Characterization of Soil Fulvic Acids

FAs were isolated from A-horizons of five chemically different soils: Haplic Fluvisol, Haplic Chernozem, Mollic Gleysol, Haplic Cambisol and Stagnic Luvisol using the procedure recommended by the International Humic Substances Society (IHSS) [88 ]. The degree of sample purity was evaluated by the analysis of ash content (A). In order to characterize the structure of FAs, the atomic ratios of H/C, O/C and O/H were determined using the CHNS/O 2400 analyzer (Perkin Elmer, Waltham, MA, USA). The internal oxidation degree (ω) was calculated with the Zdanov equation ω = [(2O + 3N) − H]/C [27 (link)]. The parameters of Q465/Q668 and Q254/Q436 expressing the humification degree and content of the compounds resistant to humification were calculated as a ratio of absorbance of aqueous FAs solution (40 mg dm⁻³) in 0.05 M NaHCO₃, respectively at 465 and 668 nm, as well as at 254 and 436 nm, using an UV-Vis spectrometer (Jasco V-520, Tokyo, Japan). The value of absorbance recorded at 280 nm was used to evaluate structure aromaticity. Three replications were performed for each measurement, and the obtained results were averaged. The content of carboxylic (COOH) and phenolic (OH) groups was determined and described earlier using the method proposed by Dragunowa and Kucharenko [35 (link)].