Chemical and Molecular Profiling of Dissolved Organic Matter within Varied Compost: A Comprehensive Spectroscopic Study

Limited information is available regarding the chemical and molecular compositions and structures of dissolved organic matter (DOM) originating from diverse matured composts. In this study, the compost DOM extracted from composts of chicken manure co-composted with wheat straw (WS), rice husk (RH), and Chinese herbal residue (HR) underwent comprehensive characterization using UV–vis absorption, excitation-emission matrix fluorescence coupled with regional integration analysis (EEM-FRI), high-performance size exclusion chromatography (HPSEC), fourier-transform infrared (FTIR) spectroscopy. The observations in optical indices (i.e., SUVA254, E2/E3, and SR) of compost DOM consistently revealed a decrease in aromaticity and polymerization degree in the order of WS > RH > HR DOM. EEM-FRI analysis indicated that both WS and RH DOM contained more humic-like substances (Region III + V, 45.4–51.1%) than HR DOM (37.9%), while the latter one enriched more protein-like substances (Region I + II, 48.3%). This implied that WS and RH DOM exhibited a more pronounced humification degree, which is supported by observation reflected by the humification Index (HIX) values. HPSEC revealed broad molecular weight (MW) distributions, encompassing a range of 0.1 to 300 kDa, across the three compost DOM. In comparison, both the weight-average and number-average MW followed the order of WS > RH > HR DOM. FTIR spectra revealed that the three compost DOM mainly comprised intricate mixtures of aromatic and aliphatic species, along with O-containing functional groups (e.g., hydroxyl, carboxyl, and carbonyl). Notably, WS and HR DOM exhibited a stronger presence of aromatic and aliphatic groups, whereas RH DOM presented a greater abundance of polar O-containing structures. In summary, this study has pointed out that diverse compost DOM manifest distinct chemical and molecular characteristics. These findings offer a foundational framework for assessing compost maturity and for the judicious application of varied matured compost types as organic fertilizers into soil systems.