Visualization of local concentrations of mobile soil colloids: advancing the Diffusive Gradients in Thin Films Method

Mobile colloids of clay, oxides and organic matter can play a major role in nutrient and contaminant mobility in soil, though their true role remains unclear due to biases in sampling methods. Sampling colloids in soil is challenging because current methods disrupt the soil structure, colloids are retained in sampling devices and/or extraction methods change the in situ environment.

The Diffusive Gradients in Thin Films (DGT) method allows in situ sampling of solutes in soil. The DGT consists of a hydrogel diffusive layer for controlled diffusion to an analyte-specific binding layer that accumulates solutes when applied in soil. Analyzing the binding layer with a spatially-resolved method, like Laser Ablation (LA)-ICP-MS, reveals the 2D distribution of solutes in soil solution at micrometre resolution.

This study aims to develop the imaging DGT technique to sample not only solutes, but also mobile colloids, in undisturbed soils.  We speculate that this method will improve our understanding of the migration of compounds in soil compared to established methods because it visualizes local colloid and solute concentrations at sub-mm scale. This was tested in the plough pan of soil from a long-term field trial with manure application where we anticipated identifying hotspots of colloidal phosphorus (P) release associated with anaerobic microsites, in line with previous research. The DGT setup consisted of a 9 µm thick membrane, which provides a short diffusion length to increase the sensitivity for colloids, which have slow diffusion rates. The large membrane pore size cutoff of 1 µm allows unrestricted passage of colloids (< 200 nm), which we showed are retained in hydrogel diffusive layers. The DGT used a zirconium oxide-based binding layer previously developed for phosphate, which we identified as the best-performing binding layer for organomineral iron (Fe) colloids.

Sampling was done in winter 2023-2024 when the annual drainage was at a record high due to high rainfall and low evapotranspiration. We unexpectedly detected mobile clay mineral colloids on the DGT by using advanced LA-Time-Of Flight (TOF)-MS. This fast non-target elemental analysis allowed us to identify clay colloids from the co-localisation of Al, Si, Rb and Cs (and not P) and is the first image of mobile colloids in soil. The presence of clay colloids is underpinned by colloid analysis of pore water extracted from the same soil using Field Flow Fractionation analysis. The low Ca concentration (< 1 mM) in soil solution related to prolonged winter rainfall, not the presence of anaerobic microsites, likely explained the nature of these mobile colloids.

Further experiments are currently being undertaken to understand clay colloid uptake on the DGT binding layer.  The DGTs are deployed in suspensions with purified native clay colloids and pore water colloids from the plough layer at various deployment times to assess clay colloid accumulation and the method’s suitability for measuring mobile clay colloids in soil.