ELEMENTAL COMPOSITION OF HUMIC ACIDS ISOLATED FROM CHERNOZEM, VERTISOL, REGOSOL, PLANOSOL AND HISTOSOL

Abstract

Humic substances, including humic acids (HAs), are the most abundant fractions of soil organic matter. Their composition, structure and properties, determined by soil formation conditions, can be used as indicators of pedogenetic processes. In this study, elemental composition as very important part of HAs characterization was determined for humic acids isolated from the soils of Serbia developed under different pedogenetic conditions (Chernozem (CH), Smonica/Vertisol (VR), Rendzina/Regosol (RG), Pseudoglej/Planosol (PL) and Prelazni treset/Histosol (HI) and related to soil properties and origin. Soil samples were collected at 0 to 15-30 cm depth. Soil texture, organic C (soil organic matter for HI), and pH were determined by common methods. HA samples were isolated using a modified IHSS method. The C, H, and N contents of HA samples were determined using elemental analyzer (CHNS 628, LECO Corporation, USA) after drying the samples over P2O5 under vacuum. Their percentages were calculated on the ash-free basis. The oxygen content was obtained as the difference. The ash content was determined by a dry combustion method. The C/N, O/C, H/C and O/H atomic ratios were used as indicators of variations in HA properties as a function of HA origin. Internal oxidation degree (ω) was calculated by the equation: ω = (2O+3N-H)/C, where: O, N, H and C are element contents (atomic %). Elemental compositions of investigated HAs fall within the range of average values reported for soil HAs. According to the humification degree obtained, HAs studied are ranged as follows: CH>VR>RG>HI>PL. Both correlations done (H/C versus O/C and H versus C) separate HAs in three groups: CH and VR - the most dehydrogenated and demethylated, i.e. the most stable; RG - the most oxidated; and HI and PL - the most hydrogenated. HAs elemental composition is related to soil organic C and pH, but not related to soil clay. Internal oxidation degree is used as an indicator of plant residue humification progress. Positive ω values point out well drained soils with prevailing oxidizing conditions and negative values reflect anaerobic soil conditions. Internal oxidation degree values are obviously lower in hydromorphic HI and PL HAs compared to terrestrial RG, CH and VR HAs, but positive values indicate predominance of aerobic under anaerobic conditions in surface layer of hydromorphic soils. Results obtained in this study are in agreement with the literature data, indicating characteristics of HAs as particularly dependant on environmental conditions.