Effect Of Iron On Methane Oxidation In Two Rice Soils With Different Iron Contents

Rice fields are a major source of methane（CH₄）emissions due to long-term flooded rice cultivation.Fe is a metalswith high content in the earth’s crust and has active redox reaction properties.The reduction process of iron in flooded paddy fields can be coupled with CH₄ oxidation,thereby reducing CH₄ emissions from paddy fields.Therefore,investing the effect of iron reduction process on CH₄oxidation in flooded paddy fields can provide a scientific basis for reducing CH₄ emissions from paddy fields.Furthermore,alleviating environmental problems such as greenhouse effect.In the present study,two rice soils which with high Fe content(5.21 g Fe kg^-1)and low Fe content(2.66 g Fe kg^-1)were selected from Jingmenand Huangshi in Hubei province,and dealt them with the following treatments:added organic carbon（glucose and sodium acetate）,one inorganic nitrogen（potassium nitrate）,and two valence levels of active Fe（Fe Cl₂and Fe Cl₃）,and two treatments of added active Fe concentration gradients(1 g Fe kg^-1 and 2 g Fe kg^-1),and incubated under flooded conditions.The oxidation characteristics of CH4 during incubation and the changes of soil dissolved organic carbon（DOC）,trivalent iron[Fe（Ⅲ）],ferrous iron[Fe（Ⅱ）],amorphous iron（Fe_o）,and complexed iron（Fe_p）contents were analyzed to investigate the effect of iron on methane oxidation in rice soils.The main findings were as follows:（1）Compared with the low Fe content soil:the CH₄ emission was 76.27%lower in the high Fe content soil,and its oxidation rate was 1.32 times higher than the former.Both CH₄emissions and oxidation rates of the two test soils were significantly correlated with reactive iron content（P<0.05）.The CH₄oxidation increased with the increase of active iron content,which was consistent with the change pattern of Fe（III）reduction to Fe（II）in the soil.There was a significant linear correlation between Fe（II）content and CH₄ oxidation（P<0.01）,and the interaction between Fe（III）,DOC and nitrate nitrogen on the CH₄oxidation of the two paddy soils also had a significant effect on CH₄ oxidation.The resultindicats that Fe can act directly on the CH₄ oxidation process and also interact with organic carbon and inorganic nitrogen on the CH₄ oxidation process.（2）The addition of two exogenous carbon sources,glucose and sodium acetate,supplemented the soil with soluble carbon sources,provided carbon substrates for CH₄production,thenincreased CH₄ emissions from both soils.Nitrate nitrogen addition enhanced the nitrate reduction coupled methane oxidation process and promoted methane oxidation.These two organic carbons competed with CH₄ as DOC in the soil as electron donors,thereby inhibiting CH₄ oxidation,with sodium acetate having a greater inhibitory effect as a direct substrate for CH₄ production than glucose.Exogenous carbon addition also complexed with Fe（III）to Fe_p,thus reducing the Fe（III）content that can couple with CH₄ oxidation in rice soils and weakening the Fe（III）reduction-coupled CH4 oxidation process,thus reducing CH₄ oxidation.The CH₄ oxidation of high Fe content soils was significantly higher（P<0.01）than the other soils under the addition of the same exogenous carbon,which was due to the high content of background Fe（III）and Fe_o that promoted CH₄ oxidation.Among them,the direct promotion of CH₄ oxidation by Fe（III）was dominant,and Fe_o can complex with soil DOC to reduce the soil easily available organic carbon content,which facilitates the oxidation of CH₄ as an electron donor in a low carbon environment.Therefore,thisinferres that the carbon source is not the cause of the difference in CH₄ oxidation between high and low Fe content soils,and Fe influences CH₄ oxidation in rice soils through the process of Fe（III）reduction to produce Fe（II）.（3）The addition of exogenous iron soil redox potential increases,creating an environment unfavorable to CH₄ production thereby inhibiting CH₄ emission in high and low iron soils.Exogenous Fe（Ⅱ）addition inhibited the reduction process of Fe（III）,thus weakened the CH₄ oxidation capacity of rice soils,which reduced the CH₄ oxidation rates of high and low iron soils by 30.50%and 5.18%.However,the CH₄ oxidation rates of high iron soils were still higher than those of low iron soils under the same concentration of exogenous ferrous iron.The effect of adding exogenous trivalent iron on the CH₄ oxidation of the two paddy soils differed.Exogenous Fe（III）mainly complexed with DOC to produce Fe_p,which inhibited the process of its reduction to Fe（II）and thus inhibited the CH₄oxidation process in the high Fe soil.1 g kg^-1 Fe（III）addition did not affect the CH₄oxidation in the low Fe soil,and 2 g kg_-1 Fe（III）addition promoted the CH₄ oxidation process in the low Fe soil,indicating that supplementation of Fe（III）to soils with low Fe content could promote CH₄oxidation.In summary,the higher CH₄ oxidation in high iron content soils than in the other soils is mainly achieved through the process of Fe（III）reduction to Fe（II）coupled with CH₄oxidation to CO_2.Organic carbon will complex with Fe（III）,therefore inhibiting CH₄oxidation,and Fe_o in soil can promote CH₄ oxidation by reducing soil DOC.CH₄ oxidation in high Fe soils was higher than that in low Fe soils with or without the addition of exogenous carbon,and the ratio of Fe（III）to exogenous carbon remained a key and straight factor in the CH₄ oxidation process.The addition of exogenous ferrous iron inhibits the reduction of Fe（III）to Fe（II）,thereby reducing CH₄ oxidation.Adding low concentration of Fe（III）to soils with low background Fe content did not affect CH₄ oxidation,and adding high concentration of Fe（III）promoted CH₄ oxidation.Nevertheless,the soils with high background Fe content were added exogenous Fe（III）then inhibited CH₄ oxidation,suggesting that there may be a threshold value of Fe（III）concentration for the promotion of Fe（III）on CH₄ oxidation in paddy soils.