| A pot experiment was carried out with flue-cured tobacco K326 by completely randomized trial design to study the effects of water and nitrogen coupling on the growth of tobacco and physicochemical and biological properties of soil.An appropriate model of water and nitrogen coupling was proposed through modeling and optimization for rational supplies of water and nitrogen resources in tobacco cultivation.Following are the main results obtained:1.The contents of organic matter,total N,and available N and P in tobacco-grown soil were reduced as the growing period prolonged in experimental treatments,Soil pH,however,was increased early and decreased lately.Large number of actinomycete was foumd,followed by bacteria, and fungus was lowest at tobacco harvest,and the content of microbial carbon was detected higher than microbial nitrogen at the same time.2.Total and available N in soil were increased with nitronge application increased.Appropriate irrigation application benefited the accumulation of total and available N in soil.They were decreased in case of insufficient nitrogen fertilizer.Organic matter was accumulated with appropriate nitronge fertilized into soil and behaved otherwise as more water was irrigated.In regard of pH in soil,it was reduced by over supply of nitrogen fertilizer but increased by irrigation.3.The numbers of soil microorganisms,such as bacteria,actinomycetes and fungi,were increased in some extent with nitronge fertilizer increased at tobacco harvest.The contents of microbial carbon and nitrogen were increased by irrigation with appropriate nitronge fertilized; however,activities of the three microorganisms in soil were inhibited by over irrigation with less nitrogen supplied.4.When 3-7 g N·plant-1 was fertilized together with 60-80 L water·plant-1 was irrigated,the height,stem diameter,leaf expansion and weight of tobacco leaves at variable locations were increased.The higher leaves located on stems,the higher the chlorophyl content and the activity of nitrate reductase(NR) were detected at mature period.In addition,the content of chlorophyl was increased as more nitronge and water supplied.The activity of NR was stimulated by nitronge fertilization and inhibited by excessive irrigation.5.The content of total N was observed highest in upper leaves of tobacco,followed by middle leaves,and lowest in lower leaves.The content of total P chaged the the sequence:lower leaves＞ middle leaves＞upper leaves.The content of total K in lower leaves was significantly higher than the middle and the upper leaves.N in tobacco leaves of various loactions was increased with the increment of nitrogen fertilized but reduced with more water irrigated.More P and K were accumulated in tobacco plants with appropriate N supplied.Irrigation benefited the absorption of K in less nitrogen fertilized but excessive irrigation decreased the contents of P and K in tobacco leaves. The shortage of nitrogen could increase the proportion of P and K in the leaves.3-7 g·plant-1 fertilized together with with 60-80 L water·plant-1 irrigated could result in the appropriate content and absorption of nutrient.6.The contents of nicotine,protein and chloride ion were increased from lower leaves to the upper ones in general.The content of reduced sugar was highest in middle leaves,followed by lower leaves,and lowest in upper leaves.Nicotine and protein increased with the increment of nitrogen fertilized and decreased with more water irrigated.Reduced sugar increased with the increment of less nitrogen fertilized but reduced at over supplies of nitrogen.Reduced sugar also decreased at less nitrogen with excessive irrigation.Similarly,the concentration of chloride was reduced at appropriate nitrogen fertilized but increased as more water irrigated.7.The ratio of nitrogen to nicotine in various leaves changed little,if any.The ratio of sugar to nicotine varied in lower and middle leaves was found higher than upper leaves.The ratio of potassium to chloride in lower leaves was found higher than middle and upper leaves.The ratio of nitrogen to nicotine in middle and upper leves was increased by the increment of nitrogen fertilization but decreased by irrigation.Appropriate irrigation could increase the ratio of sugar to nicotine but reduced by high amount of nitrogen fertilized in the taobacco leaves.The ratio of potassium to chloride,however,increased as nitrogen increased and behaved otherwise in as more water supplied.8.3-7 g N·plant-1 and 60-80 L water·plant-1 supplied is appropriate in tobacco cultivation in our present experiment.The effects of nitrogen and water on dry weight of leaves was obtained highest for nitrogen fertilization,then irrigation,and lowest for water-nitrogen coupling.There was a positive interaction trend of water and nitrogen.The best amount of fertilization and irrigation was 5.62 g·plant-1 for nitrogen and 78.19 L·plant-1 for water,and the highest dry weight of tobacco leaves(94.91 g.plant-1) could be obtained in this case.
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