Effects Of Nitrogen And Phosphorus Additions On Soil Organic Carbon Fractions And Aggregate Stability In A Subtropical Evergreen Broad-leaved Forest

Soil organic carbon?SOC?pool has the function of carbon sink,which is important in slowing down the increase of global CO₂ concentration.Soil organic carbon is composed of highly heterogeneous organic carbon components with different stabilities and turnover rates.The change of different organic carbon components will indirectly affect the stability of soil organic carbon pool,and then change the carbon sink of terrestrial ecosystem.Meanwhile,soil aggregates directly affect soil carbon cycle,and the decline of soil aggregate stability will weaken the capacity of soil carbon sequestration.Nitrogen?N?and phosphorus?P?deposition rates induced by human activities increase continuously,affecting the soil organic carbon composition and aggregate structure by changing plant fine roots,soil microorganisms and soil physical and chemical properties.Therefore,to explore the effects of nitrogen and phosphorus deposition on soil organic carbon composition and aggregate structure change is vital for the deep understanding of soil organic carbon dynamics and the comprehensive assessment of carbon sequestration potential of terrestrial ecosystems.Based on the long-term N and P addition experimantal plots in a subtropical evergreen broad-leaved forest in Tiantong,Zhejiang Province,this study addressed the effects of N and P addition on the chemical and physical components of soil organic carbon and the stability of aggregate.The organic carbon fractions with different layers?0-10 cm;10-20 cm?by methods of sulfuric acid acidification were quantified.In addition,structural equation model was used to analyze the regulation mechanism of biological factors?fine root,GRSP,arbuscular mycorrhizal fungi?on the stability of aggregates under different fertilization treatments.The main results are as followings:?1?The effect of nitrogen and phosphorus addition on the composition of soil aggregateUnder P addition treatment,the content of SM and mM-silt+clay at the depth of10-20 cm decreased significantly?p<0.05?by 18.02%and 40.72%respectively,suggesting the negative effect of P addition on the soil physical structure.?2?The effect of nitrogen and phosphorus addition on the organic carbon associated with soil aggregateThe P addition treatment significantly?p<0.05?increased the SOC concentrations of soil aggregate with different size?mM-POM,mM-silt+clay,Fm-silt+clay?and physical frations?mM-POM,mM-silt+clay,Fm-silt+clay?,and also significantly?p<0.05?increased the C/N ratio of soil aggregate with different size?SM,NA-silt+clay?;The NP addition significantly?p<0.05?increased the C/N ratio of soil aggregate with different size?LM,Fm,NA-silt+clay?.This results indicate that+N and+NP treatment could increased the sequestration of organic carbon in aggregate,and also suggest that the physical and chemica stabilization mechanisms is very important for the stability and sequestration of SOC pool.?3?The regulation process of biological factor on the stability of soil aggregates under nitrogen and phosphorus additionArbuscular mycorrhizal fungi?AMF?indirectly?p<0.05?promoted the stability of soil aggregates?MWD?mainly through the effect of E-GRSP,and the effect of fine roots on the stability of soil aggregates was not significant.Moreover,the P treatment could indirectly affect the stability of soil aggregates through other path with a negative correlation?p<0.05?.?4?The effect of nitrogen and phosphorus on SOC fractionsThere was no significant difference among the effects of fertilization on the LOC and ROC concentrations,and P addition significantly?p<0.05?increased the DOC concentration by 79.02%at depth of 10-20 cm.In conclusion,our study found that the addition of N and P did influence the soil structure and the content and quality of organic carbon fractions related to aggregates,revealed that the potential impacts of exogenous N and P addition on SOC pool,and we also explored the regulation mechanism of biological factor on soil aggregates through the structural equation model.The findings of the present study help improve our understanding of the responses of terrestrial ecosystem C pool to N and P addition.