Evolution of Retisol impacted by artificial drainage: What can we learn from stable Fe isotope ratios?

Abstract

Iron oxides are one of the most reactive mineral phases in soils. As a consequence, their transformations can considerably affect the dynamics of the adsorbed elements and of the associated soil ecosystem services. Understanding the dynamics of Fe oxides in soils is therefore a key issue for the evolution of soil and associated ecosystem services. A potentially powerful tool to study the transformations of Fe-oxides in soil is stable Fe isotopes. However, there are still important gaps in our knowledge of Fe isotope fractionations.In order to examine the Fe isotope fractionations related to each process occurring in soils, we focused on a drainage-influenced sequence of Retisols, a soil type characterized by clay translocation and subsequent degradation by redox processes inducing a strong spatial Fe segregation in contrasted soil volumes. We combined the isotopic approach at the scale of a bulk horizon and at the scale of the different soil volumes, with mineralogical analyses and mass balance calculations in order to investigate the consequences of the drainage on Fe isotope fractionation. We showed that while there were no Fe isotope fractionations at the profile scale, Fe isotopic signatures varied significantly among soil volumes (δ56Fe values from −0.49 ± 0.05 to 0.29 ± 0.06‰). These variations suggest that redox processes are the main mechanisms responsible for the Fe redistribution among the volumes, and particularly that Fe accumulation during Mn oxide precipitation makes a significant contribution to Fe isotopic fractionation, during these Retisol differentiation. In contrast, drainage-induced eluviation does not result in further Fe isotope fractionations in soil volumes in these Retisols. The isotopic signatures of the different mineral phases present in the volumes were calculated using the mass balance approach and suggest that the iron oxides (goethite, ferrihydrite) have δ56Fe values close to 0‰, while the clay minerals are enriched in heavy Fe isotopes and the Mn oxides in light Fe isotopes. This study provides insight into the dynamics of Fe minerals in hydromorphic soils and offers a new perspective on stable Fe isotope fractionation in soils.