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United States Department of Agriculture

Agricultural Research Service


item Derner, Justin
item Schuman, Gerald
item Reeder, S
item Morgan, Jack

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: December 3, 2002
Publication Date: December 3, 2002

Technical Abstract: Rangelands occur on approximately 60% of the earth's terrestrial surface, thereby representing a substantial area for storage of soil organic carbon (SOC). Because of their extent, small changes in rangeland carbon storage and dynamics can greatly nfluence the global carbon cycle. Livestock grazing is one of the most prevalent land uses of the world's rangelands, and has the potential to alter carbon storage and dynamics. Grazing is a land management practice that influences ecosystem carbon storage through modifications in plant species composition, rate of turnover/decomposition of aboveground biomass and nutrient cycling. Unfortunately, there is not a general consensus as to the relationships between grazing and soil carbon storage in rangelands. Some studies have reported no effect of grazing on soil carbon, several others have reported increases in soil carbon with grazing and a few have reported decreases in soil carbon with grazing. These inconsistencies may be the result of differences in climate, inherent soil properties, landscape topographical position, plant community composition, prior management history, and varying grazing management practices and length of the treatment (Reeder and Schuman 2002). These inconsistencies may result from soil variation within individual studies, differences in the depth of soil profile being evaluated (Schuman et al. 1999) and in the growth form of the vegetation (Derner et al. 1997). Proper grazing management of U.S. rangelands, however, may result in carbon sequestration rates ranging from 0.1 to 0.3 Mg C/ha/yr (Table 1, Schuman et al. 2002). Despite inconsistencies in the effect of grazing on soil carbon storage in rangelands, generalizations can be made with respect to the influence of grazing on soil carbon storage. First, grazing or rangeland management improvement strategies that increase forage production or nitrogen status of the soil have the potential to increase carbon storage (Conant et al. 2001, Mortenson et al. 2002). Second, grazing strategies that induce changes in species composition that result in greater root-to-shoot ratios, thereby increasing carbon allocation belowground, may also increase carbon storage (Schuman et al. 1999). Collectively, the shortgrass steppe and northern mixed-grass prairie ecosystems represent the vast majority of remaining rangelands in the North American Great Plains. Both ecosystems evolved under grazing by large herbivores, and today are predominately grazed by domesticated livestock. Interestingly, these two ecosystems intersect near the Wyoming-Colorado border; thereby presenting a unique research opportunity to address the influence of livestock grazing on carbon storage in contrasting semi-arid rangelands. The shortgrass steppe is predominantly comprised of warm-season grasses (>70%) (dominated by blue grama, Bouteloua gracilis), has a summer-dominated precipitation pattern with an annual average precipitation total of 325 mm (12.8 inches), and an average aboveground production of approximately 600 kg/ha (675 pounds/acre). In contrast, the northern mixed-grass prairie is comprised of cool-season grasses (55%) (western wheatgrass, Pascopyrum smithii and needle-and-thread, Stipa comata) and the dominant warm-season grass blue grama (25%), has a spring-dominated precipitation pattern with an annual average precipitation of 366 mm (14.4 inches), and an average aboveground production of 1,100 kg/ha (1,230 pounds/acre). The Rangeland Resources Research Unit of the USDA/ARS has research locations at Cheyenne, Wyoming (southern end of the northern mixed-grass prairie) and at Nunn, Colorado (northern end of shortgrass steppe) with long-term grazing system/stocking rate studies implemented to assess plant/soil responses to livestock grazing. Recent research has evaluated the effects of livestock grazing on soil carbon in these two semi-arid rangelands. Grazing treatments included season-long grazing at light, moderate and heavy stocking rates and non-grazed exclosures. Treatments had been in place for 12 years in the northern mixed-grass prairie and for 55 years in the shortgrass steppe at the time of sampling. Grazing in the northern mixed-grass prairie, at either a light or a heavy stocking rate, increased the mass of SOC in the surface 30 cm of soil profile by 21% compared to non-grazed exclosures (Schuman et al. 1999). However, SOC storage to a depth of 60 cm was increased by only 4% with light stocking rate (not significantly different) and by 15% with heavy stocking rate compared to the non-grazed exclosures. In the shortgrass steppe, SOC storage to a depth of 30 cm was 31% greater with moderate grazing compared to non-grazed exclosures (Derner et al., unpublished data). Likewise, there was 19% more SOC to a depth of 60 cm in the moderate than non-grazed exclosures. SOC storage was not significantly different at depths of 30, 60 or 90 cm with light grazing compared to non-grazed exclosures, whereas heavy grazing increased SOC storage by 16, 14, and 12%, respectively (Figure 1, Reeder and Schuman 2002). The principal driver increasing SOC storage with livestock grazing was the change in vegetation composition to a greater abundance of blue grama with heavy stocking rates. This species has a higher root-to-shoot ratio and transfers more carbon belowground than do the cool-season grasses it replaces (Coupland and Van Dyne 1979). Other studies reporting increased soil carbon storage with grazing have reported a significant increase in blue grama at the expense of cool-season grasses (Smoliak et al. 1972, Frank et al. 1995). This grazing-induced change in plant species composition shifts the dominant photosynthetic pathway from C3 to C4, combined with grazing stimulating carbon and nitrogen cycling from aboveground vegetation components to the soil by enhancing physical breakdown, soil incorporation and rate of decomposition of litter, has implications to several ecosystem characteristics that affect ecosystem structure and function. Until recently, essentially all studies evaluating the effects of grazing management on soil C storage have focused on measuring changes in SOC and have not evaluated changes in soil inorganic C (SIC), carbonates. Recent evaluation of the soil samples from the shortgrass steppe grazing study revealed the effect of heavy grazing on soil carbon was greater for SIC than for SOC (Reeder and Schuman, in prep). Total soil carbon was significantly higher by 23.8 Mg/ha in the 0-90 cm soil profile with long-term heavy grazing compared with non-grazed exclosures, with 68% (16.3 Mg/ha) attributed to an increase in SIC, and 32% (7.5 Mg/ha) due to an increase in SOC. The higher SIC content coincides with a consistently higher soil pH below 30 cm depth with heavy grazing, and appears to coincide with the rooting pattern of blue grama. These results emphasize the importance of including SIC in the assessment of the effects of management on soil carbon sequestration. Soil nitrogen is commonly the second most limiting factor, after water, influencing forage production on rangelands. Introduction of a legume into rangeland plant communities may mitigate this limitation by increasing forage production through greater availability of nitrogen fixed by the legume. A yellow-flowering falcata alfalfa (Medicago sativa ssp. falcata) was interseeded into northern mixed-grass prairie in 1965, 1987 and 1998. SOC, to a depth of 1m, increased by 17%, 8% and 4% in the 1965, 1987 and 1998 site, respectively (Schuman et al. 2003). This resulted in carbon sequestration rates of 0.33 Mg C/ha/yr (1965 interseeding), 0.65 Mg C/ha/yr (1987 interseeding) and 1.56 Mg C/ha/yr (1998 interseeding). Nitrogen content of native forage species increased by 8-33% with the interseeded legume and forage production increased. Introduction of this legume into northern mixed-grass prairie can help mitigate rising atmospheric CO2 through increased carbon sequestration and carbo

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