to 448 kg N/ha

caused significant reduction in non-extractable N; the greatest per-

cent reduction appears to have occurred in the Leon soil and the

least in the Plummer +D, Urea-N fertilization also caused a signi-








Table 15. Statistical analysis of parameters measured in Experiment 3: Greenhouse soil columns


A. Significance of the effects of soil moisture regimes, urea fertilization, soil types, and
soil depths on soil parameters


Soil moisture
Parameters regimes (M)a

d
NH -N volatilile losses A

Leaching losses
Urea-N *
NH -N **
NO -N **

Non-ext. Ne NS

KCl-ext. N
NH -N NS
NO -N NS
Or4. N NS

Final soil pH NS


Urea-N
added (N)

**





NS



**

**

**


Soil
type (S)c

NS


NS
NS
**

**


**
**
**

**


Soil
depth


MxN Mx S

NS


- *
- *
NS

** NS


NxS

NS


NS
NS
NS

**




NS
..*

**


B. The effects of soil moisture regimes on volatilization and leaching losses of N

Soil moisture regimes (cm of water)
170 500
Parameters o09 448 0 448

NH -N (%) O.Oa 0.54a O.Ob 4.20a

Leaching losses (%)
Urea-N O.Ob 0.93a O.Oa O.13a
NH -N 0.09b 0.21a 0.04a 0.07a








Table 15. continued.


C. The effects of soil types and soil moisture regimes on leaching losses of NO,-N

Soil types
Leon Bladen Plummer Plummer + D
Parameters 170 500 170 500 170 500 170 500

Leaching losses'
of NO -N (%) 0.07a 0.06a 0.54a 0.08b 0.10a 0.02a 0.04a 0.02a


D. The effects of soil types and urea fertilization on non-extractable and KC1-extractable
N and soil pH

Soil types
Leon Bladen Plummer Plummer + D
Parameters 0 448 0 448 448 0 448

Non-ext. N (%) 98.Oa 73.7b 95.8a 83.1b 97.3a 92.7a 98.1a 92.3b

KCl-ext. N
NH4-N (ppm) 4.7a 93.4a 28.5a 156.9a 70.7b 270.2a 51.6b 423.3a
Org. N (%) 37.8a 4.8b 18.2a 2.4b 9.8a 2.8a 26.8a 1.8b

Final soil pH 4.6b 6.1a 4.5b 5.0a 3.7b 4.3a 3.7b 4.3a




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