Coupled shifts in DOM composition and microbial community structure lead to variable biodegradation rates in thermokarst-affected permafrost stratigraphies

Abstract

Permafrost thaw can drastically alter dissolved organic matter (DOM) composition within fluvial networks, and simultaneously affect the microbial communities that degrade DOM. However, it is unclear how coupled thaw-induced change in DOM and microbes might affect microbial decomposition of permafrost-origin DOM (biodegradation), and therefore possible mineralization to carbon dioxide. Here, we use a series of incubations to explore how biodegradation varies with DOM and microbe source, and how microbial community composition changes following incubation with thaw-origin DOM. We undertake this work using leachates from different stratigraphic units across a series of retrogressive thaw slumps on the Peel Plateau, Canada, and microbial communities from upstream of, and draining, slumps. DOM composition and biodegradation varied by stratigraphic unit and across sites that were only tens of kilometers apart, but situated along different recessional fronts of the Laurentide Ice Sheet. Permafrost leachates from paleo-active layers were generally more biolabile than leachates from deeper, unmodified tills, and both were more labile than active layer leachates. Biodegradation also tended to be slightly greater for incubations inoculated with microbes from unimpacted stream water. These results emphasize that permafrost thaw-derived DOM composition and biolability will vary across stratigraphic, landscape, and regional scales, and that the composition of the recipient microbial community may play a role in determining immediate DOM fate.