Fourier Transform Infrared, UV-Visible, and X-Ray Diffraction Analyses of Organic Matter in Humin, Humic Acid, and Fulvic Acid Fractions in Soil Exposed to Elevated CO2 and N Fertilization

Abstract

FT-IR, UV-visible, and X-ray diffraction analyses were used to investigate the nature of organic matter in three fractions of soil humic substances (humin, HU; humic acid, HA; and fulvic acid, FA) extracted at a depth of 0–10 cm from the soil under rye grass ( Lolium perenne) culture. FT-IR and XRD analyses showed that the extracted HAs still contained the clay matrix, but the extracted FAs, which were purified by adsorption-desorption from a macroreticular nonionic resin, were free from the clay mineral constituents. The data obtained in the present study shed more light on the chemical structure of the HU, HA, and FA fractions in the infrared absorption band region of 400–1000 cm−1 where the assignments of bands are still lacking in the literature. The bands characteristic of organic functional groups, especially benzene rings and condensed aromatics, which were masked by the strong bands of the clay matrix in the HU and HA fractions, were clearly shown in the FA fraction, which was free from the clay mineral constituents. While the UV-visible spectra of HAs and FAs were similar, the E465/E665 ratios of HAs were about three times lower than those of FAs. Amounts of carboxyl groups, determined by titration, were about four times higher in the FA than in the HA fraction, which is in agreement with the FT-IR analysis. The HU, HAs, and FAs fractions extracted from the soil under the same culture and exposed to elevated CO2 levels (60 Pa pCO2) under different N fertilizer application rates (100 and 420 kg N/ha) were also studied. The results show that there was no significant change in the spectra of HU, HAs, and FAs extracted from soils exposed to elevated CO2 for 2 years. In contrast, elevated CO2 combined with a high N fertilization rate resulted in a remarkable increase of the acidic OH groups of the FA.