58, Winter 2005
Soil management for drylands
by Gyami Shrestha, Peter D. Stahl, Larry C. Munn, Elise G. Pendall, George F. Vance and Renduo Zhang
"[Results of our study] are consistent with the proposal that well-managed grazing may be an option for sequestering carbon in grasslands."
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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 vast expanses of grazing lands in the US for enhanced carbon sequestration and storage could prove very valuable both for offsetting present carbon imbalances and for preserving resources by maintaining overall soil health.
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 involving grazed 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 (Derner 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 the alternative hypothesis (H1) was that it exerts a significant effect on them.
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Study sites were established at four exclosures:
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) (Shon 8 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 of forage required by one mature cow and her calf (or the equivalent, in sheep or horses, for instance) for one month, average stocking rates (years 1993-2003) outside the four exclosures (rotational grazing system) were approximately 0.12, 0.08, 0.013 and 0.013 AUM/acre for GM, UGD, Shon 8 and Shon 9 exclosures respectively (1 acre = 0.4047 hectare).
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After air-drying and sieving through a 2 mm (~0.078 in) sieve for rock removal, carbon analysis was conducted as follows:
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 treatments, microsites and soil depths within a study site. Tuckey's post-hoc analysis was used for significant interactions and for significant factor comparisons. 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.
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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 Shon 9 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 (~0-2 in) depths, under shrubs and in ungrazed treatments than in soil from 5-15 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 not 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 that after 4 years of controlled grazing in a Sahelian rangeland, SOC decreased slightly compared to ungrazed control but after 9 years of intensive grazing, SOC did not decrease further. Our results may have been different from these prior studies because our ungrazed sites had been grazed prior to exclusion and our grazed sites had been grazed for years. Hiernaux et al. (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/ha) 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 hypothesis with regard to SMBC. Specifically, we observed significantly greater SMBC in the ungrazed soil than in the grazed soil in two of our four study sites (GM and Shon 9). These results are similar to Fliessbach and Maeder (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 SOC 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 better environments for microbial proliferation in the Shon 8 soil, resulting in faster microbial assimilation of available SOC in the grazed soil than in the ungrazed soil. It could also be due to differences in soil texture--Shon 8 has 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 as discussed above.
It may be that more than 40 years of grazing exclusion is required for SOC to exhibit significant changes. This could be a reason why significant differences in SOC were not observed between soils inside and outside the grazing exclosure, in spite of significant interactions of treatment, microsite and/or depth. The average bulk densities of soil in the study sites did not show significant difference between grazing or non-grazing treatments. There was no evidence of compaction due to grazing. Since soil 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:
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 grassland ecosystems, stimulating aboveground production as well as root respiration 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. 1997, Yong-Zhong et al. 2005) because of differences in stocking rates, climate, 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 the soil vary widely from one area to another, depending on the productivity of the vegetation recovering from environmental changes, physical and biological conditions in the soil and the history of soil organic carbon input 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 (Schumann et al. 2001).
Within each microsite, greater SOC was often observed in the 0-5 cm (~0-2 in) depths of soil. This trend supports other similar findings like Derner et 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 grazing 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 than 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:
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Carbon sequestration, in addition to reducing the amount of carbon dioxide 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 resource 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 Marrakesh Accords in 2001. Further studies in this area should include studies of long term grazing exclusion from sites which have not been grazed previously.
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Gyami Shrestha (firstname.lastname@example.org) is presently a Ph.D. student of Environmental Systems at University of California Merced. This study was conducted while she was an M.S. student at University of Wyoming, Laramie. Peter D. Stahl and Elise G. Pendall are Associate Professors at University of Wyoming, Laramie. Larry C. Munn and George F. Vance are professors there. Renduo Zhang, previously professor at the same university, is currently at Sun Yat-sen University, China.
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