Oyster shell-functionalized biochar enhanced compost humification during the co-composting of pig manure with rice straw

Abstract The lower humification efficiency during composting is an emerging challenge that attenuates compost quality. Although biochar has the potential to enhance humification, its effectiveness is limited by factors such as its alkaline pH and the absence of functional groups. Here, Ca-modified biochar was prepared via co-pyrolysis of coconut shell and oyster shell (1:1 ratio) at 600 °C and added to pig manure composting at a 10% (w/w) dose. The result of spectroscopy analysis of the developed compost revealed that carboxyl and carbonyl functional groups facilitated the adhesion of oyster shells to the biochar surface. The addition of Ca-modified biochar in compost not only increased the concentrations of humic acid (+ 8.6%) but also improved the seed germination index (GI) (+ 18.9%) compared to the control group. The excitation-emission matrix analysis indicated that the modified biochar facilitated the transformation of protein-like molecules, thereby enhancing the humification of organic matter by 4.92% compared to the control group. Microbial analysis indicated that Proteobacteria and Bacteroidetes were predominant in the modified biochar group, exhibiting a 107% higher prevalence, as compared to the control group. These enriched microbial taxa, known for their role in lignin degradation, are inferred to inhibit nitrogen volatilization, generate precursor substances, and catalyze the conversion of organic matter into stable humus. The correlation analysis further revealed a significant positive correlation between Proteobacteria and GI ( p < 0.05). These findings demonstrated that Ca-modified biochar enhances physicochemical and microbiological processes, offering a scalable solution for improving humification and supporting sustainable compost production. Graphical Abstract