Free and occluded colloidal transport dynamics: a novel approach using stable Fe-isotope labeling

To accurately assess nutrient dynamics and contaminant transport in soil ecosystems, understanding colloidal transport is crucial, but its role in the nutrient cycle has not yet been sufficiently explored. In this study, we examined the composition, isotopic enrichment, and transport of free and occluded colloids isolated from a ⁵⁷Fe-labeled agricultural soil. Elemental analyses showed differences in elemental composition between the different types of colloids. Free colloids were richer in Ca, while occluded colloids were higher in Fe and K. Isotopic analyses showed, that ⁵⁷Fe enrichment was concentrated in the fine free fraction (<20 µm; up to +690 ‰), while occluded colloids demonstrated only minor shifts (+90 ‰). Column experiments under saturated flow conditions showed no detectable ⁵⁷Fe breakthrough, with the isotopic enrichment confined to the upper 1 cm of soil, consistent with strong retention and dilution of the label. This work also highlights the potential and limitations of stable metal isotope labeling for tracing natural colloids in soils. Mass-balance calculations demonstrated that ⁵⁷Fe tracer detectability is governed primarily by isotopic enrichment rather than total Fe added, and an enrichment of approximately 0.255 g ⁵⁷Fe kg^-1 soil would be required for dependable detection of colloids above analytical precision.