Effects Of Straw Returning Under Maize-soybean Rotation On Key Enzyme Activities Of Mollisols

China is a large agricultural country and produces a large amount of various types of crop straws annually.As one of the four contiguous Black soil region in the world,the Northeast region is an important grain production base in China.The total amount of straw produced each year accounts for a large proportion of the country,but the current straw utilization rate is low and the utilization method is not reasonable,which not only puts pressure on the ecological environment,but also wastes a lot of precious resources.Straw,as the carrier of energy and nutrients,is rich in carbon,nitrogen,phosphorus,potassium,and micronutrients,which is essential for maintaining or enhancing soil fertility and stabilizing grain output.Although a lot of meticulous work on the theme of returning straw to the field have been carried out,the efficacy of continuous straw returning to the field requires further understanding from a systematic perspective under crop rotation cycle.This study used the established corn-soybean rotation with different fertilization experiments,and selected 3 of the 11 treatments,namely normal soybean fertilization and normal corn fertilization(CF);soybeans without chemical fertilizer+straw return to the field,normal corn fertilization+straw return to the field(R);no fertilizer applied to soybeans,and no fertilizer applied to corn(NOF).The core treatment was that all straws were crushed and returned to the field after corn and soybean harvest in each rotation cycle.In the 6th and 7th year of the rotation system,the key enzyme activities at different depths of the soil during growing season and nutrient changes in the rhizosphere during the key growing period were analysed,and the mechanism of straw returning on crop productivity was highlighted from the changes in soil enzyme activities under rotation system.The main research results were as follows:(1)Returning straw to the field increased the soil enzyme activities during the reproductive stages of soybean.Among them,the soil urease activities in the 0-30 cm soil layer at the R3,R6 and R8 stage,the soil phosphatase activities in the 0-10 cm soil layer at the R3,R6 and R8 stage and in the 20-30 cm soil layer during the 4 reproductive stages,and the soil invertase activities the 0-30 cm soil layer at the R1,R3 and R8 stages all increased significantly.(2)Returning straw to the field increased soil enzyme activities at different growth stages of maize.Among them,soil urease activities and soil invertase activities in the 0-20 cm soil layer at the 4 growth stages,and soil urease activities the 20-30 cm soil layer at milk and mature stages,and soil invertase activities at the grain-filling and mature stage,and soil phosphatase activity at the jointing stage all increased significantly.(3)Returning straw to the field enhanced the phosphatase and invertase activities in the rhizosphere soil at R6 of soybean,and the urease and invertase activities in the rhizosphere soil at milk stage of maize.The total carbon,total nitrogen,ammonium nitrogen,nitrate nitrogen and available potassium contents in the rhizosphere soil of soybean and corn were spontaneously increased.(4)Compared with normal fertilizer application,returning straw to the field had no effect on soybean yield,but significantly increased corn yield by 16.7%,mainly due to the increase in the grain number per ear and grain size.(5)In non-rhizosphere soil,phosphatase activity was positively correlated with carbon-to-nitrogen ratio,ammonia nitrogen,nitrate nitrogen and available potassium(P<0.01),and negatively correlated with total nitrogen(P<0.01)but not correlated with total carbon and available phosphorus.Urease activity had a very significant and positive correlation with carbon-to-nitrogen ratio,ammonia nitrogen,nitrate nitrogen,available potassium and available phosphorus(P<0.01),but not significantly correlated with total carbon and total nitrogen.Invertase activity was positively correlated with total nitrogen,total carbon,available phosphorus and available potassium(P<0.05),and extremely negatively correlated with ammonium nitrogen(P<0.01),but not significantly correlated with carbon-to-nitrogen ratio and nitrate nitrogen.