Soil Humic Substances Research Articles

Substâncias húmicas como suporte à classificação de solos brasileiros

As substâncias húmicas que compõem a matéria orgânica do solo participam de forma ativa dos processos pedogenéticos do solo, em especial dos horizontes diagnósticos O e H hístico, A chernozêmico, A húmico e B espódico. Este trabalho teve como objetivo determinar o teor de C orgânico das substâncias húmicas e avaliar sua influência na separação de horizontes diagnósticos. Foram utilizados 56 horizontes diagnósticos de diferentes regiões do Brasil, 52 superficiais (H hístico, A chernozêmico, A proeminente, A húmico e A moderado) e quatro subsuperficiais (B espódico). Nesses solos, foram feitas a caracterização química e física, e também de C orgânico, da fração ácidos fúlvicos (C-FAF), fração ácidos húmicos (C-FAH) e humina (C-HUM), as relações C-FAH/C-FAF e C-EA/C-HUM (C-EA = C-FAF + C-FAH) e a percentagem das frações em relação ao C orgânico total (COT), sendo as variáveis submetidas à avaliação por análises multivariadas. Nos horizontes orgânicos, o H hístico apresentou maiores valores e equilíbrio entre o C-HUM e o C-FAH. Na maioria dos horizontes minerais, observou-se predomínio do C-HUM, seguido pelo C-FAH nos horizontes A chernozêmico e A húmico e pelo C-FAF nos horizontes A proeminente e A moderado. Nos horizontes B espódico, foi observado predomínio variável de C-FAF e C-FAH. Com a análise de variáveis canônicas, identificaram-se as variáveis que poderiam ser utilizadas para a separação dos tipos de horizontes diagnósticos. Usando a análise de agrupamento, separaram-se os tipos de horizontes com menor influência da matéria orgânica. Pela análise discriminante, obtiveram-se resultados satisfatórios na classificação dos horizontes diagnósticos com base nas variáveis das substâncias húmicas, quando comparados à classificação original. Os resultados das análises multivariadas indicam que a distribuição do C das substâncias húmicas pode ser utilizada para separar os tipos de horizontes diagnósticos ricos em C orgânico.

ISOLATION AND CHARACTERIZATION OF HUMIC ACIDS IN IDAHO SLICKSPOT SOILS

Naturally occurring humic substances in soils hold promise as a factor related to restoring native plants. We have characterized the humic acids (HA) from inside and outside slickspots of two sites in the Juniper Butte Range soils on the Snake River Plain in Southwest Idaho. These HA may promote the establishment of native slickspot peppergrass (Lepidium papilliferum) seedlings. The HA were extracted from the silt, vesicular, and clay layers using a modified International Humic Substances Society method, and isolated by membrane filtration, precipitation, dialysis, and freeze-drying. Elemental analyses showed minor differences between HA; the inside surface silt samples had higher C and N values than the outside or the deeper samples. Differences were also noted between sample sites and between the samples for carbohydrates, amino acids, and acidity functionalities. Fourier transform infrared and fluorescence, and especially cross-polarization magic angle spinning 13C-nuclear magnetic resonance spectroscopies, provided compositional information and highlighted differences between the samples.

Influence of organic matter on the sorption of aromatic hydrocarbons by peat and chernozem

The interaction of water-soluble monoaromatic hydrocarbons with an organic matrix of chernozem and peat was studied. As a result of high levels of soil pollution by monoaromatic hydrocarbons, soil organic matter with the high sorption capacity actively adsorbs aromatic hydrocarbons. The hydrophobic properties of aromatic compounds determine the strength of the interactions between themselves and the soil organic matrix. The presence of lipids in the organic matter of chernozem decreases the sorption of aromatic hydrocarbons because of the blocking of hydrophobic centers of the soil humic substances.

Olive orchard amended with two experimental olive mill wastes mixtures: Effects on soil organic carbon, plant growth and yield

Amendments of olive orchard soil with two different preparations of olive mill solid waste (OMWMs) at the rate of 9 ton ha −1 per year for five years in two different plots were compared with an industry standard soil amendment using urea. Both the OMWMs amendments showed significant increases in total organic carbon and humic substances in soil of approximately 40% and 58%, respectively, without negative effects on tree growth and yield. This work has shown that olive oil mill waste (OMW) can be recycled safely using the bioremediation system used in this study. We suggest that this system is particularly beneficial to organic farming and is an alternative solution to direct spreading of raw OMW on farm lands.

Studio della componente organica del suolo di faggete trentine in correlazione al trattamento selvicolturale

Since the last 50 years, areas where beech coppices remain unmanaged are increasing in Italy. In more recent years some of these coppices underwent conversion to high-forest. Little information exists on the consequences of this management choice for ecosystem properties, such as soil organic matter and soil humic substances. This study consists in a quantitative and qualitative characterization of soil organic matter as a function of different management and pedoclimatic conditions in beech coppices in Trentino, Northern Italy. The study was conducted in four beech coppices, two calcicolous mesic typical beech forests and two calcicolous xeric beech forests. For each forest type we selected one unmanaged coppice (over the last 40 years) and one coppice stand converted to high forest by releasing a limited number of standards. The qualitative features of humic substances were examined through gel-filtration and Diffuse Reflectance Infra Red Spectroscopy. Results show that the two types of forest respond in a different way to the conversion treatment. In mesic beech forest, humification parameters appear to slow down after silvicultural treatment; in xeric beech forest, a stronger polycondensation of humic compounds was observed after the silvicultural treatment.

Sorption-Desorption Processes of Metals and Metalloids in Soil Environments

Trace elements may be present in solution with positive or negative charges and in different redox state. They occur predominantly in cationic form (Pb, Cu, Zn, Ni, Cd, Hg, Cr(III), Co) but some trace elements are present in anionic form (As, Se, Cr(VI), Mo and B). In soil environments, sorption/desorption reactions on/from inorganic and organic sorbents control the concentration of metals and metalloids in solution and affect their bioavailability, leaching and toxicity. Sorption-desorption processes of elements in cationic form differ greatly from those in anionic form. These reactions are affected by many factors, such as pH, nature of the sorbents, presence and concentration of organic and inorganic ligands, including humic and fulvic acids, root exudates and nutrients. Furthermore, redox reactions, both biotic and abiotic, are of great importance in controlling the oxidation state and thus, the mobility, the phytoavailability and the toxicity of many elements, such as Cr, Se, Co, Pb, As, Ni and Cu (Kabata-Pendias, 2001; Huang and Germida, 2002; Violante et al., 2002; Sparks, 2003; Huang and Gobran, 2005; Violante et al., 2007). The aim of this presentation is to provide information on the factors which affect the mobility of trace elements. Special attention is devoted to the influence of inorganic and organic ligands, including nutrients and root exudates, on the sorption/desorption processes of trace elements in cationic and anionic forms on/from soil components, soils and selected synthetic minerals (double layered hydroxides, the so called “anionic clays”). The soil components responsible for trace element sorption include, soil humic substances, phyllosilicates, carbonates and variable charge minerals (constituents such as Fe, Al, Mn and Ti oxides, short-range ordered aluminosilicates as well as phyllosilicates coated by OH-Al or OH-Fe species whose charge varies with the pH of the soil solution). Recently, evidence on the sorption of heavy metals on microorganisms have been reported (Jackson, 1998). Soil components differ greatly in their sorption capacities, their cation and anion exchange capacities, and the binding energies of their sorption sites. Trace element sorption kinetics depend on the type of surface and trace element, but generally are rapid. Half times for bivalent cations, such as Pb, Zn, Cu and Cd, sorption on peat range from 5 to 15 seconds. Except for some noncrystalline minerals that have very high specific surface charge density with highly reactive sites, humic substances appear to have the greatest capacity for sorption of trace elements in cationic form. A body of evidence has demonstrated that humic matter and metal oxides are much more effective scavengers of trace elements in cationic form, than even the most efficient sorbent among phyllosilicates, indicating that specific sorption and other complexation processes are the dominant binding mechanisms (Jackson, 1998; Huang and Germida, 2002; Spark, 2003). Complexation reactions have the following effects: i) metal ions are prevented from being precipitated; ii) complexing agents can act as carriers for trace elements in soil solution; iii) their toxicity is often reduced by complexation.

Distinguishing black carbon from biogenic humic substances in soil clay fractions

Most models of soil humic substances include a substantial component of aromatic C either as the backbone of humic heteropolymers or as a significant component of supramolecular aggregates of degraded biopolymers. We physically separated coarse (0.2–2.0 μm e.s.d.), medium (0.02–0.2 μm e.s.d.), and fine (> 0.02 μm e.s.d.) clay subfractions from three Midwestern soils and characterized the organic material associated with these subfractions using 13C-CPMAS-NMR, DTG, SEM-EDX, incubations, and radiocarbon age. Most of the C in the coarse clay subfraction was present as discrete particles (0.2–5 μm as seen in SEM images) of black carbon (BC) and consisted of approximately 60% aromatic C, with the remainder being a mixture of aliphatic, anomeric and carboxylic C. We hypothesize that BC particles were originally charcoal formed during prairie fires. As the BC particles aged in soil their surfaces were oxidized to form carboxylic groups and anomeric and aliphatic C accumulated in the BC particles either by adsorption of dissolved biogenic compounds from the soil solution or by direct deposition of biogenic materials from microbes living within the BC particles. The biogenic soil organic matter was physically separated with the medium and fine clay subfractions and was dominated by aliphatic, anomeric, and carboxylic C. The results indicate that the biogenic humic materials in our soils have little aromatic C, which is inconsistent with the traditional heteropolymer model of humic substances.

Effect of some heavy metals and soil humic substances on the phytochelatin production in wild plants from silver mine areas of Guanajuato, Mexico

Phytochelatins (PCs) were determined in the wild plants, focusing on their relationship with the levels of heavy metals and humic substances (HS) in soil. Ricinus communis and Tithonia diversifolia were collected from several sites in Guanajuato city (Mexico), which had long been the silver and gold mining center. The analysis of PCs in root extracts was carried out by liquid chromatography (derivatization with monobromobimane). Total Ag, Cd, Cu and Pb in plant roots and in soil samples, as well as soil HS were determined. The association of metals with HS in soils was evaluated by size exclusion chromatography (SEC) with UV and ICP-MS detection. The results obtained revealed the induction of PCs in R. communis but not in T. diversifolia. The levels of Cd and Pb in plant roots presented strong positive correlation with PC-2 ( r = 0.9395, p = 0.005; r = 0.9573, p = 0.003, respectively), indicating that these two metals promote PCs induction in R. communis. On the other hand, the inverse correlation was found between soil HS and metal levels in roots of R. communis (Cu > Pb > Cd > Ag), in agreement with the decreasing affinity of these metals to HS. Importantly, the inverse correlation between soil HS and plant PC-2 was observed ( r = −0.7825, p = 0.066). These results suggest that metals strongly bound to HS could be less bioavailable to plants, which in turn would limit their role in the induction of PCs. Indeed, the SEC elution profiles showed Pb but not Cd association with HS and the correlation between metal in soil and PC-2 in plant was statistically significant only for Cd ( r = 0.7857, p = 0.064). Based on these results it is proposed that the role of heavy metals in PCs induction would depend on their uptake by R. communis, which apparently is controlled by the association of metals with soil HS. This work provides further evidence on the role of environmental conditions in the accumulation of heavy metals and phytochelatin production in plants.

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

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.

Influence of humic fractions on retention of isoproturon residues in two Moroccan soils

The influence of different fractions of soil organic matter on the retention of the herbicide isoproturon (IPU) has been evaluated. Water and methanol extractable residues of 14C labeled isoproturon have been determined in two Moroccan soils by β -counting–liquid chromatography. The quantification of bound residues in soil and in different fractions of soil humic substances has been performed using pyrolysis/scintillation-detected gas-chromatography. Microbial mineralization of the herbicide and soil organic matter has been also monitored. Retention of isoproturon residues after 30-days incubation ranged from 22% to 32% (non-extractable fraction). The radioactivity extracted in an aqueous environment was from 20% to 33% of the amount used for the treatment; meanwhile, methanol was able to extract another 48%. Both soils showed quantities of bound residues into the humin fraction higher than humic and fulvic acids. The total amount of residues retained into the organic matter of the soils was about 65 % of non-extractable fraction, and this percentage did not change with incubation time; on the contrary, the sorption rate of the retention reaction is mostly influenced by the clay fraction and organic content of the soil. Only a little part of the herbicide was mineralized during the experimental time.

Effects of molecular weight of natural organic matter on cadmium mobility in soil environments and its carbon isotope characteristics

We investigated the role of natural organic matter in cadmium mobility in soil environments. We collected the dissolved organic matter from two different types of natural waters: pond surface water, which is oxic, and deep anoxic groundwater. The collected organic matter was fractionated into four groups with molecular weights (unit: Da (Daltons)) of < 1 × 10 3, 1–10 × 10 3, 10–100 × 10 3, and > 100 × 10 3. The organic matter source was land plants, based on the carbon isotope ratios ( δ 13C/ 12C). The organic matter in surface water originated from presently growing land plants, based on 14C dating, but the organic matter in deep groundwater originated from land plants that grew approximately 4000 years ago. However, some carbon was supplied by the high-molecular-weight fraction of humic substances in soil or sediments. Cadmium interacted in a system of siliceous sand, fractionated organic matter, and water. The lowest molecular weight fraction of organic matter (< 1 × 10 3) bound more cadmium than did the higher molecular weight fractions. Organic matter in deep groundwater was more strongly bound to cadmium than was organic matter in surface water. The binding behaviours of organic matter with cadmium depended on concentration, age, molecular weight, and degradation conditions of the organic matter in natural waters. Consequently, the dissolved, low-molecular-weight fraction in organic matter strongly influences cadmium migration and mobility in the environment.

Cycling of extracellular DNA in the soil environment

Upon entering the soil environment, extracellular DNA is subjected to dynamic biological, physical, and chemical factors that determine its fate. This review concerns the fate of both recombinant and non-recombinant sources of DNA. A schematic of DNA cycling coupled with genetic transformation is presented to understand its behavior in soil. Extracellular DNA may persist through cation bridging onto soil minerals and humic substances, be enzymatically degraded and restricted by DNases of microbial origin, and/or enter the microbial DNA cycle through natural transformation of competent bacteria. Lateral gene transfer may disseminate DNA through the microbial community. An understanding of DNA cycling is fundamental to elucidating the fate of extracellular DNA in the soil environment.

Analysis of the variable charge of two organic soils by means of the NICA‐Donnan model

SummaryWe have tested to see if the generic set of NICA‐Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back‐titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non‐ideal, competitive‐adsorption NICA‐Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H+ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, VD, varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log Kiint) and the heterogeneity of the site (mi), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by 13C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.

Influence of mineral and organic fertilization on soil fungi, enzyme activities and humic substances in a long-term field experiment

Changes in microfungal communities, fungal activities and humic substances (HS) in agricultural soils kept under different fertilization regimes were observed and their causal relationships were investigated in a long-term field experiment. Fertilization did not change the abundance of HS-utilizing microfungi and, except for organic amendment alone, total culturable microfungi were also unaffected by this factor. Organic fertilization increased activities of manganese peroxidase (MnP) and proteinase, but decreased endo-1,4-beta-glucanase activity compared to the corresponding control without organic fertilization. In soils treated with mineral fertilizers, the activities of MnP, endo-1,4-beta-glucanase and proteinase were higher than in control without any mineral treatment. Both the aromaticity of fulvic acid and the molar mass of humic acid was lower in soil with organic fertilization, which may be a result of oxidative degradation mediated by higher MnP activity observed in treatments with organic fertilization.

Sorption of Tetracycline and Chlortetracycline on K- and Ca-Saturated Soil Clays, Humic Substances, and Clay−Humic Complexes

Tetracycline (TC) and chlortetracycline (CTC) are used extensively for growth promotion and therapeutic purposes in livestock production. The sorption of TC and CTC on clays, humic substances (HS), and clay-humic complexes (clay-HC) derived from two agricultural soils was quantified using dilute CaCl2 (Ca) and KCI (K) as background solutions. In all systems, the soil components sorbed > 96% of added tetracyclines. Strongest sorption was observed for clays, followed by HS, and then clay-HC. Greater sorption by the Ca systems than the K systems and decreased sorption with increasing pH suggests that cation bridging and cation exchange contribute to sorption. X-ray diffraction analysis showed that TC and CTC were sorbed in the interlayers of smectites and that the presence of HS reduced interlayer sorption of tetracyclines by smectites in clay-HC. The results indicate that tetracyclines are dominantly sorbed on soil clays and that HS in clay-HC either mask sorption sites on clay surfaces or inhibit interlayer diffusion of tetracyclines.

Role of Humification Processes in Recycling Organic Wastes of Various Nature and Sources as Soil Amendments

AbstractRecycling the large amounts of organic wastes produced by agriculture, forestry, urban and industrial activities as soil, organic amendments are the most popular and efficient option for avoiding their dispersion in the environment and restoring, maintaining, and/or improving the content of soil organic matter. Chemical stability and biological maturity are two important factors for the successful use of organic wastes in agriculture with limited risk for the surrounding environment. Stabilization and maturation of raw organic wastes inherently imply the achievement of an extensive humification, that is, a wide conversion of easily degradable organic matter to refractory organic compounds that resemble native soil humic substances (HS). Soil HS are the most important components of soil organic matter responsible of several soil functions and processes. As a consequence, the amount and quality of HS‐like fractions in any organic amendment are believed to be of primary importance for its agronomic efficacy, environmental safety and economic value. The first part of this review focuses on the chemical and physico‐chemical changes occurring in the humic substances (HS) ‐like fractions of organic wastes of various nature and sources subjected to common treatment processes aimed at producing environmentally‐safe soil amendments with beneficial agronomic properties. The second part discusses the composition, structure, and chemical reactivity of the HS‐like components in organic amendments of various origins and nature, and their effects on native soil HS. The review concludes by highlighting the need for innovative research targeted mainly to achieve a better fundamental understanding of the molecular structure and reactivity of soil HS and HS‐like fractions in organic amendments, the mechanisms of HS formation and transformations in the natural environment and during the treatment processes of raw organic wastes, the interactions with metals and organic xenobiotics, and the direct physiological effects that HS may exert on plants.

Reduction capability of soil humic substances from the Rio Negro basin, Brazil, towards Hg(II) studied by a multimethod approach and principal component analysis (PCA)

This paper characterizes humic substances (HS) extracted from soil samples collected in the Rio Negro basin in the state of Amazonas, Brazil, particularly investigating their reduction capabilities towards Hg(II) in order to elucidate potential mercury cycling/volatilization in this environment. For this reason, a multimethod approach was used, consisting of both instrumental methods (elemental analysis, EPR, solid-state NMR, FIA combined with cold-vapor AAS of Hg(0)) and statistical methods such as principal component analysis (PCA) and a central composite factorial planning method. The HS under study were divided into groups, complexing and reducing ones, owing to different distribution of their functionalities. The main functionalities (cor)related with reduction of Hg(II) were phenolic, carboxylic and amide groups, while the groups related with complexation of Hg(II) were ethers, hydroxyls, aldehydes and ketones. The HS extracted from floodable regions of the Rio Negro basin presented a greater capacity to retain (to complex, to adsorb physically and/or chemically) Hg(II), while nonfloodable regions showed a greater capacity to reduce Hg(II), indicating that HS extracted from different types of regions contribute in different ways to the biogeochemical mercury cycle in the basin of the mid-Rio Negro, AM, Brazil.

Soil Humic Substances : their Chemical Structures and Functions

Soil Humic Substances : their Chemical Structures and Functions

Pyromorphite formation and stability after quick lime neutralisation in the presence of soil and clay sorbents

Environmental context. Questions remain regarding the potential risk of human Pb exposure from metal-contaminated soils. Studies show that the risk of human exposure is more accurately linked to the toxicity of the Pb species in soil than the total quantity of Pb. This work explores the practicality of converting Pb to a less toxic, less bioavailable species called pyromorphite in the presence of soil. Abstract. Soluble Pb is immobilised in pure systems as pyromorphite by adding sources of P, but doubts remain about the effectiveness of this approach in natural soil systems, particularly given the ability of soil humic substances to interfere with Pb-mineral formation. In addition, recent thermodynamic modelling predicts that pyromorphite formed by the addition of phosphoric acid to Pb-contaminated soils, followed by neutralisation with quick lime (Ca(OH)2) will destabilise the mineral, reverting the Pb back to more soluble species such as cerussite or anglesite. In this paper, we describe experiments to form pyromorphite in the presence of two different sorbents: a reference smectite called Panther Creek Bentonite, and a commercially available, organically rich potting mixture. We present X-ray diffraction (XRD) evidence suggestive of pyromorphite formation, yet, like similar studies, the evidence is less than conclusive. Linear combination fits of Pb X-ray absorption fine-structure spectroscopy (XAFS) data collected at the Advanced Photon Source at Argonne National Laboratory show that pyromorphite is the major Pb species formed after the addition of phosphoric acid. Furthermore, XAFS data shows that neutralising with quick lime enhances (as opposed to reducing) pyromorphite content in these systems. These results call into question relying solely on XRD data to confirm or deny the existence of minerals like pyromorphite, whose complex morphology give less intense and more complicated diffraction patterns than some of the simpler Pb minerals.

A comparative survey of recent results on humic-like fractions in organic amendments and effects on native soil humic substances

The main scope of the wide use of organic amendments of various origin and nature in modern agriculture is to increase and/or restore the organic matter content in organically poor or depleted soils in order to maintain and/or increase crop production and reduce soil exposure to degradation, erosion, desertification and pollution. The agronomically efficient and environmentally safe use of any organic amendment requires, however, an adequate control not only of the chemical quality of the humic substance (HS)-like fractions contained, which is an important indicator of the maturity and stability achieved by organic matter in the amendment, but also of the effects that these HS may have on native soil HS. In this review, the most recent results obtained on this topic by the research groups operating in Bari, Italy, and in Madrid, Spain, are discussed comparatively with results recently published by other research groups. Overall, HS-like components of organic amendments are characterized by higher aliphatic character and molecular heterogeneity, lower contents of O, acidic functional groups and organic free radicals, and lower degrees of aromatic ring polycondensation, polymerization and humification than native soil HS. These differences are less evident for composted materials. The composition, structure and functionalities of HS in amended soils may be affected in different ways and at various extents on dependence of the nature, origin and rate of amendment. In general, these properties are intermediate between those of the unamended soil HS and the HS-like fractions in the amendment, but generally resemble more the former than the latter, especially with increasing time after amendment application.