Metagenomic Analysis of Thawing Permafrost Highlights Links Between Carbon and Nitrogen Cycling Processes in Abrupt Thaw Simulation

Abrupt permafrost thaw events are projected to contribute up to 40% of permafrost carbon (C) release to the atmosphere. They involve sudden hydrological shifts within the soil column; however, the exact microbial functional pathway shifts induced by these events remain cryptic. To investigate how C and nutrient cycling processes differ in thaw scenarios, we conducted metagenomic analyses in a pilot study on soil and water subsamples (two replicates per depth group, and treatment) after experimental thaw of intact soil columns (1 m tall mesocosms) and soil horizons in isolation (120 mL vial incubations). The microbial community structure response was masked by high depth dependency, with large shifts in functional pathways within the permafrost soil horizon under the “abrupt” treatment. Most pathways of C cycling remained similar under both “abrupt” and “gradual” thaw, while denitrification and sulfate functional pathways were stimulated within the “abrupt” treatment. This is likely caused by thaw water providing more energetically favorable terminal electron acceptors for metabolic pathways such as denitrification, thus inhibiting the terminal stages of C degradation that result in methane production. This is supported by the 90% decrease in production under “abrupt” thaw simulation. This pilot study of “abrupt” and “gradual” thaw simulations highlights the potential for abrupt thaw to enhance carbon-nitrogen interactions by modifying redox conditions. Abrupt thaw events are expected to increase in warming permafrost regions, and results from our study offer a novel perspective on the close interactions that lowland permafrost soils have when cycling C and nutrients.