| Continuous cropping is the main obstacle factors for flue-cured tobacco production in the northeast,and how to overcome the obstacle is the key to increased yield and quality of flue-cured tobacco.This paper takes Longjiang 911 flue-cured tobacco as material,studied the influence of different continuous cropping years(normal crops(CK),1 year of continuous cropping,5 years of continuous cropping and the stubble of wheat)soil pH,soil enzymes activity(urease,acid phosphatase,invertase and catalase),soil physico-chemical properties and microbial populations in soil(fungus,bacteria and actinomycetes).Results showed as follows:1 pH on 0-10cm soil layer is higher than CK in stubble of wheat,1 year of continuous cropping and the 5 years of continuous cropping,the significant differences on soil pH were found in tobacco-growing land between 5 years of continous cropping and CK on 0 cm and 10 cm soil layers,no significant differences were found on 20 cm soil layer. Continuous cropping of flue-cured tobacco significantly increased 0-10 cm soil pH,while the 20 cm deepth soil pH less affected,it is indicated that continuous cropping affect the growth of tobacco roots and root exudates.2 The significant differences on the urease activity were found between 1 year of continuous cropping and CK at the last growth stage on the 10 cm soil layer,and no significant differences between 5 years of continuous cropping and CK,and no significant differences between 1 year of continuous cropping and 5 years of continuous cropping,it showed that there are no influence after 5 years of continuous cropping.The acid phosphatase activity were higher than CK between 0-20 cm soil layers in 1 year of continuous cropping and 5 years of continuous cropping, and 1 year of continuous cropping had the highest activity;compared with the CK,the available P content showed a varying degrees of deficiency on 0-20 cm soil layers,and the lowest available P content was in 1 year of continuous cropping.Soil invertase activity had a big variations in the various treatments,the CK was higher than others;invertase activity on 0 cm soil layer was increased at the squaring stage,and at the last growth stage Ocm and 20 cm soil layer have the same trends. Soil catalase also had the same trends on 0-20cm layers,early in the flourishing stage had a higher enzyme activity,and had the same enzyme activity level from the squaring stage,it showed that there is no influence between the differrent treatments.The results showed that:continuous cropping has little effect on soil urease and catalase,but increased the activity of acid phosphatase,and bring down the soil invertase activity.3 The alkali-hydrolyzable N content was higher than the CK processing,the content of available P and available K were lower than the CK processing besides the 5 years of continuous cropping processing;the exchangeable Ca content of CK was higher, and lower in 5 years of continuous cropping;and the content of exchangeable Mg have no influence between different soil layers,the total content was 1 year of continuous cropping higher than the others; there was a higher content of available S,Zn,B,Cu in CK,and the 5 years of continuous cropping have the lowest content;but the content of available S was in a high level in 5 years of continuous cropping,available Fe and Mn content effectively with the contrary,the 5 years of continuous cropping was higher and CK was lower; Cl content of soil was higher in CK and lower in the stubble of wheat,and the content of Cl was higher in 5 years continuous cropping than in 1 year of continuous cropping. So it indicate that continuous cropping significant reduced the fertility level(the content of organic matter)of tobacco-planting soil,and the ability of supply K and P was decreased,let down the efficiency of essential mineral elements,harmful elements of the Cl content of flue-cured tobacco increased,continuous cropping lead to tobacco plant stem soft,leag curl(Ca deficiency),plant height,leaf thin,lower quality flue-cured tobacco.4 The content of three types of soil microbiological was bacateria> actinomycetes> fungi, and the content was higher on 20 cm than 10 cm in different treatments.Along with the growth period,CK had the most bacteria than others,the 4 years of continuous cropping have the lowest content;actinomycetes and fungi had a high content in stubble of wheat,the least content of actinomycetes was in 4 years of continuous cropping,and the least content of fungi was in 1 year of continuous cropping. Aerobic cellulose decomposing bacteria and ammonifiers quantity were higher on 10 cm than 20 cm soil layer,and aerobic azotobacter was reversed;three physiological groups of microorganisms in 4 years of continuous cropping was lowest,besides aerobic cellulolytic bacteria content in 1 year of continuous cropping was most,the other two have the largest number of microorganisms to CK.By Biolog test we also concluded that: microbial activity was highest on CK,followed with the stubble of wheat,1 year of continuous cropping and 4 years of continuous cropping.The long-term tobacco continuous cropping does not benefit the soil ecosystem to develop healthily.Continuous cropping of flue-cured tobacco had influence of tobacco-planting soil pH, enzymes activity,microbial function and soil physico-chemical properties:pH on 0-10 cm soil layer increased;soil urease and acid phosphatase activity increased;the invertase activity were lower than CK, there were no different in catalase between each treatment;continuous cropping also changed the content of soil nutrient,the quantity of soil microbiological had a lower number to 4 years of continuous cropping and higher to CK,the Biolog test had the same conclusion.
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