Biotic and abiotic effects of manganese salt and apple branch biochar co-application on humification in the co-composting of hog manure and sawdust

This study aimed to understand the effects of using apple branch biochar (ABC) and manganese (Mn(II)) to enhance the humification process when sawdust and hog manure are being composted aerobically. Four treatments of sawdust and hog manure were established, namely, CK (no additives), ABC (5 % ABC), Mn (0.5 % MnSO4), and ABC-Mn (5 % ABC + 0.5 % MnSO4). The results from the 42-day composting showed that ABC-Mn treatment was the most effective. It increased the total organic matter reduction by 19.24 %, decreased CH4 emissions, produced a high level of humic acid (HA) (60.45 g/kg), and significantly elevated the degree of polymerization (HD) to 3.15 (P < 0.05) upon the end of the composting process. In addition, the presence of ABC abiotically reduced Mn-(II) salt stress and promoted compost maturation. Spectroscopic examination revealed that ABC-Mn treatment facilitated the breakdown of polysaccharides and the production of stable, aromatized humuslike materials throughout the composting process. The addition of Mn(II) and ABC also changed the microbial community composition of the compost substrate, stimulated the activities of laccase and dehydrogenase, and raised the relative abundances of Ascomycetes, Firmicutes, Proteus, Actinomycetes, and Basidiomycetes. ABC addition accelerated the transformation of Mn(II) to reducible Mn(II) and Mn(IV), potentially enhancing the abiotic degradation of organic matter and the formation of humus-like substances through redox reactions. In conclusion, this study highlighted the biotic and abiotic effects of adding ABC and Mn(II) to accelerate humification during the co-composting of hog manure and sawdust.