Changes in cotton rhizosphere soil bacterial community diversity and structure induced by chlorpyrifos

Indoor pot experiment simulating the natural environment conditions along with high efficiency liquid chromatography (HPLC) method and terminal restriction fragment length polymorphism (T-RFLP) techniques was conducted to investigate the effect of chlorpyrifos on bacterial community diversity and structure of cotton rhizosphere soil. Chlorpyrifos doses in the experiment were 5 mg·kg^-1 (recommended dose), 10 mg·kg^-1, 15 mg·kg^-1 and 20 mg·kg^-1, with no chlorpyrifos addition as the control treatment. Diversity indices (including Shannon-Weiner index and Simpson index) were used to evaluate bacterial community diversity. Principal component analysis (PCA) was conducted to determine changes in bacterial community structure. Then the MiCA web tool was used for taxonomic interpretation of TRFs. The results suggested that the degradation half-life of chlorpyrifos doses of 5 mg·kg^-1, 10 mg·kg^-1, 15 mg·kg^-1 and 20 mg·kg^-1 were 10.04 d, 11.36 d, 11.55 d and 12.16 d, respectively. After 60 days of treatment, chlorpyrifos was almost completely degraded. At the end of the experiment (60 days after treatment), cotton biomass significantly decreased with increase of chlorpyrifos dose. Plant root growth was also significantly inhibited by chlorpyrifos. The soil rhizosphere bacterial community diversity under chlorpyrifos treatments decreased significantly after 10 d and 30 d of treatment compared with that of control. There was no significant difference between the 10 d and 30 d treatments in terms of diversity indexes under all chlorpyrifos treatments. Soil rhizosphere bacterial community diversity of chlorpyrifos application treatments almost recovered to the normal level after 60 days. With the concentration of chlorpyrifos increasing, bacterial community diversity decreased and recovery time became longer. The bacterial community structure was significantly different between chlorpyrifos treatments and the control, respectively, at 10 d, 30 d and 60 d after treatment, even though chlorpyrifos was almost completely degraded at 60 days. The difference between chlorpyrifos treatments and the control was increased at 60 d after treatment, and 20 mg·kg^-1 chlorpyrifos treatment had the most significant difference. Also 60 d after treatment, Nitrospina sp. and Cellulophaga sp. were inhibited whereas Bacillus sp. and Streptomyces sp. enhanced by the presence of chlorpyrifos. This showed that rhizosphere soil bacterial community was restructured after treatment with chlorpyrifos. In conclusion, the study presented a comprehensive evaluation of the effects of chlorpyrifos on cotton rhizosphere soils bacterial community diversity and structure. It suggested that recommended or higher doses of chlorpyrifos resulted in cotton rhizosphere soil bacterial community restructuring with a significantly adverse effect on cotton growth and soil bacterial communities, which was need to pay close attention to ecological security of chlorpyrifos using.