Effects of intercropping with soybean on bacterial and nitrogen-fixing bacterial diversity in the rhizosphere of sugarcane

Aims In order to investigate the effects of intercropping with soybean on the diversities of bacteria and nitrogenfixing bacteria in the rhizosphere of sugarcane(Saccharum officinarum), to collect and exploit nitrogen-fixing bacterial resources, and to screen for efficient sugarcane-associative nitrogen fixation system, an experiment was conducted by cultivating three sugarcane cultivars ‘ROC22', ‘GT21', and ‘B8' with a soybean(Glycine max) cultivar ‘Guizao 2'. Methods Rhizospheric soil bacteria and nitrogen-fixing bacteria were measured in the rhizosphere of sugarcane intercropped with soybean by using a nested-PCR combined with denaturing gradient gel electrophoresis(DGGE) technique. The specific bacterial 16 S rRNA gene fragments and nitrogen-fixing bacteria nifH gene fragments were amplified for phylogenetic and diversity analyses. Important findings Cluster analysis of bacterial communities showed that the sugarcane-soybean intercropping changed the community composition of rhizospheric soil bacteria and nitrogen-fixing bacteria, with the effect being greater on nitrogen-fixing bacterial communities than on the soil bacterial communities. The intercropping system had a significant effect on the diversity of the rhizospheric bacteria and nitrogen-fixing bacteria; the impact on the diversity of nitrogen-fixing bacteria was greater than on the soil bacteria, and was only minor on the dominance of the bacterial species. Analyses of Shannon-Wiener index and Simpson index showed that thenitrogen-fixing bacteria differed in diversity among the treatments with different sugarcane varieties, and that the sugarcane cultivar ‘ROC22'-soybean intercropping had the greatest diversity. The diversity of soil bacteria in the rhizosphere of sugarcane varied greatly among different stages of crop growth; the booming stage of soybean had the greatest diversity and variations in the diversity among treatments in soil bacteria in the rhizosphere of sugarcane, and thereafter the two variables decreased. In conclusion, the sugarcane-soybean intercropping system had significant effects on the community structures and diversities of soil bacteria and nitrogen-fixing bacteria in the rhizosphere of sugarcane. This is of help for understanding the beneficial effects of sugarcane-soybean intercropping and for identifying efficient sugarcane-associative nitrogen fixation systems.