The rhizosphere soils were obtained by five point sampling method from brown tobacco soils with different fertilities in Yishui County of Shandong Province. The tobacco variety was k368. Rhizosphere bacteria, fungi and actinomyces in samples from fertile, medial fertile, and infertile soils were isolated and cultivated by the plate cultivation method with selective media. Specifically, azotobacter, phosphobacteria and potassium-bacteria were isolated and identified by their morphology, growth, and physiochemical characteristics, and by sequence analysis of their 16S rDNAs. The main results were as follows: the quantities of total rhizosphere bacteria, azotobacter, phosphobacteria and potassium-bacteria were the largest in fertile soils and were the lowest in infertile soils. In soils of the same fertility, the quantity of potassium-bacteria was the largest, that of phosphobacteria the second, and that of azotobacter the least. The quantities of rhizosphere fungi in infertile soils were significantly more than those in medial fertile or fertile soils, and the quantity of actinomyces showed a declining trend along with the decrease in soil fertility.
Azotobacter, phosphobacteria and potassium-bacteria isolated from all three types of soils consisted of mainly Gram-negative bacilli, belonging to ten genera. The azotobacter fell into four genera: Bacillus, Azotobacter, Beijerinckia and Azomonas. The phosphobacteria were from genera Bacillus, Azotobacter, Mycobacterium, Micrococcus and Pseudomonas. The potassium-bacteria were from genera Bacillus, Erwinia, Sinorhizobium, Pseudomonas, Proteus Hauser and Micrococcus.
The diversity index and evenness index of potassium-bacteria in fertile soils and in medial fertile soils were the largest; the diversity index of azotobacter in all three types of soils was the lowest. These results were consistent with tobacco plants’ high demand for potassium. The diversity index and richness index of azotobacter decreased along with the decrease in soil fertility, accompanied by an increase in the domination index. The diversity index and evenness index of phosphobacteria decreased as soil fertility decreased. The diversity index and the evenness index of potassium-bacteria were greater in medial fertile soils than in the other types.