Biomass and nitrogen responses to grazing intensity in an alpine meadow on the eastern Tibetan Plateau

This study was conducted to examine the seasonal dynamics of biomass and plant nitrogen (N) content under three grazing intensities (light grazing - LG: 1.2, moderate grazing - MG: 2.0, and heavy grazing - HG: 2.9 yaks ha(-1)) in representative alpine meadow on the eastern Tibetan Plateau. Differentiation in grazing intensity in the study area started since 1997 and has continued to the present time. Plant samples were collected in the middle of June, August and September. The highest aboveground biomass occurred at the MG site for both August and September. Over the growing season, below-ground biomass (0-30 cm) increased as grazing intensity increased. The total below-ground biomass averaged over all sampling dates was 1226, 1908 and 2244 g m(-2) for LG site, MG site and HG site, which accounted for 75, 81 and 88% of total biomass, respectively. The results suggested that grazing intensity changed biomass allocation pattern between aboveground and belowground parts of plants. Higher grazing intensity resulted in higher N concentration in both live and dead aboveground biomass over the study period. Increased grazing intensity tended to increase plant N content averaged over all sampling dates, which were 17.9 g m(-2), 23.8 g m(-2) and 27.6 g m(-2) in LG site, MG site and HG site. The results indicated that higher grazing intensity had a potential to increase the ecosystem pool of plant N.