Effects Of Nitrogen Reduction On Yield Stability And Gene Structure Of Nitrogen Cycling Function In Sugarcane//Soybean Intercropping System

This study was conducted at the experimental center of South China Agricultural University in Guangzhou,China,to study the effects of reduced nitrogen application and intercropping of soybean on crop yield,sugarcane quality and soil fertility of the system through an 11-years（2009-2019）field positioning experiment.Using isotope labeling and DNA-SIP method,as well as q PCR and Illumina Miseq high-throughput sequencing technologies,three nitrogen levels was studied by pot experiment(0,reduced rate 300 kg·hm^-2and conventional rate 525 kg·hm^-2)on sugarcane//soybean intercropping system transfer nitrogen,nitrogen cycle six core functional genes（nos Z,chi A,amo A,nir S,nir K and nif H）influence of community structure,the influence of designed to analyze the yield stability of sugarcane//soybean intercropping system,clarify different nitrogen levels in the soil nitrogen cycle core function effect of microbial community diversity.The main conclusions are as follows:1.The results of field experiments show that the yield of sugarcane,soybean and the total system under different treatment showed obvious annual dynamic changes and was significantly affected by different years and planting patterns.Nitrogen application level did not affect the yield of sugarcane,soybean and the system yield significantly.The LER of all intercropping systems was greater than 1,and the intercropping pattern improved the land utilization rate among all treatments.There was no significant difference in W²and SYI for sugarcane yield,but the CV%of MS-N1 was significantly lower than that of MS-N2.Meanwhile,the W²value of monocropping soybean was significantly higher than that of intercropping pattern,and the yield stability of monocropping soybean was lower than that of intercropping soybean.The CV%and SYI values of soybean yield showed that nitrogen application in intercropping mode had no significant effect on the stability of soybean yield,and SB2-N1 had planting advantages.There was no significant difference in W²value of total system yield under different treatments,indicating that the intercropping and nitrogen application of sugarcane//soybean did not affect the stability of system yield.The sucrose content and gravity purity of sugarcane juice was decreased with the increase of cultivation years,and the sucrore content and fiber content of bagasse,the sugar brix,sugar pol,apparent purity of sugarcane juice was not influenced by cropping year.In addition,the quality of sugarcane was not influenced by different cropping patterns and nitrogen rates in the same year,which indicated that sugarcane quality was stable under different treatments.From 2009 to 2019,soil p H,organic matter and total nitrogen were decreased significantly,but there was no significant difference in total phosphorus,available phosphorus and available potassium in all treatments except for the soybean monoculture treatment.Sugarcane continuous cultivation and continuous application of chemical fertilizer led to the decrease of soil organic matter and p H value.Reducing nitrogen application and intercropping soybean are sustainable and green production models for efficient utilization of resources and stable system yield in sugarcane producing areas in Guangdong,China,but organic fertilizer application and promotion of sugarcane leaf returning technology are needed to improve sugarcane field fertility.2.The results of pot experiment showed that the biomass of sugarcane root treated with conventional nitrogen application was significantly higher than that without nitrogen application,and there was no significant difference between conventional nitrogen application and reduced nitrogen application.The dry weight of soybean root nodules treated with nitrogen was significantly lower than that without nitrogen.The nitrogen uptake of sugarcane and soybean plants without nitrogen application was significantly lower than that of nitrogen application,indicating that nitrogen application could promote the nitrogen absorption of sugarcane and soybean plants.The nitrogen fixation rate,nitrogen transfer rate and nitrogen transfer rate were 88.90%,1.65%and 819.80 mg/kg respectively,which were significantly higher than that of no nitrogen application and conventional nitrogen application.The results showed that some nitrogen was transferred from soybean to sugarcane in intercropping system to meet the demand of sugarcane for nitrogen,and the reduced application of nitrogen effectively promoted soybean to fix more nitrogen to meet the demand of sugarcane for nitrogen.3.（1）The abundance of chi A gene community in sugarcane rhizosphere soil was significantly decreased by conventional nitrogen application.Nitrogen application significantly reduced the amo A gene community richness of sugarcane rhizosphere soil.Reduced nitrogen application significantly increased the nir S gene community richness of sugarcane rhizosphere soil.Nitrogen application significantly increased the gene community richness of nir K and nos Z in soybean rhizosphere soil.Nitrogen application significantly reduced the diversity of amo A gene community of sugarcane rhizosphere soil and significantly increased the diversity of nif H,nir K and nos Z gene community of soybean rhizosphere soil.（2）Nitrogen application reduced the number of nif H,chi A,amo A,nir K,nir S and nos Z gene communities of sugarcane rhizosphere soil and the number of amo A gene communities of soybean rhizosphere soil,and increased the number of nif H,chi A,nir K,nir S and nos Z gene communities of soybean rhizosphere soil.（3）The gene community structure and relative abundance of six core functional genes in sugarcane and soybean rhizosphere soils were significantly affected by different nitrogen application.（4）Conventional nitrogen application significantly affected the gene community structure of chi A,amo A,nir K and nos Z in sugarcane rhizosphere soil,and nitrogen application significantly affected the gene community structure of nif H and nir S in sugarcane rhizosphere soil.Different nitrogen application levels significantly affected the gene community structure of nif H,amo A and nos Z in soybean rhizosphere soil,while conventional nitrogen application significantly affected the gene community structure of chi A gene in soybean rhizosphere soil,while nitrogen application significantly affected the gene community structure of nir K and nir S in soybean rhizosphere soil.（5）Nitrogen application significantly affected the species abundance of sugarcane rhizosphere soils nif H,chi A,amo A,nir K and nir S gene communities,while conventional nitrogen application significantly reduced the species abundance of sugarcane rhizosphere soils nif H,chi A,amo A and nir S gene communities.Different nitrogen application levels significantly affected the species abundance in the gene communities of six core functional genes of soybean rhizosphere soil.Compared with conventional nitrogen application,the different bacterial flora and species abundance under reduced nitrogen application were more and higher.（6）The contents of SOM,TN,TP,AP and AK had significant effects on the species distribution of sugarcane rhizosphere soil nif H gene community.Soil p H,AN,NO₄⁺-N and NO₃^--N had significant effects on chi A gene community structure in intercropping rhizosphere soil.Soil p H,AN and NO₃^--N had significant effects on the distribution of amo A gene in intercropping rhizosphere soil.Soil p H,AN,NO₃^--N and NO₄⁺-N contents had significant effects on the community structure of nir K gene in sugarcane rhizosphere soil.Soil p H,SOM,TP,AN,NO₄⁺-N and NO₃^--N significantly affected the community structure composition of nir S gene in intercropping rhizosphere soil.Soil p H,NO₃^--N and AN had significant effects on nos Z gene community structure in intercropping rhizosphere soil.Compared with no nitrogen application and reduced nitrogen application,conventional nitrogen application significantly affected the intensity of nitrogen fixation and denitrification of rhizosphere soil,which may inhibit the ammoniation process of intercropping system rhizosphere soil microbial community.In summary,reduced nitrogen application and conventional nitrogen application had no significant effect on the total yield of sugarcane,soybean and intercropping system,and the nitrogen application amount of 300 kg·hm^-2could meet the nitrogen demand of sugarcane,soybean and intercropping system.There was no significant difference in the effects of different planting modes and nitrogen application on sugarcane quality.Intercropping and nitrogen reduction can maintain the time stability of system yield and are a sustainable production mode.Different nitrogen application levels had significant effects on the structural composition of nif H,chi A,amo A,nir K,nir S and nos Z gene communities of sugarcane and soybean rhizosphere soils involved in nitrogen cycle.Compared with no nitrogen application and reduced nitrogen application,conventional nitrogen application significantly affected community structure and species abundance.Soil nutrient content significantly affected the distribution of functional gene community.