| The effects of soil O.M. and N fertilizer rate, applying method on tobacco leaf yield, N uptake, N partitioning, alkaloid contents and quality of flue-cured tobacco were studied by a method of 15N tracing technique and field experiments conducted in three kinds of yellow soils (in Da Fang, Gui Zhou) which O.M. content were 19.2 g.kg-1, 25.7 g.kg-1, 40.70 g.kg-1, respectively. The significance of soil O.M. and soil N mineralization in the control of alkaloid and commercial quality were discussed.Results showed that the accumulation of total N in tobacco plant and those from fertilizer N reached to peak in 11 weeks after transplant, N fertilizer applied by topdressing made the N accumulation peak appeared 2 weeks later. The amount of N uptake from soil was increased and the duration of N absorption would be lasted longer as soil O.M. changed from low to high. The total N accumulated in up ground part ranged from 92.53 N kg.hm-2～95.50 N kg.hm-2, of which there were 26.55 N kg.hm-2～28.41 N kg.hm-2 and 15.45 N kg .hm-2～17.11 N kg.hm-2 came from based and topdressing fertilizer N respectively.N accumulation ratio from fertilizer decreased with the transplanting time increased, 28.66-29.75% N accumulated came from fertilizer(based and topdressing), 15.11～16.95 % N were form topdressing. Increased soil O.M. would increase the ratio of utilization of N. 35.08～35.30%, 22.73～24.49%, 25.87～26.25% and 17.92～20.18% 15N were in upper, stem and flower, middle and lower leaves, respectively. N from fertilizer was concentrated in middle and upper leaves.The percentage of N fertilizer utilization calculated from 15N technique varied with the applying method, when N applied in based and topdressing, the N use efficiency ranged in 27.06%～32.18%; While N fertilizer applied in topdressing, the values increased to a range of 50.77%～61.73%.Tobacco nicotine content in leaves up 10th or 13th were 3.5% under the condition of N=0 kg.ha-2, and higher in the treatment of N application, content of nicotine in middle and upper leaves were high. Nicotine accumulation increased in total growth period, and more nicotine accumulated in 9-15 weeks after transplanting, 118.61～123.29 N kg.hm-2 nicotine were amassed in three O.M. soils. N contents decreased and nicotine increased during tobacco rape time, N ratio from soil was right correlation with nicotine in different position leaf, correlation coefficient was 0.87.The ratio of N in nicotine from 15N were 28.31～36.39% in lower leaves, 26.63～31.95% in middle leaves, and 24.48～25.90% in upper leaves; N in nicotine from topdressing 15N were 12.58～16.79% in lower leaves, 15.30～16.40% in middle leaves, and 14.94～15.64% in upper leaves, N from topdressing was major source of N from fertilizer in alkaloids. Inorganic N of soil were 12～84 N kg.hm-2 in the whole growing period, increased soil O.M. would increase the content of soil inorganic N and the N supplying ability; Content of inorganic N would increased with N application and maintained a higher levels after 7 weeks transplanting. The amount of soil mineralized N showed a increasing trends with the seedling age increased, and 79.54 N kg.hm-2～122.80 N kg.hm-2 were mineralized during the whole growing period.Under the conditions of experiment, higher yield and economical benefit was acquired in the treatment of N=0 N kg.hm-2, which relate to former crop; Higher contents of N and alkaloid in middle and upper leaves, lower ratio of sugar in alkaloid, unsuitable chemical components in local traditional tobacco cultivation, could be controlled by limiting N application rate and regulate ratio of topdressing N.The reasons of higher nicotine in local tobacco leaf were mainly came from as follow: (1) more mineralized N in soil during later growth time, (2) topdressing N higher use efficiency; (3) large rate N in nicotine came from topdressing N. The key measures to increase tobacco leaf quality were to regulate N application, decreased topdressing N rate, control later soil mineralization, and pay attention to former crop.
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