Effects of Warming and Grazing on N2O Fluxes in an Alpine Meadow Ecosystem on the Tibetan Plateau
Abstract 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 ℃ in 2008) and no-growing seasons (average 3.0 ℃ 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.
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