Contribution of nitrification happened in rhizospheric soil growing with different rice cultivars to N nutrition

A rhizobox with three compartments and soil slicing followed by quick freezing were used to study the spatiotemporal variations of nitrification of rhizospheric soil of Yangdao 6 (Indica) and Nongken 57 (Japonica). The results obtained revealed that ammonium () was the main N form in flooded paddy soil. A concentration gradient for was observed with the lowest concentration nearer to the root zone and the concentrations increased with increasing distance from the root zone. No concentration gradient was observed for nitrate (). The nitrification activities of both rice cultivars increased with the development of the incubation time. The nitrification activities were maximal in rhizospheric soil, followed by those in bulk soil and in the root zone. In the rhizosphere, nitrification activities decreased with increasing distance from the root zone. 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 and 2 mm away from the root zone, respectively, and they were 0.88 and 0.73 mg kg⁻¹ h⁻¹, respectively. In this experiment, the nitrification activities were significantly and positively correlated with the ammonia-oxidizing bacteria (AOB) abundance (r=0.86, p<0.01). The nitrification activity, concentration, AOB abundance, dry matter and N accumulation and leaf reductase activity associated with Indica were always higher than those with Japonica. Therefore, nitrification in rhizosphere had more important significance for N nutrition, especially for the Indica rice cultivars.