2022 journal article
Identifying Sources and Oxidation of Methane in Standing Dead Trees in Freshwater Forested Wetlands
FRONTIERS IN ENVIRONMENTAL SCIENCE, 9.
Wetlands are large sources of methane (CH 4 ), therefore it is vital to understand the pathways, mechanisms, and sources to anticipate future positive feedbacks to climate change. Plant mediated transport of CH 4 from sediment-borne gases is thought to be a major contributor in wetland ecosystems, though few studies have measured standing dead trees (snags). Snags are expected to become more common across the southeastern coast as marshes migrate into freshwater forested wetlands. In this study, our goal was to distinguish the main sources of CH 4 being emitted from snags, that is, from soil or in situ origin. The δ 2 H and δ 13 C stable isotopic composition from various sources was sampled for source determination. We measured CH 4 in various components: emissions from snag stem sides and the soil-atmosphere interface; and concentrations from snag trunk airspace at various heights from ground level (30, 60, and 120 cm), and soil porewater. Potential CH 4 production and oxidation in tree cores from two heights (60 and 120 cm) was also measured to examine the potential for CH 4 generation or oxidation in stems. We found that CH 4 concentrations inside snags (∼10–200 ppm) were 2–50 times higher than atmospheric levels, and generally decreased with increasing stem height. The stable isotopes δ 13 C and δ 2 H showed an enrichment from porewater to soils and snag stems. δ 13 C enrichment of CH 4 in snag stems suggests that CH 4 is being oxidized as it moves through snags. The tree core vial incubations showed that very few cores produced small amounts of CH 4 under anaerobic conditions ( n = 5 out of 50), and very few cores oxidized CH 4 under more aerobic conditions ( n = 5 out of 50). It is possible that a small amount of CH 4 is produced in-situ within the heartwood, but it is likely this depends on the density, porosity, and aeration of snags (degree of decay). Our results highlight that high concentrations of CH 4 can persist within the heartwood of snags long after initial decay, and that CH 4 emitted from snags is largely derived from deep wetland soils and oxidized during transport ( via diffusion) throughout the stem of snags.