Mineralogical associations with soil carbon in managed wetland soils

Abstract Carbon (C)‐rich wetland soils are often drained for agriculture due to their capacity to support high net primary productivity. Increased drainage is expected this century to meet the agricultural demands of a growing population. Wetland drainage can result in large soil C losses and the concentration of residual soil minerals such as iron (Fe) and aluminum (Al). In upland soils, reactive Fe and Al minerals can contribute to soil C accumulation through sorption to poorly crystalline minerals and coprecipitation of organo‐metal complexes, as well as C loss via anaerobic respiration by Fe‐reducing bacteria. The role of these minerals in soil C dynamics is often overlooked in managed wetland soils and may be particularly important in both drained and reflooded systems with elevated mineral concentrations. Reflooding drained soils have been proposed as a means to sequester C for climate change mitigation, yet little is known about how reactive Fe and Al minerals affect C cycling in restored wetlands. We explored the interactions among soil C and reactive Fe and Al minerals in drained and reflooded wetland soils. In reflooded soils, soil C was negatively associated with reactive Fe and reduced Fe(II), a proxy for anaerobic conditions (reactive Fe: R 2 = .54–.79; Fe(II): R 2 = .59–.89). In drained soils, organo‐Al complexes were positively associated with soil C and Fe(II) (Al R 2 = .91; Fe(II): R 2 = .54–.60). Soil moisture, organo‐Al, and reactive Fe explained most of the variation observed in soil C concentrations across all sites ( p < .01). Reactive Fe was negatively correlated to soil C concentrations across sites, suggesting these Fe pools may drive additional C losses in drained soils and limit C sequestration with reflooding. In contrast, reactive organo‐Al in drained soils facilitates C storage via aggregation and/or formation of anaerobic (micro)sites that protect residual soil C from oxidation and may at least partially offset C losses.