Effects And Mechanisms Of Iron Oxide On Nitrification And N₂O Emission In Paddy Soils

As an important source of N₂O emissions,paddy soils are worth to give a deeper insight into the process and mechanism involved in N₂O emissions.Usually,paddy soils are in the condition of frequent dry-wet alternation,and during non-flooding conditions,nitrification is the main source of N₂O emissions.In paddy soil,there is abundant iron(Fe)oxides,which are one of the most active factors in soils.The redox reaction of Fe is often coupled with the biological and abiotic conversion processes of soil N,which directly or indirectly affect N₂O emissions.It is especially significant to investigate the effect and mechanism of Fe oxides on N₂O emissions during nitrification in paddy soil for comprehensively understanding the process of N cycling in farmland soils.At the same time,the result can provide the theoretical basis to take reasonable measures for reducing N₂O emissions from farmland soil.In this study,firstly,hydromorphic and gley paddy soils with obvious spatial heterogeneity in soil Fe content were selected to compare and analyze the difference of N₂O emissions and the main influencing factors between the two types of paddy soils,and explore the effect of endogenous iron on the N₂O emission.Then,by comparing the acidic(p H5.5)and alkaline(p H7.9)paddy soils,microcosmic culture experiment was carried out to investigate the biological and chemical coupling effect of goethite on N₂O emissions in the nitrification process of paddy soils and quantify the relative contribution of ammonia-oxidizing bacteria(AOB)and archaea(AOA)to N₂O release during autotrophic nitrification.Lastly,the effects of goethite on N₂O emissions through nitrification intermediates NH₂OH and NO₂^--N in abiotic processes were quantitatively analyzed.The main finfings are as follows:1.N₂O emissions of gley paddy soil was significantly higher than that of hydromorphic paddy soil,and N₂O emission rate and cumulative emissions of the two tested soils were positively significantly correlated with content of iron(p?0.05).Cumulative emissions of N₂O from each layer of the two paddy soils at 100%and 200%of water holding capacity(WHC)were 7.0-3296.0?g N kg^-1 and 6.8-11465.6?g N kg^-1,respectively,which were significantly higher than that under the condition of 60%WHC(2.5-51.6 and 2.7-33.3?g N kg^-1,respectively)(p?0.05).Under the same water condition,N₂O emissions decreased significantly with the increase of soil depth(p?0.05),which was consistent with the trend that the abundance of soil nitrification(AOB and AOA amo A)and denitrification(nir S and nos Z)function genes decreased with the increase of soil depth(p?0.05).The contents of Fe²⁺and Fe³⁺were positively correlated with the abundance of AOB,AOA amo A,nir K and nos Z genes as well as N₂O emissions.The p H of soils also had an important effect on the abundance of the four genes in the two tested paddy soils.This indicates that Fe and p H had a direct effect on nitrification and denitrification microorganisms and N₂O emission.2.The addition of 3%goethite promoted N₂O emissions from both acidic and alkaline paddy soils,and its promoting effect was greater in alkaline paddy soil than in acidic paddy soil.Under the condition of 60%WHC,the cumulative emissions of N₂O in alkaline paddy soil increased significantly from 258.5±35.0?g N kg^-1 of the control treatment to446.7±18.5?g N kg^-1 of the treatment with 3%goethite(p?0.05).Similarly,the addition of goethite increased the cumulative N₂O emissions in the acidic paddy soil(from 33.6 up to 59.5?g N kg^-1),but the increase was not significant.The addition of goethite significantly increasesd the abundance of AOB and AOA amo A genes in both alkaline and acid paddy soils(p?0.05).The high-throughput sequencing results of 16S r DNA gene showed that the dominant species of AOB and AOA were Nitrosomonadaceae,Nitrosomonas and Nitrososphaeraceae,respectively,and goethite increased the relative abundance of Nitrosomonadaceae in alkaline paddy soil.In addition,the relationship between Fe content(including Fe²⁺and Fe³⁺)and the amount of N₂O emissions proved that Fe could react with NH₂OH or NO₂^-to affect the release of N₂O during nitrification.It could be inferred that Fe oxides promoted N₂O emissions through biological and chemical coupling effects in the two paddy soils.3.N₂O emissions of acidic and alkaline paddy soils were mainly derived from biological processes(>82%)in the condition of 60%WHC,and the contribution of abiotic reactions to N₂O emissions was relatively low.Goethite increased the N₂O production mainly through increasing the contribution of autotrophic nitrification process to N₂O emissions.In the treatment of applying ammonium sulfate,the N₂O produced through nitrification(including autotrophic nitrification and heterotrophic nitrification)from acidic and alkaline paddy soils were 14.2-17.3 and 71.4-79.7?g N kg^-1,accounting for 48%-55%and 79%-84%of the total N₂O emissions,respectively.The contribution of N₂O generated by autotrophic nitrification in alkaline paddy soil was 78%-83%,which was significantly higher than that in acidic paddy soil(38%-47%).The N₂O contribution from denitrification also took a considerable proportion(29%-49%)in acidic paddy soil.The contribution of N₂O derived from autotrophic nitrification of acidic and alkaline paddy soils increased from38%and 78%of the control to 47%and 83%of the treatment with goethite added.However,the contribution of N₂O produced from denitrification was reduced from 49%and 20%of the control to 42%and 15%of the treatment with goethite added.4.In the treatment of ammonium sulfate,the contribution of AOB to N₂O emissions in both acidic and alkaline paddy soils was higher than that of AOA,and goethite increased the contribution of AOB to N₂O emissions.The contribution of AOB to N₂O generation in acidic and alkaline paddy soils was 41%and 54%,respectively,which was significantly higher than that of AOA(15%and 45%,respectively),and the contribution of AOB to N₂O increased to 56%and 59%after adding goethite,respectively.The main genera involved in N₂O emission in the two paddy soils were Nitrosospira belonging to AOB and Nitrososphaera belonging to AOA.The addition of goethite increased the abundance of several genera in alkaline paddy soil to a certain extent.Redundancy analysis(RDA)showed that the dominant species of AOB and AOA were significantly affected by soil p H(p=0.002)and Fe²⁺content(p=0.002).PH and Fe were the main factors regulating the community structure of AOB and AOA dominant flora in the two paddy soils.5.Both NH₂OH and NO₂^--N could produce N₂O through abiotic processes,and the relative contribution of NH₂OH abiotic processes to N₂O emission in both acidic and alkaline paddy soils was higher than that of NO₂^--N.Goethite reduced the N₂O contribution of NH₂OH and NO₂^--N abiotic processes.The N₂O contribution from abiotic processes of NH₂OH was 81.2%in alkaline paddy soil,which was significantly higher than that of acidic paddy soil(38.8%).However,when NO₂^--N was used as nitrogen source,the contribution of N₂O from abiotic processes of alkaline paddy soil was lower than that of acidic paddy soil(3.7%and 27.1%,respectively).After adding goethite,the contribution of N₂O produced by the abiotic process of NH₂OH in alkaline and acid paddy soils was reduced to70.2%and 35.6%,respectively,and the abiotic contribution of NO₂^--N was also reduced to3.4%and 23.0%,respectively.In the treatment with NO₂^--N applied,the N₂O release rate was significantly negatively correlated with p H(p?0.01),and when NH₂OH was used as the nitrogen source,the N₂O release rate was significantly positively correlated with p H(p?0.01).In summary,this paper revealed that goethite promoted the release of N₂O via nitrification process of paddy soils through biological and chemical coupling effects.Under the condition of 60%WHC,the N₂O emission from paddy soils was dominated by biological processes,while the contribution of abiotic processes was relatively low.Goethite mainly increased N₂O emissions from paddy soils by promoting the AOB-dominated autotrophic nitrification,and also affected the N₂O produced by nitrification intermediates(NH₂OH and NO₂^--N)through abiotic process.