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.

Atmospheric nitrogen (N) deposition has substantial effects on forest ecosystems. The effects of N deposition on understorey plants have been simulated by spraying N on the forest floor. Such understorey addition of N (UAN) might simulate atmospheric N deposition in a biased manner, because it bypasses the canopy. We compared the effects of UAN and canopy addition of N (CAN) at 0, 25 and 50 kg N ha−1 year–1 on SLA, leaf construction costs (CC), concentrations of leaf carbon ([C]), nitrogen ([N]), phosphorus ([P]), minerals ([Mineral]), nitrate ([NO3‐]), lignin ([Lignin]), lipids ([Lipid]), organic acids ([OA]), soluble phenolics ([SP]), total non‐structural carbohydrates ([TNC]) and total structural carbohydrates ([TSC]) in six dominant understorey species in a subtropical evergreen forest after 5 years of N treatments. We found that leaf CC, [C], [Lignin], [OA], [TNC] and [TSC] were significantly affected by N addition approach and rate, but leaf [P] and [Lipid] were affected by N addition approach and N addition rate respectively; leaf CC, [C], [P], [OA] and [TNC] were significantly lower under UAN than under CAN, but leaf [TSC] and [Lignin] were significantly higher and lower, respectively, under UAN than under CAN at 50 kg N ha−1 year–1; the decline of leaf [C] and [Lignin] contributed to the significantly lower leaf CC under UAN than under CAN. Synthesis. We show that canopy and understorey N addition exerted significantly different effects on leaf traits of understorey plants. The results indicate that understorey plants in subtropical forest respond differently to UAN from those to atmospheric deposition of N. Further studies are warranted to evaluate the unbiased ecological processes and functions of forest ecosystem responding to atmospheric N deposition via both CAN and UAN experiments over a longer term.