Multi-source biochar: Effects on composting humification, soil properties and plant growth

Amid rising organic waste pressures, biochar derived from thermochemical conversion of biomass has emerged as a promising tool for enhancing compost quality and promoting sustainable soil management. This study evaluated five biochars derived from kitchen waste, pig manure, sewage sludge, distillers' grains, and biogas residue for their effects on compost performance, humification, and the soil-plant system. Composting and pot experiments were conducted to evaluate humification, soil nutrient availability, and plant responses. Results showed that biochar feedstock strongly affected composting humification process. Biogas residue biochar (BRB), with the highest specific surface area (26.48 m² g^-1), significantly enhanced compost maturity, particularly increasing the humification index by 3.72-fold. Parallel factor analysis indicated that BRB significantly enhanced the structural complexity of humus, increasing the combined proportion of humic-like and fulvic-like acids by 8.26 %. Application of 8 % pig manure biochar-amended compost significantly improved soil nutrient availability (available phosphorus, 8.55-fold) and boosted plant biomass (5.89-fold). Partial least squares path modeling analysis revealed that dose-dependent benefits of composting application, with moderate doses directly enhancing root development via biochar and humic substances, while higher doses acted indirectly by improving soil nutrients and properties. These findings highlight the feedstock-specific regulatory effects of biochar on compost quality and soil improvement, offering insights for developing tailored biochar-compost products for sustainable agricultural applications.