| A higher nitrogen amount in tobacco-growing area of Nanxiong in Guangdong province is one of main reasons that cause lower nitrogen use efficiency and usability of tobacco leaves in high quality tobacco leaves production. In situ incubation experiment and 15N-labelled experiment were designed to study the characteristics of soil nitrogen mineralization, nitrogen accumulation of tobacco, absorption and distribution of nitrogen derived from fertilizer and soil in tobacco plants, meanwhile, the effects of different amount and methods of nitrogen on tobacco growth, accumulation of the dry matter and nitrogen, physiological characteristics of carbon-nitrogen metabolism and the yield and quality of tobacco leaves in field condition was also carried out. The results were as follows:1. The results of in situ incubation experiment showed that, in the whole tobacco growth period of Nanxiong tobacco-growing area, the accumulation quantities of mineralized nitrogen at nitrogen amount of 0 was higher than at nitrogen amount of 150kg/hm2, and the differences evidently increased with growth period extending. The abilities of soil nitrogen mineralization were great in the later tobacco growth period, and the percentages of nitrogen mineralized after topping stage occupied the total nitrogen mineralized in the whole growth period approximated to 50%.2. The 15N-labelled experiment indicated that, the nitrogen accumulation in root, stem and leaf of tobacco and the whole plant were mainly finished at the vigorous growing stage, the nitrogen accumulation content in different parts of tobacco plant was arranged in order of leaf>stem>root. At the topping stage,the nitrogen accumulation content in tobacco leaf was arranged in order of medium leaf>upper leaf>top leaf, but at mature period it was arranged in order of top leaf > medium leaf > upper leaf. The results showed that the tobacco plant mainly absorbed nitrogen from fertilizer in the early growth period and from soil at maturing stage. Most nitrogen in tobacco plant derived from soil, and the nitrogen derived from soil and its ratio to total nitrogen markedly increased with growth period extending and leaf position ascending, whereas, the nitrogen accumulation content derived from fertilizer was decreased with the increased leaf position. The nitrogen use efficiency was lower at knee high stage, and it increased with growth period extending. The nitrogen from fertilizer mainly residue in the soil of 0-20cm, and the average residue ratio was 38.54%.3. In Nanxiong area, the suitable nitrogen application amount was 150kg/hm2, it was useful for high quality tobacco leaves production. The height, number of leaf, area of maximum leaf of tobacco decreased with the decreased of nitrogen application amount, the tobacco growth and development was suppressed and the growth period was shortened. At knee high stage, the tobacco growth in the treatment of nitrogen application amount was 150kg/hm2, and 70% was base nitrogen fertilizer, 30% was additional showed a superior appearance, but at the medium and later growth period, the tobacco growth was better in the treatment of nitrogen application amount was 150kg/hm2, and 30% was base nitrogen fertilizer, 70% was additional. With the increased of nitrogen dressing rate, it was useful for the tobacco growth and development.4. The effects of different amount and methods of nitrogen on distribution ratio of dry matter accumulation were not obvious, and the distribution ratio of dry matter accumulation in leaf was higher in the whole growth period. The dry matter accumulation of tobacco was obviously influenced by different amount and methods of nitrogen, it increased with the raise nitrogen application and dressing rates. The accumulation of nitrogen in leaf was higher than other organs in the whole growth period, with the raise of nitrogen application and dressing rates, the accumulation of nitrogen in the whole tobacco plant increased.5. The physiological characteristics of carbon and nitrogen metabolism were greatly influenced by different amount and methods of nitrogen. The NR activity, the contents of chlorophyll, nitrate, total nitrogen, nicotine and starch of tobacco leaves rose with the raise of nitrogen application amount and dressing rates, whereas, the content of total sugar, INR and amylase activity decreased in the contrary way. The research showed that the nitrogen metabolism of tobacco leaves enhanced with the raise of nitrogen application and dressing rates, but the carbon metabolism was weakened, the photosynthetic function of tobacco plant was greatly improved.6. With the raise of nitrogen application amount,the percentage of superior leaves, yield, average price and value of cured tobacco leaves increased. The treatment of nitrogen application amount was 150kg/hm2, and 30% was base nitrogen fertilizer, 70% was additional could obviously increase the yield and value, and get a higher economic benefits. The different amount and methods of nitrogen had a great effect on chemical components of cures tobacco leaves, the chemical components of cured tobacco leaves was coordination at the nitrogen application amount of 150kg/hm2. At the same nitrogen application amount of 150kg/hm2, the contents of total nitrogen, nicotine and starch of cured tobacco leaves at the treatment of 30% was base nitrogen fertilizer, 70% was additional were higher than that of 70% was base nitrogen fertilizer, 30% was additional, but the chemical components were in the appropriate range of high quality tobacco leaves.
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