How do tree species characteristics affect the bacterial community structure of subtropical natural mixed forests?

The effects of tree species on bacterial community structure have attracted much attention, but few studies have been done in natural mixed forests. In this study, we selected 12 sampling sites in the subtropical natural mixed forest (mainly distributed by Chinese sweet gum, chestnut, Oriental oak, Masson pine, Chinese fir, etc.). The fermentation layer (OF) and humified layer (OH) were mixed as forest floor samples, and the topsoil samples (0-10 cm) were taken. Bacterial composition was studied by 16S rRNA gene sequencing. Coniferous canopy area ratio (P_c), broadleaved and shrubby canopy area ratio (P_hwd), elevation, soil properties were tested. The objective is to reveal which soil properties are significantly affected by tree species characteristics, which soil properties significantly affect bacterial community structure, and whether the bacterial community structure is the same in forest floor and topsoil samples at the same sampling site. The results showed that: (1) P_c and P_hwd could be used to represent tree species characteristics of natural mixed forests, and they significantly (P=0.05) affected the soil C/N ratio; (2) the soil C/N ratio was the main factor affecting the soil bacterial community composition, especially for the dominant heterotrophic bacteria (Acidothermus, Variibacter, Candidatus Solibacter, Acidibacter, and Bryobacter). The relative abundance (1.11-26.27%) of the dominant heterotrophic bacteria increases with an increase in the C/N ratio (6.33-10.76) within a certain range; (3) the dominant bacteria in topsoil samples were Nitrospira, Acidothermus, Arthrobacter, Bradyrhizobium, and Variibacter, while that in forest floor samples were Jatrophihabitans, Acidothermus, Burkholderia-Paraburkholderia, and Bradyrhizobium. Although the forest floor bacteria came from the topsoil at the same sampling site, the bacterial community structure had changed significantly. This study indicated that tree species drive the change of soil bacterial community by changing the soil C/N ratio, which may provide a new perspective for maintaining the stability of regional ecosystem structure.