Soil iron oxides as geochemical filters in floodplain restorations: Key drivers, dynamics, and links to nutrients and metals.

Soil iron (Fe) oxides play an important role as sorbent and catalyst for nutrient and metal cycling and transformations, but few stream and floodplain restorations have evaluated their role for nutrient and metal removal. Here, we assessed the unrestored versus restored changes in soil Fe oxides and the key soil properties that influenced these changes, for 11 floodplain restoration sites representing a post-restoration period of 0-22 years. Relationships between Fe oxides, nutrients, metals, and denitrification rates were also investigated to quantify their potential role as geochemical filters. Poorly crystalline (Feo and Fep), crystalline (Fec), and total (Fet) Fe oxide forms were quantified using standard chemical extractions. Poorly crystalline or amorphous Fe oxides increased in floodplain soils post-restoration and were strongly and positively correlated with soil moisture and organic carbon content. Soil pH and % sand, silt, and clay contents were important determinants for Fe oxides. These oxides also showed strong correlations with select soil metals and nutrients post restoration. Our observations indicated that restoration approaches and associated changes in site conditions may have greater effects on floodplain soil characteristics and Fe oxides than time since restoration. Knowledge of Fe oxides and the controlling factors along with their effects on nutrient and metal concentrations will help us better design and implement floodplain restoration practices that can maximize Fe-driven nutrient/pollutant removal and thus enhance the effectiveness of these practices.