Effects Of Wheat Straw Combined With Nitrogen Fertilizer On Soil Organic Carbon Content And Bacterial Community Structure In Apple Root Zone

China is a major producer of crop straw and a major user of nitrogen（N）fertilizer.Straw returning combined with inorganic N fertilizer is usually used in agricultural managments.Apple is one of the major economic trees in China,but the basic soil fertility and soil organic matter content are restricted in the dominant areas of apple.In addition,the inorganic N fertilizer addition is usually greater,while with the lower N use efficiency than other countries.Therefore,in this study,the wheat straw was taken as organic carbon source,and urea as N source.The effects of wheat straw(6 t·hm^-2)combined with different amounts of N fertilizer(0 mg·kg^-1,S;200 mg·kg^-1,SN1;300 mg·kg^-1,SN2;500 mg·kg^-1,SN3)on organic carbon mineralization,soil biochemical properties,and bacterial community structure were analyzed.Soil samples without wheat straw and N fertilizer as control（CK）.The results gained in this study could provide theoretical reference for improving soil organic carbon storage and soil fertility in apple root zone.The main results are as follows:（1）A laboratorial incubation experinment was conducted in order to investigating the effects of wheat straw combined with different amounts of N fertilizer on organic carbon mineralization.The results showed that the mineralization rate of organic carbon in each treatment showed a gradually decreasing trend of treatment time,and the organic carbon mineralization rates in wheat straw combined with N fertilizer treatments were significantly higher than that under S treatment and CK.The organic carbon mineralization showed a trend of increasing first and then decreasing during the incubation times.Compared with CK,the cumulative organic carbon mineralization under S and wheat straw combined with different amounts of N fertilizer treatments were increased significantly by 47.28%,85.04%,99.25%and 101.86%at the end of incubation,respectively.The cumulative organic carbon mineralization in SN1,SN2 and SN3 treatments was increased by 25.64%,35.29%and37.06%,as compared with that of S treatment,respectively.The cumulative organic carbon mineralization in SN2 and SN3 treatments was significantly higher than that in SN1 treatment.（2）The effects of wheat straw combined with N fertilizer on soil biochemical properties of apple orchard were studied by microplot experiments.Compared with CK,S,SN1,SN2 and SN3 treatments were effectively increased soil organic carbon and microbial biomass carbon contents.This promotion was higher in wheat straw combined with N fertilizer treatment than wheat straw alone,while no significant difference was found among different N levels.Wheat straw combined with N fertilizer significantly increased particulate organic carbon content.S treatment increased dissolved organic carbon content,whiel it decreased in wheat straw combined with N fertilizer after 210 days.At the early stage of treatment,wheat straw and it combined with N fertilizer treatments significantly increased the content of readily oxidized organic carbon,and increasing with the increased of N fertilizer rates.After 210 days,there was no markedly effect on readily oxidized organic carbon content.Wheat straw and it co-application with N fertilizer increased the activities ofβ-glucosidase,sucrase,cellulase,alkaline phosphatase and dehydrogenase in soil.The cellulase activities showed an increasing trend with the increase of N rates.After 150 days of treatment with S,SN1,SN2 and SN3,soil urease activities were increased,as compared with CK.Wheat straw combined with N fertilizer treatment significantly increased soil available nitrogen and available potassium contents,but decreased soil available phosphorus contents.Compared with CK,S treatment increased soil Cl^-content,wheat straw combined with N fertilizer significantly reduced soil p H values,but increased soil electrical conductivity and Cl^-contents.（3）Wheat straw addition increased Ace and Chao1 indexes,while wheat straw combined with N fertilizer had no effect on these indexes.The results showed that straw returning increased the richness of soil bacterial community,while the wheat straw combined with N fertilizer could decreased the richness of bacterial community.Wheat straw and it combined with N fertilizer significantly changed soil bacterial community structure,while no significantly difference among N levels.Compared with CK,S treatment increased the relative abundance of Firmicutes and Prteobacteria in phlum.The wheat straw combined with N fertilizer improved the relative abundance of Prteobacteria,Chloroflexi,Actinobacteriota,Chloroflexi and Gemmatimonadota,while reduced the relative abundance of Acidobacteriota and Bacteroidaota.Acidobacteria showed a trend of decreasing with the increased of N rates.Compared with S,wheat straw combined with N fertilizer decreased the relative abundance of Acidobacteria,especially high N treatment,while increased the relative abundance of Actinobacteriota,Chloroflexi and Gemmatimonadota.Compared with CK,S treatment increased the functional relative abundance of carbon fixation pathways in prokaryotes.Wheat straw combined with N fertilizer increased the relative abundance of functional pathways related to glucose metabolism and amino acid metabolism.Compared with S,wheat straw combined with N fertilizer increased the functional genes of amino acid metabolic pathways.According to comprehensive analysis,wheat straw combined with N fertilizer improved soil bio-chemical properties,increased soil organic carbon contents,enzyme activities and nutrient contents,and increased the relative abundance of soil Proteobacteria and soil bacterial functional metabolism,while no significant difference among different concentrations of N.Therefore,wheat straw returning(6 t·hm^-2)combined with N fertilizer(200 mg·kg^-1N)can effectively reduce the mineralization loss of soil total organic carbon,reduce the loss of N fertilizer,and improve the efficiency of soil carbon sequeeration in apple root zone.