Microbial necromass accelerates humic acid formation by reshaping DOM transformation pathways during composting

Humification governs organic matter stabilization during composting but humic acid (HA) accumulation is often constrained by the availability and reactivity of precursors in raw materials. Microbial necromass is increasingly recognized as a significant source of stable organic carbon, yet the direct molecular link between necromass and compost humification remains unclear. In this study, anthropogenically prepared microbial necromass was incorporated into laboratory-scale composting to directly investigate its effect on HA formation and dissolved organic matter (DOM) molecular transformation pathways. Necromass addition significantly enhanced compost humification, increasing humification index by 23.08%, and polymerization degree by 44.49%. Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that necromass addition intensified DOM key biochemical transformations (oxidation, decarboxylation, demethylation, delignification, and Maillard condensation), leading to a higher proportion of oxygenated compounds with higher unsaturation and aromaticity. Partial least squares structural equation modeling further demonstrated that microbial necromass indirectly enhanced HA formation by regulating DOM molecular intermediates and condensed aromatic structures rather than through straightforward carbon input. These insights provide molecular-scale evidence that microbial necromass accelerates stable HA formation by reshaping DOM reaction pathways in composting.