Dissolved organic carbon, a critical factor to increase the bioavailability of phosphorus during biochar-amended aerobic composting

•The activation mechanism on P attributed to DOC was investigated.•NaHCO3-Pi, NaOH-Pi and HCl-Pi were main P-fractions.•Biochar promoted the composting humification and increased the P-bioavailability.•DOC prevented phosphate binning with calcium ions and increased NaHCO3-P content. Considerable research efforts have been devoted to increase phosphorus (P) availability during aerobic composting. However, there is little discussion weather the dissolved organic carbon (DOC) controls the transformation among P-fractions. Thus, we investigated the changes in DOC compositions and P-fractions during biochar-amended composting (wet weight basis, 5% and 10%). TP content continuously increased since the ‘concentration effect’ during aerobic composting. NaHCO3-Pi, NaOH-Pi and HCl-Pi were main P-fractions, and biochar can improve P-bioavailability by transforming NaOH-Pi and HCl-Pi into NaHCO3-Pi. Structure equation models (SEMs) indicated that biochar enhanced the P-bioavailability through regulating the decomposition of DOC. Our results at least hint that the activation mechanism on P under the influence of DOC during biochar-amended composting. [Display omitted]