Review article: Terrestrial dissolved organic carbon in northern permafrost

As the permafrost region warms and permafrost soils thaw, vast stores of soil organic carbon (C) become vulnerable to enhanced microbial decomposition and lateral transport into aquatic ecosystems as dissolved organic carbon (DOC). The mobilization of permafrost soil C can drastically alter the net northern permafrost C budget. DOC entering aquatic ecosystems becomes biologically available for degradation as well as other types of aquatic processing. However, it currently remains unclear which landscape characteristics are most relevant to consider in terms of predicting DOC concentrations entering aquatic systems from permafrost regions. Here, we conducted a systematic review of 111 studies relating to, or including, concentrations of DOC in terrestrial permafrost ecosystems in the northern circumpolar region published between 2000 and 2022. We present a new permafrost DOC dataset consisting of 2845 DOC concentrations, collected from the top 3 m in permafrost soils across the northern circumpolar region. Concentrations of DOC ranged from 0.1 to 500 mg L - 1 (median = 41 mg L - 1 ) across all permafrost zones, ecoregions, soil types, and thermal horizons. Across the permafrost zones, the highest median DOC concentrations were in the sporadic permafrost zone (101 mg L - 1 ), while lower concentrations were found in the discontinuous (60 mg L - 1 ) and continuous (59 mg L - 1 ) permafrost zones. However, median DOC concentrations varied in these zones across ecosystem type, with the highest median DOC concentrations in each ecosystem type of 66 and 63 mg L - 1 found in coastal tundra and permafrost bog ecosystems, respectively. Coastal tundra (130 mg L - 1 ), permafrost bogs (78 mg L - 1 ), and permafrost wetlands (57 mg L - 1 ) had the highest median DOC concentrations in the permafrost lens, representing a potentially long-term store of DOC. Other than in Yedoma ecosystems, DOC concentrations were found to increase following permafrost thaw and were highly constrained by total dissolved nitrogen concentrations. This systematic review highlights how DOC concentrations differ between organic- or mineral-rich deposits across the circumpolar permafrost region and identifies coastal tundra regions as areas of potentially important DOC mobilization. The quantity of permafrost-derived DOC exported laterally to aquatic ecosystems is an important step for predicting its vulnerability to decomposition.