Inferences from the vertical distribution of Fe isotopic compositions on pedogenetic processes in soils

The isotopic compositions of major elements in soils can help understand the mechanisms and processes that control the evolution of soils and the nature and dynamics of the soil constituents. In this study, we investigated the variations of the Fe concentrations and isotopic compositions combined with classical soil parameters, such as granulometry, pH, and C and N concentrations. We selected three soils submitted to different hydrodynamic functioning along a toposequence: a well-drained Cambisol and two hydromorphic soils, an Albeluvisol and a Gleysol. In the Cambisol, the isotopic variations were small indicating little redistribution of Fe which we attributed to centimetric-scale exchanges from the Si-bound to the weakly-bound iron pools and insignificant subsurface Fe export. In contrast, the hydromorphic soils showed an overall variation of 0.37‰ for δ56Fe (δ56Fe (‰) = [(56Fe / 54Fe)sample / (56Fe / 54Fe)IRMM-014 − 1] × 1000) and an inverse correlation between the Fe isotopic compositions and the oxide-bound Fe concentrations. We suggest that, in the uppermost horizon, the mobilisation of oxide-bound Fe was due to the reducing conditions and predominantly involved the light Fe isotopes. Similarly, within the Bt horizon of the Albeluvisol, the fluctuations of the water table level induced changes in the redox conditions and thus Fe dissolution and transport of isotopically light Fe. The Fe isotopic composition profile in the B/C horizon of the Gleysol is dominated by the signature of the parental material. Overall, the variations of the underground water table combined with topography-driven water flow were suggested to be the main mechanisms of Fe translocation in these hydromorphic soils. Finally, the comparison between Fe isotope profiles in worldwide soils allows us to show that Fe isotopic variations can help discriminate between various mechanisms and scales of Fe transfer in soils and, accordingly, provide information on the evolution of soils, when used in combination with pedological, geochemical, geographical, and environmental characterisations.