Nitrogen managements of late japonica rice in double-cropping rice area

【Objectives】The indica rice has been planted as the major variety of late-season rice in double-cropping rice regions in South China. In recent years, due to the gradually increasing demand of japonica rice observed in most provinces of China, the project of indica to japonica in late season has been proposed in order to produce more japonica rice, which may benefit from the sufficient temperature and light in South China. To achieve high yield, good quality and high efficiency of japonica rice production, it is critical to determine the most effective split application of fertilizer nitrogen at the tillering and panicle stages. 【Methods】Therefore, field experiments were conducted at the farm of Zenxi Town, Shanggao County, Jiangxi Province, China in late rice-growing seasons in 2011-2012. Two hybrid japonica rice cultivars, i.e., Changyou-5 and Yongyou-8, were grown by applying basal-tillering-fertilizer and panicle-fertilizer in different proportions (i.e., 10∶0, 8∶2, 7∶3, 6∶4, 5∶5, 4∶6 and 3∶7), respectively at the same gross nitrogen application rate of 225 kg/ha. Subsequently, the effects of the nitrogen application strategies on yield and its formation, leaf area index(LAI), stems and tillers dynamic change, dry matter production and accumulation, nitrogen use efficiency and rice quality of late japonica rice were analyzed and compared. 【Results】It was found that the number of panicles decreased while the filled-grain percentage, 1000-grain weight and spikelets per panicle first increased and then decreased with decreasing basal-tillering-fertilizer proportion, and panicle-fertilizer in the proportions of 6∶4 and 7∶3 led to larger panicle population, bigger panicles, larger spikelet population, more stable filled-grain percentage and 1000-grain weight, and higher rice yields (as high as 9985 kg/ha). When the two treatments of 6∶4 and 7∶3 were compared, before jointing stage, the population of stems and tillers increased slowly to an appropriate peak value, and then decreased gently after jointing stage. The spike rate was significantly higher than that in the other treatments. During middle and late growth stages, the population had significantly higher dry matter production and accumulation with larger LAI and photosynthetic potential. It as also found that, when the ratio of basal-tillering fertilizer to total nitrogen decreased, the total N accumulation in the mature stage, the apparent nitrogen use efficiency (ANUE), agronomic efficiency (AE) and partial factor productivity of applied N (PFP) increased first and then decreased while the nitrogen requirement for 100 kg grain decreased first and then increased. The treatments with the proportions of 6∶4 and 7∶3 led to significantly higher total nitrogen accumulation, apparent nitrogen use efficiency (ANUE), agronomic efficiency (AE) and partial factor productivity of applied N (PFP) as well as lower nitrogen requirement for 100 kg grain than the other treatments from jointing stage to panicle stage. The relation between ANUE and ratio of basal-tillering-fertilizer could be represented by a quadratic curve opening downward. In addition, the application of basal-tillering-fertilizer and panicle-fertilizer in the proportions of 6∶4 or 7∶3 can also improve the cooking, eating and nutritional qualities of rice although the two treatments increased rice chalkiness grain percentage and degree of chalkiness, and they cannot improve the RVA profile characters. 【Conclusions】In general, the treatments with the proportions of 6∶4 and 7∶3 can achieve high-yield and good quality japonica rice production with high N use efficiency in double-cropping rice regions.