Chemical characteristics of humic acids from five soils in Kenya

Humic acids (HAs) isolated from five soils (Luvisols, with 460–600 mm rainfall, Nitosols and Ferralsol, with 1060–1950 mm rainfall) from different ecological regions of Kenya were characterized by ¹³C NMR and IR spectroscopy, elemental analyses and optical measurements. The data suggest that excessive rainfall in sub-humid regions may lower the molecular weights of the HAs as well as their aromaticities. Concentrations of aliphatic components, including aminoacids and carbohydrates, in the HAs from the sub-humid regions were higher (40–42 wt/wt %) than those in the HAs isolated from soils of the semi-arid regions (29–32 wt/wt %). Chemical characteristics of the soils from the sub-humid regions of Kenya resembled those of the HAs from temperate regions but differed from those of HAs originating from arid regions. Annual rainfall and vegetation appeared to affect the composition of HAs but effects of elevation and temperature were less clear.     

Characterization of humic acids from tundra soils of northern Western Siberia by electron paramagnetic resonance spectroscopy

Humic acids from polar soils—cryozems (Cryosols), gleyezems (Gleysols), and peat soils (Histosols)—have been studied by electron paramagnetic resonance spectroscopy. First information was acquired on the content of free radicals in humic acids from polar soils for the northern regions of Western Siberia (Gydan Peninsula, Belyi Island). It was found that polar soils are characterized by higher contents of free radicals than other zonal soils. This is related to the lower degree of humification of organic matter and the enhanced hydromorphism under continuous permafrost conditions. The low degree of organic matter humification in the cryolithozone was confirmed by the increased content of free radicals as determined by electron paramagnetic resonance, which indicates a low biothermodynamic stability of organic matter.     

Complex formation between humic acid and clays as revealed by gel filtration and infrared spectroscopy

Complex formation between pure silicate clay minerals and humic acid (HA) extracted from a Eustis loamy sand (Psammentic Paleudult) was investigated by i.r. spectroscopy and Sephadex G50 and G25 gel filtration. Gel filtration of humic acid after treatment with clay resulted in the separation of normal amounts of high m.w., but reduced concentrations of low m.w. HA. This observation suggested that only the low m.w. fraction of HA was adsorbed by silicate clays. As compared to i.r. spectrograms of original HA, humic acid remaining in solution after interaction with clay exhibited i.r. curves with new bands at 940 and 840 cm^?1 for O-Al-OH vibrations and increased absorption in the 1050, 1400 and 1620 cm^?1 regions for Si-O and COO^? stretching vibrations, respectively. The absorption band for Si-O was retained by the i.r. spectrum of the high m.w. fraction of HA, and the O-Al-OH absorption by that of low m.w. HA. The latter showed in addition the strong band for COO^? stretching vibrations.     

Chemical studies on soil humic acids

Seventeen samples of soil humic acids, two fractions of soil fulvic acid sample, and several related compounds such as lignin, tannin, flavonoid and artificial humic substances were decomposed in conc. KOH solution at 180°C. Succinic acid, glutaric acid, phloroglucin, p-hydroxybenzoic acid, vanillic acid, protocatechuic acid, 3,4-dihydroxy-5-methoxybenzoic acid, and gallic acid were detected in the degradation products of humic acids. The amounts of these degradation products were discussed in relation to the degree of humification or the sources of the humic acid samples. Succinic acid also resulted from glucose, polymaleic acid, and the humic acid and humin prepared from glucose, but glutaric acid resulted only from glucose humic acid and glucose humin but not from glucose and polymaleic acid. Succinic acid and glutaric acid were supposed to result from the same structural portions in humic acids because of the very significant positive linear correlation between their amounts. p-Hydroxybenzoic, vanillic, protocatechuic, and 3,4-dihydroxy-S-methoxybenzoic acids were presumed to result mainly from lignin structure in humic acids. Soil humic acids yielded small amounts of gallic acid although the yields by hydrolysable tannins were in large amounts. The yields of above-mentioned degradation products from humic acids decreased with increasing degree of humification. Phloroglucin resulting from ftavonoids including condensed tannins were also found in the degradation products of humic substances. Its yield showed no linear correlation with RF value of humic acid, and is presumed to be rather related to the vegetation at the sites of soil sampling.     

Chemical studies on soil humic acids

The levels of alcoholic and phenolic hydroxyl, methoxyl, carboxyl, and carbonyl groups of 33 to 38 humic acids obtained from various types of soils were determined to analyze the relationships between the amounts of these functional groups and the degree of humification or the types of soils. The amounts of various oxygen-containing functional groups examined were all proven to be significantly different among the various types of humic acids by analyses of variance. During humification. generally, the carboxyl and carbonyl groups increased while alcoholic and phenolic hydroxyl and methoxyl groups decreased. Linear and logarithmic regression analyses of carboxyl group contents on RF values (optical density of the alkaline solution of humic acids at 600 nm) gave very significant positive correlations. The carboxyl group contents of Rp type humic acids and humic acids from calcareous soils were largely distributed in the upper side of the regression curve. The carbonyl group contents showed a very significant linear positive correlation with carboxyl group contents. and both of them showed high linear positive correlation with RF values. Phenolic hydroxyl group contents decreased with humification in Rp(l). B. and A type humic acids. Alcoholic hydroxyl group contents showed a significant negative linear association with _RF values. Methoxyl group contents decreased rapidly with increasing humification in low humified humic acids, and their negative correlation with _RF values were proven to be very significant by logarithmic regression analysis. The complicated relationship between oxygen content and RF value which was reported previously (7) has been accounted for by the results obtained in the experiments conducted here.     

Chemical studies on soil humic acids

Abstract

Relationships between elementary compositions and types of humic acids or the "degrees of humification were studied statistically using 39 humic acids prepared from various types of soils. Mean values of C%. H%. N%, 0%, H/C, N/C, and O/C of the different types of humic acids (A. B. Rp(l), Rp(2). and Po) were compared. The elementary compositions of various types of humic acids were proven by variance analyses to be significantly distinct. Linear regression analyses of C%, H%, N%. 0%, H/C, N/C, or O/C on RF or AlogK values were also carried out on 35 humic acids excluding the P_+~+++ type humic acids. The linear associations were found to be significant between C%-RF (5% level), H%-RF, H%-?logK, H/C-RF and HIC-?logK (0.1% level), N/C-RF(O.1% level). N%-RF(l% level), N%-?logK N/C-?logK(5% level), and ?logK (1% level), while no significant relationship was found with regard to C%;-?logK, 0%-?logK (both 5% level) and O/C-RF and O/C-?logK (l0% level). Carbon and oxygen contents of humic acids may be apt to reflect the different conditions of soils. The deeper the visible light absorption of humic acids and so the higher the degrees of humification, the lower the hydrogen contents of humic acids were. Though nitrogen content showed a trend similar to the trend of hydrogen content against RF values as a whole. the nitrogen content of less humified humic acids (Rp type) varied from very low to very high values. suggesting the enrichment of nitrogen into humic acid molecules in the early stage of humification. In the H/C versus O/C diagram, humic acids occupied an area reserved for the oxydation products of lignins. The area was fairly wide and “J”-shaped, that is, in the early to the middle stage of humification humic acids were arranged in the direction of dehydrogenation or demethanation, and in the later stage they were arranged in the direction of dehydration.     

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.     

Determination of DTNB-reactive sulphhydryl groups in soil humic acids: their enrichment in humic acids from volcanic acid soils

Thirteen acid soils were collected from typical volcanic regions in Japan (S content: 0.9–4.1, mean = 2.2g kg^?1; pH (H₂O): 2.81–3.93, mean = 3.33), as well as nine reference soils (S: 0.6–1.7, mean= 1.1 gkg^?1; pH(H₂O): 4.10–4.74, mean = 4.47). Humic acids were extracted from the soils separately with 0.1 M NaOH and precipitated by acidification (pH = 2, HCl). After purification, the humic acids were subjected to colorimetric analysis using a DTNB reagent [5,5′-dithiobis(2–nitrobenzoic acid] for the active -SH group. Since humic acids have significant absorption at 4I2 nm, the coloured compound (5–mercapto-2–nitrobenzoic acid) was separated from the humic acids by ultrafiltration or solvent extraction prior to the colorimetric measurement. Humic acids also caused discoloration of the coloured compound when they coexisted in the reaction solutions. Thus, the reproducible determination of -SH was accomplished by employing a standard addition technique (-SH standard: cysteine). Although -SH contents obtained by the ultrafiltration method were considerably higher than those by the solvent-extraction method, probably due to the denaturation of humic acids by the higher buffer concentration used in the ultrafiltration method, they correlated well with each other. The humic acids from acid soils contained apparently higher concentrations of -SH (120–510, mean = 270mg S kg^?1 by the ultrafiltration method; 8–110, mean = 38mg S kg^?1 by the solvent-extraction method) compared to those from reference soils [20–260, mean = 90mg S kg^?1 by the former; not detectable (<5)-34, mean = 11 mg S kg^?1 by the latter]. This -SH enrichment in the humic acids from acid soils may result from the degradation and subsequent humification of S-rich debris of plants and micro-organisms and/or direct incorporation of volcanic acid gas (H₂S) into the humic acids.     

13C NMR characterization of humic acids from soils of a development sequence

Humic acids were isolated from nine topsoils in New Zealand tussock grasslands. Cross-polarization ¹³C NMR spectra of solid samples were used to estimate fractions of carbon contained in different types of chemical functional groups. The degree of oxygen substitution of aromatic rings showed a strong negative correlation with soil development. Aromaticities greater than 0.25 were found in humic acids from only the two least-developed soils.     

Methane suppression by iron and humic acids in soils of the Arctic Coastal Plain

Methane–climate interactions are reasonably well understood; the biogeochemical controls on net methane fluxes are less so. Within anoxic soils, alternative electron acceptors such as iron and humic substances influence microbial metabolic function, and thus affect the amount of carbon lost as methane (CH₄). We present three years of data from wet sedge tundra landscapes near Barrow, Alaska that show an inverse relationship between dissolved iron and CH₄ concentrations. We found increasing organic layer thickness related to increases in active layer organic matter content, and decreases in both bulk density and extractable iron. Organic layer depth was also a good proxy for carbon dioxide (CO₂) and CH₄ dynamics, with increasing organic layer depths relating to lower dissolved iron, higher amounts of dissolved CH₄, and lower CO₂:CH₄ ratios in the upper active layer. Net CH₄ fluxes were also significantly suppressed following the experimental addition of iron and humic acids. Iron and humic acid treatment effects were indistinguishable for CH₄ net flux; in contrast, post-treatment CH₄ fluxes were an average of 0.74-fold the control treatment flux rates. These results suggest that in-situ CH₄ production is tied to alternative electron acceptor availability, and that organic layer thickness is a good predictor of biogeochemical controls on CH₄ fluxes in wet-sedge Arctic Alaskan tundra.     

Sorptive interactions of pesticides in soils treated with modified humic acids

Different structural changes in peat humic acid (HA) were induced through treatments leading to chemical derivatization and/or the removal of functional groups. Eleven humic products were obtained after acetylation, amidation, ammonia fixation, methylation, nitration, oximation, sulphonation, selective degradations and ‘fixation’ of stearic acid. The sorption of 12 pesticides (alachlor, atrazine, carbofuran, 2,4-D, 2,4-DB, methyl parathion, metoxuron, monouron, prometryne, propanil, fenoprop and chloranil) on the above products was determined by ultraviolet derivatographic spectroscopy. In addition, pesticide sorption was evaluated on samples from neutral and calcic soils treated with the corresponding potassium humates. The most intense interactions occurred in samples from the neutral plot, whereas the greatest effect of the humic matter in enhancing pesticide sorption was found with the calcic soil. As expected, the sorption patterns in HA-pesticide systems were poorly correlated with those in soil-HA-pesticide systems. In the former case, the sorption performance was influenced mainly by hydrophobic interactions, whereas in the latter it depended on the participation of multiple parameters suggesting that the molecular volume and the van der Waals energy of the pesticide molecule play a chief role. The HA characteristics most frequently correlated with pesticide sorption were those connected with the aromatic/aliphatic ratio and the particle size. In the calcic-soil-HA-pesticide systems, the base saturation of the exchange complex correlated negatively with pesticide sorption.     

Interpretation of heteronuclear and multidimensional NMR spectroscopy of humic substances

Mapping the chemical structures and organization of humic substances is vital for a fundamental understanding of their roles and interactions in the soil. One‐dimensional nuclear magnetic resonance (NMR) techniques have advanced our awareness of the composition of humic materials, but modern developments in two‐dimensional NMR are soon likely to make obsolete reliance on one‐dimensional spectra alone. The advantages of using heteronuclear two‐dimensional NMR spectroscopy are illustrated in this paper in studies of the structural units in a fulvic acid fraction isolated from the Bh horizon of a Podzol. The structures identified from the NMR (at 500 MHz) experiments can be summarized as: mono‐ and dicarboxylic acids (in about equal amounts), with an average chain length of about 10 carbon atoms (these are easily the major components); smaller amounts (about 10–20% of the acids) of esters and alcohols or ethers; some carbohydrate and amino acid residues (evidence from chemical shift data would suggest that these were likely to be in the form of chains); and very small amounts of 1,2‐, 1,4‐, and 1,3,4‐substituted benzenes and of cinnamic acids. The results suggest that applications of heteronuclear and multidimensional NMR spectroscopy will allow considerable progress to be made in understanding the nature of the structural units and their connectivities in humic molecules provided that the heterogeneity of the humic mixtures can first be decreased significantly.     

Persulfate oxidation of humic acids extracted from three different soils

Three soil humic acids were degraded by a new mild chemical oxidation method using potassium persulfate at pH 2.0. The method degraded 30–40% of the starting material. The type and relative quantities of the products or fragments released as determined by GC/MS varied with the source of the polymers. Humic acid extracted from an Inceptisol yielded fatty, benzenecarboxylic and phenolic acids; humic acid from a Typic Chromoxerert produced largely dialkyl phthalates, and humic acid from an Humic Haplorthod gave only branched and straight-chain fatty acids. It is suggested that the method may be used to advantage as a first oxidant in a sequential degradation.     

Structure of humic acids in zonal soils from <Superscript>13</Superscript>C NMR data

The structure of humic acids (HAs) in zonal soil types—soddy-podzolic soils (two samples), gray forest soil (one sample), and chernozems (two samples)—was quantitatively studied by ¹³C NMR spectros-copy. In the series considered, the content of unsubstituted carbon in the aromatic fragments of HAs increased, and the fraction of unsubstituted aliphatic structures decreased. HAs of soddy-podzolic soils were found to be enriched with carbohydrate fragments compared to HAs of chernozems and gray forest soil. The carbon skeleton of HAs from typical rich chernozem contained significantly more aliphatic and carbohydrate fragments compared to typical chernozem, which probably reflected the lower degree of HA transformation in rich chernozem.     

Temporal changes in humic acids in cultivated soils with continuous manure application

Abstract

Although the application of manure to upland fields is believed to induce changes in the quality of humic substances in soil as well as the quantity, the direction and extent of these changes have not been elucidated. To understand temporal variations in humic acids, periodically collected soil samples from two fields, a Typic Hapludult (Togo) and a Pachic Melanudand (Kuriyagawa), with cattle manure and chemical fertilizer (CF) were examined. The content and degree of humification (darkening) of the humic acids were distinctly greater in Kuriyagawa than in Togo soil. Corresponding to the difference in the degree of humification, molecular size distribution, elemental composition, infrared (IR) spectra, and ¹³C cross polarization/magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectra of humic acids differed between the two soils. Manure application at 40 Mg ha^?1 year^?1 for 16 years (Togo) and at 80 or 160 Mg ha^?1 year^?1 for 19 years (Kuriyagawa) resulted in greater humic acid content compared with plots with CF only because of its increase in the manured plots and/or decrease in the CF plots. Manure application at an extremely high rate (160 Mg ha^?1 year^?1) resulted in higher H content and greater signal intensities of alkyl C, O-alkyl C and amide C=O in the ¹³C CPMAS NMR and/or IR spectra. Although humic acids with larger molecule sizes increased in all the manured plots, differences between the humic acids from the plots with and without manure applied at practical levels in the elemental and spectroscopic analyses were small or scarce. These results were considered to be because of the similarity between the indigenous soil humic acids and the manure-derived ones in Togo soil (a low degree of humification) and because of the abundance of highly-humified humic acids in Kuriyagawa soil.     

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.     

Effects of pig slurry application on soils and soil humic acids

The effect of three annually consecutive additions of pig slurry at two rates (90 and 150 m3 x ha(-1) x year(-1) on soils and soil humic acids (HAs) was investigated in a field experiment under semiarid conditions. Soils and pig slurries were analyzed by standard methods. The HAs were isolated from soils and pig slurry by a conventional procedure based on alkaline extraction, acidic precipitation to pH 1, purification by repeated alkaline dissolutions and acidic precipitations, water washing, dialysis, and final freeze-drying. The HAs obtained were analyzed for elemental (C, H, N, S, and O) and acidic functional group (carboxylic and phenolic) composition, and by UV-vis, FT-IR, fluorescence, and ESR spectroscopies. With respect to the control soil, the pig slurry amended soils had greater pH and electrical conductivity, slightly larger total N content, and smaller values of C/N ratio. A decrease of total organic C was observed only in soils amended for 2 and 3 years at the higher slurry rate. With respect to control soil HA, pig slurry HA was characterized by larger contents of S- and N-containing groups, smaller acidic functional group and organic free radical contents, a prevalent aliphatic character, extended molecular heterogeneity, and smaller aromatic polycondensation and humification degrees. Amendment with pig slurry HA determines a number of modifications in soil HAs, including increase of C, S, and COOH contents, C/N ratios, and aliphaticity and decrease of extraction yields and N, O, phenolic OH, and organic free radical contents. These effects are generally more evident after the first year of slurry application and tend to disappear with increasing number of treatments. Most probably, over the years the slightly humified slurry HA is mineralized through extended microbial oxidation, whereas only the most recalcitrant components, such as S-containing, phenolic, and aliphatic structures, are partially accumulated by incorporation into soil HA.     

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.     

A comparative study of elemental composition of water-soluble humic substances,humic acids,and fulvic acids extracted from sod-podzolic soils

Elemental analysis of water-soluble humic substances extracted from three sod-podzolic soils was carried out. Data on elemental composition were compared to those of humic and fulvic acids extracted from the same soils.