Insights into Fe⁰, Fe₂O₃, and Fe₃O₄-Mediated humification and nitrogen transformation during composting

This study evaluated Fe⁰, Fe₂O₃, and Fe₃O₄ as regulators of humification during composting of chicken manure (CM) and wine grape pomace (WGP). Results showed that Fe₂O₃ and Fe₃O₄ substantially enhanced humification, raising H₂O₂ (15-47 %) and ·OH (3-16 %) and thereby driving aromatic CC formation and hastening HA production and aromatization. Specifically, Fe₂O₃ upregulated hydroxylamine dehydrogenase and the nitrogenase molybdenum-iron protein. Correspondingly, nitrification and nitrogen fixation increased by 4.9 % and 6.7 %, respectively, with enrichment of key taxa (e.g., Oceanobacillus, Thermobifida). Furthermore, Fe₂O₃ accelerated HA functional-group formation, increasing HA yield by 14.9 %. In contrast, Fe⁰ inhibited humification by scavenging reactive oxygen species, attenuating oxidative polymerization of organic matter, limiting CO cross-link formation among precursors, and delaying microbial colonization. In addition, Fe₂O₃ improved N, P and K retention and HA yield, returning an expected 108.6 RMB t^-1. These results clarify iron-mediated composting and guide efficient technologies for valorizing nitrogen-rich organic waste.