Carbon sequestration from grazing exclusion as a land use change
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Transcript of Carbon sequestration from grazing exclusion as a land use change
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
No. 58, Winter 2005
il management for drylands
Soil carbon and microbial biomass carbon after 40 years of grazing
exclusion in semiarid sagebrush steppe of Wyoming
y Gyami Shrestha, Peter D. Stahl, Larry C. Munn, Elise G. Pendall, George F. Vance and Renduo Zh
[Results of our study] are
onsistent with the proposal
hat well-managed grazing
may be an option for
equestering carbon in
rasslands."
● Introduction
● Study sites
● Research methods
● Summary of results and discussion
●
Conclusion ● Acknowledgments
● References
● Author information
Introduction
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The sagebrush steppe of Wyoming is classified by the World Wildlife
Fund as a desert and xeric shrubland biome with vulnerable conservation
status. It is arid or semiarid and has a surface area of 132 400 km2 (~51
120 mi2). Dominated by sagebrush ( Artemisia tridentata), fescue (Festu
spp.) and wheat grasses ( Agropyron spp.), it faces disturbances like seve
winds, grazing, burning and variations in precipitation and temperature
(WWF 2001). Between 1959 and 1965, with the objective of studying th
effect of grazing on sagebrush steppe vegetation, approximately 100
exclosures were established in central Wyoming as part of a cooperative
agreement between the University of Wyoming's Range Management
Section and the Bureau of Land Management. Soil organic carbon (SOCand soil microbial biomass carbon (SMBC) were studied in four of those
grazing exclosures between 2003 and 2005. Our research was conducted
with the objective of evaluating the impact of long-term grazing exclusio
on SOC and SMBC in Wyoming sagebrush grasslands. SOC, measured
a percentage by weight, provides an estimate of how much organic matte
there is in the soil. SMBC generally only represents about 1-3% of total
soil carbon, but it is an important indicator of nutrient turnover and stora
in a given soil-plant association because it is highly labile (that is, it can
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
undergo rapid changes). Both of these measurements provide an indicati
of overall soil health as well as a way of determining how much carbon i
being sequestered.
Managed grazing on previously degraded grasslands was added to the
Kyoto Protocol's list of Land Use, Land Use Change and Forestry
(LULUCF) activities for carbon sinks (that is, sequestration) during the
Marrakesh Accords in 2001 (UNFCCC 2005). Managing the available vexpanses of grazing lands in the US for enhanced carbon sequestration a
storage could prove very valuable both for offsetting present carbon
imbalances and for preserving resources by maintaining overall soil heal
Review of past and current literature indicates that soil carbon pool,
microbial biomass and soil structure are interlinked during carbon
accumulation and storage by the soil (Sparling 1992; Breland and Eltun
1999; Fließbach and Mäder 2000; Nilsson 2004). Milchunas and
Lauenroth (1993) did a detailed literature review of 34 studies involvinggrazed and ungrazed sites around the world and reported both decrease
(40%) and increase (60%) in soil carbon as result of grazing exclusion.
More recent studies of grazed soils worldwide have also shown both
increases (Schuman et al. 1999; Reeder et al. 2004) and decreases (Dern
et al. 1997; Yong-Zhong et al. 2005) in carbon storage and accumulation
compared to adjacent ungrazed soils.
The null hypothesis (H0) for this study was that long-term grazing
exclusion has no effect on the aforementioned soil parameters, while thealternative hypothesis (H1) was that it exerts a significant effect on them
Study sites
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The four study sites, each comprising an exclosure and some immediatel
adjacent grazed soil, were located in the semiarid sagebrush steppe regio
of Fremont County, Wyoming. The vegetation type in all the exclosures
sagebrush steppe grassland (Monger and Martinez-Rios 2001) withWyoming big sagebrush (Artemisia tridentata Nutt. ssp. Wyomingensis
Beettle and Young) as the most dominant shrub. Studied exclosures had
minimal external disturbance and fence breaches since time of
establishment.
Study sites were established at four exclosures:
1. Granite Mountain (GM),
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
2. Upper Government Draw (UGD),
3. Shoshoni Ant # 8 (Shon 8) and
4. Shoshoni Ant # 9 (Shon 9).
Soil textures at the sites ranged from sandy loam to clay loam. Average
annual precipitation (years 1990-2003) for all four sites approximated 20
21 cm [~7.9-8.3 in] while altitudes ranged from 1600 m (~5250 ft) (Sho
and Shon 9) to 1800 m (~5906 ft) (UGD) and 2100 m (~6890 ft) (GM).Using the standard that 1 Animal Unit Month (AUM) equals the amount
forage required by one mature cow and her calf (or the equivalent, in she
or horses, for instance) for one month, average stocking rates (years 199
2003) outside the four exclosures (rotational grazing system) were
approximately 0.12, 0.08, 0.013 and 0.013 AUM/acre for GM, UGD, Sh
8 and Shon 9 exclosures respectively (1 acre = 0.4047 hectare).
Research methods
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The basic experimental design was a one-factor experiment with three
levels (treatment, microsite, depth). At each study site, soil sampling wa
conducted separately both inside and outside the grazing exclosures, alo
three 100 m (~328 ft) long transects at three points 20 m apart from each
other. These transects were either entirely inside or outside the exclosure
Soil from inside an exclosure was the "ungrazed soil" treatment while th
soil outside was the "grazed soil" treatment. At each sampling point alon
a transect, samples were collected from under three microsites--sagebrusgrasses and bare interspaces--and at two depths (0-5 cm [~0-2 in] and 5-
cm [~2-6 in]). The total number of samples per treatment in each study s
was 54 (3 transects per study site*3 replicate points on each transect*3
microsites at each point*2 depths at each microsite).
After air-drying and sieving through a 2 mm (~0.078 in) sieve for rock
removal, carbon analysis was conducted as follows:
1. A Carlo Erba Carbon Nitrogen Analyzer was used to measure totcarbon and nitrogen concentrations for samples from the GM and
UGD exclosure sites. An Elementar Variomax Carbon Nitrogen
Analyzer was used for the samples from Shon 8 and Shon 9
exclosure sites.
2. The modified pressure calcimeter method, as described by Sherro
et al. (2002), was used for analysis of inorganic carbon in fine-
ground soil samples.
3. Soil organic carbon content (SOC) was calculated by subtracting
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
the inorganic carbon content from the total carbon contents in eac
soil sample.
4. The SOC/ha was calculated from SOC and soil bulk density data
(Madden 2005) from the study sites.
5. The chloroform fumigation-extraction method described by Vanc
et al. (1987) and Howarth and Paul (1994) was used for extractin
SMBC.
6. Organic carbon concentration in each SMBC extract wasdetermined using the Phoenix 8000 UV-Persulphate TOC Analyz
Statistical analyses, except correlation matrices, were conducted using
Minitab 13.1 statistical software (MINITAB 2000). The general linear
model (GLM) was employed for analysis of variance between treatment
microsites and soil depths within a study site. Tuckey's post-hoc analysis
was used for significant interactions and for significant factor compariso
The a-value was set at 0.05. For analysis of overall means across all
microsites and depths within treatments, a 2-sample T-test was employed
Summary of results and discussion
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SOC concentration in the studied soils ranged from 3.67 mg/g dry soil
(0.37%) to 53.8 mg/g dry soil (5.4%). Statistically significant difference
in SOC concentrations were not observed in soils inside and outside any
the four sampled grazing exclosures, although significant interactions of
grazing treatment with microsite and depth were observed as follows:
● At GM exclosure, a significant 3-way interaction of treatment,
microsite and depth on SOC and SOC/ha was observed, indicatin
that long-term grazing exclusion exerted greater influence on soil
at 0-5 (~0-2 in) depth under shrub canopies than on soil under oth
microsites and depths.
● At UGD exclosure, a significant interaction of microsite and dept
on SOC concentration was observed, as would be expected, with
greater SOC at upper soil depths and under shrubs and/or grassescompared to lower soil depths and bare interspaces. This
occurrence is explained further below. However, no significant
difference due to treatment was observed.
● A significant 2-way interaction of depth and treatment on SOC w
observed at Shon 9 exclosure. Grazing exclusion was seen to
influence soil at 0-5 cm (~0-2 in) more than soil at 5-15 cm (~2-6
in), as expected.
● Significant 2-way interactions of microsite and depth were
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
observed on SOC at UGD, Shon 8 and Shon 9 exclosures, as
expected.
SMBC concentrations ranged from 99 to 1011 µg/g dry soil in the study
sites. Greater SMBC was observed in the ungrazed soils of GM and Sho
sites than in their grazed soils. In all of the study sites except for Shon 8,
greater SMBC concentrations were generally observed in soil from 0-5 (
2 in) depths, under shrubs and in ungrazed treatments than in soil from 515 cm (~2-6 in) depths, from other microsites and in grazed treatments.
Overall, our study showed lack of significant difference due to grazing
exclusion alone on SOC and SOC/ha between grazed soil and soil not
grazed for more than 40 years. In other words, this particular study was n
able to disprove the null hypotheses with regard to SOC. Kieft (1994)
observed similar results with lack of significant differences in SOC
between grazed lands and land not grazed for 11 and 16 years.
Our results disagree with a similar study by Schumann et al. (1999), who
found greater SOC in grazed soil than in soil not grazed for 40 years.
There are minor similarities with Hiernaux et al. (1999) who observed th
after 4 years of controlled grazing in a Sahelian rangeland, SOC decreas
slightly compared to ungrazed control but after 9 years of intensive
grazing, SOC did not decrease further. Our results may have been differe
from these prior studies because our ungrazed sites had been grazed prio
to exclusion and our grazed sites had been grazed for years. Hiernaux et
(1999) conducted their grazing studies on previously ungrazed soils.Dormaar and Wilms (1998) observed no change in total soil carbon
percentage (11.4% in no grazing and 11% in light grazing of 1.2 AUM/h
after 42 years of light grazing in Canadian fescue grasslands, although it
was seen to decrease with increase in grazing pressure.
Although our study data indicated that less than 40 years of grazing
removal did not significantly affect SOC, SMBC did exhibit significant
changes because it is a more labile fraction of soil organic matter. Thus,
this experiment did provide significant support for our alternate hypothewith regard to SMBC. Specifically, we observed significantly greater
SMBC in the ungrazed soil than in the grazed soil in two of our four stud
sites (GM and Shon 9). These results are similar to Fliessbach and Maed
(2000) who observed early system-induced changes on SMBC while
comparing conventional and organic farming systems, whereas no such
changes were observed on total soil organic matter (SOM), of which SO
is a significant component. Contradictory to our results, Kieft (1994) did
not observe significant and consistent changes on SMBC due to grazing
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
exclusion in grasslands of New Mexico.
Shon 8 was the only site in our study that demonstrated slightly but
insignificantly greater MBC in grazed soil than in ungrazed soil. This
could be attributed to greater protection of SOM by aggregates and bette
environments for microbial proliferation in the Shon 8 soil, resulting in
faster microbial assimilation of available SOC in the grazed soil than in
ungrazed soil. It could also be due to differences in soil texture--Shon 8has much higher proportions of sand than GM and UGD sites. However,
this latter explanation may need further investigation since these same
parameters at Shon 9 exclosure, with its similar soil texture, were
differently influenced by grazing exclusion.
In most cases, soils under shrubs also had the highest SMBC
concentration, followed by soil under grasses and bare interspaces. Kieft
(1994) observed similar results, attributing them to the canopy effect for
both grasses and shrubs and to the "resource island" effect for shrubs asdiscussed above.
It may be that more than 40 years of grazing exclusion is required for SO
to exhibit significant changes. This could be a reason why significant
differences in SOC were not observed between soils inside and outside t
grazing exclosure, in spite of significant interactions of treatment,
microsite and/or depth. The average bulk densities of soil in the study sit
did not show significant difference between grazing or non-grazing
treatments. There was no evidence of compaction due to grazing. Sincesoil porosity and soil bulk density are inversely related and porosity is
directly related to SOC concentrations, insignificant differences in bulk
density and porosity between grazed and ungrazed treatments might be
responsible for insignificant differences in SOC and SOC/ha.
Compared to studies that showed changes due to grazing exclusion, the
lack of significant differences in SOC concentration between grazed and
ungrazed soils in our study sites may be due to:
1. historical grazing practices and grazing intensities before and afte
grazing exclusion; and/or
2. grazing by small mammals throughout inside the grazing exclosu
(Kieft 1994).
As mentioned above, recent grazing history in our study sites indicates
very light and well-managed grazing outside the four studied grazing
exclosures. Well-managed grazing improves nutrient cycling in grasslan
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
ecosystems, stimulating aboveground production as well as root respirat
and exudation rates (Schuman and Derner 2004). Our results may have
differed from prior results showing grazing-induced SOC increase
(Schumann et al. 1999, Reeder et al. 2004) or decrease (Derner et al. 199
Yong-Zhong et al. 2005) because of differences in stocking rates, climat
soil and/or vegetation types in our study sites. As mentioned by Post and
Kwon (2000), the length of time and the rate of carbon accumulation in
soil vary widely from one area to another, depending on the productivitythe vegetation recovering from environmental changes, physical and
biological conditions in the soil and the history of soil organic carbon inp
and physical disturbance. Differences in methodology of sampling and
analyses, plant response to grazing, and photosynthesis of grazed plants
may also be responsible for different results of grazing on SOC (Schuma
et al. 2001).
Within each microsite, greater SOC was often observed in the 0-5 cm (~
2 in) depths of soil. This trend supports other similar findings like Derneet al. (1997) who found that maximum SOC accumulation was restricted
vertically to the 0-5 cm (~0-2 in) depth. The surface layers of soil tend to
accumulate more SOC than the deeper layers, due to deposition and
decomposition of litter at the surface. When left undisturbed due to graz
exclusion, this process can presumably occur more easily than when
grazing occurs, because grazing may be affecting the amount and rate of
litter deposition and decomposition on the surface soil.
Greater SOC concentration was mostly observed under shrubs rather tha
under grasses or in bare interspaces. Soils under grasses, in turn, had
higher SOC concentrations than bare interspaces. In all study sites, soil
from the interspaces had the lowest SOC concentration. These overall
results probably reflect the following:
● Soil resources tend to accumulate under shrubs, which have more
extensive root systems and canopies than do grasses (Bird et al.
2002).
● Soils beneath plant canopies receive greater organic matter inputs
contain more water, are protected from disturbance (Bird et al.
2002), and hence can accumulate more organic carbon than
interspace soil devoid of plants. Canopy effects due to grasses an
shrubs were also observed by Kieft (1994) on SOC and MBC in t
semiarid grasslands and shrublands of New Mexico, during studi
on grazing and plant canopy effects.
● Grasses tend to accumulate more SOC than bare interspaces, due
shoot and root organic matter input and deposition of plant litter
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
redistributed from surface soils between plants during their life sp
(Derner et al. 1997).
Conclusion
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Data from this study indicate no significant difference in SOC between
grazed soils and soils not grazed for more than 40 years. Significantdifferences in SMBC, however, indicate that grazing exclusion may hav
induced some differences in carbon dynamics and nutrient cycling in the
soil. Microbial activity and health may also have improved throughout th
years of exclusion. Further, managed grazing outside the exclosures in o
study sites seems to have had a stabilizing impact on the soil organic
carbon accumulation. These results are consistent with the proposal that
well-managed grazing may be an option for sequestering carbon in
grasslands.
Carbon sequestration, in addition to reducing the amount of carbon diox
in the atmosphere, increases organic matter in soil leading to greater
structural stability, water retention, pollutants fixation and toxicity
reduction (FAO 2001). Managing the vast expanses of available grazing
lands in the US for enhanced carbon sequestration could prove very
valuable for offsetting the present carbon imbalance as well as for resour
preservation. The organic carbon content in our study sites ranged from
approximately 6 Mg/ha to 16 Mg/ha. Our findings support the idea of
managed grazing as a carbon sink as proposed during the MarrakeshAccords in 2001. Further studies in this area should include studies of lo
term grazing exclusion from sites which have not been grazed previously
Acknowledgements
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Special thanks to the Department of Renewable Resources of the
University of Wyoming, Dr. Snehalata Huzurbazaar, Dr. Ann Hild, Dr.
Lachlan Ingram and all the colleagues and technical help of University oWyoming. We also thank the Lander Bureau of Land Management staff
particularly Mr. John Likins, for their help in gathering secondary
information for the grazing exclosure study. This study was funded by th
University of Wyoming Agricultural Experiment Station Competitive
Grants Program.
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ALN No. 58: Shrestha: Soil carbon and microbial biomass carbon after 40 years of grazing exclusion...
(Back to top)
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