Responses Of Soil Carbon And Nitrogen Dynamics To Straw Incorporation In An Upland Red Soil Under Long-term Fertilization

Soil organic carbon（C）and nitrogen（N）not only affect farmland ecosystem productivity,but also impact the environmental issues such as greenhouse gas emissions.Fertilization has significant effects on soil C and N status.However,how different long-term fertilization affects soil C and N dynamics and the resulting environmental consequences is not fully understood.It is generally believed that the addition of crop residues such as straw can significantly increase soil organic carbon mineralization.However,increasing evidence suggests that this priming effect may also depend on the initial soil fertility,but direct evidence is still poor.Taken advantage of a long-term fertilization experiment（36 years）in the Red Soil Research Station in Jinxian County,Jiangxi Province,this study designed two experiments to investigate the impact of long-term fertilization on soil C and N dynamics as well as the legacy effect on responses of soil carbon and nitrogen dynamics to new straw addition.The first experiment（Experiment 1）examined the effects of different long-term fertilization practices on soil carbon and nitrogen dynamics,such as the aggregate distribution of carbon and nitrogen,carbon and nitrogen mineralization rate,soil N₂O emissions.Soils were taken from field plots of three treatments:the no fertilization control（CK）,NPK fertilization（NPK）,and organic manure fertilization（OM）).Major results were:（1）compared with no fertilizer treatment for a long time,the long-term NPK fertilizer significantly reduced the mineral in the soil carbon（Fe/Al-C）content,significantly reduce the soil microbial biomass,and reduce the aggregate carbon,nitrogen content and the trend of soil total carbon,but significantly increased soil NO₃^-content and increase soil N₂O release,is not conducive to the stability of red soil carbon,nitrogen and sequestration.（2）Long-term application of organic fertilizer significantly increased the soil aggregate C and N contents,and also significantly increased the soil microbial biomass C and N,and had a tendency to increase soil Ca/Mg-C,and then significantly increased soil carbon sequestration;In addition,long-term application of organic fertilizer can significantly promote the release of soil N₂O and intensify the environmental effects of nitrogen conversion.The second experiment was a 360-day incubation experiment（Experiment 2）to examine how straw addition affect soil C and N transformations.Soils were sampled at 30,90,180 and 360 days after straw addition.Mineral-associated soil C pools,soil microbial biomass C and N,dissolved organic C and N,and CO₂ and N₂O emissions were examined to investigate soil C and N retention.（1）Straw addition significantly changed the contents of Ca and Mg bound carbon（Ca/Mg-C）and Fe and Al bound carbon（Fe/Al-C）in different long-term fertilization soils.Straw addition promoted the formation of Ca/Mg-C in soil with long-term N,P and K fertilizer,but inhibited the formation of Ca/Mg-C in soil with organic fertilizer.Straw addition promoted the formation of Fe/Al-C in soil with long-term application of NPK and organic fertilizer,and the accumulation of Fe/Al-C in soil with long-term application of NPK was significantly higher than that of soil with long-term application of organic fertilizer after 180 days of cultivation.（2）After straw addition,the increase of N mineralization rate and nitrification rate of soil treated with organic fertilizer was significantly higher than that treated with N,P,K and no fertilizer.However,with the increase of culture time,the accumulation of organic nitrogen in soil treated with organic fertilizer increased and the rate of soil N2O emission decreased gradually.In conclusion,long-term application of NPK fertilizer reduced soil C retention through decreased soil Fe/Al-C and aggregate-C,while long-term application of organic fertilizer increased soil C sequestration but increased the emission of N₂O.However,straw addition significantly increased the soil mineral-associated C and N rentention under all fertilization treatments（although significantly in the NPK treatment）,and reduced N₂O emissions from the OM soil.