Millimeter Scale 2D Colorimetric Imaging of Available Ammonium in Soils Determined with the Diffusive Gradient in Thin Films Technique.

This study developed a novel approach to visualize in situ ammonium (NH4+) concentrations in soils at millimeter resolution using the Diffusive Gradients in Thin Films Technique (DGT). Available (diffusible) NH4+ is captured with a zeolite 13X binding gel and visualized through two staining gels utilizing Berthelot color reagents. The NH4+ accumulation is linear up to 3 μmol of NH4+ cm-2; however, the color intensity remains linear only up to 0.06 μmol cm-2, beyond which the color decreases. An adapted method achieves linearity up to 10-fold higher concentration at the expense of sensitivity. A linear relationship was found between NH4+ porewater concentrations and color intensity, with 1.6-fold variation in slope among contrasting soils, reflecting cation competition and local buffering effects. The spatial resolution is about 3 mm, which is highly dependent on the deployment or staining times. This technique was field-tested, identifying NH4+ depth profiles and local heterogeneities in wheat rhizosphere after fertilization.