PowerSoil Pro Kit (Qiagen, Cat. No. 47016). To estimate bacterial DNA content, the 16S rRNA gene copy number was quantified for each sample using quantitative PCR (qPCR). The V3-V4 region of the bacterial 16S rRNA gene was amplified using the primer pair V4_515F_Nextera and V4_806R_Nextera for library preparation.