Enhancement of N2O emissions by grazing is related to soil physicochemical characteristics rather than nitrifier and denitrifier abundances in alpine grassland

Grazing can degrade grassland soil and increase nitrous oxide (N2O) emissions, but the underlying mechanism is poorly understood. We investigated the relationship of soil N2O emission with nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and the abundances of N2O-producing and N2O-reducing functional genes, in response to three grazing intensities (none, light, intensive) over two growing seasons on an alpine grassland on Kunlun Mountain in semi-arid North China. Compared to the non-grazed control, light and intensive grazing treatments increased the 2-yr total N2O emissions by 27.5% and 68.1%, respectively. Daily N2O flux rate correlated positively with soil water-filled pore space (WFPS), temperature, and dissolved organic carbon (DOC) but not with the abundances of nitrifiers (AOB, AOA, Nitrobacter-like nxrA), nitrate reducers (narG) and denitrifiers (nirS, nirK, and nosZ). These results suggest that soil environmental factors rather than nitrifier, N2O-producer and N2O-reducer abundances were more important in determining the grazing enhancement of N2O emissions from the alpine grassland in semi-arid areas. The abundance of AOB and Nitrobacter-like nxrA but not AOA correlated positively with NEA, indicating they are good indicators for the potential nitrification in this arid alpine grassland. Reducing grazing intensity from intensive to light resulted in more above-ground biomass and N uptake in both years and less N2O emissions in 2018. These results highlight the importance of establishing a proper grazing intensity to reduce N2O emissions from the degraded grassland while maintaining productivity for livestock.