Role of mineral-organic matter interactions in carbon cycling in permafrost peatlands

Permafrost peatlands, primarily occurring at northern latitudes, are a major stock of organic carbon. Although these peatlands have historically acted as a carbon sink, they are expected to transition to a carbon source due to the mobilization and rapid decomposition of accumulated organic carbon by microorganisms upon thawing. Predictions of the timescale of this transition are limited by insufficient understanding of the controls on organic carbon decomposition. One major control on microbially mediated decomposition and release of greenhouse gases carbon dioxide (CO2) and methane (CH4) is the interaction of organic carbon with minerals (predominantly high surface area iron minerals). Although well studied in common soil systems, the role of organic carbon-mineral interactions in carbon cycling is poorly understood in permafrost peatlands. This knowledge gap is particularly critical in the case of permafrost thaw, during which redox conditions may switch from oxic to anoxic due to extensive waterlogging. In this work, we investigated the interaction between organic carbon and minerals and their effect on carbon cycling in thawing permafrost peatlands. We chose Stordalen mire near Abisko, Sweden, as a representative field site as it includes a thaw gradient from intact permafrost plateaus to thaw ponds and fully thawed wetlands. Specifically, we investigated whether interaction with iron minerals protected organic carbon from microbial decomposition and release as CO2 and CH4 over thaw in field manipulation and laboratory microcosm experiments. To do this, we synthesized iron mineral-organic carbon phases using organic matter from the peatlands and added them to the soil. We then followed dissolved organic carbon dynamics as well as CO2 and CH4 release. In addition, we tracked the fate of the minerals over thaw. In a complementary study, we characterized mineral-organic carbon phases present within the soil and thaw ponds. Upon addition of the mineral-organic carbon phases to the soil, we observed a small decrease in greenhouse gas release over the short term, consistent with the results from other soil systems. However, over the timescale of weeks, the greenhouse gas release increased substantially relative to control experiments, especially in the case of CO2 (up to 130%). This increase could be attributed to the use of iron within the mineral-organic carbon phases as an electron acceptor by microorganisms, promoting organic carbon decomposition. The results of this work suggest that (a) iron mineral-associated organic carbon in permafrost peatlands may not be protected from microbial decomposition, and (b) increased water levels in permafrost peatlands due to thaw may result in increased greenhouse gas release, particularly CO2.