Coupled iron cycling and organic matter transformation across redox interfaces

Soils and sediments are major reservoirs of organic matter (OM), whose dynamic turnover has a major impact on carbon cycling and global climate. OM in soils and sediments is predominantly associated with minerals, which decelerate OM decomposition and could help store carbon. However, iron (Fe) minerals could also degrade OM and release a fraction of OM to the atmosphere as CO2 and CH4, but the coupling of these processes is only partly understood. In this Review, we describe the mechanisms and importance of coupled iron-carbon (Fe-C) cycles. Oxygenation of structural Fe(II) in minerals generates reactive oxygen species, which either degrades or synthesizes OM. Reactive oxygen species can also either decrease or increase extracellular enzyme activity and microbial activity, thus indirectly transforming OM. In addition, Fe(III) reduction contributes to OM oxidation through anaerobic respiration. By contrast, OM affects the redox properties of Fe minerals by serving as electron donor, acceptor, shuttle, buffer or conductor and by co-precipitation and complexation with Fe minerals. These feedback mechanisms can result in complex interconnected Fe-C cycling processes; hence, future work must focus on attaining the net impact of combined Fe-C cycles. Coupled Fe-C cycles are important considerations for carbon sequestration, soil fertility and ecosystem functions. This Review explores the role of Fe minerals in stabilizing and degrading organic matter and the role of organic matter in enhancing reactivity of Fe minerals under different conditions.