Field-aged biochar enhances soil organic carbon by increasing recalcitrant organic carbon fractions and making microbial communities more conducive to carbon sequestration

Biochar has been widely proposed for carbon (C) sequestration, but its residual effects on soil organic C (SOC) accumulation and microbial characteristics are unknown. Herein, we investigated the content and components of organic C, and the abundance and community characteristics of microbes in a fluvo-aquic soil, after 4 years of biochar application at rates of 3, 6 and 12 t ha−1. Biochar application increased the contents of SOC and recalcitrant organic C fraction by 11.02–22.13 % and 18.41–32.31 % at 6 and 12 t ha−1, respectively, and the increased proportion for recalcitrant organic C fraction was greater than that for SOC. Among C functional groups, aryl C content was increased by 26.50–95.41 % at all rates, compared with 23.83–56.73 % for phenolic C at 6 and 12 t ha−1, and both increases were higher than that for SOC. Biochar application decreased bacterial abundance by 9.25–35.77 %, and altered bacterial community structure by increasing the relative abundance of Chloroflexi phylum, members of which have a low C mineralisation rate and strong C fixation ability. Dissolved organic C was the most critical factor of bacterial abundance and community structure. By contrast, 12 t ha−1 biochar application decreased fungal diversity by 12.45 %, and altered fungal community structure by increasing the relative abundances of Sordariomycetes and Tremellomycetes classes, both of which favoured SOC formation. The C/N ratio was the most important variable affecting fungal diversity and community structure. Overall, our results suggest that field-aged biochar, especially at a high-dose rate, accelerates organic C accumulation by increasing aryl and phenolic C functional groups of recalcitrant organic C fractions and shifting microbial communities more conducive to C sequestration in the fluvo-aquic soil.