Phosphorus addition decreases soil fungal richness and alters fungal guilds in two tropical forests

Soil microbes regulate key functions of terrestrial ecosystems, but the effects and underlying mechanisms of elevated nutrient inputs on soil microbial diversity in tropical forests remain unclear. We evaluated the effects of seven-year nitrogen (N) and phosphorus (P) additions on soil microbial diversity in two tropical montane rainforests (P-limited primary forest and N-limited secondary forest). The results showed that soil bacterial richness was not sensitive to N or P addition, whereas soil fungal richness decreased under P and combined N and P additions. Furthermore, P inputs shifted soil fungal community composition: the relative abundance of mutualistic fungi increased and that of saprotrophic fungi decreased. The underlying mechanisms of these effects differed in the two forests. In the P-limited primary forest, the reduction in soil fungal richness was mainly related to a negative microbial interaction between microbial species and a stimulation in plant productivity. In the N-limited secondary forest, P inputs increased soil N and P imbalance, which caused a decline in the fungal richness. Overall, anthropogenic P enrichment could reduce soil fungal richness and alter fungal functional guilds in tropical forests, which will have divergent consequences on plant productivity and microbial functioning.