Soil Temperature Dynamics Modulate N2O Flux Response to Multiple Nitrogen Additions in an Alpine Steppe

Emissions of nitrous oxide (N2O) contribute to global warming and stratospheric ozone depletion. Anthropogenic N2O emissions predominately result from the addition of synthetic nitrogen (N) fertilizers to terrestrial ecosystems. Usually, an exponential increase in N2O emissions occurs as N addition rates increase to exceed plant demands. However, most evidence to date is from temperate areas, with little information available for alpine ecosystems. Here we examined the changes in N2O flux under eight N addition levels and the mechanisms regulating these changes in a Tibetan alpine steppe. Our results showed that N2O emission rate increased linearly with increasing N additions. Even when soil N availability exceeded plant N uptake, no sharp N2O emissions were observed. The likely explanation was that decreased soil temperature limited the growth of nitrification-related microorganisms, mainly ammonia-oxidizing archaea, which further attenuated the positive response of N2O emissions to excess N supply. These findings suggest that the N-induced changes in soil temperature regulate the growth of nitrifying microorganisms and the subsequent N2O fluxes in this alpine steppe, and the exponential N2O emission-N rate relationship observed in warm regions may not be simply extrapolated to alpine ecosystems.