Wetland roots as soil reducers – Insights from a Wadden Sea salt-marsh study

Abstract The redox potential in wetland soils exerts strong control over microbial decomposition and consequently carbon cycling. Wetland plants influence redox by supplying both terminal electron acceptors (i.e. oxygen) and electron donors (i.e. organic matter) to the soil. However, quantitative insight into plant effects on wetland soil redox are scarce. We investigated plant effects on soil reduction using IRIS (Indicator of Reduction in Soils) sticks. Vegetated and unvegetated treatments were created along salt-marsh flooding gradients both in a mesocosm facility and in situ. We show that the presence of plants resulted in both increased and decreased soil reduction compared to non-vegetated controls in the mesocosm study and that the direction of the plant effect (i.e. reducing versus oxidizing) depended on the background redox conditions observed in non-vegetated controls. When background soil reduction was high, plants tended to act as net oxidizers, whereas under more oxidizing conditions plants acted as net reducers of the soil. High-resolution oxygen profiling via planar optode imaging further supported these findings. Plant effects on soil reduction in the field study were negative across all elevational zones. We attribute this effect to the comparably well aerated soils of Wadden Sea salt marshes. Reduction correlated positively with belowground biomass, indicating that a higher availability of plant-derived electron donors increased soil reduction. Challenging the dominant paradigm that wetland plants primarily act as soil oxidizers, our study reveals their potential to exert a net reducing effect.