Nitrogen accumulation, remobilization and partitioning in rice (Oryza sativa L.) under an improved irrigation practice

Improving the fertilizer-nitrogen use efficiency in the rice crop is an important issue in China and the world. Field experiments were carried out during 2003 and 2004 at the research farm of China National Rice Research Institute, Hangzhou, China (30°05′N, 119°56′E, altitude 6 m). The experiment was conducted in a randomized complete block design with two factors viz., two irrigation practices and two rice genotypes. N fertilizers in the form of urea plus compost were applied at the rate of 210 kg N ha−1. Half the N fertilizer (compost plus urea) was applied at transplanting, 30% N (urea) was top-dressed at 5 days after transplanting, and 20% N (urea) was top-dressed at 35 days after transplanting. Grain yields were higher for irrigation to shallow water depth with wetting and drying (SWD) than traditional irrigation with 5–9 cm standing water (TRI) in both years of the experiment. The amount of N partitioned to the grain relative to total aboveground N at maturity, remobilized N and leaf N content at 20 days after flowering (DAF) of the TRI were lower than that of the SWD. The difference in total N accumulated between TRI and SWD was significant at flowering. During the grain-filling period (GFP), the SWD accumulated significantly higher N than the TRI. Values for remobilized N together with values of N accumulated during the GFP matched fairly well with grain N content. There were consistent differences between SWD and TRI for the amount of N remobilized from vegetative tissue to grain. There were also significant differences between SWD and TRI in their ability to accumulate N during the GFP. A comparison of the correlation coefficients between the amount of N remobilized from the vegetative tissue, or N accumulated during the GFP, with grain yield suggested that the total N accumulation during the GFP could be the greater contributor to yield improvement. Although the remobilized N was the largest contributor of N to the grain, the amount of N remobilized was statistically similar for both the SWD and TRI. Therefore, the results of the present experiment suggested that SWD does not change the ratio of accumulation of N during the GFP and remobilization of N from the vegetative tissue in grain N. Longer leaf area duration was correlated with higher yields. Final grain yield was correlated significantly with leaf area index (LAI) at 20 DAF in 2 years. Yield was better correlated with leaf N concentration at 20 DAF than at flowering stage, and LAI at 20 DAF was better correlated with leaf N concentration at 20 DAF than at flowering.