Effects Of Inhibitors On Nitrification Driven By Soil Ammonia Oxidizers And Researchs Of Screening

Nitrification in agroecosystems has long been an area of focus for scientific research,and mitigating nitrification can efficiently improve nitrogen use efficiency in crops and reduce environmental pollution.During fertilization,nitrogen cycle is accelerated by a large amount of urea or ammonium applied to the soil,which lead to nitrate（NO₃^-）loss and produce the greenhouse gas nitrous oxide（N₂O）.N₂O emissions and nitrate NO₃^-leaching in agroecosystems are main contributors to non-point source pollution and reduced nitrogen use efficiency,both of which are driven by soil nitrification.The combination of urea and nitrification inhibitors（NIs）in soils to regulate the community composition of ammonia-oxidizing microorganisms（AOM）is a very active research area.But most NIs on the market today are inhibitors for ammonia-oxidizing bacteria（AOB）.With the continuous development of molecular ecology techniques,there are three varieties of microorganisms that dominate the ammonia oxidation process in soil,including AOB,ammonia oxidizing archaea（AOA）and complete ammonia oxidizing bacteria（Comammox）,their niche in soil and contribution to ammonia oxidation have great differences.Therefore,it is necessary to study nitrification inhibitors that can inhibit AOA and Comammox to prevent their effects on soil nitrification and N₂O production,so as to provide a direction for screening new biological nitrification inhibitors,in order to achieve the purpose of regulating nitrification,reducing N₂O emissions,and improving fertilizer efficiency.In this study,dicyandiamide（DCD）,carboxy-PTIO（PTIO）,and sodium chlorate（Na Cl O₃）were used as NIs of AOB,AOA,and Comammox respectively,and their effects on nitrification,N₂O emission,and abundances of ammonia-oxidizing microorganisms in long non-nitrogen application soil（N₀P₂K₂）and long-term constant nitrogen application soil（N₂P₂K₂）were researched.Combined with the ¹⁵N isotope technique,the effects of NIs on nitrification and N₂O emissions were analyzed.In addition,we explored the possibility of olive pomace（OP）,the soapstock waste of olive processing（OL）as a new type of biological nitrification inhibitor（BNIs）,and DCD was used as a control to briefly explored the effect of them on urea transformation in soil.The main findings are as follows:（1）The treatment of urea combined with DCD（U+DCD）and urea combined with sodium chlorate（U+Na Cl O₃）could inhibit the nitrification of N₀P₂K₂ and N₂P₂K₂ soil,and DCD had a greater inhibitory effect than Na Cl O₃.The treatment of urea combined with PTIO（U+PTIO）accelerated the nitrification process in N₀P₂K₂ soil,and had no inhibitory effect on the nitrification of N₂P₂K₂ soil.（2）The level of nitrogen in the soil significantly affected the soil N₂O emissions,and the N₂P₂K₂ treatments at day 7 and 28 were higher than the N₀P₂K₂treatments in the same period.Among the three inhibitors,only DCD significantly reduced the cumulative N₂O emissions in N₀P₂K₂ and N₂P₂K₂ soils by 78.9%and 95.4%,respectively.In N₀P₂K₂ soil,U+PTIO treatment accelerated the nitrification process,so its cumulative N₂O emission was significantly higher than that of only apply urea treatment（U treatment）on the 7th and 28th days of incubation;In N₂P₂K₂ soil,U+Na Cl O₃ treatment inhibited the nitrite oxidizing bacteria（NOB）abundance,promoted the denitrification process,and the cumulative N₂O emission was significantly higher than other treatments.Although Na Cl O₃ could inhibit nitrification,it had no inhibitory effect effect on N₂O production.（3）U+DCD significantly inhibited the abundance of AOB amo A gene in N₀P₂K₂ and N₂P₂K₂ soil,while increasing the abundance of AOA amo A gene.In N₀P₂K₂ soil,U+PTIO treatment significantly inhibited the abundance of AOA amo A gene by 51%at day 7 and 73.3%at day 28,while increasing the AOB amo A gene abundance by 113.3%～359.9%;In N₂P₂K₂ soil,U+PTIO treatment had no inhibitory effect on AOA amo A gene abundance and therefore had on effect on AOB.After incubation for 7 days,Na Cl O₃ and PTIO had the effect on inhibiting Comammox amo A gene,but the growth of Comammox could be restored at day 28.Compared with U treatment,U+Na Cl O₃ treatment significantly reduced the abundance of NOB functional genes in N₀P₂K₂ and N₂P₂K₂ soil samples.（4）AOB abundance was highly negatively correlated with AOA abundance（r=-0.82,P<0.001）,and the niche competition between AOB and AOA was more active in long-term nitrogen deficient soil.The cumulative N₂O emission was positively correlated with AOB amo A gene abundance and soil NO₃^--N content.After incubation for 28 days,the correlation between cumulative N₂O emissions and other factors became weaker.（5）Phylogenetic analysis showed that among the three treatments of U,U+DCD and U+PTIO,there were nine AOB OUTs with significant difference,which were closely related to Nitrosomonaceae and Nitrosospira.In N₀P₂K₂ soil,Nitrosospira abundance were significantly inhibited by DCD,which may be a key factor affecting N₂O emission.（6）Compared with DCD,urea combined with olive oil press soapatock deep processing waste liquid can largely retain soil nitrogen in the form of NH₄⁺in the soil.It is speculated that the fat-soluble substances in the soapstock waste of olive processing can effectively retain soil nitrogen.The development of BNIs will have long-term practical significance for the development of green agriculture in China,promoting the sustainable utilization of resources,and promoting the development of new green high-efficiency nitrogen fertilizer products.