Microsensor analysis of oxygen and pH in the rice rhizosphere under field and laboratory conditions

O(2) and pH microsensors were used to analyse the microdistribution of O(2) and pH inside and outside roots of lowland rice (Oryza sativa L.). The roots of 3-week-old transplants had O(2) concentrations of about 20% air saturation at the surface, but due to a high rate of O(2) consumption in the rhizosphere, the oxic region only extended about 0.4 mm into the surrounding soil. Also the fine lateral roots created an oxic zone extending about 0.15 mm into the soil. The O(2) concentration within the roots approached air saturation close to the base, but only about 40-60% of air saturation in a region about 8 cm below the base where lateral rootlets were present. A shift from air to N(2) around the leaves led to a drop of 50% in the O(2) concentration after 12 min at a distance of 8.5 cm from the base. Flowering plants did not export O(2) to the soil from the majority of their roots, but high microbial activity was present in a very thin biofilm covering the root surface. A brown colour around the thin lateral roots indicated some O(2) export from these also during flowering. No oxidized zone was present around the roots at later stages of crop growth. The roots caused only minor minima in pH (< 0.2 pH units) in the rhizosphere as compared to the bulk soil. Illumination of the plants had no effect on rhizosphere pH.