Effects Of Cover Cropping And Nitrogen Fertilization On Soil Physico-chemical Properties And Aggregate-associated Carbon And Nitrogen Contents In A Dryland Winter Wheat Field

Cover cropping during summer fallow period can provide soil organic matter and improve soil health,but its effect on on soil physical properties,carbon and nitrogen components and soil aggregates are not clear.In this study,a 4-year field experiment was conducted to study the interaction of cover crop and N addition on soil water-related properties and aggregate-associated carbon and nitrogen contents.Three cover crops as soybean（SB）,Sudan grass（SG）,soybean+Sudan grass mixture（SS）,and no cover crop（CK）were included during the summer fallow period,followed by N application to winter wheat at rates of 0,60,and 120 kg N ha^-1.Soil bulk density,porosity,water holding capacity,saturated infiltration coefficient,soil organic carbon（SOC）,soil total nitrogen（STN）,particulate organic carbon（POC）,particulate organic nitrogen（PON）,C/N ratio（C/N）,soil aggregate composition,mean weight diameter（MWD）,aggregate-associated carbon and nitrogen contents and their contribution rates were measured.The main results are as follows:（1）Soil water-related physical properties varied with cover crops and N rates at 0-10 cm soil layer.Compared with CK,three green manure significantly reduced soil bulk density,increased capillary porosity,total porosity and water holding capacity,and SB and SS significantly improved physical properties than SG.Compared with no N addition,N addition significantly increased soil surface bulk density and water holding capacity.Low N rate significantly increased soil saturated infiltration coefficient,while high N rate significantly reduced capillary porosity and total porosity.Furthermore,soil bulk density was negatively correlated with porosity,water holding capacity and saturated infiltration coefficient.Soil porosity was positively correlated with water holding capacity.Soil saturated infiltration coefficient was positively correlated with capillary porosity,total porosity and water holding capacity.Overall,no nitrogen addition combined with SS was better to enhance soil water-related physical properties.（2）The SOC,STN,POC,and PON at 0-40 cm soil layer were affected by the interaction of green manure and N rate,but soil C/N was significant only only at 0-20 cm.Compared with CK,SB and SS significantly increased the contents of SOC,STN,POC,and PON at 0-10 cm,but only SB significantly increased C/N at 0-10 cm.Compared with no N addition,N addition significantly increased SOC,STN,POC,and PON contents in soil surface,and the effect of higher N rate on improving SOC and POC contents was significantly better than lower N rate.Overall,higher N rate combined with SB was beneficial to improve SOC,POC,and C/N,while higher N rate combined with SS could help to increase STN and PON.（3）Soil aggregates composition,MWD,aggregate-associated SOC,STN,POC,PON and their contribution rates at 0-40 cm depths varied with the interaction of green manure and N rates.Compared with CK,planting green manure significantly increased the proportion of>5mm aggregates,MWD,SOC and POC contents of>5 mm aggregates and their contribution rate,and STN contribution rate of>5 mm aggregates at 0-10 cm.Only SG and SB increased STN and PON content of>5 mm aggregates Compared with no N addition,higher N rate significantly increased the proportion of>5 mm aggregates,and nitrogen addition significantly increased MWD and SOC,STN,POC and PON contents and their contribution rate of>5 mm aggregates in the soil surface.In addition,the contents of SOC,STN,POC and PON in>0.25mm aggregates were the highest.The contents of carbon and nitrogen in both2-5 mm and<0.25 mm were negatively correlated with corresponding aggregate composition,while<0.25mm was positively correlated.Overall,higher·N rate combined with SB can enhance soil aggregate structure and increase soil aggregate-associated carbon and nitrogen contents.In conclusion,including cover crops during summer fallow and N fertilization had important impacts on soil water-related physical properties,carbon and nitrogen contents and soil aggregation in the Loess Plateau,especially for planting Changwu soybean or mixture of soybean and Sudan grass.