Variation Of Methane Flux In Soil And Stem Of Poplar Plantation And Its Microbial Mechanisms

Methane（CH₄）is the second largest greenhouse gas,contributing approximately 23%to global warming.Forest ecosystems have both CH₄ sources and sink functions.Upland forest soils are generally considered to be atmospheric CH₄ sinks,however,more and more evidence is challenging this traditional view.The CH₄ sources in forests are far more extensive than currently recognized,and CH₄ emissions from tree stems have become a“new member”of sources in forests.The forest CH₄ flux has a wide spatial and temporal heterogeneity.Soil and stems are the main atmosphere exchange interfaces in forests.It is of great significance to determine the spatiotemporal variation characteristics,driving factors,and microbial mechanisms of CH₄ flux,to accurately estimate the forest CH₄ budget.Plantations play an important role in coping with climate change.China has the largest number of artificial forests in the world,63%of which are distributed in the subtropical regions,which experience heavy rainfall every summer,often accompanied by severe hydrological changes.As a result,the CH₄ flux in the forests in these regions tends to be highly uncertain.Poplar grows fast,has strong adaptability,can grow in dry and moist habitats,and is widely distributed around the world.It is recognized as a model tree species for studying the plant-microbe relationship.However,the magnitude,mode,driving factors,and microbial mechanism of CH₄ flux in the soil and stems of poplar plantations are not completely clear.In this study,poplar plantations in the Huaihe River basin were taken as the research object,combing field in situ monitoring,lab incubation experiments,and microbial sequencing methods,to monitor the temporal and spatial characteristics of methane flux in soil and tree stems,explore the effects of climate and soil factors on methane flux,and reveal the microbial mechanism of methane flux in soil and tree stems.The main results are as follows:（1）Soil cumulative and average CH₄ flux in Sihong(cumulative flux by 2.02±0.65 mg m^–2 h^–1;average flux by 0.07±0.02 mg m^–2 h^–1)and Dongtai forest farms(cumulative flux by 0.44±0.24 mg m^–2 h^–1;average flux by 0.03±0.02 mg m^–2 h^–1)show net sources,and show weak sinks(–0.007～–0.01 mg m^–2 h^–1)in some individual sampling activities.Soil CH₄ flux showed a unimodal diurnal variation pattern,which may depend on the temperature change.The seasonal difference in soil CH₄ flux is mainly attributed to the difference in precipitation and soil moisture.Soil texture has a significant effect on CH₄ flux,and the CH₄ emission in clay soil is higher than that in sand soil.Soil CH₄ flux increased at a different rate with air temperature,precipitation,soil temperature,and soil water content.（2）The diversity of bacterial and archaeal communities decreased with soil depth.In general,the relative abundance of potential methanotrophs—Proteobacteria and Verrucomicrobia decreased with soil depth,while the relative abundance of potential methanogens—Euryarchaeota increased with soil depth.The transition zone of microbial community composition between the surface and subsurface occurs at 20 cm.Soil bulk density（BD）,p H,soil organic carbon（SOC）,total nitrogen（TN）,and NO₃^-–N at different depths had significant effects on the composition of bacterial and archaeal communities.（3）The lab incubation experiment showed that the cumulative CH₄ emissions from the surface（0–20 cm）and subsurface soil（20–50 cm）increased with the increase in temperature and moisture.The relative abundance and diversity of methanotrophs in surface soil decreased with the increase of moisture when incubated at 5℃but showed a reverse when incubated at 15 and30℃,indicating that the activity of methanotrophs may be stimulated by the elevated temperature,at the same time the inhibition of moisture lost the dominant effect.The relative abundance and diversity of methanotrophs in subsurface soil decreased with the increase in moisture.The relative abundance of methanogens in surface and subsurface soil increased with the increase of temperature when incubated under 150%water-filled pore space（WFPS）,indicating that high temperature and moisture provided a more suitable habituating for methanogens.Soil p H,dissolved organic carbon and nitrogen（DOC and DON）,microbial biomass carbon（MBC）,NO₃^-–N,and NH₄⁺–N had significant effects on the methanotrophic community composition in soil samples incubated under 60%WFPS and on the methanogenic community composition in soil samples incubated under 150%WFPS,respectively.（4）Poplar stem shows net CH₄ emission(0.4～18.1 mg m^–2 stem d^–1),which can offset the soil CH₄ uptake by 4%～31%in the same period.Stem CH₄ emission increased significantly with the increase in temperature,humidity,soil water content（SWC）,and soil CH₄ flux,but decreased with the increase of stem sampling height.The hydrogen trophic CO₂ reduction pathway and methyl trophic pathway are the main CH₄ production pathways in tree stem tissue.Not only methanogenic communities but also methanotrophic communities were detected in the stem tissue.Their coexistence provides a new direction for exploring tree CH₄ exchange/source-sink conversion.In conclusion,soil CH₄ flux in poplar plantations showed unimodal diurnal variation patterns and inconsistent seasonal variation characteristics.Soil texture significantly affected CH₄flux by controlling gas diffusion and microbial activities.Fully understanding the temporal variation dynamics of CH₄ flux would contribute to narrow/reduce the CH₄ budget estimation bias in the ecosystem.Soil temperature and moisture affect the abundance and diversity of methanotrophs and methanogens by regulating soil substrate availability and finally affecting CH₄ flux.Stem CH₄ can be produced by methanogens inhabiting tree tissues through the reduction of CO₂ and methyl compounds,but the significant positive correlation between stem CH₄ emission and soil water content and soil CH₄ flux indicates the potential soil source of stem CH₄.This study provided a basis for the scientific assessment of methane dynamics in poplar plantations,and also provides scientific knowledge for accurate methane budget.