Redox control of chromium in the red soils from China evidenced by Cr stable isotopes

With chromium isotopes, we study the intricate dynamics of adsorption and redox processes in soil ecosystems, focusing on chromium's behaviour, in red soil profiles enriched with iron-manganese nodules (FMNs) in South China. Key findings reveal that the primary geological source of chromium in the red soil profiles is the weathering of colluvium parent minerals. FMNs have higher chromium concentrations (325-1451 µg/g) compared to surrounding soils (95-247 µg/g) and display stable δ⁵³Cr values (0.78 ± 0.17‰), indicating their role as stable chromium repositories, reflecting historical processes. Furthermore, by isolating chromium associated with iron oxides (FeO) and silicate minerals (ReS) within FMNs and surrounding soils using CBD extractions, we show that FeO predominantly carry chromium, particularly in FMNs. The δ⁵³Cr values of FeO fractions consistently exhibit heavier signatures than ReS fractions, suggesting the sequestration of isotopically heavy chromium (VI) during Fe oxide precipitation. Fluctuations in soil's redox, rather than land use, play a pivotal role in controlling the precipitation of Fe oxides in surrounding soils and the formation of FMNs, thus influencing chromium mobility. This highlights the significance of these factors when utilizing chromium isotopic techniques for source tracking in soil systems, contributing to our understanding of chromium's behaviour in soil environments.