Deciphering the origin of electron exchange capacities in floodplain sediments

Floodplain sediments play a central role in redox-driven biogeochemical cycles through their electron exchange capacity, yet its origin remains unclear because of strong chemical heterogeneity. Here we quantified the electron-accepting and electron-donating capacities of 45 sediments from two Yangtze River floodplains using mediated electrochemical analysis. We identified that electron-accepting capacity was controlled by acid-extractable reactive ferric iron associated with iron (oxyhydr)oxides and expandable phyllosilicates. Electron-donating capacity was mainly contributed by terrigenous humic fractions (12.9–61.2%) and reactive ferrous iron (38.8–87.1%). Lignin-like compounds with low double bond equivalent were the dominant organic contributors to electron-donating capacity. For ferrous iron, its contribution to sediment electron-donating capacity was much smaller than predicted by chemometric analysis because a large fraction of ferrous iron associated with phyllosilicates was redox-inert due to chemical and structural constraints. The composition of electron exchange capacity dictated microbial selection of terminal electron acceptors, driving the conversion of high-redox-potential electron-accepting capacity into electron-donating capacity. This transformation of electron exchange capacity has important implications for suppressing methanogenesis in floodplains and for oxidant consumption during contaminated aquifer remediation. Floodplain sediment redox behavior depends on the chemical form not the amount of iron and organic matter, with impacts on microbial activity, methane suppression, and contaminant remediation, according to mediated electrochemical measurements of electron exchange capacity.