Soil Organic Matter Stabilization in Floodplain Soils: Role of Particulate Terminal Electron Acceptors

Abstract:

Floodplains are active sites for transport, transformation, and storage of carbon and therefore play a critical role in the carbon cycle. Under oxygen-limited conditions, organic matter mineralization is often coupled to the reduction of particulate terminal electron acceptors (TEAs), such as iron minerals. Organic matter mineralization is potentially limited by the reactivity of these particulate phases toward accepting electrons from the microbial oxidation of organic carbon.

Here, we discuss relationships between the redox reactivity of particulate TEAs and soil organic matter mineralization in the East River floodplain, Colorado, U.S.A. We examined soils from two sites that differ in redox conditions: a permanently saturated oxbow pond, and a fine-grained river meander. We used a mediated electrochemical approach to quantify microbial reduction of particulate TEAs during anoxic soil incubations and to characterize the reactivity toward reduction of these TEAs. Anoxic incubations of oxbow soils showed no microbial reduction of particulate TEAs and low reactivity toward reduction of these TEAs; no carbon dioxide was produced. In contrast, anoxic incubations of meander soils showed microbial reduction of particulate TEAs and higher reactivity toward reduction of these TEAs. Both the microbial reduction of particulate TEAs and their reactivity toward reduction decreased over time and along soil depth; the decrease was accompanied by a decrease in carbon dioxide production. Organic matter mineralization in the incubations was therefore likely limited at least in part by the low redox reactivity of particulate TEAs.