High-latitude river basins export disproportionately large amounts of terrigenous DOC to the Arctic Ocean when compared to other major river basins. As climate warms, the amount and chemical composition of DOC exported from these basins are expected to change. Clearly, the leaching of plant litter/upper soil horizons and the leaching of deeper soil horizons produce different biogeochemical fingerprints, which can then be sought in the concentration and chemical composition of organic C species in subarctic rivers, and be used as a proxy for hydrological and permafrost dynamics in these basins.

Production and fate of DOC in warmed terrestrial compartments of permafrost terrains is largely influenced by the interaction between increased microbial activity, vegetation changes, and degradation of permafrost barrier to deep infiltration of solutes. Increased DOC concentrations in Arctic rivers might be supported by enhanced terrestrial primary production, a shift from tundra to forest, increased production in organic topsoil, and release from melting permafrost.

Nevertheless, though DOC fluxes are supposed to increase, driven by warmer air temperatures, through temperature-related processes of DOC production, the increasing retention time of DOC in deep mineral soil will most likely lead to the net decrease of DOC export to rivers due to its stabilization in soils. Under this scenario, warming in high latitudes may result in the increased accumulation of C resistant to biodegradation in deep subsoil. Thus, the adsorptive properties of thawing soils distributed across the subarctic area exert the major control on this process. Another temperature-related factor influencing DOC export is wildfires, likely increased with warming. By reducing the C pool in upper organic layers and increasing the active layer thickness, fires greatly decrease the DOC output from watersheds underlain by permafrost.

Acknowledgements Studies in Central Siberia were supported by the Russian Fund for Basic Research (no. 03-04-48037 and no. 05-05-64208) and INTAS postdoctoral fellowship (YS-06-10000014-5732).

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