Effects Of Different Returning Models On The Transformation Of New Straw Carbon And Soil Carbon Pool Under Long-term Straw Returning Conditions

Soil physicochemical properties and soil biological community structure can be caused by different models of long-term straw returning,but the response of mineralization and sequestration of newly added straw carbon to this difference remain unclear.Therefore,in this research,four treatments were selected from the 9-year long-term positioning experiment of straw returning in Xiangyang,Hubei Province:CK（unfertilizer and no straw returning）,NPK（chemical fertilizer without straw returning）,NPKS（chemical fertilizer+straw returning directly）and NPKM（chemical fertilizer+straw returning through animal’s belly）.The differences in soil physicochemical properties and soil biological community structure were compared.The mineralization process of newly added straw carbon and the response of soil original organic carbon to newly added straw carbon were also quantified by adding ¹³C straw to different treatments.The results were as follows:（1）After 9 years of long-term straw returning in different models,the content of soil organic carbon（SOC）,dissolved organic carbon（DOC）,microbial biomass carbon（MBC）,light fraction organic carbon（LFOC）and particulate organic carbon（POC）were increased significantly in NPK,NPKS and NPKM treatments,compared with CK treatment.On the opposite,soil p H value was significantly reduced（p<0.05）.The content of SOC,available K（AK）and total nitrogen（TN）in NPKS treatment were significantly higher than those in NPK and NPKM treatments（p<0.05）.Consequently,different models of long-term straw returning exhibited different effects on soil carbon pool components.（2）After 9 years of long-term straw returning in different models,the carbon use efficiency of NPKS and NPKM treatments was significantly higher than that of CK and NPK treatments（p<0.05）,and the carbon use efficiency types of NPKS and NPKM treatments were similar,which showed significant differences in PCA analysis with CK and NPK treatments.Further analysis of high-throughput sequencing showed that the soil bacterial community structure of CK treatment was similar to that of NPK treatment,while different from that of NPKS and NPKM treatments;the fungal composition of NPK treatment was similar to that of NPKS treatment,while different from that of CK and NPKM treatments.Therefore,different models of long-term straw returning changed the microbial community structure in soils.（3）After the incubation experiment with ¹³C straw,compared with CK treatment,the CO₂produced by straw carbon mineralization in NPK,NPKS and NPKM treatments were increased by 8.08%,7.32%and 113.53%respectively.However,there was no significant difference between NPK and NPKS treatments.After incubation,the proportion of straw carbon residue in NPK,NPKS and NPKM treatments decreased by 8.29%,6.61%and11.77%respectively,compared with CK treatment.The proportion of straw carbon transformation allocated to POC,LFOC and MBC was significantly higher than that of DOC.Compared with other three treatments,the CO₂produced by soil original carbon mineralization in NPKS treatment increased significantly during the incubation process（p<0.05）,but there was no significant difference between NPK and NPKM treatments.（4）After the incubation experiment with ¹³C straw,the soil total carbon content of all treatments increased compared with that before incubation,and the net carbon content increased the most in CK treatment and was significantly higher than that of NPKS treatment（p<0.05）.（5）Proteobacteria,Bacteroidetes,Mortierellomycota and functional genes related to carbon degradation（amy A,chitinase and CBH2）in soils were closely related to straw decomposition.（6）Soil microorganisms and SOC content played dominant roles in straw carbon mineralization and in soil original organic carbon mineralization,respectively.