Insights into the roles of DOM in humification during sludge composting: Comprehensive chemoinformatic analysis using FT-ICR mass spectrometry

Biological humification is the core process in sludge composting for organic matter stabilization. Characterizing dissolved organic matter (DOM) would be a promising way for tracking humification, but still needs fundamental understanding. This study evidenced the role of DOM in deciphering humification via comprehensive and expansive chemoinformatic analyses based on Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Results showed that formula-derived indices including half reaction of carbon oxidation (ΔG0Cox), oxygen to carbon ratios (O/C), double bond equivalent to carbon ratios (DBE/C) and modified aromaticity index (AImod) (except for molecular lability boundary (MLB) index) from the nitrogen (N)-containing compounds were highly correlated with the fluorescence index representing humic and fulvic acids. Besides, the identified dominant reaction types (oxidative demethylation, deamination, and desulfurization, etc.) in different stage of composting, key molecules based on paired mass distance (PMD) network, and nexus molecules (e.g., C10H14N5O7P, C9H14N3O8P, C14H18N4O3, etc.) from DOM-microbes associated network further supported the important roles of N-containing compounds in humification. Furthermore, as the precursors for humification, releasing of biogenic N-containing compounds from sludge and providing thermodynamically available CHO compounds would be important for controlling humification. The fates of N-containing precursors and stable CHO precursors in determining the degree of humification need more attentions in future research. This study provides fundamental understanding that exploring DOM composition, properties and molecular transformation is alternative to track humification in composting.