Alpine Grassland CO₂Exchange and Nitrogen Cycling: Grazing History Effects, Medicine Bow Range, Wyoming, U.S.A

Our study examined carbon dioxide exchange and nitrogen cycling over two consecutive years (winter and summer) in a grazed alpine grassland and in an embedded long-term grazing exclosure to ascertain whether grazing history had resulted in divergent soil carbon attributes, CO2 exchange rates, and different vegetation C and N and soil N processes. Soil C and N concentrations and masses were significantly higher in the grazed than in the ungrazed area, though grass leaf N was higher in the ungrazed area, as was vegetation biomass. Detectable amounts of CO2 were lost from the grazed and ungrazed areas of this grassland during the winters of 1998, 1999, and 2000, and at 6 of 15 winter flux sample dates, CO2 efflux was greater in the grazed area than in the ungrazed area. The ungrazed area consistently gained more C during the summer months than the grazed area, with net CO2 exchange peaking in mid-July 1998 at nearly 5 μmol m−2 s−1 in the ungrazed area compared to <2 μmol m−2 s−1 in the grazed area. During the 2-yr study period, the grazed area was a carbon source of 170 g C m−2, while the ungrazed area was a carbon sink of 83 g C m−2. Lower N mineralization rates early and late in the summer (1999) in the grazed site at Libby Flats corresponded to reductions in net CO2 exchange and lower plant N content compared to the ungrazed exclosure. Based on these results, we suggest that: (1) long-term grazing in high-altitude rangelands can alter annual CO2 exchange and N dynamics; (2) temporal synchrony in C and N processes occur during the summer; that is, increased C exchange rates accompany increased N mineralization rates; and (3) integrative (total soil C and N) and instantaneous (CO2 exchange and vegetation N) measures of C and N dynamics may not necessarily lead to the same interpretation regarding C sequestration and N cycling in alpine grasslands.