Nitrogen addition stimulates soil aggregation and enhances carbon storage in terrestrial ecosystems of China: A meta-analysis

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 76 N 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. 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.中国正经历高水平的大气氮沉降（atmospheric nitrogen (N) deposition），该过程对陆地生态系统（terrestrial ecosystems）的土壤碳动态（soil carbon (C) dynamics）具有显著影响。土壤团聚体（soil aggregation）有助于维持土壤结构稳定性，并促进土壤碳固存（soil C sequestration）。尽管已有诸多研究报道了氮富集（N enrichment）对原状土壤碳动态（bulk soil C dynamics）的影响，但阐明土壤团聚体如何响应氮富集的内在机制仍不明确。本研究针对中国陆地生态系统中76项氮调控实验的数据开展荟萃分析（meta-analysis），以此评估氮富集对土壤团聚体及其碳固存能力的影响。整体而言，氮富集可使土壤团聚体的平均重量直径（mean weight diameter）显著提升10%。氮富集可显著提升大团聚体（macroaggregates）占比6%，同时使粉粒-粘粒组分（silt-clay fraction）占比降低9%。在所有陆地生态系统中，大团聚体碳（macroaggregate C）对氮富集的响应幅度（+15%）显著高于原状土壤碳（+5%）。但氮富集对微团聚体碳（microaggregate C）与粉粒-粘粒组分碳的影响较小。氮富集对土壤团聚体的影响幅度随生态系统类型与施肥模式的不同而存在差异。此外，土壤pH值持续降低，且与土壤团聚体碳含量显著相关。综合来看，本次荟萃分析结果表明，氮富集可通过提升大团聚体碳含量与酸化土壤，促进颗粒态有机碳（particulate organic C）的积累。与之相反，土壤团聚体占比的提升可抑制微生物介导的土壤有机质分解过程，进而使矿质结合态有机碳（mineral-associated organic C）的变化幅度极小。本研究结果显示，大气氮沉降或可增强中国陆地生态系统中土壤团聚体的形成能力及其碳固存潜力。