Soil acidification drives the negative effects of nitrogen enrichment on soil microbial biomass at the global scale

Nitrogen (N) deposition is a global driving factor that affects microbial biomass and ecosystem processes. However, it remains unclear whether N enrichment impacts microbial biomass in soil across ecosystems. In this study, we synthesized 125 studies worldwide to evaluate the effects of N addition on soil microbial biomass. We found that N enrichment substantially suppresses total microbial biomass, including bacterial, fungal, and arbuscular mycorrhizal fungi biomass. Importantly, we found strong negative effects on microbial biomass globally caused by N-enrichment-induced soil acidification. Moreover, N enrichment mainly shapes the acidification of soil through the increased concentration of ammonium and by changing exchangeable cations in soil, including magnesium, calcium, sodium, manganese, and aluminum ions. Overall, our findings demonstrate that N-enrichment-induced soil acidification is a crucial factor to consider for preserving microbial communities and ecosystem function under future trends of increasing N enrichment.