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: R² = .54-.79; Fe(II): R² = .59-.89). In drained soils, organo-Al complexes were positively associated with soil C and Fe(II) (Al R² = .91; Fe(II): R² = .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.

Keywords: agricultural soils; aluminum; carbon loss; carbon sequestration; drained wetlands; iron.