| The average nitrogen (N) fertilizer in China applied to rice was 180 kg N ha-1, which is 75% higher than the world average. In some regions with high rice productions, such as Jiangsu Province, the common N application rate in rice season was 270-300 kg ha-1, and even 450 kg N ha-1 was applied in some area. N recovery rate from fertilizer (REN) was relatively low when applied in high doses, and thus it is reported that the REN by rice in China was around 30-40%, or even less than 20% could be found in Tai lake regions, where over N fertilization was often observed. Low REN did not increase the cost of the crop production, but resulted in serious environment pollution. Thus, the REN and net incomes of farmers could increase substantially and N loss could decline significantly when optimal N fertilizer application was practiced. Field experiments and micro-experiments were carried out from 2003 to 2006 at Agricultural Ecological Experiment Station and some farmer's field in Changshu County, Jiangsu Province, and pot experiment was conducted in green house at Nanjing agricultural university in 2007. The objectives of this study are:(1) To determine the Regional Mean Optimal N application Rate (RMOR) for rice and wheat production with a good consideration of increased REN and minimized N loss; (2) To identify the effects of N application rate and timing on rice growth, grain yield, N uptake, translocation and REN; (3) To assess the effects of improved REN by planting new rice cultivar with high REN and applying water surface film-forming material in the field. The results were showed as follo wings:1. In order to study the RMOR to rice and wheat in Taihu Region, field experiments were conducted with different N application rates from 2003 to 2006. RMOR was calculated based on the average of the optimal N rate in each field. They were 167 kg N ha-1 for rice and 212 kg N ha-1 for wheat. The N application rate for the rice-wheat rotation system at RMOR was only 379 kg N ha-1 per year, which result in 21.7% and 36.8% reduction per year in N application compared to Regional Mean Maximal Rate (RMMR) and local N application rate, without negative effect on crop yield and farmers' net income. Furthermore, RMOR greatly decreased N loss and increased N use efficiency.2. Field and micro-plot experiments were conducted under different N application rates to study the effects of N application rate on N uptake and translocation of two rice cultivars (4007, a new rice cultivar, and Wuyunjingl5, WJ15, a native one). The increases in grain yield with N fertilizer (100,150,200 and 250 kg N ha-1) over the control were 20.6%, 33.6%,37.3% and 34.8% in 4007, while they were 9.41%,14.3%,20.3% and 19.4% in WJ15. N accumulation in rice increased at each growth stage as the enhanced N application rate at both cultivars. The largest N uptake by plants was found between midtillering and initiation and it was 37%-39% of the total N uptake in whole growth period. N translocation to grains increased with enhanced N application rate, ranging from 72.0 kg N ha-1 to 104.4 kg N ha-1 for 4007, and 57.0 kg N ha-1 to 96.5 kg N ha-1 for WJ15, and N translocation efficiency was 66%-51% and 57%-47% for 4007 and WJ15, respectively. 65%-88% of grain N came from the existing N reserves of vegetative tissues acquired before flowering, and only 12%-35% uptaken from soil at reproductive stage. The 15N distribution in grains decreased significantly with increased N application rates, while there was a reverse trend in straw, and no difference in root of both cultivars among the N application rates. The 15N distribution rate in grains of 4007 was much higher than that in WJ15, but lower in straw of 4007 in comparsion with WJ15 under the same N application. Clearly, the higher 15N distribution rate in grains was the main reason for the high grain yield and REN of 4007.3. Fertilizer-N uptake and distribution of rice were studied using 15N tracer technique. 23.1%,8.3% and 19.9% of N uptake in mid-tillering, initiation and anthesis were from 15N labeled fertilizer applied in base (N1), tillering (N2) and initiation (N3), respectively. The 15N translocation from anthesis to maturity was in the order of N3> N1> N2, but the 15N translocation efficiency was higher in N1 (base fertilizer treatment) than in the other two treatments. At maturity, the 15N distribution in straw in the treatments of N1, N2 and N3 was only 24.3%,26.7% and 30.4%, respectively. No matter what time the N fertilizer was applied, the 15N uptake was mostly distributed in leaves, then in the sheath, the least in stem, and 15N distribution in spike increased with the increased 15N translocated from vegetable organs to spike after anthesis. The study also showed that the 15N uptake at maturity in N1, N2 and N3 treatments was 10.6%,5.9% and 12.4%, respectively. The results indicate that when soil N content is not high, the base fertilizer application is important to rice growth, and optimal increment may help increase tillerings, and improve rice quality; the initiation fertilizer significantly promotes quantities during grain filling, and thus application of N fertilizer in initiation is of considerable advance in increasing N harvest index (NHI).4. Effect of applying water surface film-forming material on rice yield, ammonia volatilization, N uptake and utilization were also assessed between the two N application rates of 200 and 250 kg ha-1 in field condition. When 200 kg N ha-1 was applied, water surface film-forming material resulted in little differences in grain yields and ammonia volatilization, but much N uptake in rice leaf and stem before anthesis were found, which was benefitable in N translocation from vegetable organs to seeds in the reproductive growth stage. Accordingly, the N accumulation in seeds (treated by water surface film-forming material) was improved by 13.2% compare to control. When 250 kg N ha-1 was applied, the grain yields in the treatment of water surface film-forming material was increased by 9.9% compared to control and this was mainly contributed by more numbers of tillering. In addition, ammonia volatilization was decreased by 4.15%. Except the tillering stage, higher N uptake was found in leaves and stems during the all growth stage, but no difference was observed in seeds. Whatever the N application rate was, application of water surface film-forming material was benefitable in N accumulation in every part of rice plants, and the total N accumulation was increased by 3.8%-5.9%; Moreover, application of water surface film-forming material could make much more 15N left in the surface of the soil, and make less N lost, and thus contribute a lot to decreased pollution to environment.
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