Anthropogenic soil acidification severely decreases microbial diversity and functionality

Soil acidification, a pervasive yet often underestimated issue in the Anthropocene, poses a critical threat to soil health. Although microorganisms are the cornerstone of soil health, the impact of anthropogenic soil acidification on microbial communities remains unresolved. Here, we leverage 455 experimental studies to disentangle how soil acidification affects microbial traits in different scenarios (anthropogenic acidification versus natural pH gradients). Nitrogen-induced acidification significantly reduces microbial biomass, richness, Shannon index, and functionality (enzyme activities involved in carbon, nitrogen, and phosphorus cycling) by 4.4%, 5.1%, 2.7%, and 5.0%, respectively, and similarly, acid addition decreases microbial biomass and functionality by 8.8% and 13.0%, respectively. By contrast, natural pH gradients at local scales exert only minimal effects on these microbial traits. Disturbingly, croplands experience more severe acidification (by 10.0%) compared with natural ecosystems (by 6.5%), leading to markedly steeper declines in microbial traits. Reductions in microbial biomass, rather than diversity loss, primarily drive the decline of soil functionality. These findings underscore the imperative to mitigate anthropogenic soil acidification to sustain soil health, particularly in intensively managed croplands.