Combination of warming and N inputs increases the temperature sensitivity of soil N2O emission in a Tibetan alpine meadow

Abstract

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.

Keywords: Alpine ecosystem; Climate change; Greenhouse gas emission; Temperature sensitivity.