Responses of soil microbes and their interactions with plant community after nitrogen and phosphorus addition in a Tibetan alpine steppe

Nitrogen (N) and phosphorus (P) additions are the widely used restoration management for degraded grasslands. However, soil microbial community responses to N and P additions are still not well understood, especially multiple levels or/and combined N and P additions in high-altitude grasslands. In this study, the single and interactive effects of N and P additions on the soil microbes were explored. A field experiment with two factors (N 0, 7.5, and 15 g N m−2 year−1; P 0, 3.27, and 6.55 g P m−2 year−1) was conducted by a completely randomized block design in a Tibetan plateau steppe. Total and specific microbial biomass (bacteria, fungi, G+/G− bacteria, saprotrophs, actinomycetes, and arbuscular mycorrhizal fungi) were analyzed by using phospholipid fatty acids (PLFAs). Additionally, based on their fixed weight composites, we used a composite variable (phenotype) to represent the microbial community. The results showed that N addition decreased the microbe phenotype, total microbial biomass, and specific microbial biomass (total bacteria, G+, and G− bacteria). P application and its interaction with N application all increased the microbial phenotype, total microbial biomass, and specific microbial biomass (total bacteria, G+, G−, saprotrophs, and actinomycetes). Besides, the microbial phenotype was positively correlated with P application rates and not correlated with N application rates. In conclusion, P was the key limiting factor for soil microbes, and its interaction with N is also important for some specific microbes. In addition, long-term responses of soil microbes after N and P amendments need further investigation in the Tibetan alpine steppe.