The rhizosphere soils were sampled by three-point-sampling method from mulberry fields of different fertility levels in Taian, Shandong Province. The mulberry variety planted was Husang 32 and of 8 years′ wood age. Rhizosphere bacteria, fungi and actinomyces in both fertile and infertile soils were isolated and cultivated by plates prepared with selective media, and the populations of each kind of microbes were examined by CFU counting. Rhizosphere bacteria interested included total bacteria, azotobacter, phosphate-dissolving bacteria and potassium-dissolving bacteria. BOX polymerase chain reaction (BOX-PCR) was used to fingerprint different rhizosphere bacteria. The main results were as follows: the rhizosphere bacteria, azotobacter, phosphate-dissolving bacteria, potassium-dissolving bacteria and actinomyces in fertile soils were more abundant than those in infertile soils, whereas the fungi in fertile soils were less than those in infertile soils. In soils of the same fertility level, phosphate-dissolving bacteria were most abundant, followed by potassium-dissolving bacteria, while azotobacter ranked the least.
Based on the BOX-PCR fingerprints cluster analysis of rhizosphere bacteria, the DNA homology of rhizosphere bacterial isolates from fertile soils was higher than that of infertile soils, the genetic evolutionary distance of rhizosphere PGPR isolates from fertile soils was close to that of infertile soils. The rhizosphere bacteria in fertile and infertile soils were divided into 71 and 33 clusters at the dissimilarity of 0.2 with BOX-PCR fingerprints, respectively. The PGPR in fertile soils were divided into 33 clusters, including 10 clusters of azotobacter, 12 clusters of phosphate-dissolving bacteria, 14 clusters of potassium-dissolving bacteria at the dissimilarity of 0.2 with BOX-PCR fingerprints. The PGPR in infertile soils were divided to 28 clusters, including 11 clusters of azotobacter, 11 clusters of phosphate-dissolving bacteria, 10 clusters of potassium-dissolving bacteria at the dissimilarity of 0.2 with BOX-PCR fingerprints. Therefore, the soil fertility significantly influence the diversity and abundance of rhizosphere bacteria, more population and higher biodiversity of rhizosphere bacteria and PGPR occurred in fertile soils than in infertile soils.
The diversity index, richness index and evenness index of rhizosphere bacteria, phosphate-dissolving bacteria and potassium-dissolving bacteria in fertile soils were higher than those in infertile soils, but the dominance index was lower than those in fertile soils. The diversity index, evenness index and dominance index of azotobacter in fertile soils were lower than that of fertile soils, but the richness index was higher. These results showed that soils fertility influenced bacterial distribution. The fertile soils provide amicable micro-environments for growth of most bacterial genera, and bring on larger bacterial number of different genera and lower dominance. In infertile soils, relatively few bacterial genera can colonize due to the lack of nutrition and lead to a higher dominance.