To what extent do iron organic carbon interactions attenuate C release from permafrost thaw?

Opfergelt, Sophie [UCL] Roux, Philippe [UCL] du Bois d'Aische, Eléonore [UCL] Villani, Maëlle [UCL] Thomas, Maxime [UCL] Osy de Zegwaart-Favart, Cécile [UCL]

Enhanced thawing of the permafrost in a warming Arctic exposes previously frozen soil organic carbon (OC) to microbial decomposition, leading to the release of soil C as greenhouse gases. Depending on temperature and moisture environmental variables, a centennial to millennial-yearold C pool can be reached, thus accelerating the feedback to climate change. Iron-OC interactions in soils and sediments contribute to stabilize OC (by adsorption onto Fe oxides or forming Fe-OC complexes), thus mitigating permafrost C emissions. However, their formation and stability are dependent on soil pH and redox conditions. The heterogeneous soil moisture conditions and drastic changes in soil water pathways upon permafrost thaw make the significance of Fe-OC interactions in attenuating permafrost C emissions uncertain. Using radiogenic Sr isotopes, we show that, in saturated layers, Fe-OC interactions can remain undissociated and preserved since their formation. In contrast, we highlight that at the redox interface, processes of dissolution and precipitation of the Fe-OC interactions occur, changing the OC stabilization potential. Given the implications for overall long-term ecosystem C storage, we will discuss an approach to estimate at the landscape scale in the Arctic: (i) the proportion of permafrost soils Fe with potential for interactions with OC (reactive Fe), and (ii) the locations which are the most sensitive to changes in Fe-OC interactions.