PNNL

Abstract

Coastal environments are dynamic interfaces that mediate carbon and nutrient exchanges between terrestrial landscapes and open water bodies, and understanding the biogeochemical factors controlling these exchanges, particularly iron (Fe) redox transformations, is crucial for predicting coastal ecosystem functions. Here, we investigated the mechanisms controlling Fe speciation changes across upland-to-shoreline gradients in freshwater and estuarine soils, using Fe K-edge X-ray absorption spectroscopy, solids and porewater composition analysis, and 16S rRNA sequencing analysis. We show that Fe transformations vary across gradients and depend on site-specific inundation patterns. In unsaturated uplands, Fe occurs as Fe(III)-oxyhydroxides, mainly goethite (a-FeOOH), Fe(II,III)-phyllosilicates, and Fe(III)-organic species. Soils influenced by estuarine waters exhibit sulfide in the porewater and an abundance of S-cycling bacteria and pyrite (FeS2), indicating that sulfur-driven redox dynamics control Fe transformations. In lacustrine wetlands, Fe(III) reduction, likely driven by Fe(III)-reducing bacteria and potentially coupled with cryptic S-cycling, is indicated by increasing porewater Fe(II) concentrations and the formation of Fe(II,III)-(hydr)oxides as green rust, vivianite (Fe3(PO4)2•8H2O), and adsorbed Fe(II) species. In clay-rich soils, EXAFS results also suggest the partial reduction of structural Fe(III) to structural Fe(II) in phyllosilicates. Fe(II) re-oxidation was indicated above/near the water table by the presence of poorly ordered ferrihydrite or lepidocrocite (?-FeOOH). Furthermore, little to no reduction of Fe(III) or sulfate was observed at some water-saturated sites located at the upland-wetland transition, suggesting that surface water or groundwater inputs impacted redox processes. Overall, our results highlight the importance of considering both Fe-speciation and hydro-biogeochemical dynamics when predicting Fe cycling at coastal interfaces.