Combination of Warming and N Inputs Increases the Temperature Sensitivity of Soil N2O Emission in a Tibetan Alpine Meadow.

Many high-elevation alpine ecosystems have been experiencing significant increases in air temperature and, to a lesser extent, nitrogen (N) deposition. These changes may affect N-cycling microbes and enhance emissions of nitrous oxide (N2O, a potent greenhouse gas) from soil. However, few studies have investigated whether and how the resulting changes in N-cycling microbes may affect the temperature sensitivity (Q10) of N2O emission and in turn feed back to N2O emissions. We conducted two incubation experiments to examine N2O emissions and their temperature sensitivities in soils that had experienced 3-yr field treatments of warming, N inputs and their combination in a Tibetan alpine meadow. Our results showed that neither N inputs nor warming alone affected the rate or Q10 of soil N2O emission, but combining the two significantly increased both parameters. Also, combined N and warming significantly increased the abundance of ammonia-oxidizing bacteria (AOB), corresponding with high soil N2O emission. In addition, N2O emission from nitrification accounted for 60-80% of total emissions in all soils, indicating that nitrifying microbes dominated the N2O production and its temperature sensitivity. Using random forest (RF) and structural equation model (SEM) analyses, we further evaluated the effects of various soil characteristics on soil N2O emissions and Q10. We identified soil moisture, pH, N mineralization and AOB abundance as the main predictors of the Q10 of N2O emissions. Together, these findings suggest that alterations in soil moisture, pH and ammonia-oxidizing bacteria induced by long-term N inputs and warming may increase temperature sensitivity of soil N2O emissions, leading to a positive climate feedback in this high-altitude alpine ecosystem.