Experimental evidence that organo-mineral interactions regulate dissolved organic matter composition and lability across permafrost landscapes of northwestern Canada

Abstract. Increased land-water connectivity of northern landscapes driven by permafrost thaw is shifting the bioavailability of dissolved organic matter (DOM) in surface waters, with implications for northern food webs and regional and global carbon balances. However, sorption of DOM to previously frozen sediments has received little attention as a mechanism of regulating the bioavailability of organic matter in thaw-affected freshwater ecosystems. Using batch sorption experiments, we assessed sorption potential, water-extractable dissolved organic carbon (DOC) concentration, and the impact of sorption on DOM composition of six different permafrost sediment types common throughout northwestern Canada, reflecting variation in geologic and permafrost histories. A principal component analysis revealed that sediment biogeochemical characteristics reflected geologic origin, and past thaw increased within-type variation. Sorption was positively correlated with organo-reactive forms of Al and Fe and negatively correlated with sediment pH. Proportion of bulk sediment organic carbon released as water extractable DOC ranged from 1.0 % to 62.0 %, with yedoma sediments from the Klondike region releasing substantially more than sediments from other regions. Preferential sorption of larger, humic-like compounds and displacement of mineral-bound small, aliphatic molecules enriched the DOM pool in labile compounds. Bio-incubations verified that exposure to sediments increased rates of biodegradation, corresponding with shifts in DOM composition and increased nutrient concentrations. Our experiments demonstrate that organo-mineral interactions have the potential to decrease DOC concentrations while increasing DOM bioavailability following exposure to permafrost-origin sediments, but that the strength of this response varies with sediment characteristics that are reflective of landscape history.