| N2O is one of the important greenhouse gases,and farmland soil is an important source of atmospheric N2O emission.As the microenvironment region with the most intense interaction among plants,soil and microorganisms,the root zone is not only the only way for nutrients and water to enter the plant system from soil,but also an important part to study N2O emission in plant root-soil interface.In this study,summer maize,a high-yield farmland crop in North China,was taken as the main research object,combined with indoor culture and field experiments,using N2O microelectrode method,silica gel tube method,and with the help of high-throughput sequencing,qPCR soil molecular microbiology and stable isotope analysis techniques.The aim of this study was to explore the effects of long-term different nitrogen fertilizer regimes and different soil fertility levels on soil N2O emission fluxes from rhizosphere to non-rhizosphere and root dense zone to row,the growth and decline of soil nutrients and extracellular enzymes in root zone and the response mechanism of soil N2O emission by microbial community.The main results are as follows:(1)The calibration method of microelectrode for in situ measurement of N2O in soil rhizosphere microenvironment was improved by indoor culture experiment.Using the improved calibration method for in-situ monitoring,it was found that a large amount of N2O was released from rhizosphere and non-rhizosphere soil after the application of urea,and the N2O emission from rhizosphere soil accounted for 60%of the total emission.Therefore,the N2O emission from rhizosphere soil should be fully considered when studying the response of agricultural soil greenhouse gas emissions to global change.In addition,the field experiment results showed that the soil N2O concentration in the root dense area under different long-term nitrogen application treatments was significantly higher than that between rows during the peak emission period.Throughout the whole growth period,there was no statistically significant difference between them.This may be due to the limitation of measuring N2O flux by silica gel tube method.Therefore,the effect of in-situ quantitative monitoring of crop root growth on N2O emission from rhizosphere soil needs to be further studied.(2)The nutrient indexes of high fertility(Convention)soil were significantly higher than those of low soil fertility(Barren)soil,and the soil nutrient index contents of the two soil fertility conditions increased with the increase of nitrogen application.Fertilization could supplement soil nutrient content and significantly affect the activities of C.N and P nutrient acquisitive enzymes.In addition,there were significant differences in the stoichiometric ratio of enzyme activities between non-rhizosphere soil and rhizosphere soil.With the increase of nitrogen application rate,nutrients in rhizosphere soil also stimulated microorganisms to secrete more C,N and P enzymes.At the same time,C,N and P in rhizosphere soil may become limiting factors with the level of nutrient supplement and the competition of roots on soil nutrients,but the nutrient limiting effect of ConventionN400 treatment at high soil fertility level is the weakest.The limiting factors of low soil fertility level without nitrogen application(BarrenN0)may be caused by the insufficient supply of C,N and P nutrients.The partial least square regression analysis showed that the difference of N2O concentration between root zone and non-root zone was due to the significant difference in substrate and energy material between them.(3)The copy numbers of soil nitrification and denitrification genes were significantly increased after fertilization at different soil fertility levels.The production of N2O in soil was significantly correlated with ammonia oxidizing bacteria amoA,denitrifying bacteria nirS and nosZ,but not with the copy number of NRK gene of denitrifying bacteria.Long-term application of urea nitrogen fertilizer could significantly increase the richness and diversity of soil amoA nitrifying bacteria,nirS and nosZ denitrifying bacteria,but after planting corn,the common species of amoA nitrifying bacteria,nirS and nosZ denitrifying bacteria in rhizosphere soil were lower than those in non-rhizosphere soil.The principal component analysis showed that both fertilization effect and rhizosphere effect could change the community structure of denitrifying bacteria in nirS and nosZ.Long-term application of urea and nitrogen fertilizer significantly increased the abundance of Nitrosovibrio in soil amoA nitrifying bacteria,Burkholderiaceae of NIRS-type β-Proteus and slow-growing Rhizobium of nosZ denitrifying bacteria(Bradyrhizobiaceae).In addition,the Zoogloeaceae abundance of NIRS denitrifying bacteria in rhizosphere soil increased significantly,but the number of Chromobacteriaceae bacteria decreased at the same time,but there was no significant difference among different fertilization treatments,and the relationship between nirS denitrifying bacteria level in rhizosphere soil was more complex than that in non-root zone,and the relationship between nosZ denitrifying bacteria level in rhizosphere soil was simpler than that in non-root zone.In addition,most of the microflora in the rhizosphere soil will change with the change of soil available nutrients. |
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