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. The effects of biochar application on soil microbial communities and functional characteristics and their correlations with soil fertility properties were explored in a double rice cropping system three to four years after a single biochar amendment. Three treatments including a control, a low (24 t ha−1), and a high (48 t ha−1) application rate of straw-derived biochar were constructed. Biochar amendment significantly increased the abundance of bacteria and fungi by up to 102 % and 178 %, respectively, which might be probably caused by the increases in soil total organic carbon (TOC), total nitrogen, and rice biomass as compared with the control. However, the abundance of archaea was only slightly elevated after biochar amendments. Bacteria/fungi ratios were significantly decreased by up to 61.4 % in the biochar treatments, probably because fungi were the dominant decomposers of increased recalcitrant carbon from biochar and rice biomass. Biochar stimulated the relative abundance of Acidobacteria, which favours soil organic carbon accumulation. Biochar increased the relative abundances of Mortierella and Westerdykella, which are more beneficial to plant growth and TOC degradation. Furthermore, potential phytopathogens of Athelia and Penicillium were decreased with biochar amendment. The results demonstrate that biochar application should be sustained as an effective measure for improving the microbial characteristics of paddy field by ameliorating its soil properties.