Tobacco Growth And Activities Of Soil Enzymes In Differential Soil Particle Size Under Continous Cropping Conditions

Tobacco, an important cash crop in our country, is one of the major sources of fanners' income in tobacco-grown areas and national and local revenues. Tobacco cultivation also benefits to increase farmers' proceeds, to employ the surplus of labors in countryside, to promote the rural development and to form a harmonious social atmosphere. In the case of gradual reduction of cultivated lands in our country year by year, however, continuous cropping of tobacco is now quite common in tobacco-grown areas, which leads to the great depression of yields and qualities of tobacco. Related investigations are necessary to carry out in order to solve the problems produce in continuous cropping of tobaccoGreat achievements have been obtained to solve the problems produced by continuous cropping such as soybean, corn, peanut and wheat so on. But the results obtained are quite different and scare with tobacco. Moreover, much research work focuses on the changes in physical and chemical properties of continuously cropped soils in contrast to the activities of enzymes in the soils, especially in variable soil particles, which is hardly reported. Therefore, Pot experiment was carried out to investigate the yields and roots of tobacco (cultivar: K326) as well as the activities of enzymes in variable soil particles. Experimental soils were two yellow soils derived from lime stones (suitable for continuous cropping of tobacco) and clays formed in quaternary period (suitable for continuous cropping of tobacco), respectively, which are typical tobacco grown soils in Guizhou province.The growth of tobacco varied with soils and cropping. The yield and biomass of tobacco were decreased significantly in lime stone derived soil by continuous cropping. In the first cropping season, they were as much as 2.14 times (yield) and 1.96 times (biomass) in the second and 1.73 times (yield) and 1.82 (biomass) in the third. However, the yield and biomass of tobacco were increased significantly in clay derived soil by continuous cropping, which increased by 94.8% (yield) and 73.2% (biomass) in the second cropping season and 54.8% (yield) and 46.2% (biomass) in the third compare to those in the first one.Tobacco roots grew poorly and the weight and length of the roots decreased in the case of continuous cropping. The fresh weight of tobacco in lime stone derived soil reduced by 18.9% in the second cropping season and 44.7% in the third, respectively, compared to that in the first one. The lowest fresh weight was observed in the second cropping season in clay derived soil and then...