Co-applying biochar and manganese ore can improve the formation and stability of humic acid during co-composting of sewage sludge and corn straw

[Display omitted] •Co-applying biochar and manganese ore promoted the formation of humic acid (HA).•Increased HA resulted from enhanced conversion of humus precursors and fulvic acid.•Co-applying biochar and manganese ore improved HA stability.•Improved stability of HA due to increased high molecular weight components content. This study examined the effects of corn straw biochar (CSB) and manganese ore (MO) on the abiotic formation and stability of humic acid (HA) during sewage sludge composting. Co-applying CSB and MO (106%) induced a higher increase in HA content of final compost product than those of no or single applications (32.6–85.1%). This positive change was achieved by promoting the conversion of humus precursors and fulvic acid to HA through abiotic pathway, respectively, in the early and later stages of composting. The co-application of CSB and MO also exhibited a higher capacity to improve HA stability than those of single applications. In sum, this study confirmed a clear synergistic effect of CSB and MO on improving the formation and stability of HA in compost product, which could further enhance the multi-benefits (e.g., carbon sequestration and soil quality improvement) of compost soil application.