carbon sequestration from grazing exclusion as a land use change

8/9/2019 Carbon sequestration from grazing exclusion as a land use change

http://slidepdf.com/reader/full/carbon-sequestration-from-grazing-exclusion-as-a-land-use-change 1/11

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

(Back to top)

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

ttp://ag.arizona.edu/oals/ALN/aln58/shrestha.html (1 of 11) [4/20/2010 10:58:29 AM]

http://ag.arizona.edu/oals/ALN/aln58/aln58toc.html


http://ag.arizona.edu/oals/ALN/ALNHome.html




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

(Back to top)

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),





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

(Back to top)

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





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

(Back to top)

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





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





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





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





redistributed from surface soils between plants during their life sp

(Derner et al. 1997).

Conclusion

(Back to top)

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

(Back to top)

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.

References





(Back to top)

Bird, S. B., J. E. Herrick, M. M. Wander and S. F. Wright. 2002. Spatial

heterogeneity of aggregate stability and soil carbon in semiarid rangelan

Environmental Pollution 116: 445-455.

Breland, T. A. and R. Eltun. 1999. Soil microbial biomass and

mineralization of carbon and nitrogen in ecological, integrated and

conventional forage and arable cropping systems. Biology and Fertility oSoils 30: 193-201.

Derner, J. D., D.D. Briske and T.W. Boutton. 1997. Does grazing media

soil carbon and nitrogen accumulation beneath C4 perennial grasses alon

an environmental gradient? Plant and Soil 191(2): 147-156.

Fließbach, A. and P. Mäder. 2000. Microbial biomass and size-density

fractions differ between soils of organic and conventional agricultural

systems. Soil Biology and Biochemistry 32(6): 757-768.

Howarth, W. R. and E. A. Paul. 1994. Microbial Biomass. In Methods of

Soil Analysis, Part 2. Microbial and Biochemical Properties, 753-773.

SSSA Book Series No. 5. Madison, WI: Soil Science Society of America

Kieft, T. L. 1994. Grazing and plant-canopy effects on semiarid soil

microbial biomass and respiration. Biology and Fertility of Soils 18: 155

162.

Lecain, D. R., J. A. Morgan, G. E. Schuman, J. D. Reeder and R. H. Har

2000. Carbon exchange rates in grazed and ungrazed pastures of

Wyoming. Journal of Range Management 53: 199-206.

Madden, S. Long-term effects of domestic livestock removal from

Wyoming big sagebrush dominated rangelands: vegetative diversity and

soil stability. (Master's thesis, University of Wyoming, 2005).

Milchunas, D. G. and W. K. Lauenroth. 1993. A quantitative assessmentthe effects of grazing on vegetation and soils over a global range of

environments. Ecological Monograph 63: 327-366.

Monger, H. C. and J. Martinez-Rios. 2001. Inorganic carbon sequestratio

in grazing lands. In The potential of U.S. grazing lands to sequester carb

and mitigate greenhouse effect . R. F. Follett, J. M. Kimble, and R. Lal,

eds., 87-118. Boca Raton, FL: CRC Press.





Nilsson, K. S. 2004. Modeling soil organic matter turnover. (Ph.D. diss.,

Swedish University of Agricultural Sciences, 2004). Acta Universitatis

Agriculturae Sueciae, Silvestria 326. Online: http://diss-epsilon.slu.se/

archive/00000654/01/SNfin0.pdf .

Reeder, J. D., G. E. Schuman, J. A. Morgan, and D. R. Lecain. 2004.

Response of organic and inorganic carbon and nitrogen to long-term

grazing of the shortgrass steppe. Environmental Management 33(4):485

95.

Schuman, G. E. and J. D. Derner. 2004. Carbon sequestration by

rangelands: Management and potential. Paper presented at the Western

Regional Cooperative Soil Survey Conference. Online: http://soils.usda.

gov/partnerships/ncss/conferences/westregion2004/agenda.html.

Schuman, G. E., J. D. Reeder, J. T. Manley, R. H. Hart and W. A. Manle1999. Impact of grazing management on the carbon and nitrogen balanc

of a mixed-grass rangeland. Ecological Applications 9(1): 65-71.

Schuman, G. E., J. E. Herrick and H. H. Jansen. 2000. Soil C dynamics o

rangelands. In C Sequestration Potential of US Grazing Land , eds. R. F.

Follet, J. M. Kimble and R. Lal, 267-290. Chelsea, MI: Ann Arbor Press

Sherrod, L. A., G. Dunn, G. A. Peterson, and R. L. Kolberg. 2002.

Inorganic carbon analysis by modified pressure-calcimeter method. SoilScience Society of America Journal 66: 299-305.

Sparling, G. P. 1992. Ratio of microbial biomass carbon to soil organic

carbon as a sensitive indicator of changes in soil organic matter. Austral

Journal of Soil Research 30: 195-207.

U.N. Framework Convention on Climate Change (UNFCCC). 2005. Lan

use, land use change and forestry (LULUCF). Online: http://unfccc.int/

methods_and_science/lulucf/items/3060.php.

U.S. Department of Agriculture, Natural Resources Conservation Servic

(USDA-NRCS). 2005. Soil Survey Geographic (SSURGO) database for

Fremont County, Wyoming, East Part and Dubois Area. Online: http://

SoilDataMart.nrcs.usda.gov/ .

Vance, E. D., P. C. Brookes and D. S. Jenkinson. 1987. An extraction


http://diss-epsilon.slu.se/archive/00000654/01/SNfin0.pdf


http://soils.usda.gov/partnerships/ncss/conferences/westregion2004/agenda.html


http://unfccc.int/methods_and_science/lulucf/items/3060.php


http://soildatamart.nrcs.usda.gov/













method for measuring soil microbial biomass C. Soil Biology and

Biochemistry 19: 703-707.

World Wildlife Fund. 2001. Wyoming Basin shrub steppe (NA1313).

Online: http://www.worldwildlife.org/wildworld/profiles/terrestrial/na/

na1313_full.html.

Yong-Zhong S., L. Yu-Lin, C. Jian-Yuan and Z. Wen-Zhi. 2005.

Influences of continuous grazing and livestock exclusion on soil propert

in a degraded sandy grassland, Inner Mongolia, northern China. CATEN

59 (3): 267-278.

ack to top)

bout the Arid Lands Newsletter

http://www.worldwildlife.org/wildworld/profiles/terrestrial/na/na1313_full.html


http://ag.arizona.edu/oals/ALN/about-aln.html


http://ag.arizona.edu/oals/ALN/ALNHome3.html

http://ag.arizona.edu/oals/ALN/ALNHome2.html

http://ag.arizona.edu/oals/ALN/ALNHome.html

http://ag.arizona.edu/oals/ALN/about-aln.html

mailto:[email protected]



carbon sequestration from grazing exclusion as a land use change

Documents