Compositional and functional responses of soil microbial communities to long-term nitrogen and phosphorus addition in a calcareous grassland

Increased nitrogen (N) input into the ecosystem, which is mainly caused by anthropogenic activities, is usually not paralleled by a similar increase in phosphorus (P) input, and thus, can shift the ecosystem from N limitation to P limitation. Although the effects of N enrichment on ecosystem components have been intensively evaluated, the impacts of altered P resource availability and the interactive effects of N and P on the biomass, composition, and function of soil microbial communities are not well understood. Here, based on a 9-year field experiment, we investigated the responses of soil biotic and abiotic properties to N and P addition in a semi-arid calcareous grassland in northern China. We documented a significant increase in the relative abundance of bacteria (copiotrophic group in a broad ecological meaning) and a decrease in the relative abundance of fungi (oligotrophic) and the fungi/bacteria (F/B) ratio under N addition. The proportion of arbuscular mycorrhizal fungi (AMF) decreased under both N addition and P addition. N addition and P addition inhibited N-acquisition enzymes (protease, PRO) and P-acquisition enzymes (alkaline phosphomonoesterases, Alka PME), respectively. N fertilization inhibited most of the soil enzymatic activities by reducing soil pH and microbial biomass. P addition alleviated the negative impacts of N addition on substrate induced respiration (SIR), peroxidase (PER), and Alka PME, likely by reducing the available N content in soils, but strengthened the effects of N on the soil total carbon (TC) content, which might contribute to increased plant productivity. We also found that the overall changes in soil microbial enzyme profiles in response to nutrient addition were mainly caused by eutrophication (changes in NO3--N and Olsen-P), whereas variations in the broad community structure were driven by soil acidification. Overall, the application of N and P in this natural steppe will cause serious environmental issues and impact ecosystem service and function through changing the compositional and functional profiles of soil microbial communities.