Peatlands Versus Permafrost: Landscape Features as Drivers of Dissolved Organic Matter Composition in West Siberian Rivers

West Siberia contains some of the largest soil carbon stores on Earth owing to vast areas of peatlands and permafrost, with the region warming far faster than the global average. Organic matter transported in fluvial systems is likely to undergo distinct compositional changes as peatlands and permafrost warm. However, the influence of peatlands and permafrost on future dissolved organic matter (DOM) composition is not well characterized. To better understand how these environmental drivers may impact DOM composition in warming Arctic rivers, we used ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry to analyze riverine DOM composition across a latitudinal gradient of West Siberia spanning both permafrost-influenced and permafrost-free watersheds and varying proportions of peatland cover. We find that peatland cover explains much of the variance in DOM composition in permafrost-free watersheds in West Siberia, but this effect is suppressed in permafrost-influenced watersheds. DOM from warm permafrost-free watersheds was more heterogenous, higher molecular weight, and relatively nitrogen enriched in comparison to DOM from cold permafrost-influenced watersheds, which were relatively enriched in energy-rich peptide-like and aliphatic compounds. Therefore, we predict that as these watersheds warm, West Siberian rivers will export more heterogeneous DOM with higher average molecular weight than at present. Such compositional shifts have been linked to different fates of DOM in downstream ecosystems. For example, a shift toward higher molecular weight, less energy-rich DOM may lead to a change in the fate of this material, making it more susceptible to photochemical degradation processes, particularly in the receiving Arctic Ocean. West Siberia is warming faster than other regions and contains vast areas of peatlands and permafrost, which contain vast stores of carbon. This carbon is transported off the landscape by rivers and the composition of this exported carbon is likely to change with continued warming, but there is no consensus on exactly what changes will occur. To study these potential changes, we used ultrahigh resolution mass spectrometry to analyze molecular-level organic matter composition across a gradient of permafrost influence and peatland cover in West Siberian watersheds. Warm permafrost-free watersheds had organic matter that was more diverse, of higher molecular weight, and had unique molecular composition compared to cold permafrost-influenced watersheds. We also found that while peatland cover explained much of the compositional diversity between rivers, permafrost ultimately controlled the influence of peatland cover on dissolved organic matter composition, effectively acting as a switch on the compositional signal from peatlands. We predict that as West Siberia warms, the fate of organic matter transported by rivers in the region will thus change and the role of photochemical degradation processes may become more important. West Siberian watersheds exhibit distinct dissolved organic matter (DOM) composition related to permafrost influence and peatland cover Permafrost acts as a switch controlling the influence of peatland cover on the molecular composition of watershed DOM Warming West Siberian watersheds may export more heterogeneous DOM with ramifications for its fate in the Arctic Ocean