Soil Environmental Factors Dominate over Nitrifier and Denitrifier Abundances in Regulating Nitrous Oxide Emissions Following Nutrient Additions in Alpine Grassland

Nutrient additions including nitrogen (N) and phosphorus (P) are widely considered as an important strategy for enhancing grassland productivity. However, the effects of these nutrients additions on soil nitrous oxide (N2O) emissions and the underlying mechanisms remain debated. We conducted a two-year field experiment in an alpine grassland on Kunlun Mountain in northwestern China to assess the effects of N and P additions on N2O emissions, in relation with nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and key functional genes abundance responsible for nitrification (amoA and Nitrobacter-like nxrA) and denitrification (narG, nirS, nirK and nosZ). Compared to the Control without nutrient addition (CK), N addition alone substantially increased cumulative N2O emission (ƩN2O) by 2.0 times. In contrast, P addition or combined N and P (N+P) addition did not significantly affect ƩN2O, though both treatments significantly increased plant aboveground biomass. Such results indicate that P addition may mitigate N-induced N2O emission, likely by reducing soil N availability through enhanced plant and microbial N uptake. Compared to CK, N or N+P addition significantly elevated NEA but did not affect DEA. Structural equation modeling (SEM) indicated that NEA was directly influenced by the gene abundances of ammonia-oxidizing bacteria (AOB) and Nitrobacter-like nxrA but not by ammonia-oxidizing archaea (AOA). However, SEM also revealed that soil environmental variables including soil temperature, pH, and water-filled pore space (WFPS) had a stronger direct influence on N2O emissions than the abundances of nitrifiers. These results demonstrate that soil environmental conditions play a more significant role than functional gene abundances in regulating N2O emissions following N and P additions in semi-arid alpine grasslands. This study highlights that the N+P application can potentially decrease N2O emissions than N addition alone, while increasing productivity in the alpine grassland ecosystems.