Methane Fluxes From Wetland Trees: Sources, Sinks And Microbial Diversity

Gomez, Carla (2022). Methane Fluxes From Wetland Trees: Sources, Sinks And Microbial Diversity. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00014def

Abstract

The contribution of trees to the methane (CH₄) wetland budget is still uncertain. CH₄ emissions from tree stems need to be investigated with respect to hydrological fluctuations that can affect both CH₄ production and oxidation in soil. The present study focuses on characterising stem CH₄ fluxes at low water tables, the presence and distribution of CH₄ production and oxidation in tree stems and the CH₄ related microbial diversity potentially responsible for these processes.

The study was conducted on Shorea balangeran and Xylopia fusca in a tropical site and Alnus glutinosa and Betula pubescens in a temperate site. These species had previously been investigated in wet soil conditions. Analyses were performed at multiple heights including stem CH₄-CO₂ coupled flux measurements, and CH₄ production and oxidation activity rates. Moisture, density and pH of wood cores were measured as potential controlling factors. The microbial community was characterised from the tree stems, bark and soil.

Stem CH₄ fluxes were significantly lower compared to previous studies, and the presence of CH₄ uptake suggested that wetland trees can potentially switch from sources to sinks with lower water tables. The low stem and soil fluxes reflected the low CH₄ production in drier soils. The CH₄-emission pattern along the length of the tree was heterogenous, with no clear pattern with stem height.

CH₄ production was mostly at the stem base. Both low and high affinity oxidation were detected in all tree species at both sites. A diverse, although sparse community of methanogens and methanotrophs was detected in trees, which were potentially involved in CH₄-cycling. Their diversity and proportion may shift across different seasons, so future studies should consider measuring both the flux and microbial processes year-round, including following extreme weather events if we are to estimate CH₄ emissions accurately in upscaling exercises, process-based models and the global CH₄ budget.

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