Insight into the molecular transformation pathways of humic acid in the co-composting of bagasse and cow manure after adding compound microorganisms

The inoculation of compound microorganisms effectively processes lignocellulosic biomass and promotes the formation of humic acid (HA). However, the basic mechanism of HA formation in compost remains unclear. In this study, we comprehensively characterized the transformation of HA molecules during co-composting. The addition of compound microorganisms enhanced the degradation of proteinoids and lignocellulose in the dissolved organic matter (OM), thereby accelerating the humification of bagasse cow dung co-compost. Elemental and UV–Vis spectral analyses showed that inoculation with the compound microorganisms enhanced the oxidation level and aromatization of the HA structure. Furthermore, two-dimensional Fourier-transform infrared spectroscopy correlation analysis suggested that the addition of compound microorganisms accelerated the decomposition and oxidation of OM. Accordingly, premature aggregation of HA was avoided, and the side chains bonded to HA were more abundant, promoting the evolution of HA to higher-order structures. By influencing the degree of aromaticity, molecular weight, and the C2 fluorescence peak of HA, the inoculation of compound microorganisms promoted HA synthesis during bagasse composting. These results provide an important basis for revealing the mechanism by which compound microorganisms are inoculated to promote HA formation at the molecular level and serve as a reference for the production of high-quality and high-maturity compost.