Seasonal fluctuations of the rusty carbon sink in thawing permafrost peatlands

In permafrost peatlands, up to 20% of total organic carbon (OC) is bound to reactive iron (Fe) minerals in the active layer overlying intact permafrost, potentially protecting OC from microbial degradation and transformation into greenhouse gases (GHG) such as CO2 and CH4. During the summer, shifts in runoff and soil moisture influence redox conditions and therefore the balance of Fe oxidation and reduction. Whether this “rusty carbon sink” is stable or continuously dissolved by Fe(III) reduction and reformed by Fe(II) oxidation during redox shifts remains unknown. We exposed ferrihydrite (FH)-coated sand in the active layer along a permafrost thaw gradient in Stordalen mire (Abisko, Sweden) over the summer (June to September) to capture changes in redox conditions and quantify formation and dissolution of reactive Fe(III) (oxyhydr)oxides and associated OC. We found that Fe(III) minerals formed under the constantly oxic conditions in palsa soils overlying intact permafrost over the full summer season. In contrast, in fully-thawed fen areas, conditions were continuously anoxic and by late summer 50.4% of the original Fe(III) (oxyhydr)oxides were lost via dissolution while 44.7% and 4.9% of the Fe remained as Fe(III) and Fe(II) on the sand, respectively. Periodic redox shifts (from 0 mV to +300 mV) were observed over the summer season in the partially-thawed bog due to changes in active layer depth, runoff and soil moisture. This resulted in dissolution and loss of 47.5% of initial Fe(III) (oxyhydr)oxides and release of associated OC in early summer when conditions are wetter and more reduced, and new formation of Fe(III) minerals (34.7% gain in comparison to initial Fe) in the late summer under more dry and oxic conditions which again sequestered Fe-bound organic carbon. Our data suggests that the so-called rusty carbon sink is seasonally dynamic in partially-thawed permafrost peatlands, thus likely either promoting or suppressing carbon mineralization and leading to seasonal changes in GHG emissions.