Variation of bacterial communities in the rhizosphere soils of successive rotations Casuarina equisetifolia plantations based on high-throughput sequencing analysis

Based on the IonS5TMXL next-generation sequencing system, the rhizosphere soils sampled from three different plantations（including FCP, SCP, TCP） were used to explore the causes and mechanisms during successive rotations of Casuarina equisetifolia plantations. The results indicated that a total of 338560 effective bacterial 16 S rRNA gene（V4） sequences were obtained from 3 rhizosphere soil samples, which were assigned to 17627 OTUs. Alpha diversity indices（Simpson, Chao1 and ACE） revealed that the bacterial communities in the rhizosphere soils decreased with the aggravation of successive rotations. The observed species and Shannon indices increased first and decreased afterwards. Beta diversity indices showed that the bacterial community of FCP was significantly different from that of SCP. UPGMA cluster analysis displayed that there were significant differences between FCP, SCP and TCP. Based on the heat map analysis of the dominant bacteria at the genus level, there were some differences among the structure of the bacterial communities under the successive rotations of C. equisetifolia. The relative abundance of Bradyrhizobium in rhizosphere soil increased with the increase of successive rotations. In contrast, Acidothermus, Rhizomicrobium, Acidibacter, and Bryobacter decreased with the increasing years of successive rotations. Among them, the relative abundance of Rhizomicrobium, which was related to nutrient cycling, decreased by 311.49% and 282.16%, and the Acidibacter decreased by 176.07% and 284.54%. We concluded that the consecutive monoculture problem of C. equisetifolia might be caused by the imbalance of the microbial community structure in the rhizosphere soil, thereby reducing the forest ecological benefits, and decreasing the biomass and net productivity. The results have certain significance to illustrate the reasons of consecutive monoculture problem of C. equisetifolia.