Effects of high concentrations of sulfate on dissolved organic matter in paddy soils revealed by excitation-emission matrix analyzing

Abstract

The problem of sulfate pollution is becoming increasingly serious in freshwater and wetlands. Since paddy fields are the largest constructed wetland in Earth’s surface, the increased sulfate input may have great effect on dissolved organic matter (DOM) in paddy soils. To understand these effects, a 24-day anaerobic incubation experiment was conducted with four Chinese paddy soils amended with high concentrations (0, 10, 25, 50, and 100 mmol L−1) of Na2SO4. Dissolved organic carbon (DOC) and chlorophyll a (Chl a) concentrations were determined after incubation. Parallel factor analysis (PARAFAC) of the excitation–emission matrix (EEM) spectra was used to analyze the DOM composition. In all four soils, DOC concentrations generally increased with increasing sulfate concentration, while the Chl a concentrations decreased. The EEM spectra of DOM were resolved into four components by PARAFAC. With increasing sulfate concentration, the proportion of the ultraviolet C humic acid-like compound decreased and the tyrosine-like compound increased in two algae-rich soils (Sichuan and Tianjin). No obvious variation was observed in the humification index (HIX) or the ratio of peak β to peak α (β:α) in any soils with added sulfate. Specific ultra-violet absorbance at 254 nm (SUVA254) decreased with increasing sulfate concentration in Jilin, Tianjin, and Ningxia soils, and the fluorescence index (FI) decreased in two algae-rich soils. In conclusion, although sulfate addition increased the DOC concentration, the DOM composition depended more strongly on soil type and physicochemical properties than sulfate. Sulfate addition only affected soil DOM origin and composition by inhibiting algal growth in algae-rich paddy soils.