Carbon and nitrogen dynamics of an alpine grassland: Effects of grazing history and experimental warming on carbon dioxide flux and soil properties

Ecosystem CO2 flux patterns and soil C and N properties in adjacent grazed and ungrazed alpine grasslands, under ambient and experimentally warmed conditions, were investigated during winter and summer of 1998 at Libby Flats (Medicine Bow National Forest, southeast Wyoming). Both winter CO 2 efflux (loss of C to the atmosphere) and net summer ecosystem CO 2 flux were significantly affected by grazing history. Winter loss of C was greater in the grazed area than the ungrazed area on 3 of 10 dates. The net flux of CO2 was greater in the ungrazed area during the summer as compared to the grazed area. Experimental warming had no significant effect on net C flux, although respiration rates were negatively affected on some dates. Total soil C and N were significantly (P<0.05) greater in the upper soil horizon of the historically grazed area as compared to the ungrazed area, although winter mineralizeable N was unaffected by grazing regime. This study indicates that long-term grazing has altered soil C and N pools and has affected both winter and summer C flux dynamics in these high altitude grasslands. Contrary to our expectations that simulated climate warming would affect ecosystem C balance, no consistent effect was observed.