Folia Geographica 10. sējums - Latvijas Universitāte

DABAS PĒTĪJUMIsources on the structure and properties of humic substances. Humic substances from soils have ahigher degree of aromaticity and molar absorption than those from water.Light Absorbance Properties and Aromaticity of Humic SubstancesHumusvielu sorbcijas raksturojumsTable 2Molar absorption,L (mole C) -1 cm -1 E4/E6 Aromaticity, % M wFA Missouri River a 247 13.9 20.4 -FA Ohio River a 274 17.2 24.3 -FA Minnesota groundwater a 122 3.53 12.6 -FA Suwannee River a 389 20.7 24.8 880FA Lake Fryxell a 150 - 13.0 -FA Coal Creek b 401 20.7 27.4 -FA Lake Burtnieku b 214 8.6 17.4 2200FA Lake Liepājas b 189 9.4 16.1 1650HA-soilBrown soilsPodzolic soilsAnthrosolsHydromorphic soilsa – Chin et al., 1994;b – Kļaviņš et al., 19975014442696815.46.57.85.641.436.732.456.31300180035001500Direct evidence of the structural features of humic substances provides their 13 C NMRspectra. The signals in these NMR spectra represent different types of carbon atoms. The13 C NMR spectra can be divided into several chemical shift ranges (Figure 3), indicating thepresence of different major structural elements into structure of the humic substances. Region A(0-50 ppm) consists primarily of aliphatic carbon resonances (methyl, methylene and methinecarbons). Region B (50 to 60 ppm) consists of methoxylcarbon (-OCH 3 ) resonance signals.Region E (110 to 140 ppm) is assigned to the resonance of unsubstituted and alkysubstitutedaromatic carbons. Region F (140 to 160 ppm) consists of the resonance of the aromatic carbonssubstituted by oxygen and nitrogen (e.g., phenols, aromatic ethers or amines). This analysisallows the direct evaluation of the substantial differences among humic substances of differingorigins and the substantially lower aromaticity of aquatic humic substances in comparison withcommercial humic substances.It can be concluded that the properties of humic substances depend on the origin of thehumic matter. The similarities existing among humic substances reflect not so much theenvironment from which they has been isolated but rather the conditions of their formation.These differences may be explained by the proportions of production and transformationprocess intensities in the relevant environments.The distinctive distribution of functional groups and the major building blocks of humicsubstances may reflect their production and structure alteration due to microbial degradation andgeochemical alteration. Fulvic acid formation may occur through condensation reactionsinvolving compounds derived from their precursor organic materials or through the productionof residual recalcitrant compounds in the microbial degradation of their precursor organicmaterial. For peat humic acids, lignin is not included in the set of precursor materials, andcarbohydrates and aliphatic structures are relatively abundant. Carbohydrate structures,however, are the most labile and are rapidly degraded by microorganisms. Similarity betweenhumic substances from peat and soil indicate either the dominance of allochtohonous peathumus or the deep transformation of autochthonous humic material.4. ConclusionsThe elemental, functional, mass-molecular and spectral characterisation of humicsubstances stresses the role of their origin on the properties of these humic substances. Humicsubstances from different soil, water and peat types differ in their properties. The similarity inthe structure and properties of humic substances suggests the importance of the intensity of52