Adaptation mechanisms of the soil microbial community under stoichiometric imbalances and nutrient-limiting conditions in a subtropical nitrogen-saturated forest

Purpose Anthropogenic activities have increased the nitrogen (N) inputs in terrestrial ecosystems, thereby altering both the carbon (C) and phosphorus (P) availability along with resource stoichiometry. Stoichiometric deviations between microbial biomass and resources availablity cause stoichiometric imbalances and nutrient limitations for microbial activity. However, whether N deposition will further aggravate the existing P limitation is unknown. Furthermore, how soil microbes respond to these conditions, along with the biogeochemical cycles they mediate, still remains unclear. Methods To answer these questions, a 7-year N addition experiment (+0, +50, +150 kg N ha −1 yr −1 ; CK, LN, HN) has been conducted in a subtropical evergreen-broadleaved forest in the Rainy Area of West China, where received the highest background N deposition in the world and the highest precipitation in inland China. Soil-available nutrients, microbial biomass, C-, N-, and P-acquiring enzyme activities, microbial community composition and diversity were measured. Results With increasing N addition, the soil DOC:AP and AN:AP ratios increased significantly, whereas microbial biomass C:P and N:P ratios decreased significantly, resulting in increased C:P and N:P imbalances between the soil microbes and resources, thereby aggravating the existing P limitation of microorganisms in this subtropical N-saturated forest. Microbial communities maintained stoichiometric homeostasis by increasing ACP enzyme activity (by 8.99% to 19.28% under N treatment) and threshold elements ratio (TER) at P-limited levels. The aggravated imbalance of C:P and N:P caused by N addition decreased the bacterial and fungal community alpha-diversity. Interestingly, changes in the beta-diversity of the bacterial rather than the fungal community responded more strongly to stoichiometric imbalance. Bacterial communities transitioned from coprophilous (Proteobacteria-dominated) to oligotrophic (Actinobacteriota-dominated) under N addition treatment. Conclusions This study not only highlights the importance of stoichiometric imbalances in regulating the soil microbial community structure and enzymatic activity, but also may help in understanding how global N deposition-induced resource stoichiometry changes affect the terrestrial C and nutrient flows.