Controlled soil monolith experiment for studying the effects of waterlogging on redox processes

Climate change induced mild and rainy winters may expose soils to more frequent and prolonged waterlogging in boreal regions. Resulting oxygen depletion induces reductive dissolution of iron (Fe) oxides further altering the stability of Fe-associated organic matter. Thus far, the impact of waterlogging on the coupled cycling of Fe and carbon (C) in upland arable soils remains unknown. We constructed a monolithic experimental system with 32 soil profiles (l = 63 cm, d = 15.2 cm) collected from two agricultural fields (silty clay, sandy loam) to study the effects of off-season waterlogging, overwintering cover crop and soil type on soil redox potential (E_h), Fe solubility, and movement of C and nitrogen (N) within the soil–plant-atmosphere continuum. Soil moisture, temperature, electrical conductivity, and E_h were continuously monitored, and soil pore water samples were collected at three soil depths. Here, we assess the systems suitability for studying coupled Fe and C dynamics in boreal climate, and investigate the treatment impacts on soil E_h, pH, reductive Fe dissolution and N concentration in pore water. Waterlogging led to reducing conditions in both soils down to 30 cm depth at the soil temperature (+4 to 12 °C) matching those of spring and autumn in southern Finland. The declining E_h and the slightly rising Fe concentration in porewater (max ∼ 10 µmol l⁻¹) suggest that reductive dissolution of Fe could proceed even during mild winters if the duration of waterlogging exceeds 1–2 weeks. The study demonstrated that cover crops may accelerate the drop in soil E_h by removing bioavailable N_, and hence controlling the availability of alternative electron acceptors (nitrate) in the soil. Thus, the simultaneous effects of cover crops on C inputs, and on N and water dynamics, all influencing redox reactions, emphasize the importance of incorporating vegetation into studies exploring the impacts of waterlogging on coupled dynamics of Fe and C.