Distribution of heavy metals by the molecular-weight fractions of humic acids in the soils of long-term field experiments

The influence of different agricultural treatments on the contents of Cd, Zn, and Co in the organic matter and humic acids of soddy-podzolic and dark gray forest soils was studied in long-term field experiments. The use of gel chromatography proved the complicated molecular-weight composition of the humic acids in all the studied soils. The contents of heavy metals and their distribution by the molecular-weight fractions of humic acids were determined in the experimental soils and in the virgin podzolic soils. The complex nature of organomineral compounds of Cd and Zn with humic acids was shown by means of gel filtration. The thermodynamic stability of the organomineral complexes increased with the increasing weight of the initial molecular-weight fractions of humic acids.     

Dissolved organic matter in lysimetric water of mountain forest soils in the southern Sikhote Alin

The fractional composition of dissolved organic matter and the chemical nature of humic and fulvic acids were studied in lysimetric waters from forest soils of different altitudinal zones in the Sikhote Alin Range. The elemental composition, infrared absorption spectra, concentrations of acid functional groups, and pK spectra of humic and fulvic acids were determined. Fulvic acids predominated in the upper soil horizons, and fraction of nonspecific dissolved organic substances predominated in the lower mineral horizons. The portion of humic acids in the humus horizons markedly decreased from the low-mountain soils to the high-mountain soils; the nitrogen content of humic and fulvic acids decreased in the same direction. Three classes of carboxyl and phenolic groups were determined in pK-spectra of humic and fulvic acids. The soils of high-mountain zones had stronger acidic properties of humic and fulvic acids in comparison with the soils of low-mountain zones. The determined characteristics of the composition of dissolved organic matter and the trends of their changes contribute to our knowledge of pedogenetic processes in the altitudinal sequence of forest landscapes of the Sikhote Alin Range.     

Regularities of extracting humic acids from soils using sodium pyrophosphate solutions

Regularities of extracting humic acids from different soil types (soddy-podzolic soil, gray forest soil, and all chernozem subtypes) with sodium pyrophosphate solutions at different pH values (from 5 to 13) have been studied. It is found that, regardless of soil type, the process occurs in two stages through the dissociation of carboxylic groups and phenolic hydroxyls, each of which can be described by a logistic function. Parameters of the logistic equations approximating the extraction of humic acids from soils at different pH values are independent of the content and composition of humus in soils. Changes in the optical density of humic acids extracted from soils using sodium pyrophosphate solutions with different pH values are described in the first approximation by the Gaussian function. The optically densest humic acids are extracted using sodium pyrophosphate solutions at pH 10. Therefore, it is proposed to use an extract with pH 10 for the characterization of organic matter with the maximum possible degree of humification in the given soil.     

Effect of organic amendment on amount and chemical characteristics of humic acids in upland field soils

To assess the effect of continuous organic material (OM) application on soil humic acids, the amount and chemical characteristics of humic acids in various types of soils (n = 10) were compared between plots treated with farmyard manure (FYM) or rice straw compost (RSC) plus chemical fertilizer (CF) and plots treated with CF alone. The degree of humification (degree of darkening), molecular size distribution and ¹³C cross polarization/magic angle spinning nuclear magnetic resonance spectra of humic acids from CF‐treated soils showed wide variation among the soils. Humic acid content was generally larger in OM + CF soils than in corresponding CF soils, and the stable C isotopic ratio suggested partial replacement of indigenous humic acids with OM‐derived ones even where no apparent increase in humic acid content was observed. The rate of OM application and the indigenous humic acid content were related positively and negatively, respectively, to the apparent accumulation rate of humic acids among soils. The degree of humification of humic acids was generally smaller in OM + CF soils than in CF soils. Humic acids extracted from FYM and RSC exhibited chemical characteristics typical of humic acids having a smaller degree of humification, which suggested the contribution of OM‐derived humic acids to the differences between OM + CF and CF soil humic acids, such as larger average molecular sizes and smaller and larger proportions of aromatic C and O‐alkyl C, respectively, relative to total C in the OM + CF soil humic acids. Little change was observed in the chemical characteristics of humic acids when the degree of humification of indigenous humic acids was small. The effect of OM application on the chemical characteristics of humic acids was most conspicuous in soils containing humic acids having an intermediate degree of humification, possibly resulting from the combination of accelerated degradation of indigenous humic acids and the accumulation of OM‐derived humic acids.     

The influence of different land management practices on the transformation of humic acids in heavy loamy soddy-podzolic soils of the Cis-Urals region

The elemental composition and structure of humic acids were studied in heavy loamy soddy-podzolic soils of the Cis-Urals region under different land management practices. The humic acids in the soil of the long-term clean fallow, overgrown fallow (abandoned plot), and crop rotation plots differed in their composition and properties. The humic acids in the soils of the crop rotation with the regular application of manure and of the overgrown fallow were enriched in the components of both their central (nuclear) and peripheral parts. Such a structure ensured the active participation of humic acids in the carbon cycle with the simultaneous preservation of the soil’s fertility.     

Molecular structure of humus acids in soils

The molecular structure of humus acids remains one of the most challenging issues in the soil science. The focus of the current research lies in defining the molecular and supramolecular structure of soil humus acids. This paper provides a brief review of the current understanding of the problem and shares findings of the elemental and X‐ray structural analysis of humic and fulvic acids extracted from Chernozems and Chestnut soils in the Rostov region. This region occupies a large area of the South‐European Russia. Humic acids in the studied soils have reduced carbon content compared to average parameters for these soil types. The supramolecular organization of humic acids in three‐dimensional space contains two to four layers of condensed aromatic systems supplemented by a network of chain fragments of different regularity and length. Parameters of this structure vary for soils of different genesis and genetic horizons. However, interplanar distances are fixed in the range of 0.349 to 0.371 nm. Thickness of the carbon network band increases with the benzoidicity level of humic acids. The obtained results confirm the specific matrix structure of humic acid molecules. Considering currently available scarce data these findings are valuable and unique. Our findings are consistent with the general hypothesis about the supramolecular nature of humic substances.     

A comparative study of molecular weight distribution of water-soluble humic substances,humic acids,and fulvic acids extracted from sod-podzolic soils

The alteration of organic matter from three sod-podzolic soils in the course of the extraction of water-soluble humic substances was studied by means of size-exclusion chromatography using a combined UV-DOC detector. A comparison of water-soluble humic substances with humic and fulvic acids extracted from the same soils was carried out.     

The distribution of nitrogen in some highly organic tropical volcanic soils

The qualitative and quantitative distribution of N-compounds in 10 tropical soils, and in a number of humic materials extracted from representative samples thereof, was determined after 6 N HCl hydrolysis.Eighty to 98% of the total N in the soils and humic materials was hydrolysable by 6n HCl. Slightly less than one half the hydrolysable N in the soils and humic fractions consisted of amino acids. Well-drained soils and fulvic acids extracted from them contained unusually high concentrations of the acidic amino acids, aspartic and glutamic acids. Between 80 and 95% of the amino acids in the soils was accounted for in the humic materials + NaOH-insoluble organic residues. NH⁺₄-N released by acid hydrolysis was generally higher for the soil samples than for the humic materials. Amino sugar-N constituted relatively small proportions of the total N in the soils and humic fractions.Our data suggest that large quantities of amorphous allophanic materials coupled with relatively high enzymic activity are responsible for the observed accumulation of acidic amino acids in the well-drained tropical volcanic soils.     

Comparative characterization of humic acids from arable soils of the taiga, steppe, and semidesert zones

The parameters of the elemental composition, molecular weight distribution, infrared and proton spectroscopy, and hydrophilic-hydrophobic properties were determined for humic acids preparations isolated from arable horizons of soils from different natural zones (an ordinary chernozem, a soddy-podzolic soil, and a meadow sierozem). The general and specific properties of the humic acids from the three studied soils were revealed.     

Chemical and physical characteristics of humic substances extracted from greek soils varying in classification order and natural drainage

Abstract

Humic and fulvic acids were extracted from the surface horizons of Alfisols, Vertisols, and Entisols with good and impeded natural drainage. These profiles are located in the temperate subhumid zone. The extracted humic substances were characterized by elementary and functional group analyses, IR‐spectroscopy, differential thermal analysis (DTA) and electron spin resonance spectroscopy (ESR). Some differences in the humic substances seem to be associated with differences in the natural drainage of the soils. When data from samples of the same soil order are compared, indicate that humic (HAs) and fulvic acids (FAs) extracted from soils with impeded drainage, have higher contents of carbon and spin concentration, but less total acidity and fewer carboxylic groups (COOH), than the respective humic substances extracted from well drained soils. The IR‐spectra of the HAs extracted from soils with impeded drainage showed more aliphatic groups than those of the well drained soils. No essential differences, which could be related to the drainage conditions of the studied soils, were observed in the IR‐spectra of FAs. DTA‐diagrams shows that the thermal stability of the HAs and the FAs are related to their carbon and ash contents. Iron was the dominant element in the ash of the humic acids, aluminum and silicon were the most abundant elements in the ash of the fulvic acids.     

The humus state of the soils of locally hydromorphic landscapes in the lower reaches of the don river

The humus state of rainfed chernozems affected by local waterlogging was studied. The total humus content in the hydromorphic chernozems increases, as well as the content of fulvic acids, whereas the content of nonhydrolyzable residue (humin) decreases. A significant increase in the portions of the third fractions of humic and fulvic acids is observed. The role of the fine silt and clay fractions in the binding of humic substances increases in the lower horizons of locally hydromorphic soils. The increase in the content of fulvic acids (fulvatization) is mainly due to their predominance in the clay fraction. The latter is specified by the significant narrowing of the C_ha-to-C_fa ratio, the lower content of the nonhydrolyzable residue, and the increased content of the clay-bound (3rd fraction) fulvic and humic acids. The composition of the humus in the fine silt fraction of the studied soils is characterized by an increased amount of humic acids of the second fraction with a decrease in the relative content of fulvic acids.     

Characterization of fungal melanins and soil humic acids by chemical analysis and infrared spectroscopy

Summary Humic acids from two Brazilian topsoils under savanna grassland and five soil fungal melanins were characterized by elemental, functional group and infrared analysis. C, N, total acidity, COOH, and phenolic OH contents were within the ranges reported for several other fungal melanins and soil humic acids. Compared with the soil humic acids, the infrared spectra of the fungal melanins showed greater detail, indicative of higher aliphaticity. They were similar to the type III infrared spectra of humic acids, which are characteristically high in proteinaceous material and polysaccharides. The infrared spectra of the humic acids from the two Brazilian soils studied were classified as type I, which includes most soil humic acids. Notwithstanding the greater detail, in some areas the fungal melanin spectra were similar to those reported for other fungal melanins and humic acids of different origins. The probable contribution of the melanic fungi to the formation of soil humic polymers is discussed.     

Effects of prolonged irrigation on the humus of steppe soils in southwest Siberia

Changes in humus resulting from up to 20 years' irrigation with slightly saline water were studied in dry steppe soils in southwest Siberia. The amounts of brown humic acids and of the Pg fraction increased with the period of irrigation, and their maximum amounts occurred at greater depths in the profile. Other fractions of humus and humic acids fluctuated within the limits typical of dry steppe soils. The humus therefore remained essentially stable under irrigation for up to 20 years.     

ELECTRON-SPIN RESONANCE OF HUMIC ACIDS FROM CULTIVATED SOILS

An examination has been made of the electron-spin resonance spectra of humic acids from (i) two groups of cultivated soils from the north-east of Scotland, and the related natural soils from which the cultivated soils are considered to be derived, and (ii) some Rothamsted soils. With the Scottish soils the humic acids from the more acid natural soils (pH <4.6) show e.s.r. spectra with a four-peak structure (class I), and those of the less acid soils (pH >4.6) almost structureless spectra (class II). The cultivated soils contain humic acids with the same class of spectrum as that of the soils from which they are derived, even though they are now much less acid (_PH 5.9 –6.5). Humic acids from Rothamsted soils, sampled recently, were compared with those from the same site sampled 80 years previously. Where the soil has been allowed to revert towards a natural state but still contains free calcium carbonate the pH remained high (7.8–7.9) and the spectrum of the humic acid remains class II. Where the soil contained no free calcium carbonate the pH has fallen from 7.1 to 4.5 on reversion and the spectrum class of the humic acid has changed from II to I. The results support the hypothesis that the class of spectrum of the humic acid depends on the pH at which it is formed.     

CHEMICAL DEGRADATION OF HUMIC AND FULVIC ACIDS EXTRACTED FROM MEDITERRANEAN SOILS

Humic and fulvic acids were extracted from two Israeli and tour Italian soils and oxidized with alkaline permanganate solution after methylation. Following oxidation, the degradation products were separated by solvent extraction and chromatographic methods and identified by gas chromatography-mass spectrometry. Major oxidation products were aliphatic, phenolic and benzenecarboxylic acids. In toto, 33 oxidation products were identified. These were essentially the same compounds as those produced by the permanganate oxidation of methylated humic and fulvic acids extracted from soils formed under widely differing climatic and geologic conditions, except that yields of phenolic acids from Mediterranean humic and fulvic acids were lower than those produced under similar conditions from humic materials extracted from other soils. The information provided by chemical degradation suggests that humic and fulvic acids from widely differing soils have similar chemical structures.     

Combination reaction between humic acid and calcium ions

In the previous paper (1), liming an acid humus volcanic ash soil resulted in the formation of humic acids combined with calcium, but had no significant effect on acid brown forest soils containing humic acids of a lower degree of humification. Subsequently, the authors examined the humus composition of the soils treated with calcium acetate, and conducted some experiments on the combination reaction between humic acid and calcium ions. The results obtained are presented in this paper.     

Molecular parameters of humic acids

Several molecular parameters of humic acids isolated from the soils of the most important genetic types are discussed.The material obtained makes it possible to establish limiting values of the molecular weights of humic acids and indicate the tendencies in their changes.The selected parameters characterise the submolecular, molecular and supramolecular levels of the organisation of humic acids. It has been shown that the most important indices include the molecular-weight distribution, molecular weight and the molecular form of humic acids.     

Specific features of the humus status in soils of Buryatia

The humus content in soils of Buryatia varies significantly in dependence on the local hydrothermic conditions. All the studied soils are characterized by a relatively short humus profile, a sharp drop in the humus content down the soil profile, considerable amounts of the humin fraction and the fraction of humic acids bound with sesquioxides, and a moderate humus enrichment with nitrogen.     

Acid properties of fulvic and humic acids isolated from two acid forest soils under different vegetation cover and soil depth

We studied the acid‐base properties of 16 fulvic acids and 16 humic acids isolated from the surface (3–15 cm) and subsurface (> 45 cm) horizons of two types of acid forest soils, derived respectively from amphibolite and granite rocks, under five different types of vegetation. The observed differences between the contents of humic substances in the two types of soils were related to the degree of Al‐saturation of the soil organic matter, as indicated by the molar ratio between pyrophosphate extractable Al and C. Humic fractions were characterized in terms of elemental composition, and CPMAS ¹³C NMR spectrometry. The contents of carboxylic and phenolic groups were estimated by potentiometric titrations conducted in 0.1 m KNO₃ in a nitrogen atmosphere. The fulvic acids contained more carboxylic groups but less phenolic groups than the humic acids: the ratio of phenolic to carboxylic groups in the humic acids was 0.48 ± 0.10 and in the fulvic acids 0.23 ± 0.05. The mean values of the protonation constants of each of the humic substance fractions can be used as generic parameters for describing the proton binding properties. The fulvic acids isolated from the subsurface horizon of the soil contained between 2.6 and 23% more carboxylic groups, and the humic acids between 8 and 43% more carboxylic groups than those isolated from the surface horizon of the same soil.     

Chemical and physical characteristics of humic and fulvic acids extracted from soils of the Mediterranean region

Humic and fulvic acids were extracted from two soils from Israel and from four soils from southern Italy. Following purification, the humic substances were characterized by elementary and functional group analyses, spectrophotometric and spectroscopic methods, thermal analysis and by scanning electron microscopy.The analytical characteristics of the humic and fulvic acids extracted from the Israeli and Italian soils were generally similar to those of humic materials originating from soils formed under widely differing geographic and pedologic conditions. There were some differences in the content and composition of inorganics in the fulvic acids which were reflected in functional group analyses, IR and ESR spectra, DTA curves and SEM micrographs.