Impact of nitrogen and phosphorus additions on soil gross nitrogen transformations in a temperate desert steppe

Nutrient addition has a significant impact on plant growth and nutrient cycling. Yet, the understanding of how the addition of nitrogen (N) or phosphorus (P) significantly affects soil gross N transformations and N availability in temperate desert steppes is still limited. Therefore, a 15N tracing experiment was conducted to study these processes and their underlying mechanism in a desert steppe soil that had been supplemented with N and P for 4 years in northwestern China. Soil N mineralization was increased significantly by P addition, and N and P additions significantly promoted soil autotrophic nitrification, rather than NH4+-N immobilization. The addition of N promoted dissimilatory NO3- reduction to NH4+, while that of P inhibited it. Soil NO3--N production was greatly increased by N added alone and by that of N and P combined, while net NH4+-N production was decreased by these treatments. Soil N mineralization was primarily mediated by pH, P content or organic carbon, while soil NH4+-N content regulated autotrophic nitrification mainly, and this process was mainly controlled by ammonia-oxidizing bacteria rather than archaea and comammox. NH4+-N immobilization was mainly affected by functional microorganisms, the abundance of narG gene and comammox Ntsp-amoA. In conclusion, gross N transformations in the temperate desert steppe largely depended on soil inorganic N, P contents and related functional microorganisms. Soil acidification plays a more key role in N mineralization than other environmental factors or functional microorganisms.