Study On Soil Aerobic/anaerobic Methane Oxidation Characteristics Of Typical Paddy Fields

Methane（CH?）is an important greenhouse gas with a warming potential 25 times that of CO₂gas on a century-old scale,and the atmospheric CH?concentration continues to increase.Wetlands are one of the important sources of atmospheric CH?gas.Paddy fields belong to constructed wetlands,which have been in alternating dry and wet conditions for a long time and carry out extensive fertilization,which is an important source of CH?anthropogenic emissions in terrestrial ecosystems.It has been found that CH?emissions from paddy fields are the net combined result of CH?production and CH?oxidation.The CH?oxidation process can be carried out under aerobic and anaerobic conditions,which is the main way of CH?reduction in paddy fields.Therefore,it is of great significance to study the CH?oxidation reduction process of paddy soil in different regions,elucidate the coupling mechanism of CH?oxidation process and nitrogen conversion process and its mutual influence effect,and analyze the microbial mechanism of anaerobic/aerobic CH?oxidation process in paddy field and its correlation with CH?oxidation rate,which is of great significance for further elucidating the internal mechanism of CH?emission in paddy soil and alleviating CH?emission in terrestrial ecosystems.In this study,six typical paddy soils[Fujian（FJ）,Jiangsu Jiangsu（JJ）,Yangzhou（YZ）,Henan Yuanyang County（YY）,Henan Xinyang City（XY）,Nanjing（NJ）]were selected from different regions in China as research objects,and the regional differences in the oxidation rate of CH₄in different typical paddy soils were studied under anaerobic/aerobic conditions,and the response characteristics of CH₄oxidation rate to oxygen concentration were analyzed.The influence effect between aerobic CH₄oxidation process and ammonia oxidation process was analyzed;The response characteristics of CH₄oxidation process to electron（NO₂^-、NO₃^-、AQDS、CH₄）acceptors in paddy fields were studied by adding electron acceptors.Long-term anaerobic culture of paddy soil revealed the coupling reaction characteristics between CH₄anaerobic oxidation and N₂O reduction.The main results are as follows:（1）The aerobic CH?oxidation rate of soil in the six paddy fields showed significant regional differences.Among them,the average CH?oxidation rates of Fujian（FJ）and Northeast Jiansanjiang（JSJ）soils were 44.12 and 38.17μg·g^-1·d^-1,respectively,which were significantly higher than those in Yuanyang（YY）,Xinyang（XY）and Nanjing（NJ）paddy soils in Henan.Moreover,the response degree of aerobic CH?oxidation rate to oxygen concentration in the tested soil varied greatly.Among them,JSJ,YZ and YY soils decreased significantly with the decrease of oxygen concentration,with decreases of 46.88%,92.20%and 82.95%,respectively.However,there was no significant difference between FJ soil and three oxygen concentrations.The average net increase of ¹³C-SOC in FJ paddy field was as high as 14.79 nmol ¹³C g^-1,which was significantly higher than that of the other five rice fields.The net increase of ¹³C-SOC in the soil of six paddy fields was significantly positively correlated with its aerobic CH?oxidation rate,indicating that the process of aerobic CH?oxidation in paddy soil was the main driver of the increase of ¹³C-SOC.Correlation analysis showed that the aerobic CH?oxidation rate of paddy soil was significantly positively correlated with the pmo A copy number of its functional gene.The average emission of N₂O accompanying aerobic CH?oxidation in FJ paddy field is only 0.45μg·g^-1,which is significantly lower than that of other paddy fields,and combined with its highest aerobic CH?oxidation rate,the soil of this paddy field shows a higher greenhouse gas reduction potential.In contrast,the average N₂O emissions accompanying the oxidation of aerobic CH?in YZ,XY and YY soils have a high degree of responsiveness to oxygen concentrations,and their greenhouse gas reduction potential is greatly offset.（2）The effect of NH₄⁺addition on the oxidation rate of aerobic CH?in paddy fields was different.Among them,the oxidation rates of aerobic CH?in FJ,JSJ,YZ,XY and NJ soils increased by 31.20%,30.33%,40.62%,13.35%and 36.35%,respectively,and decreased by29.92%in YY soils under the conditions of NH₄⁺.On the other hand,the presence of CH?also had an important impact on the conversion process of N elements:the ammonia oxidation in FJ and JSJ soils decreased by 30.14%and 37.70%,respectively,while the ammonia oxidation in YY soil increased by 37.75%.（3）The background anaerobic CH?oxidation rates of the six paddy soils were significantly different,and the anaerobic CH?oxidation rates of FJ and JSJ soils were 0.59and 0.37 nmol ¹³CO?·g?1d?1,respectively,which were significantly higher than those of NJ soils.Compared with the control,the oxidation rates of anaerobic CH?in FJ,JSJ,YZ,YY,XY and NJ soils under the N₂O addition conditions were 0.71,0.60,0.47,0.59,0.57 and 0.24nmol ¹³CO?·g?1d?1,respectively,indicating that N₂O may act as an electron acceptor to promote the soil anaerobic CH?oxidation process.Moreover,the anaerobic CH?oxidation rate of soil under the addition of AQDS+CH?+N₂O was significantly higher than that of CH?+N₂O,AQDS+CH?and CH?treatments,indicating that AQDS as an electron shuttle may promote electron transport and promote the coupling reaction between anaerobic CH?oxidation and N₂O reduction.The total range of net increase of soil organic carbon in the six rice soils was 1.43～7.18 nmol ¹³C g^-1,indicating that anaerobic methane oxidation has carbon sequestration potential.After comparing it with aerobic CH₄oxidation,it was found that the net increase of anaerobic CH₄oxidation and aerobic CH₄oxidation organic carbon differed by 1 order of magnitude,indicating that CH₄oxidation in paddy fields was dominated by aerobic carbon sequestration potential.（4）The results of the 10-month anaerobic culture test showed that the anaerobic CH?oxidation rate of paddy soil increased with the extension of the cultivation time.At 10 months of culture,the anaerobic CH?oxidation rates of FJ,YY and XY soils reached 1.95,1.83 and2.06 nmol ¹³CO?·g?1d?1,respectively.Moreover,the anaerobic CH?oxidation rate of CH?+N₂O addition was significantly higher than that of CH?addition in these three paddy soils,indicating that anaerobic CH?oxidation and N₂O reduction underwent a typical coupling reaction process in the soils of these three paddy fields.The total net increase of soil organic carbon in the six rice soils ranged from 10.40～85.54 nmol13C g?1,indicating that anaerobic CH₄oxidation after 10 months of cultivation also had carbon sequestration potential.The correlation analysis showed that the oxidation rate of anaerobic CH?in the tested rice field was significantly positively correlated with the mcrA copy number of the functional gene of nitrate anaerobic CH?oxidizing archaea,but not with the pmo A copy number of the functional gene of nitrite-type anaerobic CH?oxidizing bacteria.This suggests that nitrate-type anaerobic CH?oxidizing archaea may be involved in the coupling process of anaerobic CH?oxidation and N₂O reduction in paddy soil.