More and more evidence shows that nitrification happened in rhizosphere soil of rice is of significance to nitrogen (N) nutrition of rice plants. Three japonica rice cultivars (4007, Wuyunjing7 and Elio) with different N use efficiencies were used to study rice grain yields, total N accumulation and the nitrification characteristics under three N treatments, such as zero N level (0 kg N/hm²), moderate N level (180 kg N/hm²) and high N level (300 kg N/hm²) in field conditions. The results obtained were described as follows. There were significant differences of rice grain yields among the three rice cultivars under different N application rates. The maximal and minimal grain yields were obtained in 4007 in the moderate N level and in Elio in the zero N level, achieved 11117 kg/hm²and 5322 kg/hm2, respectively. There were significant differences of the total N accumulation among the three N treatments, and the total N accumulation increased with the increase of the N fertilizer application rates. Significant differences were found in the fertilizer-N use efficiency and rice grain yields among the three rice cultivars under different N application rates. For example, the fertilizer-N use efficiency of 4007 was always significantly higher than those of the Wuyunjing 7 and Elio in both moderate and high N level treatments, and the average fertilizer-N use efficiency in the high N level treatment was 42.2% lower than that of the moderate N level treatment. Based on the fertilizer-N use efficiency and grain yield at zero N fertilization level, the three rice cultivars could be defined as efficient and responsive (4007), efficient and nonresponsive (Wuyunjing7) and nonefficient and nonresponsive (Elio).
Under the water management of alternation of wetting and drying during the middle-late rice growing stages, the main N form in the rice growing rhizosphere soil was nitrate (NO₃^--N) in the zero and moderate N level treatments, while ammonium (NH₄⁺-N) was the main N form in the high N level treatment. The contents of NH₄⁺-N and NO₃^--N in the rhizosphere soil increased with the increase of the N fertilizer applications. For example, the average contents of NH+4-N at the zero, moderate and high N level conditions were 0.88, 0.94 mg/kg and 13.5 mg/kg, respectively, while those of NO₃^--N were 1.61, 1.73 mg/kg and 2.33 mg/kg, respectively. The nitrification potential in the rice growing rhizosphere soil represented significant differences among the three rice cultivars, and the average values were 6.94, 5.46 g/(kg•h) and 2.42 g/(kg•h) for 4007, Wuyunjing 7 and Elio, respectively, under all the N application levels. The abundance of ammonia-oxidizing bacterial (AOB) in the rhizosphere soil of Elio was significantly lower than those of 4007 and Wuyunjing 7 under the different N application rates. The maximal abundance of AOB was 2.02×106/g soil in the rhizosphere soil of 4007 at high N level, while the minimal one was 1.89×105/g soil in the rhizosphere soil of Elio at moderate N level. The nitrification potentials in rhizosphere soil were significantly correlated with the rice grain yield at zero, moderate and high N levels (r=0.799**, 0.877** and 0.934**, respectively), and also they were significantly correlated with the N physiological efficiency at moderate N level (r=0.735*). Furthermore, the abundances of AOB in the rhizosphere soil were correlated with the nitrification potentials and the grain yields. These results inferred that there should be a relationship among rice yields, fertilizer-N use efficiency and nitrification potential in rhizosphere of rice plants.