Long-term Inorganic N Impact On Nitrogen Uptake And Translocation Of Spring Wheat And Soil Microorganisms In The Semi-arid Loess Plateau

In order to explore the effects and mechanisms of long-term application of inorganic nitrogen fertilizers on nitrogen uptake,translocation and nitrogen utilization efficiency of wheat in the semi-arid region of the Loess Plateau,this study relied on the long-term nitrogen(N)fertilizer experoment started in 2003 to study the effect of urea application on spring wheat yield,nitrogen uptake,translocation,and resorption,soil nitrogen residues,and main soil microorganisms.Treatments included a non-N-fertilized control and annual application of52.5,105.0,157.5,and 210.0 kg N ha^-1in the first two years(2003 and 2004).In the third year(2005),the four main plots with N fertilizer application were split.In one subplot,N fertilization was continued as mentioned previously,while in the other subplot,N fertilization was stopped and defined as its corresponding sub-treatments N52.5c,N105c,N157.5c,N210c.The main results were as follows:(1)Application of inorganic nitrogen fertilizer significantly increased the biological yield and grain yield of spring wheat in the semi-arid area of the Loess Plateau.Higher nitrogen fertilizer application rate ensured higher dry matter accumulation and grain yield,but it is not that the higher the N application rate,the better.When the application rate of inorganic N was about 105 kg N hm^-2,the grain yield(3062.7 kg hm^-2)and nitrogen fertilizer use efficiency(16.40%)of spring wheat were both higher,continues increase in the amount of N fertilizer did not significantly increase grain yield.Compared with non-N-fertilized control,the grain yield of 105 kg N hm^-2 treatment increased by 46.90%,and nitrogen absorption increased by 90.54%.(2)With the increase of inorganic nitrogen application rates,the NO₃-N content of the0-200 cm soil layer increased significantly,and the soil p H value dropped from 8.99 to 8.70.Compared with sowing,the soil NO₃-N content of high nitrogen treatments(N157.5 and N210)showed a significant excess at harvest,with a growth rate of 6.94–76.23%,and mainly occurred at 10–110 cm and 170–200 cm soil layer.Wheat grain yield in the third year(2005)was significantly,greater(22.57–59.53%)than the unfertilized treatment after the N application was stopped.There was a positive relationship of soil NO₃-N in the 0–50cm and 50–110 cm soil layers at sowing with grain N content and yield.The residual effect of nitrogen fertilizer increased with the increase in the amount of nitrogen fertilizer applied in the previous period,but decreased with the extension of time.(3)Long-term application of inorganic N fertilizer to wheat farmland in the semi-arid area of the Loess Plateau,the main soil bacteria were,Proteobacteria,Actinobacteria,Acidobacteria,Chloroflexi,Gemmatimonadetes,Bacteroidetes,Nitrospirae,Verrucomicrobia and Planctomycetes.The abundance of their communities was closely related to soil physical and chemical properties,NH₄-N content was the most important factor.With an increase in N fertilizer application,the total number of ammonia oxidizing archaea(AOA)communities decreased from 4.88×10⁷ to 1.05×10⁷ parts of g^-1 dry soil,while the abundance of ammonia oxidizing bacteria(AOB)increased from 3.63×10⁷ to8.24×10⁷ copy number g^-1 dry soil,the ratio of AOA to AOB decreased from 1.34 to 0.16.Nitrogen application rates significantly affects the diversity and community composition of soil AOB,but had no significant effect on AOA.Nearly 100%of the AOA gene sequence belongs to the Thaumarchaeota,and 100%of the AOB gene sequence belongs to the Proteobacteria.Soil moisture was the main soil factor that affected the AOA community(contribution rate is 72.60%),while the soil NH₄-N was the soil factor that affected the AOB community(contribution rate is 72.00%).The higher grain yield of wheat was attributed to the higher abundance of bacterial microorganisms of the Gammaproteobacteria,Sphingobacteriia and the AOB genus Nitrosospira,and the lower abundance of bacterial microorganisms of class Acidobacteria and Chloroflexia.(4)Nitrogen use effeciency decreased with the increase in nitrogen fertilizer application,and was significantly correlated with nitrogen harvest index and grain yield.Under the conditions of applying different rates of nitrogen fertilizer,the nitrogen transfer rate was 66.44–71.11%,the nitrogen resorption efficiency of leaves was 53.62–64.33%,the nitrogen resorption efficiency of stem+sheath was 34.82–54.04%.The nitrogen resorption efficiency of the sheath positively correlated with Nitrogen utilization efficiency and Nitrogen harvest index.In summary,for spring wheat in the semi-arid area of the Loess Plateau,when the inorganic nitrogen application rate is about 105 kg N hm^-2,it can ensure the highest N uptake,transpiration and leaf nitrogen resorption efficiency,while achieving lower soil NO₃-N residues and higher soil bacteria,the total number of ammonia oxidizing bacteria communities(8.24×10⁷ copies g^-1 dry soil)and their diversity,so as to obtain higher wheat yield and nitrogen use efficiency.