Impact of Seasonal Waterlogging on Organic Carbon Stabilisation and Microbial Communities in Cultivated Mineral Soils

ABSTRACT Waterlogging of aerobic soils can induce microbially catalysed reductive dissolution of iron oxides and the subsequent mobilisation of associated carbon. Additionally, the resulting shift in microbial metabolism and community structure may eventually affect the accrual of microbially derived carbon into soils. It remains, however, unknown whether temporary anaerobic conditions significantly influence soil carbon cycling in cultivated boreal mineral soils, which are increasingly prone to off‐season waterlogging due to climate change. We investigated the effects of off‐season waterlogging on carbon–mineral associations and microbial carbon immobilisation and community structure in a 1.5‐year greenhouse experiment conducted with intact soil profiles (monoliths) from two agricultural fields in southern Finland. The experiment comprised three cycles of alternating warm growing seasons and cold off‐seasons. Spring barley ( Hordeum vulgare ) was cultivated in all monoliths for the growing seasons, while Tall Fescue ( Festuca Arundinacea ) was under‐sown into half of them as an overwintering cover crop. During the off‐seasons, half of the monoliths were subjected to waterlogging for 7 weeks. We found that waterlogging induced shifts in microbial communities favouring anaerobic taxa capable of iron reduction. Despite this, repeated waterlogging had no significant impact on the iron‐associated or the total mineral‐associated carbon, nor the microbial residue carbon content in either soil. However, waterlogging reduced belowground carbon inputs by limiting cover crop root growth in the subsoil. Overall, our findings suggest that off‐season waterlogging of cultivated mineral soils in a cold humid climate does not compromise their ability to stabilise carbon in mineral‐associated forms.