Characterization Of Anaerobic Oxidation Of Methane In Wetland Soil Driven By Different Electron Acceptors

Methane(CH₄)is the second greenhouse gas after CO₂,and the global warming potential(GWP)per unit mass CH₄is 28 times that of CO₂.In nature,the metabolic pathways of CH₄are mainly divided into two types:Aerobic methane oxidation(AMO)and anaerobic methane oxidation(AOM).Methane anaerobic oxidation can not be ignored for methane emission reduction.Methane emissions from paddy wetlands account for 12?26 percent of global anthropogenic CH₄emissions and are one of the major anthropogenic sources of methane in the atmosphere.Reclamation is an important land use method.Coastal tidal flat wetland forms field through reclamation,which changes its original physical and chemical properties,and the anaerobic oxidation process of methane may be affected.In this study,taking the coastal reclamation paddy wetland and Nanjing paddy wetland as the research objects,anaerobic culture experiments were carried out using ¹³CH₄stable isotope labeling,to study the influence of different electron acceptors on the the possible mechanism of methane anaerobic oxidation and the effect of different electron receptors on soil organic carbon accumulation.Main conclusions are as follows:1.Anaerobic oxidation of methane occurs in paddy wetlands of different reclamation years.The AOM potential of Guangtan wetland,27 years paddy field and 86 years paddy field were 29.9?91.7 nmol ¹³C g^-1soil?82.3?393.7 nmol ¹³C g^-1soi,100.3?284.4 nmol ¹³C g^-1soil,respectively.The reclamation paddy field is significantly higher than the Guangtan wetland.The addition of electron receptors(NO₃^-,N₂O,SO₄^2-,Fe³⁺,AQDS,HA)significantly increases AOM potential.With the exception of SO₄^2-,Fe³⁺treatments,soil sample WK27have the greatest potential,all other electron receptor mediated AOM potential increased with the increase of reclamation years.The net increase of ¹³C-SOC in soil during AOM process was 11.4.4?710.0 nmol ¹³C g^-1soil,indicating that the AOM process may be one of the sources of organic matter in soil.2.Anaerobic oxidation of methane occurs in soil samples of different depth of Reed wetland and reclamation paddy wetland,AOM potential is 39.0?230.4 nmol ¹³C g^-1soil,82.4?398.3 nmol ¹³C g^-1soil,respectively.The wetland of reclamation paddy field is higher than that of reed wetland.Electron receptor treatments(NO₃^-,N₂O,SO₄^2-,Fe³⁺)of reed wetland has the highest AOM potential in deep soil samples.For reclamation paddy wetlands,Fe³⁺treatment AOM potential increases significantly with depth,however there was no significant difference in the AOM potential of other electron receptors with depth.Addition of each electron receptor promotes the AOM process.123.1?623.4 nmol ¹³C g^-1soil ¹³C-CH₄were assimilated to soil organic carbon by anaerobic oxidation of methane.It is beneficial to the accumulation of soil organic carbon.3.Methane anaerobic oxidation process was confirmed by isotope labeling in paddy fields of Nanjing and coastal reclamation.The anaerobic oxidation rate of methane in Nanjing paddy soil is 0.45?1.84 nmol ¹³C g^-1soil day^-1,the anaerobic oxidation rate of methane was0.16?0.82 nmol ¹³C g^-1soil day^-1in coastal reclamation paddy wetland.Note that N₂O can also act as electron receptors to drive methane anaerobic oxidation in two soils,and the AOM rate of Nanjing paddy wetland was significantly higher than that of coastal reclamation paddy wetland.Also,more than 60%of ¹³C-CH₄assimilate into soil organic carbon.Net transformation in Nanjing paddy soil sample is 120 nmol ¹³C g^-1soil,1.4 times higher than wetland soil samples in reclamation paddy fields,indicating that the methane anaerobic oxidation process promotes the accumulation of soil organic carbon.4.The organic electron acceptor AQDS(anthraquinone-2,6-disulfonate),MV(methyl violet)can not only act as terminal electron acceptor to drive the methane anaerobic oxidation process,but also act as an electron shuttle to promote methane anaerobic oxidation coupling N₂O reduction.The AOM rates of AQDS and MV as electron receptors were 1.09 and 1.03nmol ¹³C g^-1soil day^-1,respectively.There was no significant difference with the AOM rate of N₂O as electron receptor.AQDS and MV as electron transporters increased the AOM rate of N₂O as electron receptors by 142%and 56%,respectively.58.9?62.5 percent of ¹³C-CH₄assimilated into soil organic carbon.These results suggest that humic acids(AQDS congeners)in soil humus play an important role in mitigating greenhouse gas(CH₄and N₂O)emissions and increasing soil organic carbon accumulation in paddy fields.5.AQDS can be used as terminal electron acceptor for methane anaerobic oxidation in paddy soil.The addition of AQDS significantly increased the nitrate AOM rate and the amount of ¹³C-CH₄assimilation into soil organic carbon,while it significantly reduced nitrite type AOM rate and the amount of ¹³C-CH₄assimilation into soil organic carbon.The results showed that AQDS significantly increased the total denitrification AOM rate and the amount of ¹³C-CH₄assimilation into soil organic carbon.These results suggest that quinones in natural organic matter can act as electron transporters to reduce methane emissions by facilitating AOM coupling with denitrification processes.