The response of humic substances to soil acidification and freezing

Abstract

The effects of freezing and acidification on humic substances (HS) of tundra, soddy podzolic, and grey forest soils have been studied in a laboratory experiment. Samples were acidified to decreased the initial pH of soil suspension by a 1 pH unit and were kept at +9 or −12°C for two months. HS were sequentially extracted by two cation exchange resins (fractions HS-1 and HS-2) in distilled water, followed by alkali (HS-3). HS functions, as well as humic and fulvic acids isolated from them, were examined by fluorescence spectroscopy in the emission, excitation, and synchronous-scan excitation modes. Electrophoretic mobilities and E 4/E 6 ratios of HS-1, 2, and three fractions were analysed. On the basis of C HA/C FA and E 4/E 6 ratios and electrophoretic and fluorescence behaviour, HS fractions from the same sample showed distinct differences while corresponding fractions of different origin displayed evident similarities. HS of soils studied differed in their response to acidification and freezing. HS of treated and untreated soils were distinguished by fraction composition, peak wavelength, and intensity of fluorescence in excitation and synchronous-scan excitation spectra.