3.2 Microbial Community Composition and Structure
For rhizosphere bacterial community in different treatments, the dominant bacterial phyla (> 4% in any sample) were Proteobacteria, Actinobacteria, Firmicutes, Acidobacteria, and Bacteroidetes (Figure 3a). These phyla were represented in all groups with different relative abundances. Proteobacteria and Actinobacteria had the highest number of reads in each rhizosphere, together making up at least 60% of the total bacteria population in each set of samples. The main effects of pesticide and fertilizer addition on the bacterial community composition of the sugarcane rhizosphere and the interaction between both were studied using PERMANOVA analysis. The addition of pesticide (SP) or fertilizer (SF) had a significant effect on the bacterial community (p < 0.05), and there was an interaction effect on SPF with almost equivalent abundance between Proteobacteria (34.7%) and Actinobacteria (30.9%). While, Actinobacteria showed significantly increased in SP (43.0%) compared with that in SF (16.5%) (p < 0.01), and Proteobacteria was significantly increased in SF (48.1%) compared with that in SP (23.2%) (p < 0.05) (Appendix Figure S4a).
At the family level, the bacterial communities in all groups were dominated by Burkholderiaceae, Moraxellaceae, Lactobacillaceae, Streptomycetaceae, Micrococcaceae, Bacillaceae, and Enterobacteriaceae (Figure 3b). The relative abundance of many families varied greatly among different groups, especially families affected by the SP and SF groups, respectively. The abundance of Micrococcaceae and Streptomycetaceae in SP were significantly increased compared with those in SF (p < 0.01), while Moraxellaceae and Bacillaceae showed significantly increased in SF compared with those in SP (p< 0.01) (Appendix Figure S4b). Variation in the family abundance of SPF was more likely affected by both SP and SF addition.
In the top 20 genera, the dominant genera had distinct differences among different treatments. The abundance of Dyella ,Chryseobacterium , Lysinibacillus , and Acidothermusin SPF showed significant higher compared with those in other groups (p < 0.05, Figure 3c). The abundance of Conexibacter , Leifsonia ,Chujaibacter , Sinomonas , and Acidipia in the SP treatment was significantly higher than that of the SA (excipients addition) and SK (blank control) (p < 0.05). It indicated that the distinctive genera in the group of pesticide addition (SP) could be relevant to the different processes of degeneration and metabolism of pesticides.
The dominant fungal phylum across all the groups was Ascomycota, followed by Basidiomycota and Mucoromycota (Figure 4a). Among the top ten phyla, Mortierellomycota, Chytridiomycota, Glomeromycota, Blastocladiomycota, Rozellomycota, Aphelidiomycota, and Kickxellomycota were minor phyla, with an average relative abundance of less than 1%. As shown in Figure 4b, the relative abundance of the fungal families diverged significantly among different groups. Specifically, the relative abundance of Chaetomiaceae was greatly increased in SP compared with that in SF (p < 0.01, Appendix Figure S4c). The relative abundance of the top 10 families were much closer between SPF and SP, compared with other groups. In the top 20 genera, the bacterial communities after the SPF treatment were dominated by Penicillium(10.4%), Talaromyces (10.3%), followed by Trichoderma(4.7%) and Myceliophthora (4.4%) (Figure 4c). Compared with other groups using ANOVA analysis, the abundance of Nigrospora ,Deconica , Fusarium , Neocosmospora , Dokmaia ,Pachykytospora , and Ceratobasidium showed significantly increased in SPF (p < 0.05). The abundance ofMycelliophthora , Corynascus , and Mortierella both in the SPF and SP groups were much higher compared with those in the SF group (p < 0.05).