Study On Nitrogen Utilization And Microbial Regulation Mechanism Under Nitrogen Fertilizer Deep Placement Modes In No-tillage Rice Fields

In recent years,the shortage of rural labor force has contributed to the promotion and application of the technology of no-tillage.However,if traditional surface broadcasting technology of nitrogen fertilizer was still taken in no-tillage paddy fields,it would make nitrogen fertilizer granules stay in the soil surface,thus resulting in increasing gaseous nitrogen loss and reducing nitrogen utilization rate and yield in no-tillage rice fields.Therefore,the study investigated ammonia volatilization,nitrous oxide emissions,nitrogen utilization rate and the corresponding microbial regulation mechanism under deep placement of nitrogen fertilizer in no-tillage rice fields,which would provide scientific basis for the popularization and research of the technology.The study was conducted in long-term no-tillage rice fields of Huaqiao Town,Wuxue City,Hubei Province from 2013 to 2016.Different placement depth treatments were set to study the effects of nitrogen fertilizer deep placement on ammonia volatilization,nitrous oxide emission and nitrogen utilization in no-tillage rice fields.The experimental treatments included no nitrogen fertilizer treatment(CK),nitrogen surface broadcasting(S),deep placement of nitrogen fertilizer at 5 cm(5P),10 cm(10P)and 20 cm(20P).Based on the analysis of soil nitrification and denitrification functional microbial communities,this study discussed the microbial regulation mechanism of nitrogen fertilizer placement depths on nitrous oxide emissions,and analyzed the impact of nitrogen fertilizer placement depths on rice root growth and nitrogen absorption through rhizospheric inorganic nitrogen transport rate model.At the same time,setting up a nitrogen fertilizer deep placement proportion pot experiment would provide theoretical basis for further improving the nitrogen fertilizer deep placement mode in no-tillage rice fields.The main test results were as follows:(1)Deep placement of nitrogen fertilizer in no-tillage rice field increased average soil ammonium nitrogen concentration by 19.4%-94.5% during the whole growth period compared with the surface broadcasting of nitrogen fertilizer,and decreased averagesurface water ammonium nitrogen concentration by 22.5%-49.4%,finally reduced ammonia volatilization by 32.4%-93.5% significantly.The inhibiting effect of nitrogen fertilizer deep placement on ammonia volatilization was enhanced with the increasing of nitrogen fertilizer placement depth.(2)Compared with the S treatment,deep placement of nitrogen fertilizer reduced the soil oxidation reduction potential,pH and dissolved oxygen observably,thus resulting in16.7%-17.3% of reduction in concentration of nitrous oxide in the soil profile during the whole period of rice growth,and 25.4%-26.5% of decrease in nitrous oxide emissions in rice fields subsequently.Compared with those under 5P and 20 P,due to the comprehensive regulation of soil reduction,nitrification and denitrification on nitrous oxide emission.the nitrous oxide emission under 10 P was reduced by 19.8%-24.1% and23.6%-26.1%,respectively.(3)Relative to S treatment,as increasing the concentration of available nitrogen substrate,deep placement of nitrogen fertilizer increased rhizospheric nitrate nitrogen transport rate by 18.5%-23.5% through promoting the metabolism of corresponding functional microbial communities of ammonia-oxidizing archaea,ammonia-oxidizing bacteria,nirK type and nirS type denitrifying bacteria.Moreover,nitrogen application at10 cm in no-tillage rice fields among other deep placement treatments could effectively improve rhizospheric nitrate nitrogen transport rate by 6.3%-20.4%,thus promoting the absorption and utilization of nitrogen fertilizer through increasing the root surface area by8.4%-23.5%.(4)Compared with S treatment,deep placement of nitrogen fertilizer increased rhizospheric nitrate nitrogen transport rate through reducing nitrogen fertilizer gaseous loss by 29.2%-30.5%.Moreover,deep placement of nitrogen fertilizer improved the utilization rate of nitrogen fertilizer by 43.2%-47.6% through promoting the growth of root system and the absorption of nitrogen fertilizer.The comprehensive analysis showed that 10 cm was the optimum application depth in no-tillage rice fields due to reducing gaseous loss of nitrogen fertilizer and improving utilization rate of nitrogen fertilizer effectively.(5)Within 70%,increasing the proportion of nitrogen fertilizer deep placed at the seedling stage could improve rhizospheric inorganic nitrogen transport rate,thereby improving nitrogen utilization efficiency through promoting root growth.When more than 70% of nitrogen fertilizer was deep placed at the seedling stage,the nitrogen demandof rice at the later growth stage was not satisfied and the nitrogen utilization efficiency was restrained.Compared with other amounts of nitrogen fertilizer,70% of nitrogen fertilizer deep placed at the seedling stage could increase the nitrogen utilization efficiency by 21.9%-29.5% and grain yield by 16.9%-28.3%.In summary,the results of the 4-year field and pot experiment indicated that deep placement of nitrogen fertilizer could significantly reduce nitrogen gaseous loss through decreasing ammonia volatilization and nitrous oxide emission,and improve rhizospheric inorganic nitrogen transport rate,root growth and nitrogen uptake,resulting in improving nitrogen utilization rate and grain yield in no-tillage rice fields finally.Moreover,for conventional hybrid medium straw rice varieties,the optimal ratio and depth to nitrogen fertilizer deep placement mode in no-tillage rice fields was that 70% of nitrogen fertilizer as basic fertilizer was deep placed at 10 cm depth.