Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Oxygen (O 2 ) supply and the related redox potential (E H ) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O 2 concentration and E H is poorly documented under aerobic soil conditions. We investigated how far O 2 consumption of roots and microorganisms in the rhizosphere is replenished by O 2 diffusion as a function of water/air‐filled porosity. Oxygen concentration and E H in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized E H electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O 2 profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O 2 concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O 2 transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O 2 transport was limited, causing a decline in oxygen partial pressures (pO 2 ) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O 2 increased considerably. This was confirmed by E H around 300 mV under aerated conditions, while E H decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO 2 and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, E H gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO 2 ) and is strongly dependent on soil moisture.