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Title: CARBON STORAGE ON SHORTGRASS AND NORTHERN MIXED-GRASS PRAIRIES

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

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 12/3/2002
Publication Date: 12/3/2002
Citation: DERNER, J.D., SCHUMAN, G.E., REEDER, S.J., MORGAN, J.A. CARBON STORAGE ON SHORTGRASS AND NORTHERN MIXED-GRASS PRAIRIES. MEETING ABSTRACT. 2002.

Interpretive Summary:

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