Towards Linking Permafrost History to DOM Composition and Biodegradation across the Western Canadian Arctic

Abstract:

There has been increasing effort to characterize carbon derived from thawing permafrost because its release may perpetuate further climate change. Although carbon in dissolved organic matter (DOM) derived from permafrost has a composition that can be highly susceptible to biodegradation (biolabile), diversity in formation and thaw modification processes create clear spatial and stratigraphic variability in permafrost. Despite this, our understanding of how the composition and biolability of DOM derived from permafrost of different origins (end-members) might vary is poor. Furthermore, few studies couple biolability measurements with assessments of the microbial community structure, despite the important role that microbes play in degrading DOM. Here, we investigate how the composition of DOM leached from diverse permafrost end-members may vary, how compositional differences may relate to biodegradation rates, and how microbial communities sourced from contrasting thaw-affected areas may enable differences in biodegradation rates.

Using Fourier transform ion cyclotron resonance mass spectrometry, we identified marked variation in DOM composition among permafrost end-member types. Tills were compositionally dissimilar to all other permafrost end-members. Compounds unique to Yedoma were predominantly aliphatic, while compounds unique to peat, lacustrine, and diamicton spanned saturation and oxygenation. In an incubation experiment, DOM compositional differences among stratigraphic units and sites were linked to varying carbon-normalized biodegradation rates. Protein-like components were preferentially consumed over humic-like components, and these DOM compositional shifts were coupled with shifts in the microbial community structure. The inocula source had a marginal effect on biodegradation rates, but influenced the direction and extent of structural divergence of the microbial communities over time. In all cases, compositional differences appear to reflect not only variation in permafrost parent materials, but also strong effects from thaw-driven modification processes. Constraining DOM composition, biolability, and microbial community structure variability will become more pressing as the spatial and stratigraphic extent of thaw increases with future warming.