Title: Shifts in stable carbon isotopic signatures in soil fractions following tree integration into pastures
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Title: Shifts in stable carbon isotopic signatures in soil fractions following tree integration into pastures
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Language: English
Creator: Haile, S.G.
Publisher: Soil and Water Science Department, College of Agriculture and Life Sciences, University of Florida
Place of Publication: Gainesville, Fla.
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Bibliographic ID: UF00091730
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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SWS 09-02


Soil and Water Science

h Research Brief


Shifts in Stable Carbon Isotopic Signatures in Soil Fractions following Tree
Integration into Pastures

S. G. Haile, V. D. Nair and P. K. R. Nair


Soil organic matter (SOM) is extremely
vulnerable to land-use change, as well as to
intensification of agricultural practices. The
functional consequences of integration of
trees into grass-dominated vegetation
include changes of above- and below-
ground productivity, modifications to
rooting depth and distribution, and changes
in the quantity and quality of litter inputs to
the soils. These changes in vegetation, litter,
and soil characteristics modify SOM
dynamics and storage, which in turn may
lead to alterations of local and regional
climate systems.

In this study, we combine physical soil
fractionation and the 13C natural abundance
technique to investigate the shifts in soil C
following the alteration in vegetation
structure due to the integration of trees into
open pasture by comparing with an adjacent
open pasture with similar background
conditions. When one type of vegetation is
replaced with another, 613C values could be
used to identify SOM derived from residues
in the native vegetation and the new
vegetation, for example, where C4 plant
(with 613C values be reported as -19 to
-9%o) grows on soil derived from a C3 plant
(13C of 35 to 20%o). Soil samples were
collected from silvopastures of slash pine
(Pinus elliottii) + bahiagrass (Paspalum
notatum), and adjacent open pasture (OP), at
six depths down to 125 cm, at four sites


representing two major soil orders
(Spodosols [Hardee and Osceola sites] and
Ultisols [Alachua and Suwannee sites]) of
Florida. At the silvopastures, soils were
sampled both near the tree (SP-T) and in the
middle of the alley (SP-A).

The open pastures on Suwannee, Osceola,
Hardee and Alachua farms, were 40, 22, 48,
and 55 years old, respectively, since pasture
establishment. Florida Flatwoods was the
land use prior to the pasture establishment in
all the sites except the Alachua site where
the land was under agriculture (corn, Zea
mays). The respective ages of silvopastures
since establishment at the four sites were:
Suwannee (40 yr), Osceola (14 yr), Hardee
(12 yr), and Alachua (8 yr). The plant
sources of C in the whole-soil (non-
fractionated) and the three soil fraction-
sizes, (250 2000, 53 250 and <53 im)
obtained using a wet-sieving process, were
traced using stable C isotope signatures.























The C3-derived soil organic carbon (SOC)
in silvopasture in the whole soil (non-
fractionated) was double at the surface and
was generally higher at the other depths as
compared to that in OP sites, particularly at
the Ultisol sites. Slash pine trees (C3 plants)
seemed to have contributed more C in the
silt- + clay- sized (<53 [im; the most stable
C fraction) fractions than bahiagrass (C4
plants), particularly deeper in the soil
profile. Spodosols sites contained more C in
the <53 tm fraction at and below the spodic
horizon (40 50 cm deep) in silvopasture
compared to OP.


U11- I s~os Spodokso to
S- 0 tb
)04
0p

,0 ~;

-.10-I-
10A


- OP
---- SP-A
-*-- SP-T


Soil dpth, cm
Changes in percent of C3-derived soil organic carbon
(SOC) in macro-sized (250 2000 ) im; a, b, c, & d ),
micro-sized (53 250 )rm; e, f, g, & h), and silt + clay (<53
jim; i, j, k, & 1) fractions with mid-points of sampled depths
at three pasture locations (silvopasture center of the alley
[SP-A] and in-between tree rows [SP-T] and open pasture
[OP]) for soils of the Alachua (a, e, & i), Suwannee (b, f, &
j), Hardee (c, g, & k), and Osceola (d, h, & 1) sites in
Florida.


The study showed an increase in total C3-
drived SOC pools following tree integration
into pasture. Compared to C4-derived SOC,
a substantial proportion of C3-derived SOC
was C found associated with silt + clay and
in the micro-sized fraction. However, the
macro-sized fraction had the largest
accumulation of C3-derived SOC of all the
fractions. The C3 plant had consistently
contributed higher proportion of SOC to the
silt + clay fraction (<53 jim) than C4 plants,
particularly in the greater depths, in all sites,
irrespective of the age of the land-use.

Soil type appears to be a major factor in
SOC content, with the accumulation being
greater in Ultisols and at the Bh horizon of
Spodosols where the clay content is greater.
In addition, prior land-use history of the
sites could have substantial effect on the
current status of C storage in soil. In the
long term, however, results suggest that tree-
based agricultural systems may help
sequester more SOC.

For information on total soil carbon storage
at these sites, see SWS 09-01. Additional
information can be obtained from:

S.G. Haile: solomonh@ufl.edu
V.D. Nair: vdn(@ufl.edu
Soil and Water Science Department
P.O. Box 110510
Gainesville, FL 32611-0510
Tel: (352) 392-1803


P.K.R. Nair: pknair(@ufl.edu
School of Forest Resources and
Conservation
Tel: (352) 846-0880




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