Dominant Factors Of Methane Anaerobic Oxidation And Their Response Mechanisms To Fe And Mn Inputs In Coastal Wetlands Of The Yellow River Delta

Anaerobic oxidation of methane（AOM）is an essential way to reduce the emissions of methane（CH₄）and mitigate the greenhouse effect,which results in a significant reduction of CH₄produced in marine and freshwater environments.Coastal wetland soils are rich in organic matter and various electron acceptors,which provide good conditions for the occurrence of the AOM reaction.However,the reaction is influenced by a variety of factors,and the exact process and related mechanisms of the reaction remain unclear due to the complex interactions among multiple influencing factors in nature.In addition,Fe and Mn are important variable-valence metal elements in coastal wetland environments,and the metal-reducing bacteria-mediated metal-dependent AOM processes are coupled with carbon and nitrogen cycles and have significant thermodynamic advantages,playing an important driving role in biogeochemical cycling processes.However,the effects of Fe and Mn oxides addition on AOM are still unclear in the current study,and the mechanisms of their effects vary greatly.In this study,the coastal wetlands of the Yellow River Delta were used as the study area,and laboratory incubation methods combined with molecular biology techniques were used to investigate in depth the drivers of AOM processes in coastal wetlands and their response mechanisms to Fe,Mn inputs.These are important for reducing CH₄and mitigating global warming,and provides a microbial ecological perspective for efforts to promote the cause of carbon neutrality in China.The conclusions are as follows:（1）Pearson correlation analysis showed that soil depth,pH,ORP,EC,TN,NO₃^-,SO₄^2-,and N₂O fluxes were important environmental factors influencing the AOM process in coastal wetlands.When multiple electron acceptors co-exist,NO₃^-and N₂O might be the key electron acceptors driving the AOM process.The results of PLS-PM showed that soil N substances was significantly and positively correlated with the AOM rate,and the N substances would further influence AOM processes by affecting key functional microorganisms.Given the large N inputs to coastal wetland environments from current and increasing human activities,N cycle-mediated AOM processes will become a more critical CH₄ sink in coastal wetlands and will play a more prominent role in limiting global climate change in the future.（2）High-throughput sequencing of 16S rRNA and qPCR results indicated the widespread presence of microorganisms associated with the AOM process in soil samples,which provided suitable molecular biological conditions for the occurrence of AOM reactions in coastal wetland soils.The results of correlation analysis,on the other hand,indicated that denitrifying communities（nar G,nir S,nir K,nos ZI,and nos ZII）and ANME-2d were important functional microorganisms affecting the AOM process,and PLS-PM further demonstrated that denitrifying communities were the key microorganisms playing a dominant role in the AOM process.（3）AOM rates responded differently to Fe³⁺input in various habitats,and Fe³⁺input did not always lead to a promoted response of AOM rates.Different TN content in the environment was an important environmental factor contributing to the difference in AOM response to Fe³⁺input,i.e.,lower background values of soil TN content positively influenced the response of AOM following Fe³⁺input,while the opposite was true for higher TN content.Given the environmental context of increasing anthropogenic N input to coastal wetlands and Fe input to estuaries globally,these findings implied that the ability of Fe³⁺cycle-mediated AOM may gradually weaken,while its response process would become more complex and more strongly coupled between multiple factors.（4）Higher soil microbial diversity had led to the positive responses of AOM to Fe³⁺input,where the Shannon index could be an important microbial diversity indicator reflecting the change of AOM response.Meanwhile,the AOM response was influenced by the participation of various microorganisms,among which Planctomycetota and Burkholderiaceae were the key microorganisms causing AOM to exhibit promoting and inhibiting effects,respectively.Whereas Anaerolineae,Anaerolineales,Anaerolineaceae,Bacillus and Halomonas resulted in no significant effect on AOM.（5）Depending on the soil environment,Mn⁴⁺input affected the AOM rate differently,i.e.,the AOM rate was not directly promoted by Mn⁴⁺input.The process may depend on the type of Mn input and the involvement of other factors in the environment,in addition to the background value of soil Mn content.TN content modulated the difference in the response of AOM,i.e.,at high N environment,the input of Mn⁴⁺made the AOM process more likely to exhibit non-inhibitory effects.In the current environmental context of increasing estuarine eutrophication and the input of Mn metal substances,the coupling link between N elements as well as Mn⁴⁺may bring about an important contribution to the future AOM process.（6）Inhibition of AOM by Mn⁴⁺input tended to occur in soils with low microbial diversity and vice versa in soils with high diversity.Among them,Shannon and Simpson indices may be important microbial indicators for the difference in AOM response.In addition,the differences in AOM response may also be related to the involvement of multiple functional microorganisms.Among them,Gemmatimonadota,BD2-11＿terrestrial＿group,Nitrosococcales,Anaerolineae,Steroidobacterales,Clostridiales,Lautropia Rhodocyclaceae,Nitrosomonadaceae,Comamonadaceae,Ellin6067,Desulfuromonas,Chitinophagales,Methylophilaceae,MND1,RB41,Pyrinomonadaceae,and Pyrinomonadales may all be key functional microorganisms that play a role in the variation in AOM response.