Anthropogenic soil acidification severely imperils microbial diversity and functionality

Soil acidification, a pervasive yet often underestimated issue in the Anthropocene, poses a critical threat to ecosystem 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), based on ‘Mill's causal inference’. Anthropogenic soil acidification caused by nitrogen and acid addition significantly decreases microbial diversity, biomass, and functionality; in stark contrast, natural pH gradients exert only minimal effects on these microbial traits. Notably, nitrogen-induced acidification significantly reduces microbial richness, Shannon index, biomass, and functionality by 5.1%, 3.1%, 4.1%, and 5.1%, respectively. Meanwhile, acid addition decreases microbial biomass and functionality by 9.4% and 15%, respectively. Disturbingly, croplands experience more severe acidification (by 10.0%) compared to 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.