Microbiological Mechanism Of Nitrogen High Efficient Utilization For Maize Intercropped With Green Manure Under Reduced Chemical Nitrogen Application

Reducing chemical nitrogen application while ensuring crop yield is a major issue in modern agricultural production.Improving nitrogen use efficiency of crop is the key.A field experiment was conducted at Hexi oasis irrigation area from 2019 to 2021.Split plot design was used for the experiment.Three cropping patterns were main plots,including maize-common vetch intercropping（M||V）,maize-rape intercropping（M||R）,and sole maize（M）.Subplots included three nitrogen levels,i.e.,zero nitrogen application（N0）,local conventional nitrogen application reduced by 25%(N1:270 kg hm^-2),and local conventional nitrogen application(N2:360 kg hm^-2).This study focused on the effects of intercropping green manure on maize yield and nitrogen use characteristics under reduced chemical nitrogen application,and monitored the characteristics of soil gaseous nitrogen loss and the response of soil key factors to maize-green manure intercropping combined with nitrogen reduction,in order to reveal the regulation mechanism of nitrogen high efficient utilization of maize by intercropping green manure combined with nitrogen reduction.The aim was to provide a theoretical and practical base for higher efficient technology of nitrogen reduction in maize.The main results are as follows:（1）Intercropping green manure could compensate for maize yield loss caused by25%nitrogen reduction,which was beneficial to improve the nitrogen use efficiency of maize.Reduced nitrogen application decreased grain yield and biomass of sole maize by 9.9%-22.0%and 7.7%-13.9%,respectively,compared with N2.There was no difference between N1 and N2 in maize-green manure intercropping systems.The compensation effect of intercropping green manure on maize grain yield increased with the extension of location years.N1 increased nitrogen use efficiency and nitrogen partial productivity of maize by 18.5%-20.4%and 25.7%-29.1%,respectively,compared with N2.Intercropping green manure combined with nitrogen reduction further improved nitrogen use efficiency,nitrogen agronomic efficiency,and nitrogen partial productivity of maize.Compared with MN2,M||VN1 was increased by 17.7%-31.3%,15.2%-21.6%,and 32.3%-36.2%,M||RN1 was increased by 14.8%-25.1%,21.5%-25.4%,and 34.6%-39.8%,respectively.（2）Intercropping green manure combined with reduced chemical nitrogen application decreased soil gaseous nitrogen loss in maize fields.During the whole stage,soil NH₃ volatilization and N₂O emissions under the MN2 treatment were 15.7-16.9 kg hm^-2 and 1.34-1.56 kg hm^-2,respectively.Compared with MN2,the two indexes of M||VN1 were decreased by 44.0%-45.0%and 34.4%-41.5%,the two indexes of M||RN1 were decreased by 46.4%-47.3%and 27.8%-37.1%,respectively.Under reducing nitrogen application,intercropping common vetch was more conducive to soil N₂O emission reduction in maize farmlands,M||VN1 was lower 7.1%-9.1%than M||RN1.（3）Under nitrogen reduction,intercropping green manure could ensure nitrogen supply and reduce the risk of deep nitrogen leaching during the maize growth stage,and M||V was the most prominent.Intercropping green manure increased the average nitrate nitrogen and ammonium nitrogen contents at the 0-110 cm soil layer of the maize strip before green manure clipping,but decreased it after green manure clipping and there was not different between intercropped and sole maize.With the extension of years of positioning experiment,intercropping green manure combined with reduced nitrogen application increased soil nitrate nitrogen and ammonium nitrogen contents in the topsoil layer of the maize strip,but decreased it below the 50 cm soil layer.Intercropping common vetch was more beneficial to increase soil nitrogen content of the green manure strip in the 0-50 cm soil layer under the nitrogen reduction.（4）Under nitrogen reduction,intercropping green manure could maintain the higher soil microbial biomass nitrogen and urease activity in maize farmlands,but decreased the activities of nitrate reductase and nitrite reductase in soils.M||V was the most significant.Intercropping green manure combined with reduced nitrogen application decreased soil urease activity at the green manure flowering stage,M||VN1and M||RN1 were reduced by 6.2%-20.1%and 16.0%-24.8%,respectively,compared with MN2.There was no significant difference on the activities of nitrate reductase and nitrite reductase in soils among M||VN1,M||RN1,and MN2.Before the crops sowing,the maize silking stage,and after the maize harvesting,intercropping green manure combined with reduced nitrogen application had no effect on soil microbial biomass nitrogen and urease activity,but reduced nitrate reductase and nitrite reductase activities.At the maize silking stage and after harvesting,M||VN1 was lower soil nitrite reductase activity by 7.3%-11.5%and 7.0%-10.2%than M||RN1,respectively,which was beneficial to soil N₂O emission reduction.（5）Under nitrogen reduction,intercropping green manure could change the composition of denitrifying bacteria community in maize fields,which was beneficial to inhibit soil ammoxidation,weaken nitrite reduction process,and reduce soil gaseous nitrogen generation.M||V was more prominent.Compared with MN2,intercropping green manure combined with reduced chemical nitrogen application decreased the gene abundance of nitrogen cycle functional microorganism in the maize strip,but increased the Chao1 indices.Among them,the Chao1 indices of nir K,nir S,and nos Z genes under M||VN1 in the maize strip were increased by 8.2%,29.7%,and 19.0%,respectively,compared with those of M||RN1.Under nitrogen reduction,M||V was more conducive to the decrease of gene abundance of nitrogen cycle functional microorganism and the increase of Chao1 index in the green manure strip.The dominant phylum of AOA was Thaumarchaeota,and that of AOB and denitrifying bacteria were Proteobacteria.At the genus level,intercropping green manure combined with nitrogen reduction significantly affected the community structure of denitrifying bacteria,and had no effect on AOA and AOB.In conclusion,intercropping green manure combined with reduced chemical nitrogen application improved nitrogen uptake and utilization of maize and reduced soil gaseous nitrogen loss in maize fields,which could be considered as an efficient maize production technology in oasis irrigation area.Intercropping maize with common vetch integrated 25%chemical nitrogen reduction was the optimum practice.