Changes in Chemical Composition and Spectral Characteristics of Dissolved Organic Matter From Soils Induced by Biodegradation

Dissolved organic matter (DOM) provides the main energy and C source for soil microorganisms; however, relatively little is known about the dynamics of individual DOM fractions during microbial decomposition. Our objective was to investigate the changes in chemical composition and spectral characteristics of DOM from soils under four different vegetation types, including evergreen broad-leaved forest, coniferous forest, dwarf forest, and alpine meadow along an elevation gradient in the Wuyi Mountains of southeastern China during a 90-day incubation with UV, fluorescence, and Fourier-transform infrared spectroscopic techniques. In synchronous fluorescence spectra, the relative fluorescence intensity (RFI) of fulvic acid–like materials in DOM decreased for evergreen broad-leaved forest and dwarf forest soils after incubation, indicating that fulvic acids, especially when abundant at high initial content, seemed to be an important C source for soil microorganisms. The RFI of humic acid–like materials in DOM increased for all the samples after incubation, indicating that humic acids were resistant to biodegradation. The RFI of protein-like materials in DOM increased for all cases after incubation. Fourier-transform infrared spectroscopy showed that the ratio of absorption intensity of aromatic C=C to that of carbohydrate C-O in DOM increased by twofold to threefold after incubation for all the samples. Increasing values of UV254 absorbance and Humification Index of DOM were found for soils under most vegetation types after incubation. Carbohydrate C concentrations showed a larger fluctuation throughout the incubation, with a change of between 4.5 and 10.4 mg C L−1. Phenol C concentrations increased significantly for all cases. These results indicate that aromatic compounds with an enhanced degree of condensation were accumulated gradually during DOM biodegradation, and that there were considerable amounts of carbohydrates and protein-like materials in the residual DOM.