Fe(III)-(II) Mineral Transformation and Associated Organic Matter Stabilization Processes in a Tidal Marsh Soil.

Iron (Fe) minerals are key agents in organic carbon (OC) stabilization in soils, yet their function under fluctuating redox conditions in tidal marshes remains poorly constrained, particularly for Fe(II) phases. We conducted a six-month in situ incubation of synthetic mineral aggregates along a tidal marsh profile of the Elbe River, Germany, to examine Fe(III)-Fe(II) mineral interactions with OC under dynamic redox conditions. Membrane cylinders containing synthetic ferrihydrite, siderite, or pyrite aggregates with clay and sand were installed at four depths (15-60 cm). A subset received sorbed 13C-labeled reed (Phragmites australis) as dissolved and particulate organic matter to trace OC retention and redistribution. Sequential Fe extraction and Mössbauer spectroscopy showed progressive reduction of ferrihydrite and concurrent formation of siderite in deeper, anoxic layers. Organic matter enhanced Fe(III) reduction and modulated mineral transformation in reducing zones to Fe(II) minerals. In oxic horizons, ferrihydrite stabilized added reed-derived OC through classical surface sorption, while siderite and pyrite retained comparable or higher fractions of mineral-associated OC in deeper, anoxic horizons, primarily within the aggregate, with partial redistribution to surrounding soil. These results demonstrate that Fe(II) minerals non-negligibly contribute to OC stabilization under reducing conditions, a mechanism likely to intensify with ongoing sea-level rise.