Effects Of Nitrogen Application Level On Spatial Microbial Composition Of Root System And The Influence Mechanism Of Nitrogen Utilization Of Silage Maize

Aiming at the inefficient utilization of silage corn nitrogen,the low biological yield,and the lack of clarity of the influence mechanism of nitrogen application level on soil biological characteristics and the composition of root spatial microbiomes in central Inner Mongolia,six nitrogen application treatments(N0,N8,N12,N16,N20,N24,0,120,180,240,300,360 kg N hm^-2)were studied from 2018 to 2019 with"Zhongxing Qingsi 1"as the test material.The differences of soil biological characteristics,plant yield and quality formation,and nitrogen transfer and utilization in silage maize field were analyzed.At the same time,high-throughput sequencing and bioinformatics were used to explore the response of bacterial and fungal communities in bulk soil,rhizosphere soil and endosphere of silage maize to nitrogen application rate,in order to reveal the nitrogen control mechanism between soil-silage maize,to clarify the optimal nitrogen application and economical nitrogen utilization efficiency of silage corn under drip irrigation conditions,and to provide theoretical basis and practical basis for the high and stable yield of silage maize in the middle mature area of Inner Mongolia.The main results are as follows:(1)The application of nitrogen fertilizer increased soil enzyme activity and microbial biomass in silage maize field,which effectively promoted soil nitrogen decomposition and microbial carbon and nitrogen fixation.With the advance of growth period,the soil enzyme activity and microbial biomass of silage maize field increased first and then decreased.The maximum value was distributed in the trumpet period–tasseling stage.Compared with 0?20cm soil layer,the deeper soil layer increased the catalase activity and decreased the urease activity and microbial biomass carbon,microbial biomass nitrogen content.N16 treatment can effectively promote the increase of soil enzyme activity and microbial biomas,excessive nitrogen application inhibited the soil enzyme activity and microbial growth and reproduction.(2)Nitrogen application significantly affected microbial abundance,diversity and community composition in different root spatial of silage maize,and the fungal community was the most sensitive microbial community to nitrogen fertilizer.With the spatial structure closer to the root,the abundance of microbial community and the diversity of bacterial community were lower,and the influence of nitrogen fertilizer on the composition of bacterial and fungal community gradually weakened with the spatial location moving inward.However,compared with the bacterial community in the bulk soil and rhizosphere soil,the bacterial community in the root was more closely linked,and the competition among communities was weakened.In this study,26.95%and 22.70%of bacteria and fungi communities were detected to be enriched under N16treatment.Under the condition of short-term application of nitrogen fertilizer,it was helpful to strengthen the connection between microorganisms and improve the adaptability to the environment,while higher nitrogen application level would weaken this connection when the nitrogen application rate was 240 kg N hm^-2.(3)The application of nitrogen fertilizer could increase the plant height,fresh weight and dry matter quality of silage maize from jointing stage to harvest stage,increase the dry matter accumulation rate of silage maize,and delay the rapid growth period.The maximum growth rate of dry matter accumulation in 2018 and 2019 was25.45%?135.91%and 22.51%?59.16%higher than that in the control,respectively.In 2018,the maximum and average growth rates of N16,N20 and N24 were 4.71?5.92g·d^-1·plant^-1 and 4.13?5.19 g·d^-1·plant^-1,respectively;in 2019,the maximum and average growth rates of N12,N16 and N20 were 4.35?5.43 g·d^-1·plant^-1 and 4.96?6.20 g·d^-1·plant^-1,respectively.From seedling stage to trumpet period,the distribution ratio of stem in dry matter was not affected by nitrogen application rate.From tasseling stage to harvest stage,N16 and N20 treatments were more conducive to the transportation of dry matter accumulated in stem and leaf to reproductive organs.In the harvest period,the biological fresh weight and biological yield of silage maize were the highest in N16 treatment.In 2018 and 2019,the biological fresh weight were 91.63t·hm^-2 and 79.06 t·hm^-2 respectively,and the biological yield were 38.75 t·hm^-2 and31.96 t·hm^-2 respectively.(4)The application of nitrogen fertilizer improved the photosynthetic index of silage maize to different extent,which was helpful to the accumulation of photosynthetic products.With the development of growth stage,leaf area index,chlorophyll content,net photosynthetic rate,transpiration rate and stomatal conductance were showed as the trend of male tasseling stage>trumpet period>jointing stage>seedling stage.With the increase of nitrogen application,the concentration of intercellular CO₂ decreased first and then increased,while the other photosynthetic indexes increased first and then decreased,and the maximum value was mainly distributed in N16 treatment.In the period of vigorous growth of silage maize,the net photosynthetic rate of N16 treatment was the highest in 2018 and 2019,which was 36.30 and 39.31?mol·m^-2·s^-1,respectively.The fresh weight and dry matter quality of silage maize are closely related to photosynthetic index,among which leaf area index is an important factor affecting the production of photosynthetic dry matter.(5)The application of nitrogen fertilizer can improve the nutritional quality of silage maize.The contents of CP,EE,starch and RFV increased with the increase of nitrogen application rate in the range of 0?240 kg N hm^-2 but when the nitrogen application rate was higher than 240 kg N hm^-2,the quality indexes decreased gradually with the increase of nitrogen application rate.The contents of ADF and NDF increased with the application of nitrogen.Fresh weight and biomass of silage maize were positively correlated with CP,EE,starch content and RFV(P<0.05),and negatively correlated with ADF and NDF.(6)Nitrogen application significantly increased the nitrogen accumulation of silage maize,and the appropriate amount of nitrogen application was helpful to plant nitrogen accumulation and soil nitrogen balance of silage maize field,reducing nitrogen loss.The soil nitrogen content of silage maize field was the highest at the trumpet period.From jointing stage to harvest stage,the nitrogen application rate of240?300 kg N hm^-2 could meet the nitrogen demand of silage maize plants.Excessive nitrogen application would reduce the nitrogen accumulation of plants and make nitrogen redundant in the soil.Nitrogen accumulation of silage maize increased gradually with the extension of growth period under different nitrogen application levels,and showed a quadratic positive correlation with growth days,in which N16and N20 treatments showed a nearly linear increasing trend.Under N16 treatment,the regression equation between nitrogen accumulation(y)and growth days(x)was y_N16=0.0125x²+1.1980x–12.1476.Agronomic nitrogen use efficiency of N16 treatment was the highest in 2018,which was 73.68%.In 2019,agronomic nitrogen use efficiency of N8,N12 and N16 treatment was higher than that of other high nitrogen treatment,and there was no significant difference between them.Therefore,N16treatment can use nitrogen fertilizer more effectively,and excessive nitrogen application will cause nitrogen loss.In conclusion,two years of continuous nitrogen localization experiment showed that nitrogen application affected the net photosynthetic rate of silage maize by changing soil enzyme activity,microbial biomass and plant nitrogen accumulation,and further changed the whole plant fresh weight,which affected the yield and feeding quality of silage maize.When N application rate was 240 kg N hm^-2,soil microbial biomass and enzyme activities were at a higher level,which was more conducive to carbon and nitrogen fixation and nitrogen accumulation in plants,thus improving the photosynthetic performance,yield and nutrient quality of silage maize,and improving the nitrogen use efficiency of silage corn.Therefore,under drip irrigation in central Inner Mongolia,the optimal nitrogen application level of silage maize was 240 kg N hm^-2.