Spectroscopy, quality variations, and chemical release characteristics of dissolved organic matter extracted from soil of a wheat field at the North China Plain

Soil organic matter (SOM) property and the desorption effect by the hydrochemical solution during rainfall or irrigation play key roles in soil carbon dynamics. The quantity and molecular characteristics of the dissolved organic matter (DOM) extracted from the soil at a wheat field in the North China Plain were investigated to understand the chemical release characteristic of the SOM and the responsible factors. The amounts of DOM desorbed from the different extraction solutions were of the following order: Na2SO4 (62.1 mg/kg) > NaCl (58.7 mg/kg) > H2O (55.7 mg/kg)≈NaHCO3 (55.6 mg/kg) > CaCl2(46.2 mg/kg). The comparative differences of spectroscopy parameters including the smaller E2/E3, larger SUVA254, higher HIX, and lower BIX and FI of the Na-derived DOM than those of the Ca-obtained DOM indicate that the sodium electrolytes were favorable for the desorption of the organic matter with larger molecular weight, higher aromaticity, and humification degrees representing the more refractory property of the DOM. The good ternary relationship of the soil capacity of exchangeable cation (CEC), HIX of soil DOM, and soil TOC (rCEC-HIX = 0.78 ± 0.07, p < 0.05; rCEC-TOC = 0.82, p < 0.05; rHIX-TOC = 0.81 ± 0.09, p < 0.05) suggests that the binding of exchangeable cation with humus were the main association of organic matter with soil. Soil TOC was highly correlated with DOC concentrations (r = 0.78 ± 0.04, p < 0.05), and both parameters were positively correlated with the HIX of the DOM (rTOC-HIX = 0.81 ± 0.09, p < 0.05; rDOC-HIX = 0.64 ± 0.08, p < 0.05). This result indicates that the soil with higher TOC content tended to release higher concentrations of DOM with humus as the main component. The opposite relationships of the sodium adsorption ratio (SAR) with E2/E3 observed in ultrapure water and sodium electrolytes suggest that the larger SAR contributed to the release of DOM with more macromolecules in the solutions. Our study demonstrated that the quantity and quality of DOM leached from soils depend on the chemistry of extraction solutions, the dominant organic components of SOM as well as the soil properties such as CEC and SAR.