Decrease in soil pH has greater effects than increase in above-ground carbon inputs on soil organic carbon in terrestrial ecosystems of China under nitrogen enrichment

Impacts of nitrogen (N) enrichment on soil carbon (C) budgets in terrestrial ecosystems have been well documented by numerous field experiments and syntheses. Although previous studies have largely attributed this phenomenon to the increased organic C inputs, the potential mechanisms of how N enrichment increases soil organic C (SOC) remain contentious. In this study, we conducted a meta-analysis comprising 234 published field N-addition experiments from multiple terrestrial ecosystems across China to evaluate the impacts of N enrichment on the SOC budget. Although the meta-analysis revealed that N-addition significantly enhanced plant biomass and SOC concentration across the selected studies, we found that SOC concentration was independent of, or even decreased with, the enhanced plant biomass due to increased soil C loss as increasing organic C inputs. The negative correlation between plant C inputs and SOC under N enrichment, meanwhile, appeared reversible by the concomitant changes in soil pH, which depended on the magnitude of soil acidification. Increased SOC in terrestrial ecosystems was closely associated with decreased soil pH, which reduced soil C losses by limiting microbial degradation. Synthesis and applications. We suggest that soil carbon (C) budget is determined by the trade-off between plant C inputs and nitrogen (N)-induced soil acidification. In contrast to other studies, our findings demonstrate that N-mediated soil acidification, rather than increased above-ground C inputs, is the main driver increasing soil organic C (SOC) under elevated N inputs in terrestrial ecosystems of China.