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

Warming temperatures are enhancing permafrost degradation, allowing the vast stores of organic matter to undergo decomposition by microbes (biodegradation). Due to differences in the composition of the substrate and structure of the active microbes, not all dissolved organic matter in permafrost is susceptible to biodegradation. Hence, it is unclear how changing dissolved organic matter and microbial community structure might affect the biodegradation rates. To address this question, we completed a 28-day incubation experiment using leachates from active layer, Holocene-origin permafrost, and Pleistocene-origin permafrost from a series of retrogressive thaw slump sites on the Peel Plateau, NT, Canada. We assessed dissolved organic matter composition and microbial community composition at the start and end of the experiment, while also tracking oxygen concentrations throughout to determine biodegradation rates. Our results emphasize the need to account for variation in dissolved organic matter composition and biolability across stratigraphic, landscape, and regional scales, and to better constrain how these factors together may further impact the fate of carbon following permafrost thaw.