Impacts of seasonal grazing on net ecosystem carbon exchange in alpine meadow on the Tibetan Plateau

Understanding the effect of grazing season on net ecosystem CO2 exchange (NEE) and its components is crucial to predict the feedback of grazing management to climate change. We estimated NEE, gross primary productivity (GPP) and ecosystem respiration (Re) under different seasonal grazing practices (i.e. no-grazing (NG), warm season grazing (WG) and cold season grazing (CG)) by sheep during the growing seasons from 2008 to 2012 on the Tibetan Plateau. Our results show that the impacts of seasonal grazing on daily GPP, Re and NEE in the alpine meadow ecosystem varied with sampling date and year. Compared with NG and CG, WG significantly reduced average seasonal NEE by 22.7 %, because grazing impact was exacerbated by drought in July in 2010. Soil temperature only explained 19-31 % of the variation in daily GPP, Re and NEE for all grazing treatments. The interannual variabilities of GPP, Re and NEE were mainly determined by root biomass and/or average soil temperature during the growing season. Our results suggest that although WG may decrease sequestration of CO2 under continuous drought conditions after grazing, it would have little impact on CO2 sequestration during the growing season under conditions of future warming with greater rainfall in alpine meadows on the Tibetan Plateau.