Global soil acidification impacts on belowground processes

With continuous nitrogen (N) enrichment and sulfur (S) deposition, soil acidification has accelerated and become a global environmental issue. However, a full understanding of the general pattern of ecosystem belowground processes in response to soil acidification due to the impacting factors remains elusive. We conducted a meta-analysis of soil acidification impacts on belowground functions using 304 observations from 49 independent studies, mainly including soil cations, soil nutrient, respiration, root and microbial biomass. Our results show that acid addition significantly reduced soil pH by 0.24 on average, with less pH decrease in forest than non-forest ecosystems. The response ratio of soil pH was positively correlated with site precipitation and temperature, but negatively with initial soil pH. Soil base cations (Ca²⁺, Mg²⁺, Na⁺) decreased while non-base cations (Al³⁺, Fe³⁺) increased with soil acidification. Soil respiration, fine root biomass, microbial biomass carbon and nitrogen were significantly reduced by 14.7%, 19.1%, 9.6% and 12.1%, respectively, under acid addition. These indicate that soil carbon processes are sensitive to soil acidification. Overall, our meta-analysis suggests a strong negative impact of soil acidification on belowground functions, with the potential to suppress soil carbon emission. It also arouses our attention to the toxic effects of soil ions on terrestrial ecosystems. (letter)