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Title: GRAZING AND ECOSYSTEM CARBON STORAGE IN THE NORTH AMERICAN GREAT PLAINS

Author
item Derner, Justin
item BOUTTON, T - TEXAS A&M UNIVERSITY
item BRISKE, D - TEXAS A&M UNIVERSITY

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2005
Publication Date: 2/15/2006
Citation: Derner, J.D., Boutton, T.W., Briske, D.D. 2006. Grazing and ecosystem carbon storage in the North American great plains. Plant and Soil Journal 280:77-90.

Interpretive Summary: The influence of long-term livestock grazing on plant community composition and ecosystem (plant+root+soil) carbon storage may differ in relatively dry and wet grassland ecosystems. Long-term livestock grazing in a dry, shortgrass ecosystem in Colorado did change plant community composition from a combination of cool- (C3 photosynthetic pathway) and warm- (C4 photosynthetic pathway) season plants to a community dominated by the warm-season grass blue grama In contrast, long-term livestock grazing did not affect plant community composition in relatively wet midgrass and tallgrass ecosystems in Kansas. Long-term livestock grazing in the dry, shortgrass ecosystem increased ecosystem carbon storage by 24% compared to the non-grazed areas, but ecosystem carbon storage decreased 8% with long-term livestock grazing in the relatively wet midgrass and tallgrass ecosystems. These results suggest that increases in ecosystem carbon storage in the dry, shortgrass ecosystem are attributable to grazing-induced shifts in plant community composition; the replacement of cool-season grasses by the warm-season grass blue grama. Although these results suggest that livestock grazing may benefit carbon sequestration in dry, shortgrass ecosystems, a negative consequence for producers and land manager is that grasslands dominated by blue grama are only about half as productive in terms of aboveground plant production compared to plant communities with greater representation of the cool-season grasses.

Technical Abstract: We assessed the impact of long-term livestock grazing on the functional group composition of plant communities, sources and magnitudes of C inputs, and ecosystem carbon storage along an east-west precipitation/productivity gradient in the central portion of the North American Great Plains. All three variables displayed inconsistent responses to long-term livestock grazing, and the pattern of response in the semiarid shortgrass community was generally different from that of the more mesic mid- and tallgrass communities. Long-term moderate livestock grazing shifted functional group composition from C3-C4 co-dominance to C4 dominance (Bouteloua gracilis) in the shortgrass community, but did not alter functional group composition in the two more mesic communities. Aboveground biomass and the relative proportion of coarse root biomass decreased, while the relative proportion of fine root biomass increased, in response to grazing at all three sites along the precipitation gradient. In grazed sites of the shortgrass community, the redistribution of fine root mass and the increase in C4 functional group composition resulted in a 25% increase in soil organic carbon (SOC) and a 24% increase in ecosystem C storage. In contrast, both mesic communities showed a marginally significant reduction of 7-8% in SOC and an 8% reduction in ecosystem C storage in response to long-term grazing. Ecosystem C storage may have been more resistant to long-term grazing in the mid- and tallgrass communities than in the shortgrass community because they both possessed 3-fold greater pools of SOC which would respond more slowly to any changes in ecosystem C fluxes due to grazing. In contrast, the semi-arid system possessed a 2-fold greater ratio of root mass (0-30 cm) to SOC compared to the two mesic systems indicating that a grazing-induced increase in fine root mass could potentially have a larger and more immediate impact on the SOC pool. Increases in SOC and ecosystem C storage in grazed sites of the shortgrass community appear to be attributable to grazing-induced shifts in the functional composition of the plant community, especially the replacement of cool-season C3 grasses by the warm-season C4 grass Bouteloua gracilis.