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.