Analysis of plant growth promoting rhizobacteria population in apple rhizosphere soils

Apple replant disease (ARD) is a complex syndrome of young apple trees in replanted orchards that causes death of fine feeder roots, stunted tree growth and low yield. Analyzing changes in the number and species of plant growth promoting rhizobacteria (PGPR) in perennial apple tree (PAT) and replanted young tree (RYT) fields could lay theoretical basis for understanding the interactions among ARD and rhizosphere microbes. In this study, rhizosphere soil samples were collected in PAT and RYT fields in Changli, Hebei Province. Rhizosphere bacteria of interest in the study included azotobacter, phosphate-dissolving bacteria, potassium-dissolving bacteria and antagonistic bacteria. While rhizosphere azotobacter, phosphobacteria, potassium-bacteria were cultivated by the selective media plate cultivation method, antagonistic bacteria (with antagonistic activity against Rhizoctonia solani or Fusarium camptoceras) were isolated using the in vitro screening technique. For soil samples from both fields, microbe species and population examined by colony-forming unit (CFU) count. Also BOX Polymerase Chain Reaction (BOX-PCR) was used to fingerprint the different PGPRs. Total rhizosphere bacteria, azotobacter, phosphate-dissolving bacteria, potassium-dissolving bacteria and antagonistic bacteria were more abundant in PAT than in RYT fields. In PAT fields, potassium-dissolving bacteria were the most abundant, followed by phosphate-dissolving bacteria and then azotobacter. Antagonistic bacteria were the least abundant. In RYT fields, phosphate-dissolving bacteria were the most abundant, followed by potassium-dissolving bacteria and then azotobacter. Antagonistic bacteria were also the least abundant. Based on BOX-PCR fingerprints cluster analysis of PGPR, there were over 1.25 dissimilarities in both PAT and RYT fields. This somehow suggested close genetic evolutionary distance among the isolates. PGPR in PAT fields were divided into 79 clusters; including 18 azotobacter, 29 phosphate-dissolving bacteria, 19 potassium-dissolving bacteria and 18 antagonistic bacteria clusters at 0.25 BOX-PCR fingerprint dissimilarity. Similarly, PGPR in RYT fields were divided into 46 clusters; including 15 azotobacter, 19 phosphate-dissolving bacteria, 8 potassium-dissolving bacteria and 9 antagonistic bacteria clusters at 0.25 BOX-PCR fingerprint dissimilarity. Replanting therefore significantly influenced rhizosphere bacteria abundance in orchard fields. Higher PGPR population and biodiversity were noted in PAT than in RYT fields. While the indices of diversity, richness and evenness of azotobacter, phosphate-dissolving bacteria, potassium-dissolving bacteria and antagonistic bacteria were higher in PAT than in RYT fields, the reverse was true for dominance index. Based on the findings, abundant microbes existed in PAT fields and with complex and stable ecological distribution of microbial community. However, only less PGPR with great vitality colonized RYT fields and with simple rhizosphere microbial community structures.