Effects Of Nitrogen Reduction By Soybean-maize Rotation On Soil Water-stable Aggregates,Nitrogen-fixing Microorganisms And Nitrogen Efficiency Of Maize

In view of the problems such as single planting structure of maize,excessive chemical fertilizer and low utilization efficiency,resulting in soil compaction,fertility degradation and optimization of grain and oil crops layout.The experiment was conducted in the Agricultural Science and Technology Park of Arong Banner,Hulunbeier,Inner Mongolia from 2018 to 2021.Maize variety A6565 and soybean variety Mengdou 30 were used as test materials.The split plot design was used.The main treatment was soybean-maize rotation（SM）and maize continuous cropping（control,MM）,and the secondary treatment was conventional nitrogen application(N_100%)and reduced nitrogen application(conventional 70%,N_70%).The physical properties and nitrogen distribution of soil aggregates,soil nitrogen-fixing bacteria community and diversity,and maize yield of spring maize under soybean-maize rotation were investigated under two rotation cycles.The effects of soybean-maize rotation on nitrogen use efficiency of spring maize were clarified.The main results were as follows:1.Soybean-maize rotation promotes soil aggregate formation.Compared with maize continuous cropping,soybean-maize rotation could significantly increase the content of water-stable aggregates of>5 mm,5～3 mm,3～2 mm and 2～1 mm,and increase the content of organic carbon and total nitrogen of aggregates of>5 mm,1～0.5 mm,0.5 mm～0.25 mm.The contribution rate of organic carbon and total nitrogen in soil aggregates（>1 mm）was increased,especially in 0～20 cm soil layer.The correlation analysis of aggregate particle size content with soil total nitrogen,organic carbon and aggregate stability index showed that aggregate particle size content greater than 1 mm was positively correlated with MWD,GMD,WR_0.25,total nitrogen and organic carbon,and negatively correlated with D and E_LT.Total nitrogen and organic carbon were positively correlated with MWD,GMD and WR_0.25,and negatively correlated with D and E_LT.Total nitrogen was positively correlated with organic carbon.2.Soybean-maize rotation improved soil nitrogen nutrients.Compared with continuous cropping of maize,soybean-maize rotation significantly increased the contents of organic carbon,total nitrogen,alkali-hydrolyzable nitrogen,ammonium nitrogen and nitrate nitrogen in each soil layer of T1 and T2 cycles at N_100%and N_70%;under different nitrogen application treatments,soil organic carbon and ammonium nitrogen in 0～20 cm soil layer,total nitrogen and alkali-hydrolyzable nitrogen in 0～60 cm soil layer and nitrate nitrogen in 0～40 cm soil layer under soybean-maize rotation and maize continuous cropping were N_100%significantly higher than N_70%.3.Soybean-maize rotation improved soil microbial diversity.Chao1 index and ACE index of nitrogen-fixing bacteria in 0～20 cm and 20～40 cm soil layers were soybean-maize rotation>maize continuous cropping,N_100%>N_70%;the Shannon index of N_100%increased significantly under soybean-maize rotation in 0～20 cm soil layer.The Shannon and Simpson index of soybean-maize rotation in 20～40 cm soil layer were significantly higher than that of maize continuous cropping.There was no significant difference between N_100%and N_70%under soybean-maize rotation,and N_100%was significantly higher than N_70%under maize continuous cropping.The dominant phyla and genera of soil nitrogen fixation under continuous cropping and rotation were similar,and the dominant phyla and genera of soil nitrogen fixation under continuous cropping and rotation were dominated by Rhizobium.Compared with continuous cropping of maize,the relative abundances of anaerobic Myxomycetes,Geomycetes,Nitrogen-fixing spironobacteria,Desulfurization bacteria and Nitrogen-fixing Vibrio in0～20 cm soil layer under soybean-maize rotation at N_100%and N_70%increased.The relative abundance of Rhizobium,Anaeromyxa,Geobacterium,Dechlorococcus,Vibrio azotobacter and Frankia increased in 20～40 cm soil layer.4.Soybean-maize rotation increased maize yield and nitrogen use efficiency.Compared with continuous cropping of maize,soybean-maize rotation significantly increased the grain number per spike,1000-grain weight and yield of spring maize;under the T1 cycle,the grain yields of N_100%and N_70%increased by 3.10%and 3.14%compared with the continuous cropping,respectively,and the grain yields of T2 cycle increased by 12.89%and 6.70%,respectively.The dry matter accumulation of soybean-maize rotation after silking was significantly higher than that of maize continuous cropping.The dry matter and nitrogen nutrient translocation,translocation rate and contribution rate of translocation to grain increased,and the nitrogen absorption efficiency and nitrogen partial productivity increased.Reduced nitrogen application had significant effect on nitrogen use efficiency of spring maize organs.Soybean-maize rotation could significantly improve nitrogen harvest index and nitrogen use efficiency in T2 cycle,and N_70%was significantly higher than N_100%.In conclusion,soybean-maize rotation can improve soil carbon and nitrogen nutrients,improve soil aggregate structure and stability,optimize soil nitrogen-fixing bacteria community structure and diversity,increase spring maize yield and nitrogen use efficiency.