Abstract: Aims Ecosystem reactive nitrogen (N) and phosphorus (P) inputs induced by recent industrial and agricultural development can alter soil carbon (C) storage capacity by affecting soil aggregate structure and stabilization. Currently, there is a lack of observational studies on soil aggregates modeling N deposition in the subtropics, and less attention has been paid to P-N interactions. Methods In order to investigate how ecosystem N enrichment and its interactions with P deposition affect C sequestration mechanisms in soil aggregates, N and P addition experiments were conducted for 7 consecutive years in a subtropical forest in southern China. Sample plots of four N addition types were established (control: CK, 0 kg N·hm-2a-1; N addition: LN，50 kg N·hm-2a-1; N addition: N, 100 kg N·hm-2 a-1; N and P: N+P，100 kg N·hm-2 a-1+50 kg P·hm-2 a-1). Soil properties, soil aggregates and their C, N fractions, and C, N stable isotopes of aggregates at each aggregate level were determined. Important findings Evergreen broadleaf forest macroaggregates (>250 μm) were the dominant soil aggregate size accounting for 83-87% of the total soil weight, and LN treatment increased macroaggregate formation, mean mass diameter (MWD) and mean geometric diameter (MGD), while N and N+P treatments marginally reduced soil aggregate stability (p>0.05). N enrichment in this region mainly increased aggregate C and N content by increasing aggregate C and N concentration at each grain level, and the added organic matter was mainly enriched in macroaggregates. The macroaggregates mainly added organic matter with high C to N ratio and rich δ13C abundance. Compared with low N addition, excessive N input was detrimental to soil aggregate structure stabilization and C sequestration. Under N treatment conditions, the addition of P did not significantly change the stability of local aggregates and their C, N components (p>0.05). Total soil C, N, and P contents of broad-leaved evergreen forests under N addition did not promote soil aggregate formation, and the decrease of pH significantly promoted the increase of C and N contents of aggregates (p<0.05). The results of this study increase our understanding of the mechanisms of aggregate change, and thus are important references for predicting the potential of soil C sinks in broadleaf evergreen forests and other areas with similar conditions under future N and P deposition.

Key words: nitrogen and phosphorus addition, broadleaf evergreen forest, soil aggregates, soil aggregate carbon and nitrogen, soil carbon and nitrogen stable isotopes