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The green fraction of humic acid (Pg) and the chloroform-extractable green fraction (CEGF) are characteristic soil organic matter (SOM) components. These alkaline solutions are green-colored due to the presence of 4,9-dihydroxyperylene-3,10-quinone (DHPQ) chromophore. While both of which are potential indicators for the effect of land use and paleoclimatic environment in the fields of soil science as well as geochemistry,  CEGF as well as its relationship with Pg in soils are not yet fully documented. In this study, we firstly investigated the chemical properties of soil CEGF fractions by ultraviolet-visible (UV-Vis) and infrared (IR) method. Two CEGF components were separated by sequential liquid-liquid extraction using aqueous ammonium hydroxide (NH4OH) followed by aqueous sodium hydroxide (NaOH). Results showed that the UV-Vis spectral shape of NH4OH-extractable component is very similar to that of DHPQ, except that it is red-shifted. The solubility and UV-Vis spectrum of the NaOH-extractable fraction were completely identical with those of synthesized DHPQ. Their IR spectral shapes were also almost the same. Subsequently, the distribution of CEGF in humic acid (HA), fulvic acid (FA) and humin (HN) from Japanese Andosols and Cambisol was quantitatively evaluated by sequential extraction. Most of CEGF was detected in the HA (60-78%) and HN (22-40%), but not in the FA. While the UV-Vis spectral shape of CEGF extracted from Andosols HAs showed a relatively higher proportion of DHPQ than its derivative, the opposite was observed in Cambisol HA, whose CEGF is similar to that of sclerotium grain (one of the possible origin of CEGF). These results suggest the diversity of CEGF-producing soil fungi. Quantitative data also indicated that 35-49% of Pg consisted of a chloroform-soluble fraction (i.e., CEGF) and the remaining 51-65% of Pg was chloroform-insoluble. Based on these results, we propose that CEGF is composed of DHPQ and DHPQ-derivatives and that CEGF is one of the major fractions of Pg. [ABSTRACT FROM AUTHOR]

Carrot ( Daucus carota L.) is a widely consumed root vegetable, whose growth and safety might be threatened by growing-medium arsenic (As) contamination. By this work, we evaluated the effects of humic acids from Leonardite and NPK mineral fertilisation on As mobility and availability to carrot plants grown for 60 days in a volcanic soil irrigated with As-contaminated water – representing the most common scenario occurring in As-affected Italian areas. As expected, the irrigation with As-contaminated water caused a serious toxic effect on plant growth and photosynthetic rate; the highest rate of As also inhibited soil enzymatic activity. In contrast, the organic and mineral fertilisation alleviated, at least partially, the toxicity of As, essentially by stimulating plant growth and promoting nutrient uptake. The mobility of As in the volcanic soil and thus its phytoavailability were differently affected by the organic and mineral fertilisers; the application of humic acids mitigated the availability of the contaminant, likely by its partial immobilisation on humic acid sorption sites – thus raising up the intrinsic anionic sorption capacity of the volcanic soil; the mineral fertilisation enhanced the mobility of As in soil, probably due to competition of P for the anionic sorption sites of the soil variable-charge minerals, very affine to available P. These findings hence suggest that a proper soil management of As-polluted volcanic soils and amendment by stable organic matter might mitigate the environmental risk of these soils, thus minimising the availability of As to biota. [ABSTRACT FROM AUTHOR]

Abstract: Sandy soils in tropical climates with low organic matter (OM) content result in degradation and fertility loss. It is important to note that humified OM accumulation and stabilization increases soil fertility. This study aimed to characterize humic acids (HAs) in sandy soils on an agroecological farm operating for more than 20 years. The structures of the HA of two Planosols under a grazing (G) and crop rotation (R) management system and of an Acrisol under an agroforestry management system (AFS) were characterized using HA extracted from a local Organosol under cassava monoculture (M) as a structural reference. Lower CHA/CFA ratio in sandy soil reveals a slightly humified OM and higher E4/E6 ratio value in HA confirming lower aromatic structure condensation. The HA in Planosols (R and G) and Acrisols (AFS) are predominantly formed by CAlq‐O,N–, CAlq‐O–, and CAlq‐di‐O structures closely related to the soil physical properties and preserved by interaction with the mineral fraction confirmed by νO‐H, νSi‐O stretching of kaolinite and hematite bands in FTIR spectroscopy. Management and vegetal carbon type (−20.65‰ AFS ‐C3) or (−15.55‰ (G) and −17.82‰ (R) ‐C4) had no effect on HA formation. Evolution of structural change in HA modeled by chemometric analyses showed that more recalcitrant structure characteristic in HA can enhance OM preservation in these sandy soils. This study elucidated the OM stabilization mechanisms of sandy soil HA, their relationship with soil properties and models structural changes for humified OM recovery and preservation, which contribute to increased fertility and agricultural production. [ABSTRACT FROM AUTHOR]

The effects of humic acid (HA) and heavy metals (Cu 2+ and Ag + ) on the sorption of polar and apolar organic pollutants onto biochars that were produced at temperatures of 200 °C (BC200) and 700 °C (BC700) were studied. Due to the plentiful polar functional groups on BC200, cationic propranolol exhibited higher levels of sorption than naphthalene on BC200 while naphthalene and propranolol showed similar sorption capacities on BC700. HA changed the characteristics of biochars and generally inhibited the sorption of target organic pollutants on biochars; however, enhancement occurred in some cases depending on the pollutants involved and their concentrations, biochars used and the addition sequences and concentrations of HA. On BC200, HA modifications mainly influenced sorption by decreasing its polarity and increasing its aromaticity, while on BC700, the surface area and pore volume greatly decreased due to the pore-blocking effects of HA. Residue dissolved HA in solution may also contribute to sorption inhibition. Complexation between polar functional groups on BC200 and heavy metals slightly enhanced the sorption of neutral naphthalene and significantly enhanced that of anionic 4-nitro-1-naphtol, while limited the sorption of cationic propranolol. Heavy metals together with their associated water molecules decreased the sorption of target chemicals on BC700 via pore-filling or pore-mouth-covering. Inhibition of heavy metals for 4-nitro-1-naphthol was found to be the weakest due to the bridge effects of heavy metals between 4-nitro-1-naphtol and BC700. The higher polarizability of Ag + led to the increase of its sorption on biochars in the presence of organic aromatic pollutants. The results of the present study shed light on the sorption mechanisms of bi-solute systems and enable us to select suitable biochar sorbents when chemicals co-exist. [ABSTRACT FROM AUTHOR]

Dissolved Fe(II) and humic acid (HA) were pre-impregnated into contaminated soil to catalyze hydrogen peroxide to remove crude oil (CO). The effects of parameters such as initial Fe(II), HA and H2O2concentrations on the oxidation of total petroleum hydrocarbon (TPH) were investigated using response surface methodology based on Box–Behnken design. The rate of hydrogen peroxide decomposition is decreased by pre-impregnating with dissolved Fe(II) + HA compared with only pre-impregnated Fe(II) and modified Fenton (MF). Oxygen evolution is the predominant route of hydrogen peroxide decomposition at natural pH. Unlike O2evolution, the kinetics of hydroxyl radical (OH•) production are clearly uncoupled from H2O2decay in these systems. The steady-state hydroxyl radical production rate is higher in the systems with pre-impregnated dissolved Fe(II) and HA, and more significance is the decrease in detectable TPH (70.84% removal efficiency) when soil is pre-impregnated with dissolved 25 mM Fe(II) + 0.7 mg/mL HA, and with the application of 700 mM H2O2, possibly due to hydrogen peroxide catalyzed by the iron of this complex (CO-HA–Fe(II)) producing hydroxyl radical in close proximity to the CO. Meanwhile, the removal efficiency of C21–C30is up to 65.69%, which is 2.6 times higher than that of the MF (25.52%). [ABSTRACT FROM AUTHOR]

The short-term effectiveness of soil-applied humic acid–like substances [naturally complexed or uncomplexed with iron (Fe)] extracted from composted poultry (HLSP) and sheep (HLSS) manures in preventing lime-induced Fe chlorosis in Eureka lemon (Citrus lemonL.) cuttings grafted on sour orange (Citrus aurantiumL.) was investigated and compared with the commonly applied ethylenediamine di-2-hydroxyphenyl acetate ferric (FeEDDHA). Treatments were untreated plants and plants fertilized with FeEDDHA, HLSP, and HLSS injected into the soil at a total rate of 25 mg Fe per plant each. Chlorophyll concentration index (CCI) of the youngest fully expanded leaves, growth vigor, and leaf Fe concentration were measured. HLSP and HLSS, similar to FeEDDHA, resulted in significantly greater CCI values, growth vigor, and leaf Fe concentration compared to untreated plants. HLSP and HLSS offer potential alternative to FeEDDHA to prevent lime-induced Fe deficiency in Eureka lemon. Concentrates of HLSP and HLSS can be commercially prepared to offer an inexpensive and environmentally safe product. [ABSTRACT FROM AUTHOR]

The results of mathematical simulation of sorption equilibria with the participation of divalent cations of heavy metals (HMs), chelant (EDTA), and insoluble forms of humic acids (HAs) are discussed. It is shown that the formation of chelates of metals with EDTA in solutions results in the decreasing sorption of the metals by humic acids. We also analyzed the effect of the acidity of the medium and the HM: EDTA: HA ratio (in a wide range) on the desorption of metals. The desorbing effect of EDTA on the metals is the highest at pH 3-5 and increases with an increase in the concentration of EDTA and a decrease in the concentration of HAs. With respect to the remobilization of metals under the impact of EDTA, the metal cations can be arranged into the following sequence: Cu(II) > Ni(II) > Pb(II) ≫ Cd(II) > Co(II) > Zn(II). The obtained data have been used to analyze the remobilization / extraction of HMs from soils with a high content of humic substances. [ABSTRACT FROM AUTHOR]

Effects of Humic acid and hydrogel with and without recommended dose of NPK in eight different treatments and four replications were studied. Humic acid revealed better results as compared to hydrogel for number of leaves, and potassium contents plant-1 whereas in all the other studied growth and yield parameters hydrogel performance was good. Overall results revealed that both humic acid and hydrogel improved nutrient's uptake efficiency as well as reduce environmental pollution in agriculture. The treatment T7 (80 mg Hydrogel treatment (two doses) + NPK) exhibited highest tuber's yield while T0 (control) reported minimum yield. [ABSTRACT FROM AUTHOR]

Most studies on the treatment of chlorinated contaminants by Fe(0) focus on aqueous system tests. However, few is known about the effectiveness of these tests for degrading chlorinated contaminants such as 1,1,1-trichloroethane (TCA) in soil. In this work, the reductive degradation performance of 1,1,1-TCA by Fe(0) was thoroughly investigated in a soil slurry system. The effects of various factors including acid-washed iron, the initial 1,1,1-TCA concentration, Fe(0) dosage, slurry pH, and common constituents in groundwater and soil such as Cl, HCO, SO, and NO anions and humic acid (HA) were evaluated. The experimental results showed that 1,1,1-TCA could be effectively degraded in 12 h for an initial Fe(0) dosage of 10 g L and a soil/water mass ratio of 1:5. The soil slurry experiments showed two-stage degradation kinetics: a slow reaction in the first stage and a fast reductive degradation of 1,1,1-TCA in the second stage. The reductive degradation of 1,1,1-TCA was expedited as the mass concentration of Fe(0) increased. In addition, high pHs adversely affected the degradation of 1,1,1-TCA over a pH range of 5.4-8.0 and the reductive degradation efficiency decreased with increasing slurry pH. The initial 1,1,1-TCA concentration and the presence of Cl and SO anions had negligible effects. HCO anions had a accelerative effect on 1,1,1-TCA removal, and both NO and HA had inhibitory effects. A Cl mass balance showed that the amount of Cl ions released into the soil slurry system during the 1,1,1-TCA degradation increased with increasing reaction time, suggesting that the main degradation mechanism of 1,1,1-TCA by Fe(0) in a soil slurry system was reductive dechlorination with 1,1-DCA as the main intermediate. In conclusion, this study provides a theoretical basis for the practical application of the remediation of contaminated sites containing chlorinated solvent. [ABSTRACT FROM AUTHOR]

Abstract: Humic substances are a major component of soil organic matter that influence the behavior and fate of heavy metals such as Cr(VI), a toxic and carcinogenic element. In the study, a repetitive extraction technique was used to fractionate humic acids (HAs) from a peat soil into three fractions (denoted as F1, F2, and F3), and the relative importance of O-containing aromatic and aliphatic domains in humic substances for scavenging Cr(VI) was addressed at pH 1. Spectroscopic analyses indicated that the concentrations of aromatic C and O-containing functional groups decreased with a progressive extraction as follows: F1>F2>F3. Cr(VI) removal by HA proceeded slowly, but it was enhanced when light was applied due to the production of efficient reductants, such as superoxide radical and H2O2, for Cr(VI). Higher aromatic- and O-containing F1 fraction exhibited a greater efficiency for Cr(VI) reduction (with a removal rate of ca. 2.89mmolg−1 HA under illumination for 3h). 13C NMR and FTIR spectra further demonstrated that the carboxyl groups were primarily responsible for Cr(VI) reduction. This study implied the mobility and fate of Cr(VI) would be greatly inhibited in the environments containing such organic groups. [Copyright &y& Elsevier]

Soil organic matter (OM) of aggregate coatings and biopore walls can affect the transport of reactive solutes during preferential flow in structured soil. As a nondestructive method, diffuse reflection infrared Fourier transform (DRIFT) spectroscopy, has been proposed for mapping the millimeter-scale OM composition of intact flow path surfaces. The surfaces of such intact soil structures, e.g., aggregate and biopore surfaces, mostly have a distinctive microtopography, however, that affects the intensity of DRIFT signals. Thus DRIFT mapping data require geometric corrections for a quantitative interpretation. This study analyzed a digital terrain model (DTM)-based approach for describing microtopography effects on DRIFT reflectance. A gypsum block model was first used for developing the concept. The surface of the gypsum block had defined channels and pores and was partly coated with defined humic acid (HA). A millimeter-scale DTM of the DRIFT-mapped gypsum block surface was obtained with a laser scanner. The signal intensities at specific wavenumbers were corrected for microtopography effects using surface elevation, slope, and aspect data of the sample surfaces. The corrections were found to be dependent on both the wavelengths and the measured substance (gypsum or HA). The method was then applied to determine the OM composition at intact structural soil surfaces. The DTM-based approach reduced microtopography effects and was compared with an alternative approach in which spectral ratios between specific absorption bands were used for corrections. The results indicated differences in OM composition and local distribution between surfaces of worm burrows and crack walls. The results suggest that DTM correction of a DRIFT-mapped intact soil aggregate surface enhanced the interpretation of the millimeter-scale OM composition. [ABSTRACT FROM AUTHOR]

A comparative investigation of the hydrophobic-hydrophilic properties of humic acids (HA) extracted with 0.1 M Na4P2O7 at pH 7 from soils under shelterbelts of different ages 14 years old (young) and 200 years old (old) and adjoining cultivated fields was carried out with the application of 13C NMR spectroscopy and hydrophobic interaction chromatography (HIC). The results determined using both methods were in line with the chemical composition of HA. Shelterbelt age was shown to be the principal factor affecting the composition and hydrophobic-hydrophilic properties of HA. HA from soil under the young shelterbelt was found to be less chemically mature and more hydrophilic than HA from the soil of the adjoining cultivated field. By contrast, HA from soil under the old shelterbelt was considerably more chemically mature and hydrophobic than that of the corresponding cultivated field and HA from the soil under the young shelterbelt. [ABSTRACT FROM AUTHOR]

Sorption by humic acids is known to modify the bioavailability and toxicity of metals in soils and aquatic systems. The sorption of cadmium(II) and copper(II) to two soil humic acids was measured at pH 6.0 using ion-selective electrode potentiometric titration at different temperatures. Sorption reactions were studied with all components in aqueous solution, or with the humates in suspension. Adsorption reactions were described using a multiple site-binding model, and a model assuming a continuous log-normal distribution of adsorption constants. Adsorption of Cu2+ was more favourable than adsorption of Cd2+. The log-normal distribution model provided the closest fit to observations and allowed parameterisation of adsorption data using a mean adsorption constant (log Kμ). Sorption of Cd2+ to dissolved humic acids increased slightly in extent and sorption affinity with increasing temperature, but the effect was small (log Kμ 2.96-3.15). A slightly greater temperature effect occurred for sorption of Cd2+ to solid-phase humic acids (log Kμ 1.30-2.08). Sorption of copper(II) to both aqueous- and colloidal-phase humates showed more pronounced temperature dependence, with extent of sorption, and sorption affinity, increasing with increasing temperature (log Kμ 3.4-4.9 in solution and 1.4-4.5 in suspension). The weaker adsorption of Cd2+ than Cu2+, and smaller temperature effects for dissolved humates than suspended humates, suggested that the observed temperature effects had a kinetic, rather than thermodynamic, origin. For any metal-to-ligand ratio, free metal ion concentration, and by inference metal bioavailability, decreased with increasing temperature. The consistency of the data with kinetic rather than thermodynamic control of metal bioavailability suggests that equilibrium modelling approaches to estimating bioavailability may be insufficient. [ABSTRACT FROM AUTHOR]

Humic acid formation and soil physical properties were studied in a Mediterranean calcimorphic vineyard after a 5-year application of wine vinasses. This treatment increased significantly (P < 0.05) soil carbon content (×3.7), humic acid (×6.8), aggregate stability (×3.5), water retention (×1.5), and permeability (×2.3) and decreased penetration resistance (×6.6). The humic acid infrared spectra in soils with wine vinasses showed enhanced protein (1640, 1540 cm-1) and aromatic (1620, 1510 cm-1) bands, suggesting rapid transformation of the water-soluble wine vinasses into melanoidin-like macromolecules. In the humic acid from untreated soils, lignin-derived structures prevailed. The second-derivative visible spectra of the humic acid showed peaks at 530, 570, and 620 nm produced by fungal-derived dihydroxyperylenequinones, the intensity of which increased with the distance in soil from the vinasses' application point. The immobilization and stabilization in soil of wine vinasses, a fairly biodegradable material, and its bearing in the local improvement of several soil properties are emphasized. [ABSTRACT FROM AUTHOR]

The article presents a comparative study on the efficiency of soluble humic substance (HS), ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA) as washing agents from long-term equilibrated urban soil. It uses multiple-step extraction and single-step extraction in extracting heavy metals in the soil and sequential extraction to examine the effectiveness of the extractions. It reveals that the characteristics and concentration of the extractant, the metal as well as solution or soil ratio determine the extraction of the cadmium, copper, and lead in the polluted calcareous soil. It further discloses that soluble HS is more appropriate washing agent for soils that are polluted by cadmium and copper compared to EDTA and NTA.

Abstract: The Mezquital Valley in Central Mexico has received wastewater from Mexico City for nearly 100 years. Wastewater brings in organic matter and nutrients but also trace metals. Humic substances, the main components of organic soil matter, are responsible for retaining and regulating the mobility of trace metals in soils. In this study, humic substances were extracted from the soil and separated into distinct fractions (humic acids, fulvic acids and humins). The particle induced X-ray emission (PIXE) technique was applied to determine the metal content in bulk soil as well as in humic acids and fulvic acids not soluble in H3PO4. In order to assess whether the long-term input of organic matter and metals modifies the metal association with these humic substances, parcels irrigated for three time periods (5, 47 and 89 years) were selected for this study. It was observed that metals such as Zn and Cu are mainly associated with the humic acids. Fulvic acids retain mainly Cr while Pb is distributed among humic and fulvic acids. It was also observed that in general, metal retention by humic substances increases with irrigation time. Depth also affects metal association with the humic substances. [Copyright &y& Elsevier]

Despite the broad consensus that humic substances (HS) can act as electron shuttles in bioreduction processes occurring in aquatic and terrestrial environments, no published studies have presented direct evidence that electron shuttling by HS plays a role in the mechanisms of Fe(III) bioreduction in soils. We conducted experiments in which two HS with very different chemical properties, a soil humic acid and a mixture of peat humic acid with aquatic organic matter, were added to an Ultisol and incubated under anoxic conditions to evaluate their effect on Fe(III) bioreduction by the indigenous microorganisms in the soil. Our experiments were designed to have the same initial reducing capacity available from the added HS but to have very different initial total carboxyl concentrations to examine the role of Fe(II) complexation by HS in the bioreduction process. The addition of HS significantly increased soluble Fe(II) production relative to that observed in control experiments without added HS. Moreover, after an initial period of incubation, soluble Fe(II) production was accelerated by the presence of a higher initial carboxyl content. These trends support the hypothesis that, at the initial stages of in Fe(III) bioreduction, electron shuttling dominates, whereas later Fe(II) complexation dominates. Our results apparently are the first to demonstrate directly that added HS can indeed enhance the bioreduction of Fe(III) minerals in soil by at least two different mechanisms. [ABSTRACT FROM AUTHOR]

Interest in the environmental fate of fluorotelomer alcohols (FTOHs) has spurred efforts to understand their equilibrium partitioning behavior. Experimentally determined partition coefficients for FTOHs between soil/water and air/water have been reported, but direct measurements of partition coefficients for dissolved organic carbon (DOC)/water (Kdoc) and octanol/water(Kow) have been lacking. Here we measured the partitioning of 8:2 and 6:2 FTOH between one or more types of DOC and water using enhanced solubility or dialysis bag techniques, and also quantified Kow values for 4:2 to 8:2 FTOH using a batch equilibration method. The range in measured log Kdoc values for 8:2 FTOH using the enhanced solubility technique with DOC derived from two soils, two biosolids, and three reference humic acids is 2.00-3.97 with the lowest values obtained for the biosolids and an average across all other DOC sources (biosolid DOC excluded) of 3.54 ± 0.29. For 6:2 FTOH and Aldrich humic acid, a log Kdoc value of 1.96 ± 0.45 was measured using the dialysis technique. These average values are ∼1 to 2 log units lower than previously indirectly estimated Kdoc values. Overall, the affinity for DOC tends to be slightly lower than that for particulate soil organic carbon. Measured log Kow values for 4:2 (3.30 ± 0.04), 6:2 (4.54 ± 0.01), and 8:2 FTOH (5.58 ± 0.06) were in good agreement with previously reported estimates. Using relationships between experimentally measured partition coefficients and C-atom chain length, we estimated Kdoc and Kow values for shorter and longer chain FTOHs, respectively, that we were unable to measure experimentally. [ABSTRACT FROM AUTHOR]

Abstract: In sandy soils the behavior of heavy metals is largely controlled by soil organic matter (solid and dissolved organic matter; SOC and DOC). Therefore, knowledge of the partitioning of organic matter between the solid phase and soil solution is essential for adequate predictions of the total dissolved metal concentration in the soil solution. At present, only a few studies have incorporated solid/liquid partitioning of organic matter in metal mobility predictions. In order to gain more insight in the behavior of DOC, we have studied the effect of the extraction solution, solid to liquid ratio (SLR), pH and storage time on the extractability of DOC and related metal concentration in the soil solution of a sandy soil. Furthermore, the composition of the DOC was measured and free metal concentrations were analyzed in the soil solution with the Donnan Membrane Technique. Extraction solution, SLR and pH affected the extracted amount of both DOC and metals. The DOC concentrations were highest in pore water and decreased further from water extracts to CaCl2 extracts. In general, with increasing SLR the metal/DOC ratio decreased, which indicated that the increased DOC at higher SLR had a lower average metal binding capacity than DOC released at the lowest SLR. Storage time of the samples and changes in the field greatly affected the extracted amount of DOC; the DOC concentration of samples taken 20 years ago and stored for a long period of time are 10 times higher than for samples from the same area and stored for less than one year. The extracted DOC is comprised mainly of fulvic acids (FA) and hydrophilic acids. Humic acids (HA) played a minor role in the DOC, which is in contrast to the often used assumptions on the DOC compositions in modeling studies. The composition of DOC is also affected by the soil pH; at low pH the contribution of FA to the total DOC concentration is lower because more FA is adsorbed to Fe-oxides in the soil. Speciation calculations assuming that DOC comprised of HA, FA and citric acid (as a model substance for hydrophilic acids) showed that for Cu the citrate-bound Cu could be up to 20% of the total Cu in solution. This study shows that more research is needed to gain more insight in the composition, concentration and behavior of DOC under field conditions in order to improve the prediction of metal leaching in the field. [Copyright &y& Elsevier]

By using a simple bubble column, the adsorption behavior of a commercial soil-humic acid (CHA) at air-water interfaces was investigated. At pH 4.0, the concentrations of the CHA exhibited clear gradients in the bubble column, and increased significantly along the column height; smaller concentration gradients were also observed at pH 6.0. These concentration profiles demonstrate the surface activity of humic acid and pH-dependent affinity toward air-water interfaces. Taking advantage of the bubble column method, we interestingly found that the adsorptive fractionation of the CHA at air-water interfaces did occur. The components with higher molecular weight and stronger UV absorptivity showed greater affinity toward air-water interfaces, despite that the fractionation pattern was reduced to a certain extent as solution pH increased. The organic carbon-normalized pyrene partition coefficient KOC values deviated from the corresponding values of original bulk solutions at both pH 4.0 and 6.0, and increased along the height of the column. Our results demonstrate the usefulness of the simple bubble column, and suggest that the adsorptive fractionation of humic acid at air-water interfaces might have implications for some natural environments and engineered systems where air-water interfaces exist extensively. [ABSTRACT FROM AUTHOR]

The most southerly ombrotrophic peat bogs in Europe are in Galicia (northwest Spain). The humic matter in them originates from chemical processes in anaerobic conditions. We studied the acid properties of fulvic acids and humic acids isolated from two peat horizons of an ombrotrophic peat bog by potentiometric titration. Solutions containing 25, 50 and 100 mg l−1 of each humic substance were titrated at ionic strengths 0.005 m, 0.01 m and 0.1 m (with KNO3 as the inert electrolyte). Charge curves were analysed with a Donnan model to determine the intrinsic proton binding parameters. The concentration of the humic substance affected the charge curves more significantly at pH exceeding 6, and tended to disappear at greater concentrations. The proton binding conditional constants decreased with increasing ionic strength, this effect being more significant in the carboxylic groups with less affinity for protons. The proton binding constant of the carboxyl groups in a fulvic acid was one order of magnitude less than the value for the corresponding humic acid, whereas for the phenolic groups the values for both fractions were similar. The total content of acid groups was approximately 2 mol kg−1 greater in the fulvic fraction than in the humic fraction. Both humic fractions from the lower horizon contained more acid groups than those from the upper horizon, mainly because the content of carboxyl groups increases with soil depth. Therefore, the humic substances in the lower horizon of the peat will be more negatively charged, which will affect their solubility and the binding of metal ions. [ABSTRACT FROM AUTHOR]

Although the chemical nature of soil organic matter (SOM) is thought to affect the mineralization rate of N bound in SOM, little direct evidence exists for such effects. To test the hypothesis that the N mineralization rate is affected by the degree of SOM humification, we added equivalent amounts of humic acid (HA) N as either the labile mobile humic acid (MHA) fraction or the more humified calcium humate (CallA) fraction to two lowland rice (Oryza sativa L.) soils, which were subsequently incubated under anaerobic conditions for 6 wk. The HA fractions had been chemically extracted from seven irrigated lowland rice soils from Vietnam and the Philippines. In both incubation soils, the amount of N mineralized from the added HA fractions decreased exponentially as the optical density of the HA at 465 nm (E4, index of humification) increased (R² = 0.94-0.98). Mineralization of humic N was also strongly negatively associated with the amount of N or C contained in each HA fraction per kilogram of soil extracted, suggesting a more recalcitrant nature of the fractions in those soils where conditions allowed them to accumulate. Nitrogen mineralization was less for both the MHA (by 26%) and Calla (by 41%) when incubated in the International Rice Research Institute (IRRI) soil compared with the Tanhoi soil. The most plausible explanation for this decrease is increased stabilization of the added HA by the relatively abundant Ca+2 in the IRRI soil. We conclude that the degree of humification of the MHA and Calla fractions plays an important role in governing the rate of N mineralization in lowland rice soils because (i) N mineralization from these fractions was affected by their degree of humification, and (ii) the fractions are a small but important component of total soil N. [ABSTRACT FROM AUTHOR]