Effects On Soil Environments Of Long-Term Succession Cropping And Allelopathy Mechanism Of Cotton In Xinjiang

Cotton is the most important economical crop in Xinjiang. Along with planting area increasing and continous cropping time extending, continuous cropping obstacles became more common and serious, the sustainable development of cotton production had been seriously restricted. We firstly detected the variation of soil physical,chemical and biochemical properties in response to long-term continuous cotton cropping, illustrated the soil environment effect of continuous cropping cotton simultaneously, The allelopathy of cotton plant extract with the other five kind of crops about cotton, wheat, tomato,rape and alfalfa were studied, then a few allelophathic substances were collected and identified in plant aqueous extract,root exudates and soil of different continuous cropping years. In addition, we also trialed the influence on cotton seedlings and verticillium wilt hypha growth by exterior addition of the four detected phonolic compounds. The main conclusions are as following:1 The soil organic matter's content and the soil porosity were increased because of long-term continuous cotton cropping, but its Bulk Density was reduced by an average0.0034 g. cm^-3, the soil salt content and pH slightly were also raised, which led to the soil structure meliorate.2 Along with the continuous cropping year increasing, the soil Alkali-hydrolyzable N content was remarkably raised,its content of 20 years continuous cotton cropping was increased by 151.8% , the soil Available P was first fell and then stable in 10-year continuous cotton cropping, but the soil Available K was remarkably increased by straight line.3 The content of residue film was increased by average 11.2 kg.hm(-2) a year and residue film mostly distributed in underlayer (15-30cm) of tilth, the shatter ratio of residue film was enhanced owing to increasing continuous cropping years. The residue film in soil can inhibit the vertical growth of the cotton roots,and force its to form okra roots and cluster roots, but cotton yield weren't affected.4 The activities of catalase,protease,invertase and phosphatase were decreased in 15-20 years continuous cropping, but those enzyme activities were increased in 15-20 years continuous cropping; The soil urease activity was the most declined in 10 years continuous cotton cropping, and then slowly increased; The soil peroxidase activity was increased with the continuous cropping year increasing.5 There had allelopathic effect of cotton plant aqueous extract and affected seed germination and seedling elongation of wheat, rape, tomato and alfalfa, the sequence of material allelopathy was alfalfa >wheat>tomato>rape.The autotoxic effect of cotton extract intensly affected cotton seedlings, and had a promoting effect on germination percentage, root length, shoot high and number of root as cropping intensity added , meahwhile SOD activities and root activities of cotton seeding were lessened, and POD activities and MDA content were enhanced.6 The main components of root exudateand and plant aqueous extract were identitified as p-hydrobenzoic, vanillic, gallic acid and ferulic acids by HPLC. Their contents in plant aqueous extract were more than that of root exudate. Four phenolic components of p-hydrobenzoic, vanillic, gallic acid and ferulic acids could be detected in soil with different continuous cropping years. The total contents of four allelopachimicals in soil were showed a stepwise accumulation from continuous cropping 1 year to 10 years, but failed in fifteen and twenty.The detected phenolic, gallic acid was gradually increased with continuous cropping years increasing, vanillic was steady in soil of different continupus cropping years, ferulic acids had more content in soil of five-year continuous cotton cropping, but remarkably reduced with ten-year continuous cropping.7 It was found that the p-hydrobenzoic, vanillic, gallic acid and ferulic acids in different comentration inhibited/stimulated the spore germination and mycelium growth of cotton verticillium wilt. It is discrepant in different allelochemicals and concentration, low concentration of the ferulic acids and vanillic could stimulate the growth of verticillium hypha, while at hight concentration, the growth vigor was restrained. The inhibitory effect of p-hydrobenzoic and gallic acid were enhanced with increasing concentration.8 The p-hydrobenzoic, vanillic, gallic acid and ferulic acids affected root length,shoot height and fresh weight of cotton seeding, Inhibition effect was raised with the comcntration increasing. The sensitivity of allelochemicals was different with the different parts of coton seedlings.The root sensitivity was higher than that of above-ground of seedlings. When cotton was dealt with different concentrations of the same allelochemical, SOD and root activity were decreased , MDA content and POD activity were increased with the concentration increasing.9 The allelochemicals from root exudateand and plant extract were accumulat into soil, and was related to continous cropping obstacle of cotton.