Developing a diffusive gradients in thin films (DGT) technique for large-scale and high-resolution mapping of labile...

Potassium (K) is crucial for plant growth, but only a small portion of the K in soil is available to plants. Traditional methods, such as ammonium acetate (NH₄OAc) extraction, are widely used to assess available K but do not provide information on the diffusion and spatial distribution of labile K in soil. This study aimed to develop and validate a diffusive gradients in thin films (DGT) technique using a cation exchange membrane (CXM-200) for quantification and spatial distribution of K availability, using polyhalite (K₂Ca₂Mg(SO₄)₄·2H₂O) (POLY4 a granulated polyhalite product of Anglo American plc (www.poly4.com).) fertiliser applied to soil as a model system. We demonstrate that the CXM-DGT had low detection limits (9.93 μg/device), high linear binding capacity (1687 μg at 0.01 M ionic strength), and good performance across various pH levels (3.5 to 8.0) and ionic strengths (0 to 0.01 M NaNO₃). This approach also had comparable results to Amberlite IRP-69 DGT for K measurement, with the added advantage of being more user friendly and reusable after regeneration. However, the presence of major cations such as Ca²⁺ and Mg²⁺ were found to potentially influence the performance of CXM-DGT, with shorter deployment times (6 h) being required to minimise interference, particularly in soils with elevated cation concentrations. Coupling CXM-DGT with X-ray fluorescence microscopy (XFM) enabled high-resolution mapping of K availability around fertiliser bands. The results showed higher K concentrations around the fertiliser band in an Alfisol (up to 34.6 ± 9.2 mg L^-1) than in an Oxisol (∼15.7 ± 11.3 mg L^-1), with K declining rapidly within ∼20 mm in the Alfisol but diffusing further in the Oxisol; these patterns closely matched CXM-DGT and NH₄OAc-extractable K. The contrasting K behaviour reflects differences in clay mineralogy, CEC and pH, with higher-charge illite/smectite clays and higher pH in the Alfisol retaining K more strongly than the kaolinite-dominated, lower-CEC Oxisol. These findings demonstrate that CXM-DGT is a robust, high-resolution tool for quantifying the spatial distribution of labile K in soils.