Nitrification and denitrification in the rhizosphere of rice: the detection of processes by a new multi-channel electrode

N turnover in flooded rice soils is characterized by a tight coupling betwe en nitrification and denitrification. Nitrification is restricted to the mi llimetre-thin oxic surface layer while denitrification occurs in the adjace nt anoxic soil. However, in planted rice soil O-2 released from the rice ro ots may also support nitrification within the otherwise anoxic bulk soil. T o locate root-associated nitrification and denitrification we constructed a new multi-channel microelectrode that measures NH4+, NO2-, and NO3- at the same point. Unfertilized, unplanted rice microcosms developed an oxic-anox ic interface with nitrification taking place above and denitrification belo w ca. 1 mm depth. In unfertilized microcosms with rice plants, NH4+, NO2- a nd NO3- could not be detected in the rhizosphere. Assimilation by the rice roots reduced the available N to a level where nitrification and denitrific ation virtually could not occur. However, a few hours after injecting (NH4) (2)HPO4 or urea, a high nitrification activity could be detected in the sur face layer of planted microcosms and in a depth of 20-30 mm in the rooted s oil. O-2 concentrations of up to 150 mu M were measured at the same depth, indicating O-2 release from the rice roots. Nitrification occurred at a dis tance of 0-2 mm from the surface around individual roots, and denitrificati on occurred at a distance of 1.5-5.0 mm. Addition of urea to the floodwater of planted rice microcosms stimulated nitrification. Transpiration of the rice plants caused percolation of water resulting in a mass flow of NH4+ to wards the roots, thus supporting nitrification.