Riparian Cottonwood Trees and Adjacent River Sediments Have Different Microbial Communities and Produce Methane With Contrasting Carbon Isotope Compositions

Abstract

Rivers and their adjacent riparian forests are intimately linked by the exchange of water, nutrients, and organic matter. Both riparian cottonwood trees and adjacent river sediments host microbial communities including archeal methanogens, supporting methane production and emission to the atmosphere. Here we combine microbial community and in vitro stable isotope analyses to characterize the drivers of methane cycling in distinct anoxic habitats (river sediments vs. riparian cottonwood stems) associated with the Oldman River, southern Alberta (Canada). We demonstrate that, differences in the chemical characteristics of organic matter support divergent microbial communities that generate methane from distinct metabolic pathways. Organic matter in river sediments had C/N ratios approximately 50‐fold lower than in tree stems and had more diverse dissolved organic components. Contrasting substrate availability between river sediment and tree stems was likely the primary mechanism for the greater microbial diversity in river sediments than in tree stems, the significantly different bacterial communities, and the trend toward differing abundance of methanogen orders. The methane carbon isotope composition (δ13C values) differed for the tree stem (−103.6‰ to −70.6‰) and river sediment (−55.1‰ to −48.4‰) environments, suggesting that methane was primarily produced via CO2‐reduction in tree stems by Methanobacteriales, while river sediments produced more methane through acetate fermentation primarily by Methanosarcinales. This study demonstrates the importance of organic matter quality and microbial community composition in driving metabolic processes contributing to methane production and emission in rivers and adjacent riparian forests.

A diverse and distinct microbiome inside living trees

“…Source-tracking analyses revealed a limited contribution of the soil microbiome to the wood microbiome, with estimates that only 3-5% of the community originated from mineral soils and 6-13% from organic soils (SI Figure 5). While expected 93,94 given the pronounced substrate and habitat differences, this result supports a relatively minor role of soil communities in shaping the wood microbiome. In our forest-scale survey, paired soil samples (from both organic and mineral horizons) were collected adjacent to each tree to explore soil origins of the wood microbiome, and whether variations in the wood microbiome were a function of varying soil communities.…”

Wetland tree barks are dynamic hotspots for microbial trace gas cycling

“…For example, methanogenic archaea (Methanobacteriales) are highly abundant in sap and heartwoods of various high CH 4 -emitting cottonwoods (Populus sp.) 26,[28][29][30] . In subtropical wetland trees, bark-dwelling methanotrophic bacteria accounted for up to 25% of the total microbial community, and actively mitigated tree stem CH 4 fluxes to the atmosphere by ~30 -40% 31,32 .…”

Large Methane Emissions From Tree Stems Complicate the Wetland Methane Budget

“…In wetland forests, this is particularly important, as soil CH 4 is transported upwards either passively via diffusion, or actively via plant tissues and the transpiration stream, and eventually egressed via tree stems. Studies on the microbial drivers of tree stem CH 4 have also revealed methanogenic communities living within heartwood and living tissues (Feng et al., 2022; Smits et al., 2021; Yip et al., 2019) and also methanotrophic communities within the phyllosphere that is, stems, bark and leaves (Feng et al., 2022; Jeffrey et al., 2021a; Putkinen et al., 2021). Thus, it is clear that methane fluxes from trees can come from diverse sources.…”