Spatiotemporal variations of nitrification in rhizosphere soil for two different rice cultivars at the seedling stage growing under waterlogged conditions

Increasing amounts of evidence is showing that nitrification in the rhizosphere is very significant for the nitrogen nutrition of rice plants. A rhizobox with three compartments and subsequent soil-slicing after quick freezing was used to measure simultaneously the spatiotemporal variations of mineral nitrogen, nitrification and AOB in the rhizosphere soil of two rice cultivars Yangdao 6 (Indica) and Nongken 57 (Japonica) growing at the N level of 30 mg N kg -1. The results obtained can be listed as follows: the main nitrogen form was ammonium (NH+4-N) in flooded paddy soil and NH+4-N concentrations in bulk soil showed almost no changes with incubation time, but NH+4-N concentrations increased with the distance from root zone of both rice cultivars. The NH+4-N concentration of both Yangdao 6 and Nongken 57 in the zone 40 mm from the root at 51 days after sowing, for example, achieved 13.8 and 14.6 mg kg -1 soil, respectively. However, the nitrate (NO-3-N) concentration decreased significantly with the development of the incubation time, although the distribution of NO-3-N was even in the bulk soil. The average NO-3-N concentration for both cultivars was 0.05 mg kg -1 soil. When the two varieties were compared, the NH+4-N concentration was almost the same, while NO-3-N concentration was significantly different at every sampling time. The nitrification activities of both rice cultivars increased with incubation time. Maximal nitrification activities were found in rhizosphere soil, followed by those in the bulk soil and in the root zone in every sampling. In the rhizosphere the nitrification activities decreased with increased distance from the root. The maximal nitrification activity measured at 44, 51 and 58 days after sowing of Yangdao 6 and Nongken 57 rice cultivars was at a distance of 6 mm and 2 mm from root zone, respectively. The maximal nitrification activity values measured were 0.88 and 0.73 mg kg -1 h -1, respectively. The AOB in the root zone, rhizosphere (0～4 mm away from root zone) and bulk soil (4 mm away from root zone) for both rice cultivars increased during the growth periods. The AOB in the root zone was always the lowest, while that in the rhizosphere soil was the highest in both cultivars. In these experiments, the nitrification activities measured were significantly proportional to AOB (r=0.86, p0.01). The nitrate concentration, nitrification activities and AOB of Indica were always higher than those of Japonica rice.