Analyzing the effects of freeze-thaw on dissolved organic matter in alpine peat wetland soil based on EEM-PARAFAC

  Objective  Global warming has accelerated changes in soil freeze-thaw patterns, and it is important to clarify the effects of freeze-thaw on dissolved organic matter (DOM) in soils.

  Method  Experimental studies were conducted in alpine peat wetland soils under two freeze-thaw conditions (daily freeze-thaw: −3 ℃/5 ℃, 12 h/12 h, 30 times; deep freeze-thaw: −24 ℃/5 ℃, 20 d/10 d, 1 time) using simulated freeze-thaw methods. The changes in soil DOM quantity and quality were investigated based on ultraviolet-visible absorption spectroscopy (UV-vis) and three-dimensional fluorescence spectroscopy combined with parallel factor (EEM-PARAFAC) analysis.

  Result  (1) After 30 d of incubation, the soil dissolved organic carbon (DOC) content was significantly increased. The DOC content of soil samples under day freeze-thaw and deep freeze-thaw conditions was 183.26 and 187.06 mg/kg, respectively, which increased by 18.80% and 21.26% compared with control group (P < 0.05). (2) The UV-vis absorption curves of soil DOM under different freeze-thaw conditions showed little difference, with no obvious characteristic absorption peak, and the absorbance decreased exponentially with the wavelength, but the freeze-thaw samples showed obvious red shift phenomenon. (3) Both freeze-thaw treatments resulted in increased HIX (from 4.42 to 11.73 and 17.10), SUVA₂₅₄ (from 1.35 mg/(L·cm) to 1.37 and 1.40 mg/(L·cm)) and A₂₅₃/A₂₀₃ values (from 0.38 to 0.40 and 0.43) and decreased spectral slope coefficient at wavelength of 275−295 (from 13.34 μm⁻¹ to 13.23 and 12.91 μm⁻¹) of soil DOM. (4) The fluorescent components identified by EEM-PARAFAC included fulvic acid-like substances, humic acid-like substances, soluble microbial metabolites and protein-like substances. Freeze-thaw changed the composition of soil DOM and the percentage of PARAFAC-derived components. The proportions of the three PARAFAC-derived components in the control group were 37%, 32% and 31%, in the daily freeze-thaw group were 41%, 34% and 25% and in the deep freeze-thaw group were 44%, 35% and 21%, respectively.

  Conclusion  Freeze-thaw changes the quantity and quality of DOM in soils, which is manifested in the increase of DOC content and the enhancement of humification degree.