Effects of warming and grazing on N2O fluxes in an alpine meadow ecosystem on the Tibetan plateau

A great deal of uncertainty is associated with estimates of global nitrous oxide (N₂O) emissions because emissions from arid and polar climates were not included in the estimates due to a lack of available data. In particular, very few studies have assessed the response of N₂O flux to grazing under future warming conditions. This experiment was conducted to determine the effects of warming and grazing on N₂O flux at different time scales for three years under a controlled warming-grazing system. A free-air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures for both of growing (average 1.8 °C in 2008) and no-growing seasons (average 3.0 °C for 3-years) within 20-cm depth, but only warming reduced soil moisture at 10-cm soil depth during the growing season during the drought year of 2008. Generally, the effects of warming and grazing on N₂O flux varied with sampling date, season, and year. No interactive effect between warming and grazing was found. Warming did not affect annual N₂O flux when grazing was moderate during the growing season because the tradeoff of the effect of warming on N₂O flux was observed between the growing season and no-growing season. No-warming with grazing (NWG) and warming with grazing (WG) significantly increased the average annual N₂O flux (57.8 and 31.0%) compared with no-warming with no-grazing (NWNG) and warming with no-grazing (WNG), respectively, indicating that warming reduced the response of N₂O flux to grazing in the region. Winter accounted for 36-57% of annual N₂O flux for NWNG and NWG, whereas only for 5-8% of annual N₂O flux for WNG and WG. Soil temperature could explain 5-35% of annual N₂O flux variation.