Nitrogen addition weakens the positive effect of tree diversity on soil microbial biomass

Abstract Plant diversity interacts with soil microorganisms to sustain ecosystem functions. While plant diversity and soil microbial biomass are tightly linked, the impact of nitrogen (N) addition on this relationship remains unclear. This study involved a field experiment in which four levels of tree species richness (1, 4, 8 and 16 species) and N addition were factorially manipulated to assess their effects on total soil microbial biomass, including the biomass of bacteria, fungi and arbuscular mycorrhizal fungi, as well as how N addition modulates these effects. We found that tree species richness significantly increased soil microbial biomass, with stronger effects on bacterial and arbuscular mycorrhizal fungal biomass than on total fungal biomass. However, these benefits were reduced under N addition, largely because N addition diminished the positive effects of tree species richness on tree productivity and soil nutrient content. Synthesis . These results highlight that high levels of N enrichment may disrupt the pathways through which diverse tree communities support soil microbial biomass, further suggesting that tree diversity effects are context‐dependent and influenced by local soil nutrient status. We conclude that ongoing atmospheric N deposition may compromise biodiversity restoration efforts and reduce the capacity of mixed forests to sustain soil microbial biomass and associated functions.