Effects Of Antecedent Soil Moisture On Nitrification And Nitrifying Microorganisms

Nitrification is one of the key components of the global nitrogen cycle,and soil is one of the major habitats where the nitrogen cycle occurs.As the rate-limiting step of nitrification process,ammonia oxidation process has a profound impact on soil N cycle,and the nitrite produced by this process is prone to groundwater and surface water pollution as well as the production of greenhouse gas N₂O which is highly susceptible to greenhouse effect and stratospheric ozone loss.In contrast,the ammonia oxidation process is carried out with the participation of ammonia oxidizing microorganisms（AOA,AOB,Comammox）.Nitrification is influenced by antecedent soil moisture,and changes in water history affect the rate of ammonia oxidation and the activity of ammonia oxidizing microorganisms.Therefore,it is important to clarify the nitrification and the changes of AOA,AOB and Comammox gene abundance under different antecedent soil moisture conditions.In this paper,three different moisture levels（40%WHC,100%WHC,200%WHC）were artificially set as moisture pretreatment samples and incubated indoors for 180 d.At the end of incubation,the nitrification strength was measured,and then the samples were incubated indoors for 28 d at 60%WHC on the basis of moisture pretreatment,and two p H soils（neutral purple soil and acidic purple soil）were periodically measured.purple soil and acidic purple soil)nitrate nitrogen,ammonium nitrogen,nitrification potential,mineralization potential,p H,N₂O,and ammonia-oxidizing microbial abundance to determine the effect of antecedent soil moisture on nitrification and ammonia-oxidizing microorganisms in the two p H soils were measured periodically,and the results were as follows:（1）Different antecedent soil moisture（40%-60%WHC,100%-60%WHC,200%-60%WHC）had significant effects on nitrification,mineralization,and ammonia-oxidizing microbial abundance in the two p H soils.Under the same antecedent soil moisture condition,nitrification was greater in neutral purple soil than in acid purple soil,and nitrification and mineralization were strongest in neutral purple soil at 40%-60%WHC and in acid purple soil at 100%-60%WHC;nitrification was weakest in both p H soils when the antecedent soil moisture condition was 200%WHC,and soil N₂O emissions were significantly lower than in the remaining Nitrification and mineralization were strongest at 100%-60%WHC;nitrification was weakest at 200%WHC for both p H soils,and soil N₂O emissions were significantly lower than the remaining two antecedent soil moisture treatments,while p H was significantly higher than the remaining two antecedent soil moisture treatments;（2）Higher abundance of Comammox was detected in both p H soils.AOA,AOB,and Comammox amo A gene copy numbers were higher in the neutral purple soil than in the acidic purple soil.Comammox was more suitable than AOA and AOB for survival in flooded environment in both p H soils;200%WHC was more suitable for the growth of clade A,40%WHC was suitable for the growth of AOA,while AOB had higher microbial activity at 100%WHC in antecedent soil moisture while clade B had the highest gene copy number at 40%WHC but the microbial abundance decreased at the end of the incubation.decreased in abundance;（3）The addition of urea as a nitrogen source stimulated the nitrification potential and mineralization potential as well as the gene copy number growth of AOA,AOB and Comammox amo A more than ammonium sulfate under different antecedent soil moisture.Two additional wasteland soils with different parent materials（neutral purple soil and neutral tidal soil）were selected for the study,and the experimental procedure was kept consistent with p H soil to determine the effects of different antecedent soil moisture on nitrification and ammonia oxidizing microorganisms in soils with different parent materials,and the results were as follows:（1）Different antecedent soil moisture（40%-60%WHC,100%-60%WHC,200%-60%WHC）had significant effects on nitrification,mineralization and ammonia oxidizing microbial abundance in different parent soils.Nitrification and mineralization were greater in neutral tidal soils than in neutral purple soils under the same antecedent soil moisture treatment.Nitrification and mineralization were strongest in purple soils at40%-60%WHC,and in tidal soils at 100%-60%WHC;nitrification and mineralization were weakest in both soils at 200%WHC,and soil N₂O emissions were significantly lower and p H significantly higher than the other two antecedent soil moisture treatments;（2）Higher abundance of Comammox was detected in both purple and tidal soils,and the copy numbers of AOB and Comammox clade A genes were higher in neutral tidal soils than in neutral purple soils at the end of incubation.clade A was more suitable for growth at 200%WHC,AOA was more suitable for growth at 40%WHC,AOB was more suitable for growth at 100%WHC antecedent soil moisture and clade B had the highest gene copy number at 40%WHC but microbial abundance decreased at the end of the culture;（3）The addition of urea as a nitrogen source stimulated the nitrification potential and mineralization potential as well as the growth of AOA,AOB,and Comammox clade A amo A gene copy number more than ammonium sulfate under different antecedent soil moisture treatments.Taken together,the results of this paper revealed that antecedent soil moisture had an effect on different p H,parent soil nitrification,and ammonia oxidizing microorganisms.Based on this,the following conclusions were drawn:（1）Nitrification was strongest in neutral tidal soils and weakest in acidic purple soils under the same antecedent soil moisture treatment.Acid purple and neutral tidal soils had the strongest nitrification at 100%WHC,neutral purple soils had the strongest nitrification at 40%WHC,and all soil samples had the weakest nitrification and mineralization at 200%WHC;（2）antecedent soil moisture affected the abundance of ammonia-oxidizing microorganisms,and Comammox had a higher number of gene copies;（3）combined with the intensity of nitrification and different antecedent soil moisture,Comammox clade A was mainly involved in soil nitrification at 200%WHC;AOB was mainly involved in soil nitrification at 100%WHC while AOA was mainly involved in soil nitrification at 40%WHC.