Studies On Rhizospheric Microecology Of Continuous Tobacco Cropping Obstacle

Pot experiments were conducted to study effects of successive cropping non-sterilized soil and sterilization soil on the grow status, protective enzymes,soil enzyme activity of tobacco.In order to search for micro-ecological factors, which were related to successive cropping obstacle in tobacco, and a further study from two angles of autotoxins and soil microbial flora.The results were summarized as follows:(1) Growth of tobacco was restrained under the condition of successive cropping, such as plant short, leaf area decrease. Also the resistant response of tobacco was triggered.For example, the POD and SOD activities increased, the CAT activity decreased, and the MDA content increased. The activities of soil urease, acid phosphatase and catalase decrlined while the soil polyphenol oxidase activities increased first , then decreased afterwards.(2) Successive cropping sterilization soil could improve the growth of tobacco plants to greater extent. But the inhibitory effects of plant height and leaf area were increased, the SOD, POD activities of plants presented increasing trend as the successive cropping years increased under sterilization.The result showed that some factors, which hindered the growth and development of the crop plant, still exist in the sterilized soil.(3) Lettuce seedling radical growth,tobacco seed germination and tobacco seedling radical growth were all affected by aqueous extracts from the rhizospheric soil under continuosly cropping the tobacco crop.There were dual effects,i.e. low dosage-stimulative effect and high dosage–inhibition, and the inhibitory effects was increased as the continuously cropping years increased. N-hexane,chloroform and ethyl acetate extracts from the rhizospheric soil had no allelopathic effect on the radical growth of Lactuca sativa and tobacco seedlings.The methanol extract from the rhizospheric soil had the strongest allelopathic effect on the target plants under the concentration of 2g/ml. It was showed that allelopathic compounds from the rhizospheric soil were strong polar materials. The compounds in the methanol extract from he rhizospheric soil were identified by GC-MS.The results indicated that the components of the methanol extracts from the rhizospheric soil after continuously cropping the crop for one season and three seasons respectively mainly included alkane,alcohols,alcohols,acid,ester, phenol,terpene and polycyclic aromatic hydrocarbons etc.Most of which were reported as allelochemicals .(4) The Shannon diversity index, Simpson index and Margalef index of bacterial community in the rhizospheric soil were increased at first, and then decreased as the continuously cropping years increased. Diversity level of bacterial community was significantly decreased, and bacterial community structure became simple after continuously cropping for four years. The Similarity index was decreased with continuously cropping year increasing,indicating that the difference of bacterial community structure in rhizosphere soil was gradually increased, which meant the slower succession of the community. It could be speculated that the change in bacterial community might be related to the accumulation of root exudates in the soil. It became rich in some specific bacterial community, especially the pathogenic bacteria, after continuously cropping the crop.This resulted in destroyed balance of bacterial community in rhizospheric soil, and the micro-environment of tobacco was deteriorated.(5) Compasion of bacterial community in successive cropping non-sterilized soil and sterilization soil, there were a lot unique pathogenic bacteria in all non-sterilized soils, showing that sterilization could reduce the amount of pathogenic bacteria.The trend of diversity level of bacterial community in successive cropping sterilized soil was consistent with that in non-sterilized soil, the similarity index of both were higher,the common bacteria of both accounted for high proportion and had pathogenic bacteria.The result suggested that the residual secretion had epigamic stimulation to the evolution of bacterial community.The analysis of ectomycorrhizal fungi showed that there were some mycorrhizal Fungi secreted polypeptide toxin in successive cropping soil, and generated some fungi related to plant detoxification and stress-resistant.In a word, there were potential autotoxins in the successive cropping soil of tobacco. Diversity level of microbial community decreased significantly while the amount of pathogenic bacteria increased, which induced by those allelochemicals. The rhizosphere micro-environment of tobacco was deteriorated, which hindered the metabolic activity of root system and the efficiency of nutrient utilization, the growth of tobacco was restrained eventually.