Decrease of soil acidity has greater effects than increase of aboveground carbon inputs on soil organic carbon in terrestrial ecosystems of China under nitrogen enrichment

1. China has
experienced chronic increases of anthropogenic nitrogen (N) deposition. Notable
impacts of 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.

2. 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.

3. 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.

4. Synthesis and
applications. In this study, we suggest that soil C budget is determined by
the trade-off between plant C inputs and N-induced soil acidification. In
contrast to other studies, our findings demonstrate that N-mediated soil
acidification, rather than increased aboveground C inputs, is the main driver
increasing SOC under elevated N inputs in terrestrial ecosystems of China.