Changes to Modern and Aged Dissolved Organic Matter Inputs into the Kolyma River

Climate change is radically altering the Arctic. These alterations are expected to have immense and cascading implications on the carbon cycling of the region. In particular, our interest lies in the Kolyma River (KR) as it is the largest Arctic watershed entirely lined below with continuous organic-rich permafrost and is equally subject to shrubification. On one hand, it appears that permafrost is predicted to thaw with the potential to release large quantities of ancient organic carbon. On the other hand, nutrient liberation, increased moisture, and warming allow vegetation to colonize more effectively increasing modern sources of organic carbon. The decomposition of permafrost soil organic matter has been postulated to release permafrost-derived dissolved organic matter (DOM) into stream and riverine networks that is exceptionally old and aliphatic in nature. We aimed to improve the evaluation of permafrost dissolved organic carbon (DOC) inputs and fate by implementing ramped pyrolysis oxidation (RPO) and isotopic analysis coupled with ultrahigh-resolution mass spectrometry. Bioincubation analyses were fundamental in replicating the transformations that occur to DOM during riverine transport. We found that highly aliphatic molecular formulae identified in undegraded permafrost DOM contrasts with the comparatively aliphatic-poor composition of KR DOM, suggesting that limited quantities of undegraded permafrost DOM are present in the KR. RPO radiocarbon analysis of KR DOC fractions exhibited no “hidden” aged component indicative of permafrost influence. Consistent with radiocarbon results, thermostability analysis indicated limited biodegraded permafrost DOC in the KR. A mixing model allowed us to estimate permafrost inputs to the KR DOC. Ultimately, our findings highlight that export of modern terrestrial DOM is currently overwhelming any permafrost DOM signature in the KR. This emphasizes the important ramifications of vegetation changes on the composition and biolability of transported DOM in this region with increasing shrubs dominance over herbaceous vegetation and bare ground. To investigate this, we subjected leachates of representative vegetation and soils both to biodegradation incubations and analysis using ultrahigh-resolution mass spectrometry. Biolability was seen to primarily be driven by lower relative abundances of highly unsaturated and phenolic compounds and higher relative abundances of aliphatic compounds and an overall low nominal oxidation state of carbon. In particular, DOC derived from shrubs has a lower biolability when compared to non-woody vegetation, containing on average nearly half as much %BDOC. This indicates that this DOC may persist in the water column which may have large implications as we continue attempts to quantify a permafrost DOC flux. It may be that studies will need to appraise the role vegetation cover plays in altering the age and composition of DOM of a region before investigating inputs of aged sources.