Effects of tobacco-cultivated soil extracts on tobacco growth and bacterial community diversity in rhizosphere soils

Different concentrations (T1: 40 μg·mL^-1; T2: 120 μg·mL^-1; CK: 0 μg·mL^-1) of tobacco-cultivated soil extracts were used to pot-soils to determine the effects of continuous tobacco cropping on tobacco growth and bacterial community diversity in rhizosphere soils. The results indicated that soil extracts significantly inhibited tobacco growth, dwarfed tobacco plant, reduced tobacco leaf area and photosynthesis, and destroyed protective enzymes in tobacco plants. MDA enhancement in tobacco plants was noted. Tobacco MDA in T2 was 2.44 times higher than that in CK. Tobacco growth inhibition was significantly enhanced with increasing concentration of soil extracts. Analysis of T-RFs suggested that 17 plylums and 24 classes rhizospheric bacteria existed in CK soils, 14 plylums and 21 classes in T1 soils and 10 plylums and 17 classes in T2 soils. A similar trend was also noted for Margalef index. The results suggested that bacterial community diversity dropped significantly with increasing concentration of soil extracts. Microbial/plant-growth analysis showed that a large proportion of T-RFs derived from soils treated with soil extracts had negative correlations with tobacco growth, and with most T-RFs identified as pathogens. However, a large proportion of T-RFs derived from CK showed positive correlation with tobacco growth, and with several corresponding T-RFs bacteria were involved in soil nutrient recycling. In conclusion thus, tobacco-cultivated soil extracts inhibited tobacco growth, destroyed soil micro-system balance, and reduced bacterial community diversity. The results suggested that soil autotoxic allelochemical accumulation was a major limiting factor in continuous tobacco cropping.