Applications of microelectrode techniques to measure pH and oxidation-reduction potential in rhizosphere soil

Microelectrodes are powerful analytical tools that can be used to measure microscale environments in rhizosphere soil. In this study, custom-made microelectrodes were used to measure pH and oxidation-reduction potential (ORP or redox potential) distributions in vegetated and unvegetated munitions-contaminated soils. While pH was essentially uniform in unvegetated soil, in the vegetated soil the pH was lower near the root tips than at other locations around the root. The pH varied little in the radial (horizontal) direction but was highest (pH = 6.20) near the surface (0.5 cm depth) and lowest (pH = 4.99) at 6 cm below the soil surface. There was a strong correlation between the redox potential and pH. Therefore, redox potentials were normalized to pH 7 based on an experimentally obtained ORP-pH curve. In the vegetated soil, the redox potentials increased as the center of the plant was approached. The highest (ORP7 = 256 mV) and lowest (ORP7 = 191 mV) redox potentials were measured at 0.5 and 3 cm below the surface of the soil, respectively. In the unvegetated soil, redox potential decreased with depth, which can be attributed to reduced transport of oxygen from the air to soil.