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.



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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