Grazing alters ecosystem functioning and C:N:P stoichiometry of grasslands along a regional precipitation gradient

摘要

The Eurasian steppe has long been subject to grazing by domestic ungulates at high levels, resulting in widespread deterioration of biodiversity and ecosystem services. While abundant evidence demonstrates that heavy grazing alters the ecosystem structure and function of grasslands, research on how grazing specifically affects ecosystem functioning and stoichiometry on broad scales is scarce because of a lack of adequate ungrazed reference sites. We examined the effects of grazing on ecosystem functioning and C: N: P stoichiometry across a precipitation gradient along the 700 km China-Mongolia transect (CMT), covering three community types: meadow steppe, typical steppe and desert steppe. Long-term grazing has dramatically altered the C, N and P pools and stoichiometry of steppe ecosystems along the CMT. Grazing reduced the C, N and P pools in above-ground biomass and litter, while the responses in below-ground biomass and soil C, N and P pools to grazing differed substantially among community types. Grazing increased N content and decreased C: N ratios in all plant compartments, suggesting accelerated N cycling. The altered C: N: P stoichiometry may be explained by changes in the composition of species and functional groups as well as increased foliar N and P contents for the same species in grazed communities. Synthesis and applications. Plant stoichiometric responses to grazing ranged from large in the meadow steppe to small in the typical steppe to generally insignificant in the desert steppe, implying that different underlying mechanisms operated along the regional precipitation gradient. Our findings suggest that reducing the stocking rate and restoring the vastly degraded steppes are essential to sustain native steppe biodiversity, ecosystem functioning and biological capacity for mitigating the impact of climate change in the Inner Mongolia grassland. Plant stoichiometric responses to grazing ranged from large in the meadow steppe to small in the typical steppe to generally insignificant in the desert steppe, implying that different underlying mechanisms operated along the regional precipitation gradient. Our findings suggest that reducing the stocking rate and restoring the vastly degraded steppes are essential to sustain native steppe biodiversity, ecosystem functioning and biological capacity for mitigating the impact of climate change in the Inner Mongolia grassland.