Morgan, JA, DP Smith and WJ Parton*; Shortgrass Steppe LTER; USDA-ARS and *Colorado State University, Fort Collins, CO

To investigate the effect of heavy cattle grazing (75% forage utilization), recommended grazing (40% utilization) and no grazing on CO2 flux of a shortgrass steppe (SGS) at the USDA-ARS Central Plains Experimental Range in northeastern Colorado, a Bowen ratio/energy balance micrometeorological system was utilized. Flux measurements were conducted year-round from 2001 through 2003. In 2001, precipitation was 80% of the long-term average and no detectable effect of grazing intensity on C flux was evident. This null response of CO2 flux to grazing is characteristic of previous work that has shown little to no significant effects of stocking rate on seasonal CO2 fluxes. However, in 2002, precipitation was only about 50% of average, and as a result, cattle were removed from the pastures in mid-July. Nevertheless, CO2 exchange was strongly affected by grazing treatment. Net CO2 assimilation was lowest in the heavily grazed pasture and highest in the ungrazed pasture while CO2 assimilation in the moderately grazed pasture was intermediate between the other two treatments. Heavy grazing caused a net loss of C from the ecosystem. These results suggest that while the SGS may be resilient to grazing in the long-term, in the short-term, livestock removal or reductions in stocking rate may be important for maintaining a positive C balance during projected periods of drought.