Nitrogen deposition does not exacerbate phosphorus limitation of rhizosphere microbes in subalpine forests

The stoichiometric imbalance caused by nitrogen (N) deposition typically exacerbates phosphorus (P) limitation in plants. However, it remains unclear whether this effect extends to soil microbes, particularly those in the rhizosphere. Addressing this knowledge gap is important as P-limited microbes in the rhizosphere may capture P and restrict its availability for plant uptake. Here we investigated the responses of microbial P status to seven years of simulated N deposition in rhizosphere soils from two subalpine forests, where trees experience N deposition-induced P deficiency. Our findings show that rhizosphere microbial growth in both forests is limited by P under ambient N conditions, as indicated by the significant increase in the growth rate (measured as 18O-H2O incorporation into DNA) following short-term P addition. Interestingly, rhizosphere microbial P limitation was not exacerbated by N deposition. This is evidenced by the lack of impact of N deposition on the degree of P limitation with respect to both microbial growth and respiration. In support of this physiological evidence, N addition did not lead to the enrichment of microbial genes involved in P acquisition and processing, reflecting a lack of selection pressure from increased P scarcity. Unlike plants in the subalpine forests, N deposition did not exacerbate the P limitation of rhizosphere microbes. This neutral effect may reduce the likelihood of intensified P competition between plants and microbes, with important ecological consequences for the productivity of subalpine forests under N deposition.