Biochar alters soil microbial communities and potential functions 3–4 years after amendment in a double rice cropping system

•Straw biochar increased the abundances of paddy soil bacteria and fungi 3–4 years after amendment.•Biochar amendment decreased soil bacteria/fungi ratios.•Biochar improved soil bacterial composition to favor TOC accumulation.•Biochar improved soil fungal composition to favor plant growth and TOC degradation.•Biochar reduced fungal abundances of plant pathogen in paddy soil.