Development of critical nitrogen dilution curve of Japonica rice in Yangtze River Reaches

Plant-based analytical techniques of nitrogen (N) nutrition, established on the concept of critical nitrogen (Nc), can be used to diagnose the in-season N status of rice, which in turns can provide understanding of N nutrition and serve as a guide for the profitability and sustainability of agricultural production system. The objectives of present study were to develop an appropriate Nc curve for Japonica rice, to compare this curve with existing Nc dilution curve for Indica rice and to assess its plausibility to estimate the level of N nutrition for rice crop in east China. Three field experiments were conducted with varied N rates (0-360 kg N ha−1) in three Japonica rice (Oryza sativa L.) hybrids, Lingxiangyou-18 (LXY-18), Wuxiangjing-14 (WXJ-14) and Wuyunjing (WYJ) in Jiangsu province of east China. Five hills from each plot were sampled from active tillering to heading for growth analysis. The Nc dilution curve for rice, based on whole-plant N concentration, was described by the equation (Nc = 3.53W−0.28), when aboveground biomass ranged from 1.55 to 12.37 t ha−1. However, for above-ground biomass <1.55 t ha−1, the constant critical value Nc = 3.05% DM was applied, which was independent of aboveground biomass. Our Nc dilution curve was lower than the existing curve of Indica rice in tropics. The N nutrition index (NNI) and accumulated N deficit (Nand) ranged from 0.58 to 1.06 and 127 to −12 kg ha−1, respectively, during main development stages under different N treatments in 2010 and 2011. Different N fertilizations were suggested for unit NNI increase or unit Nand decrease at different stages. LXY-18 have more N assimilation tendency than WXJ-14 for satisfying rice growth for same dry matter. The Nc dilution curve, resulting NNI and Nand, adequately identified situations of limiting and non-limiting N nutrition and could be used as a reliable indicator of N stress during the growing season of Japonica rice in east China.