摘要：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, CO 2 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 CO 2 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, CO 2 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 CO 2 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 CO 2 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 CO 2 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 (CO 2 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.
