Effects Of Different Nitrogen Application Rates On The Abundance And Community Structure Of Soil Nitrifiers In Soil Under Integration Of Water And Fertilizer

Nitrification is the most critical part of the whole nitrogen（N）cycle,and nitrifying microorganisms play an important role in nitrification.Recent research found that the whole process nitrifying bacteria（Comammox）broke the traditional idea that two groups of different nitrifying microorganisms complete nitrification together,and put forward the basic problem of their relative contribution to agricultural soil ammonia oxidation.At present,the impact of different N application rates on the abundance,community composition,and diversity of soil nitrifying microorganisms and their main influencing factors are still unclear.Therefore,this paper selected five N fertilizer application rates under film mulching and drip irrigation in the semi-arid area of western Jilin Province:0 kg ha^-1（N0）,90 kg ha^-1（N1）,150 kg ha^-1（N2）,210kg ha^-1（N3）and 270 kg ha^-1（N4）,and used fluorescent quantitative PCR and Illumina Miseq sequencing to analyze ammonia oxidizing archaea（AOA）,ammonia oxidizing bacteria（AOB）The abundance and community structure of nitrite oxidizing bacteria（NOB）and whole process nitrifying bacteria（Comammox）,as well as the contribution of ammonia oxidizing microorganisms to ammonia oxidation.The main results of the study were as follows:（1）Different N application rates significantly affected soil physicochemical properties and soil enzyme activity.The content of soil organic matter（SOM）,total N（TN）,nitrate N（NO₃^--N）,and ammonium N（NH₄⁺-N）significantly increased with the increase of N application,while the p H value significantly decreased with the increase of N application.N application significantly increased soil enzyme activity.Compared with low N application rates（N1,N2）,high N application rates（N3,N4）significantly increased soil hydroxylamine oxidase（HAO）activity and soil nitrite oxidoreductase（NXR）activity;Compared with N0,high N application rates（N3,N4）significantly increased the activity of ammonia monooxygenase（AMO）.Under different N application rates,TN was an important factor driving soil enzyme activity.（2）The application of N fertilizer significantly increased soil nitrification potential（PNR）.Compared with low N application rates（N1,N2）,high N application rates（N3,N4）significantly increased soil PNR,which was significantly positively correlated with SOM,TN,NO₃^--N,NH₄⁺-N,AN.N application significantly increased soil nitrite oxidation potential（PNO）,and soil PNO showed a significant positive correlation with SOM,TN,NO₃^--N,NH₄⁺-N,and AN.N application significantly increased the abundance of AOB,NOB,and Comammox,but significantly reduced the abundance of AOA.The abundance of AOA was significantly negatively correlated with SOM,TN,NH₄⁺-N,NO₃^--N.The abundance of AOB was significantly positively correlated with SOM,TN,NO₃^--N.The abundance of NOB was significantly positively correlated with SOM,AN,TN,NH₄⁺-N,NO₃^--N,AN.The abundance of Comammox was significantly positively correlated with TN,NO₃^--N.Under different N application conditions,the relative contribution rates of AOA and AOB to ammonia oxidation are 3.03%-25.27%and 4.75%-53.22%,respectively.The relative contribution rates of Comammox to ammonia oxidation were 5.26%-91.48%.Comammox was the main contributor to soil nitrification.The relative contribution rates of traditional ammonia oxidizing microorganisms in low N application rates soils were higher than that in high N application rates soils,and the relative contribution rates of Comammox in high N application rates soils were higher than that in low N application rates soils.（3）Different N application rates significantly affected the community structure of nitrifying microorganisms.Thaumarchaeota,Nitrosomonadales,Nitrospira moscoviensis were the dominant bacteria of AOA,AOB,and NOB,respectively,and their relative abundance were significantly higher in high N application rates than in low N application rates.Nitrospira was the dominant strain of Comammox,and N4 significantly increased the relative abundance of Nitrospira.N application significantly reduced the diversity of AOA,but increased the diversity of Comammox,while N4 significantly increased the diversity of AOB and NOB.The community structure of AOA,AOB,and Comammox was mainly influenced by N application rates.There were significant differences in the community structure of AOA,AOB,NOB,and Comammox between low N application rates（N0,N1,and N2）and high N application rates（N3and N4）.The results of the Structural Equation Model（SEM）showed that SOM had the greatest impact on the community structure of AOA,NH₄⁺-N had the greatest impact on the community structure of AOB,and TN had the greatest impact on the community structure of NOB and Comammox.In summary,the application of N fertilizer significantly increased the content of soil SOM,TN,NO₃^--N,and NH₄⁺-N,as well as the yield of maize and the activity of soil HAO,AOM,and NXR.The community structure of nitrifying microorganisms was influenced by the amounts of N fertilizer applied.Within the range of 0-270 kg ha^-1of N fertilizer application,the abundance and diversity of AOB,Comammox,and NOB increased with the increasing of N fertilizer application.NH₄⁺-N had the greatest impact on the community structure of AOB,while TN had the greatest impact on the community structure of Comammox and NOB.The abundance and diversity of AOA significantly decreased,and SOM had the greatest impact on the community structure of AOA.Comammox was the main contributor to ammonia oxidation under different N fertilizer application conditions.