Trace MnFe₂O₄ Boosts Polyphenol-Maillard Reaction and Humification Process for Value-Added Composting: Integrated Effect of Chemical and Enzymatic Catalysis

Promoting humification during composting is of pivotal significance for converting organic waste to value-added fertilizer. Traditional composting additives for enhanced humification commonly suffer from low efficiency and a large dosage. Herein, we presented a novel and effective technique with great application potential to promote humification during composting via simple addition of trace MnFe2O4, behind which the essential mechanism was interpreted from both chemical and biological perspectives. Results indicated that with an economical dosage of MnFe2O4 (0.02 wt %), the content of humic acid (HA) and humification index (HI) were increased by 15.2 and 18.7% in comparison with the control group, respectively. The chemical mechanism steering such enhanced humification was revealed through analysis of precursor substances evolution and HA structural characterization. Specifically, MnFe2O4 addition catalyzed the polyphenol-Maillard reaction, leading to rapid oxidation and subsequent polymerization of the precursor substances. Meanwhile, analysis of diversity and evolution of microbial communities as well as activities of laccase and peroxidase demonstrated that MnFe2O4 addition increased the relative abundance of laccase/peroxidase-producing bacteria and thus elevated the enzymatic activities of laccase/peroxidase, which played crucial roles in catalyzing polyphenol-Maillard reaction and humification. This study demonstrates that MnFe2O4 could serve as a promising composting additive to promote humification and thereby produce value-added composts.