Effects Of Nitrogen Reduction And Deep Application On Yield And Nitrogen Transfer Of Double Cropping Rice

The increasing population means that the future global food demand will be unlimited,and under the premise of limited arable land resources,increasing nitrogen fertilizer input to increase food production is one of the effective ways to meet the food demand.Rice is one of the major grain crops in China,and more than 60%of the population is mainly fed by rice in China.In order to pursue high yield,farmers usually increase the input of nitrogen fertilizer in the rice field,resulting in a large amount of nitrogen loss.In order to solve the problems of excessive application of nitrogen fertilizer and low utilization efficiency of double cropping rice in China,field experiment and pot experiment were set up in this study.Field experiment based on the conventional fertilization as the control treatment（CF,150 kg N/hm² in the early and late season rice,respectively）,three nitrogen application levels（nitrogen reduction 30%,23%,and 16%,respectively）were set up with deep application of nitrogen fertilizer to study the effect of deep application and nitrogen reduction on rice yield;pot experiment is under the premise of reducing nitrogen 30%in conventional fertilization,set different fertilization depth（DF2:2 cm deep,DF7:7 cm deep）,research the characteristics of nitrogen migration in soil and the influence of the microbial population of nitrification and denitrification function,the result can provide theoretical basis for the reasonable cropping nitrogen reduction in our country.The main research results are as follows:（1）The results of field experiments for continuous 7 years showed that,compared with CF treatment,though applying chemical fertilizers deep application instead of traditional surface application,nitrogen reduction 30%significantly increased the number of productive ear and grains,and increased the yield and stability of double cropping rice.（2）The pot experiment further confirmed that the reduction of nitrogen fertilizer and deep application could improve rice yield.Compared with conventional fertilization,applying chemical fertilizers at 7 cm below soil surface,nitrogen reduction 30%promoted the accumulation of dry matter and nitrogen absorption in the overground part of rice growth period,but reduced the absorption of fertilizer nitrogen in the overground part of rice.Compared with CF treatment,DF2 and DF7 treatment increased rice yield by 3.39%and24.42%,respectively.The utilization efficiency of nitrogen fertilizer increased to 36.92%,39.88%,the increase was 9.80%and 12.76%,respectively.（3）The concentration of total nitrogen and NH₄⁺-N at the fertilization point was significantly increased by the deep application of reduced nitrogen fertilizer,forming a migrating and diffusing nitrogen nutrient concentration gradient centered on the fertilization point to all directions.During the growth period of rice,the horizontal migration distance of nitrogen nutrients was 7.5-10 cm,and the nitrogen nutrients were mainly distributed in the area of 3-12 cm below the soil layer in vertically direction with DF2 and DF7.In the early tillering stage and the peak tillering stage,although the nitrogen fertilizer dosage was reduced by 30%,the deep application treatment still increased or maintained the total nitrogen and NH₄⁺-N concentrations equivalent to CF treatment.In the maturation stage,there was no significant difference in total nitrogen content of soil between DF7 and CF,but the concentration of NH₄⁺-N was reduced.During the growth period of rice,the inorganic nitrogen in soil under all fertilization treatments was dominated by NH₄⁺-N,the concentration of NO₃^--N was relatively low,and there was no obvious change rule.（4）The abundance of functional microorganisms in nitrification and denitrification was significantly changed by the deep application of reduced nitrogen fertilizer.Compared with CF,DF7 reduced the abundance of AOB genes in the fertilizing site and rhizosphere soil,reduced the denitrification functional microbial population abundance of narG and nosZI genes in the rhizosphere soil,but increased the abundance of narG and nosZI genes in the fertilizing site soil.In summary,compared with the conventional treatment,applying chemical fertilizers at7 cm below soil surface,nitrogen reduction 30%increased the concentration ratio and supply concentration of nitrogen nutrient at the fertilization point,forming a nutrient concentration gradient centered on the fertilization point and diffused to the rice root system,which promoted the nitrogen absorption and improved the yield of rice.Deep application of nitrogen fertilizer reduced the abundance of AOB population,slowed down the conversion of NH₄⁺-N to NO₃^--N,reduced the nitrogen loss,and improved the utilization efficiency of nitrogen fertilizer.Therefore,deep application of reduced nitrogen fertilizer can replace the current mode of installment broadcasting in double cropping rice production,which is worthy of further study and application.