Mechanism Of N₂O Emission During The Nitrification Process Of Two Paddy Soils

As one of the main greenhouse gases in the atmosphere,the increasing N₂O largely comes from agricultural production.Dry-wet alternation of rice fields and the application of chemical fertilizers have a significant effect on the amount of N₂O releasing from the soil and its release pathway.However,there are no enough studies on the mechanism of nitrification and non-biological N₂O production in paddy soils in the process of dry-wet alternation.In this experiment,we used microcosm incubation with yellow-brown paddy soil and fluvo-aquic paddy soil to simulate the changes in water content during rice production.Two different fertilizers?urea and ammonium sulfate?were applied;meanwhile,acetylene?10 Pa?was used to inhibite autotrophic nitrification and octine was used to inhibite ammonia-oxidizing bacteria?AOB?.Fluorescent quantitative PCR,high-throughput sequencing and other relevant methods were done to study the effects of nitrification?autotrophic nitrification and heterotrophic nitrification?on N₂O emission;then,mechanisms of autotrophic nitrifying microorganisms AOA and AOB on the release of N₂O were discussed,so as to provide a scientific basis for clarifying the N₂O emission mechanism in rice fields.The main findings are as follows:?1?The increase of nitrogen application rate can significantly increase total N₂O emission rate,cumulative emissions,and N₂O emission rate of autotrophic nitrification and heterotrophic nitrification in the two paddy soils,but it has no significant effect on the N₂O emission rate of non-biological processes.The discharge rate of 24-hour urea treatment and cumulative N₂O emissions from both soils were higher than those treated with ammonium sulfate,and the cumulative release of fluvo-aquic soil was significantly higher than that of yellow-brown paddy soil.The cumulative N₂O emission from urea and ammonium sulfate in paddy soil of yellow brown soil was 166.7 ng·kg^-1 and 93.5 ng·kg^-1,respectively,which was 4.5 and 2.5times to that of non-fertilizer application.Fluvo-aquic soil was applied with urea and sulfuric acid.Their cumulative ammonium N₂O emissions were 9349.1 ng·kg^-1 and7997.8 ng·kg^-1,respectively,which were 135 and 116 times to that of no fertilizer application.?2?The change of soil moisture content significantly affected the total N₂O emission rate within 24 hours and the N₂O emission rate of nitrification and heterotrophic nitrification within 24 hours,and the discharge rate of yellow brown soil paddy soil was higher than that of fluvo-aquic soil.The total N₂O emission rate emission rate of yellow-brown paddy soil increased with the increase of water content and reached the highest peak at 160%of water holding capacity?WHC?.The total N2O emission rate of fluvo-aquic paddy soil increased at first and then decreased with the increase of water content,and reached the maximum at 100%WHC.?3?The contribution of autotrophic nitrification of two paddy soils to N₂O emission was greater than that of heterotrophic nitrification,and the contribution of non-biological effects only accounted for a small proportion.Nitrogen application increased the contribution of autotrophic nitrification to N₂O emission from both soils.The highest contribution of autotrophic nitrification in yellow-brown paddy soil was that the highest was 40%at 112%WHC after application of urea,and the fluvo-aquic paddy soil was 49%at 100%WHC after applying ammonium sulfate.Nitrogen application increased the contribution of heterotrophic nitrification in yellow-brown paddy soil.The highest contribution was 26.1%when appying urea at 160%WHC,but the nitrogen application reduced the contribution of heterotrophic nitrification in fluvo-aquic paddy soil.?4?Nitrogen application in both soils increased the contribution of AOA to N₂O emission and reduced the contribution of AOB.In the absence of nitrogen?CK?,AOB was the main contributor to N₂O;however,the AOA became the main contributor after nitrogen application.In yellow-brown paddy soil,the contribution of AOA after applying ammonium sulfate increased from 14%to 42%,and the contribution of AOB decreased from 45%to 26%.For fluvo-aquic paddy soil,after applying urea and ammonium sulfate,AOA contribution increased from 10%to 25%and 32%,respectively;meanwhile,AOB contribution decreased from 52%to 40%and 37%,respectively.Nitrogen application resulted in lower soil pH,increased AOA amoA gene copy number in fluvo-aquic paddy soil,and decreased AOB amoA gene copy number in yellow-brown paddy soil.?5?MiSeq sequencing analysis results showed that nitrogen application increased the number of OUT of AOA amoA and AOB amoA in two paddy soils,increased the AOA and AOB amoA gene diversity?Chao1?in yellow-brown paddy soils,and decreased the AOA and AOB amoA gene diversity in fluvo-aquic paddy soil.The AOA of yellow-brown paddy soil was mainly composed of Thaumarchaeota,with a relative abundance more than 99%,and the AOA Thaumarchaeota and Crenarchaeota of fluvo-aquic paddy soil accounted for about 50%,respectively.AOBs in both paddy soils were mainly composed of Nitrosospira and their relative abundance increased after nitrogen application.RDA analysis showed that the community structure of AOA and AOB in yellow-brown paddy soil was positively correlated with pH and negatively correlated with organic matter.However,the community structure of AOA and AOB in fluvo-aquic paddy soil was negatively correlated with pH and positively correlated with organic matter.