Variation in the content and fluorescent composition of dissolved organic matter in soil water during rainfall-induced wetting and extract of dried soil

Dissolved organic matter (DOM) is present in all soils, providing a readily available carbon source for microorganisms, which influences microbially mediated biogeochemical processes. Rainfall-induced wetting can alter the content and composition of soil DOM. However, conventional methods commonly used to extract DOM from soils involve air- or oven-drying followed by extraction with water, and the results vary considerably in terms of indexes used. Therefore, this study aimed to determine the variation in DOM content and composition of soil gravity water and capillary water during wetting, and establish a better method to obtain real soil DOM information. Following simulated rainfall (50 mm h⁻¹, 0–24 h), gravity water and capillary water in fresh soil samples were separated using a high-speed refrigerated centrifuge. Additionally, DOM in dried soil samples was extracted using various soil/water ratios after drying by different methods. The DOM data obtained by conventional methods were compared with capillary water data. The results showed that DOM degradation occurred mainly in capillary water close to the surface of soil particles. Among the six fluorescent components of DOM identified, a tryptophan-like component (Eₓ/Eₘ = 295/335 nm and 230/335 nm) was possibly derived from terrestrial plants, and a tyrosine-like component (Eₓ/Eₘ = 265/305 nm) was likely derived from microbial secretion. Except for little variation in the fluorescence index, dissolved organic carbon concentrations in capillary water were double those in dried soil extracted by conventional methods. The humification index and spectral slope ratio of DOM extracted by conventional methods also markedly varied, and no clear patterns were observed for the variation in specific UV absorbance at 254 nm. These findings allow real information to be obtained regarding soil DOM during wetting, and better selection of the extraction method and indexes when studying soil DOM.