Insights into the complex nature of dissolved organic matter from plant residues using multiple spectroscopic techniques

Dissolved organic matter (DOM) is the most active and labile organic fraction in soils, participating in numerous biogeochemical processes. Comprehensive understanding of DOM composition at the molecular level remains a challenge due to limited or contrasting analytical data based on a single technique or limited analysis, which may be addressed using multiple complementary techniques. In this study, we utilized five prevalent and novel spectroscopic and spectrometric techniques, i.e., XPS, FTIR, NEXAFS, NMR, and FT-ICR-MS, coupled with multivariate analysis to decipher the complexity of DOM. We extracted DOM from four decomposed plant residues − pine, eucalyptus, pasture, and wheat. The organic carbon concentration in the DOM samples followed the order: pine > eucalyptus > wheat > pasture. Eucalyptus and pasture DOM exhibited similarities in aromatic compounds but differed in saturation and O-containing groups. Pine DOM was enriched in aromatic and carboxylic compounds, whereas wheat DOM contained more lipid and aliphatic compounds. The differences in the abundance of functional groups and compound families were attributed to the influence of both the original plant residues and microbial degradation. Chemical convergence of the four DOM samples, reflected by their broadly similar spectral composition was possibly due to similar biodegradation processes but in different stages. Multivariate analysis of multiple spectroscopic data comprehensively captured the DOM composition.