China is experiencing a high level of atmospheric nitrogen (N) deposition, which greatly affects the soil carbon (C) dynamics in terrestrial ecosystems. Soil aggregation contributes to the stability of soil structure and to soil C sequestration.Although many studies have reported the effects of N enrichment on bulk soil C dynamics, the underlying mechanisms explaining how soil aggregates respond to N enrichment remain unclear. Here, we used a meta-analysis of data from 76N manipulation experiments in terrestrial ecosystems in China to assess the effects of N enrichment on soil aggregation and its sequestration of C. On average, N enrichment significantly increased the mean weight diameter of soil aggregates by 10%. The proportion of macroaggregates and silt-clay fraction were significantly increased (6%) and decreased (9%) by N enrichment, respectively. A greater response of macroaggregate C (+15%) than of bulk soil C (+5%) to N enrichment was detected across all ecosystems. However, N enrichment had minor effects on microaggregate C and silt-clay C. The magnitude of N enrichment effect on soil aggregation varied with ecosystem type and fertilization regime. Additionally, soil pH declined consistently and was correlated with soil aggregate C. Overall, our meta-analysis suggests that N enrichment promotes particulate organic C accumulation via increasing macroaggregate C and acidifying soils. In contrast, increases in soil aggregation could inhibit microbially mediated breakdown of soil organic matter, causing minimal change in mineral-associated organic C. Our findings highlight that atmospheric N deposition may enhance the formation of soil aggregates and their sequestration of C in terrestrial ecosystems in China.

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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.