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1.
The retention of dissolved organic matter in soils is mainly attributed to interactions with the clay fraction. Yet, it is unclear to which extent certain clay‐sized soil constituents contribute to the sorption of dissolved organic matter. In order to identify the mineral constituents controlling the sorption of dissolved organic matter, we carried out experiments on bulk samples and differently pretreated clay‐size separates (untreated, organic matter oxidation with H2O2, and organic matter oxidation with H2O2 + extraction of Al and Fe oxides) from subsoil horizons of four Inceptisols and one Alfisol. The untreated clay separates of the subsoils sorbed 85 to 95% of the dissolved organic matter the whole soil sorbed. The sorption of the clay fraction increased when indigenous organic matter was oxidized by H2O2. Subsequent extraction of Al and Fe oxides/hydroxides caused a sharp decrease of the sorption of dissolved organic matter. This indicated that these oxides/hydroxides in the clay fraction were the main sorbents of dissolved organic matter of the investigated soils. Moreover, the coverage of these sorbents with organic matter reduced the amount of binding sites available for further sorption. The non‐expandable layer silicates, which dominated the investigated clay fractions, exhibited a weak sorption of dissolved organic matter. Whole soils and untreated clay fractions favored the sorption of ”︁hydrophobic” dissolved organic matter. The removal of oxides/hydroxides reduced the sorption of the lignin‐derived ”︁hydrophobic” dissolved organic matter onto the remaining layer silicates stronger than that of ”︁hydrophilic” dissolved organic matter.  相似文献   

2.
Abstract

The extractant Mehlich‐1 is routinely used in Brazil for determination of soil nutrients, whereas Mehlich‐3 has been suggested as a promising extractor for soil fertility evaluation. Both were used for extraction of molybdenum (Mo) in Brazilian soils with Mo dosage by the KI+H2O2 method. The Langmuir and Freundlich isotherms were used to study soil Mo adsorption. Mehlich‐1 extracted more Mo than Mehlich‐3 in soils with high contents of organic matter, clay, and iron (Fe) oxides. Mehlich‐3 and Mehlich‐1 extractions correlated positively and significantly with amorphous Fe oxides, crystalline Fe oxides, and organic matter. Molybdenum recovering rates correlated to crystalline Fe oxides and clay contents but not to organic matter, pH, and Mo adsorption capacity. Amorphous and crystalline Fe oxides, clay, and organic matter were responsible for most of the Mo adsorption. The Langmuir isotherm described better the Mo adsorption to soil amorphous Fe oxides and organic matter than the Freundlich isotherm.  相似文献   

3.
Dissolved organic matter (DOM) in soils is partially adsorbed when passing through a soil profile. In most adsorption studies, water soluble organic matter extracted by water or dilute salt solutions is used instead of real DOM gained in situ by lysimeters or ceramic suction cups. We investigated the adsorption of DOM gained in situ from three compartments (forest floor leachate and soil solution from 20 cm (Bg horizon) and 60 cm depth (2Bg horizon)) on the corresponding clay and fine silt fractions (< 6.3 μm, separated together from the bulk soil) of the horizons Ah, Bg, and 2Bg of a forested Stagnic Gleysol by batch experiments. An aliquot of each clay and fine silt fraction was treated with H2O2 to destroy soil organic matter. Before and after the experiments, the solutions were characterized by ultra‐violet and fluorescence spectroscopy and analyzed for sulfate, chloride, nitrate, and fluoride. The highest affinity for DOM was found for the Ah samples, and the affinity decreased in the sequence Ah > Bg > 2Bg. Dissolved organic matter in the 2Bg horizon can be regarded as slightly reactive, because adsorption was low. Desorption of DOM from the subsoil samples was reflected more realistically with a non‐linear regression approach than with initial mass isotherms. The results show that the extent of DOM adsorption especially in subsoils is controlled by the composition and by the origin of the DOM used as adsorptive rather than by the mineralogical composition of the soil or by contents of soil organic matter. We recommend to use DOM gained in situ when investigating the fate of DOM in subsoils.  相似文献   

4.
The sorption and ion-exchange behavior of Co(II) and Zn in the soil-equilibrium solution system was studied for different types and varieties of native soils and their clay fractions before and after mild oxidation with H2O2 to remove the organic carbon. The parameters of the ion-exchange adsorption and the selectivity coefficients of the (Co(II), Zn)/Ca ion exchange were determined using different models for describing the relationship between the dissolved and sorbed forms of the metals. These were the empirical Langmuir and Freundlich adsorption isotherms and the model of the ion-exchange adsorption based on the acting mass law. It was found that the soil organic matter played an important role in the selectivity of the ion-exchange adsorption of Co(II) and Zn by the soils and their clay fractions. This was confirmed by an abrupt decrease (to almost 1) of the selectivity coefficients of the Co2+/Ca2+ and Zn2+/Ca2+ exchange after the treatment of the clay fraction with hydrogen peroxide.  相似文献   

5.
Measuring the specific surface area (SSA) of soils that contain much organic matter (OM) is problematic. The adsorption of p-nitrophenol (pNP) from xylene at room temperature yielded realistic values for the SSA of a wide range of clays, oxides and subsoils. Here we have extended the same measurement to some topsoils with varied OM content, texture and clay mineral composition. Specifically, we have compared the surface areas measured by adsorption of N2, and, applying the BET equation, with the values obtained by adsorption of pNP, before and after treatment of the samples with hydrogen peroxide. In all instances, the removal by H2O2 of organic matter – albeit in part only – led to a marked increase in the SSAs measured by nitrogen because of the exposure of micropores previously blocked or covered by OM. The surface areas measured by pNP were appreciably larger than those obtained by the standard BET equation, and showed little change after removal of organic matter. However, the surface area of two smectite-rich samples measured by pNP increased substantially after peroxidation, presumably because smectite crystals decomposed during treatment with H2O2. The results suggest that, under the experimental conditions used, pNP could diffuse without hindrance into and through organic matter, enabling it to adsorb on to micropore surfaces within clay aggregates (domains). In keeping with this suggestion, the relation between the surface areas measured by pNP and the corresponding values calculated from the clay and OM contents, and clay mineral composition, of the soils was close to 1:1. An even stronger relation was observed between the measured and calculated values for cation exchange capacity.  相似文献   

6.
The relationship and mechanisms among weathering processes, cation fluxes, clay mineralogy, organic matter composition and stability were studied in soils developing on basaltic material in southern Italy (Sicily). The soils were transitions between Phaeozems and Vertisols. Intense losses of the elements Na, Ca and Mg were measured indicating that weathering has occurred over a long period of time. The main weathering processes followed the sequence: amphibole, mica, volcanic glass or if ash was the primary source → smectite → interstratified smectite–kaolinite → kaolinite. Kaolinite formation was strongly related to high Al, Mg and Na losses. The good correlation between oxyhydroxides and kaolinite in the soils suggests that (macro)aggregates have formed due to physical or electrostatic interactions between the 1:1 clay minerals and oxides. The stability of organic matter was investigated with a H2O2-treatment that assumes that chemical oxidation mimics the natural oxidative processes. The ratio of C after the H2O2 treatment to the total organic C ranged from 1–28%. No correlation between clay content and organic matter (labile or stable fraction) was found. The refractory organic fraction was enriched in aliphatic compounds and did not greatly interact with the kaolinite, smectite or poorly crystalline Fe or Al phases. A part of this fraction (most probably proteins) was bound to crystalline Fe-oxides. In contrast, the oxidisable fraction showed a strong relationship with poorly crystalline oxyhydroxides and kaolinite. Surprisingly, smectite did not contribute to the stabilisation of any of the organic C fractions. The stabilisation of organic matter in the soils has, therefore, two main mechanisms: 1) the protection of labile (oxidisable with H2O2) organic matter, including also aromatic-rich compounds such as charcoal, by the formation of aggregates with oxyhydroxides and kaolinite and 2) the formation of a refractory fraction enriched in aliphatic compounds.  相似文献   

7.
With the large-scale cultivation of transgenic crops expressing Bacillus thuringiensis (Bt) insecticidal toxin in the world, the problem of environmental safety caused by these Bt crops has received extensive attention. The effects of soil organic matter (SOM) on the adsorption and insecticidal activity of Bt toxin in variable- and constant-charge soils (red and brown soils, respectively) were studied. Organic carbon in the soils was removed using hydrogen peroxide (H_2O_2). After H_2O_2 treatment, the SOM in the red and brown soils decreased by 71.26% and 82.82%, respectively. Mineral composition of the H_2O_2-treated soils showed no significant changes,but soil texture showed a slight change. After SOM removal, the cation exchange capacity (CEC) and pH decreased, while the specific surface area (SSA), point of zero charge (PZC), and zeta potential increased. The adsorption isotherm experiment showed that the Bt toxin adsorption on the natural and H_2O_2-treated soils fitted both the Langmuir model (R~2≥ 0.985 7) and the Freundlich model (R~2≥ 0.984 1), and the amount of toxin adsorbed on the H_2O_2-treated soils was higher than that on the natural soils. There was a high correlation between the maximum adsorption of Bt toxin and the PZC of soils (R~2= 0.935 7); thus, Bt toxin adsorption was not only influenced by SOM content, but also by soil texture, as well as the SSA, CEC, PZC, and zeta potential. The LC_(50) (lethal concentration required to kill 50% of the larvae) values for Bt toxin in the H_2O_2-treated soils were slightly lower than those in the natural soils, suggesting that the environmental risk from Bt toxin may increase if SOM decreases. As the measurement of insecticidal activity using insects is expensive and time consuming, a rapid and convenient in vitro method of enzyme-linked immunosorbent assays is recommended for evaluating Bt toxin degradation in soils in future studies.  相似文献   

8.
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9.
The binding of organic matter by clay minerals was studied in experiments simulating the transformation of clay rock with a high content of dioctahedral 2: 1 phases in the soil during its interaction with decomposing plant residues. Using modern methods (X-ray phase analysis, thermal analysis and Fouriertransform IR spectroscopy, and adsorption-luminescence analysis), it was shown that the binding of organic matter into a form resistant to treatment with 30% H2O2 entailed changes in the actual structure of the clay aggregates. Peculiar organic-silicate compositions with their structure disordered along the c* axis were formed, in which organic matter was localized both on the surface of the particles and in the interlayer spaces.  相似文献   

10.
Fifty-five soil samples representing Egyptian alluvial and lacustrine soils were chemically analyzed for total Se which was found to vary from 0.18 to 0.85 ppm with an average of 0.45 ppm. These levels are positively correlated with organic matter, total carbonate and clay content of the soils. Minimum variation of total Se with soil depth was found. The chemical fractionation of soil Se, expressed as percent of the total, indicates that on the average about 25.4 % exists in 0.2 M K2SO4-extractable form, 18.5 % is extracted with 0.5 N NH4OH, 9.7 % as 6 N HCl-extractable form and 13.8 % as extractable with 9 N HNO3. Amounts of K2SO4-Se and HNO3-Se in soils correlated significantly with soil organic matter, total carbonate, free iron oxide and clay content. The NH4OH-Se and HCl-Se fractions correlated significantly only with organic matter and clay content. There is also significant correlation between total Se and the studied Se fractions. Specific adsorption of Se by soils was low as expressed by the Langmiur adsorption maximum values. The high soil pH has a reducing effect on Se adsorption.  相似文献   

11.
Abstract

The influence of soil organic matter on selenite sorption was investigated in the selenite adsorption capacity and the surface particle charge change by ligand exchange reaction using the hydrogen peroxide (H2O2) treatment and the ignition treatment of two Andosols. The removal of organic carbon (C) in soils accelerated selenite sorption, implying that organic matter of soils had negative influence on the selenite adsorption on the soils. Positive charge decrease on soil particles, concomitant proton consumption, and release of silicon (Si), sulfate (SO4 2‐), and organic C were observed in selenite sorption by the soils. The development of surface particle negative charge with selenite sorption was smaller in the H2O2‐treated soil than in the original soils and was scarcely observed in the ignition‐treated soil. It can be assumed that the increase of negative charge by selenite sorption was attributed to new negative sites borne by released insoluble organic matter and negative charge development directly by selenite sorption was small.  相似文献   

12.
Aggregation, particle size, and chemical composition affect the colour of the soil. We have attempted to quantify and understand these effects in 12 Mediterranean soils. We measured the CIELAB colour variables hab, L*, and C*ab in aggregated and dispersed soil samples, and also in coarse sand, fine sand, silt, and clay samples before and after sequential removal of organic matter, carbonates, and Fe oxides. Grassmann's colour‐mixing equations adjusted by regression analysis described the colour of the dispersed soil from its particle‐size fractions with an error of 1% for hab, 4% for L*, and 9% for C*ab. This suggests that the contribution of each fraction to the colour of the dispersed soil can be accurately calculated by its colorimetric data weighted by its content and a regression coefficient, which was greatest for clay. We inferred the influence of a component within each fraction by measuring the colour changes after its removal. Iron oxides reduced hab of the silicated substrate by 19%, reduced L* by 12%, and increased C*ab by 64% in all particle‐size fractions. Carbonates and organic matter had little influence: the former because they impart little colour to the silicates and the latter because there was little of it. The CIELAB colour‐difference between dispersed and aggregated soil (mean ΔE*ab = 15.3) was due mainly to ΔL* (?14.7). Aggregation contributed to diminishing L* of dispersed soil by 34%. Scanning electron microscopy showed that Fe oxides and organic coatings cover the surface of aggregates thereby influencing soil colour.  相似文献   

13.
The specific surface areas of nine argentinian soils obtained by adsorption followed the order N2 < ethylene glycol < H2O, attributed to the presence of smectites (verified by XRD analysis) and some organic coating. The H2O2 treatment of these soils modified the former order of surface area determined by different absorbents. This order was modified because the mineral surface was evidenced and an increase of cation adsorption was produced by organic matter removal. This fact was supported by the decrease of PZC values of soils after H2O2 treatment. The specific surface area of initial soils obtained by water and ethylene glycol adsorptions showed a good correlation with carbon content and CEC of untreated soils and with the PZC of protonated soils.  相似文献   

14.
Abstract

The study aims at determining the cobalt retention properties of various soil components. Therefore, cobalt (Co) sorptions and extractions were carried out using an Oxisol sample before (untreated) and after successive removal of organic matter and active manganese (Mn) oxides (H2O2‐treated) and iron (Fe) oxides (H2O2+CBD‐treated). A synthetic goethite was included for comparison. Sorption of the four sorbents was determined over a range of Co concentrations (initially 10‐8 M to 10‐4 M), pH values (3 to 8) and reaction times (2 hours to 504 hours). The Co species sorbed was Co(ll), since oxygen exclusion during sorption had no effect on the amount sorbed. The pH‐dependent sorption curve (sorption edge) was shifted to lower pH at decreasing initial Co concentration and increasing reaction time. The displacements, in particular of the sorption edges corresponding to the lowest initial Co concentrations, to successively higher pH following removal of Mn oxides, organic matter and Fe oxides could be attributed to sorption onto sites of decreasing Co affinity [Mn oxides (and organic matter) > Fe oxides > kaolinite]. Extractions of sorbed Co at pH 5.5–7.5 with 2 M HCI showed that the extractability decreased with increasing sorption time and decreasing initial Co concentration. The untreated and H2O2‐treated soil samples retained sorbed Co at least as firmly as the synthetic goethite, whereas the H2O2+CBD‐treated sample (kaolinite) was clearly less effective. The results emphasized the importance of the soil Mn and Fe oxides for Co retention in soils but also the necessity of taken interior sorption sites into consideration.  相似文献   

15.
Abstract

To evaluate contributions of organic matter, oxides, and clay fraction to copper (Cu) adsorption in six characterized soils, adsorption isotherms and distribution coefficients were obtained by a batch experimental method. Copper adsorption isotherms from untreated soil, organic matter removed from samples, and organic‐matter‐ and oxide‐removed samples were compared with curve patterns and correlated to Langmuir and Freundlich models. Copper sorption data on untreated soils described L or H‐curves, whereas in soils deprived of any component, their curves were S‐type. Distribution coefficients allowed knowing Cu adsorption capacity of untreated soil and of organic matter, oxides, and clay fraction. Soil organic matter is the main component that affects Cu adsorption as long as soil pH is near neutrality. At acid pH, oxides are the main component that affects Cu adsorption, although to a much smaller extent than organic matter near neutral conditions. Soil pH is the main soil factor that determines Cu adsorption.  相似文献   

16.
The choice of analytical method and sample pretreatment has significant consequences for the shape of particle‐size distribution (PSD) frequency curves, and therefore for the evaluation of soil textural parameters. Here, the comparison of several granulometric methods based on different physical principles is presented: wet sieving (WS), based on gravity and the mechanical force of water, was combined with pipette (PP) sedimentation/settling velocity, Micromeritics SediGraph (MS) sedimentation/x‐ray attenuation and Coulter Counter (CC) electroresistance particle counting. The Malvern Mastersizer (MM), an instrument based on laser diffraction, was used without WS. Twelve typical temperate‐region soil types were chosen for this study on the basis of their mineral composition, organic matter content, conductivity and magnetic susceptibility. The modalities of PSD frequency curves obtained by different methods were compared. The results showed considerably larger clay‐fraction contents determined with the sedimentation techniques, PP and MS, than that obtained by CC or MM. Statistical correlation and regression models were used to compare the fractions of clay, silt and sand obtained with different methods. Linear correlations were found in almost all cases, except those comparing clay fractions obtained by CC with those obtained by MM, PP and MS methods. These correlations were non‐linear. The observed difference was attributed to the presence of aggregates and/or magnetic properties of soil particles. However, more data are needed to verify this hypothesis. The implications of sample pre‐treatment with dispersant and hydrogen peroxide (H2O2) were evaluated by the MS and CC techniques and observed by the scanning electron microscope. H2O2 was found to be an efficient disaggregating agent. The use of dispersant resulted in the increased amount of the clay fraction measured by the MS method.  相似文献   

17.
According to recent conceptual models, the organic carbon (OC) of soils can be divided into OC fractions of increasing stability from labile free OC to resistant OC associated with the soil mineral phase. In this study, we present a method for quantifying two OC fractions based on soil aggregate–size fractionation and the N2 gas–adsorption method. For this purpose, we analyzed soil material of the plow layer of a Haplic Chernozem subjected to different fertilizer treatments (no fertilizer, mineral fertilizer, mineral and organic fertilizer). The total organic‐C concentration (TOC) and the clay content of the different size fractions were determined as well as the specific surface area (SSAmineral) and the sample pore volume after thermal oxidation (OC‐free). The TOC of the different soil‐aggregate fractions was linearly related to SSAmineral. Clay‐associated OC and nonassociated OC fractions of the different soil samples were quantified using two methods based on the OC surface loading at the clay fraction. The application of organic fertilizer increased the amount of nonassociated OC but hardly affected the concentration of clay‐associated OC. This finding agrees with previous studies on C dynamics in soils and indicates a finite capacity of soil materials to sequester OC. Even without any addition of organic fertilizer, the mineral phase of the analyzed soil material appears to be C‐saturated.  相似文献   

18.
The decrease of NMR visibility of the C signal in soil samples due to the association between organic carbon (OC) and the topsoil mineral surface was investigated. CPMAS 13C‐NMR spectra were obtained for soil particle‐size fractions (< 2 μm, 2–20 μm, > 20 μm) and bulk soils from an agricultural topsoil (Chernozem) that had received three different amendments (no fertilization, mineral fertilization (NPK), mineral (NPK) and organic (cattle manure) fertilizations) at Bad Lauchstädt, Germany. The soil organic carbon content of the three soils depended on the degree of soil fertilization. There was no constant relationship between the total NMR signal intensity and the total amount of organic carbon (TOC) for all size fractions. Indeed, a key role played in the C signal intensity by the paramagnetic ferric ion from the clay content in soil fractions and bulk soils was confirmed. Thus, we describe the variations of C signal intensity by taking into account the distribution of clay‐associated OC and non‐associated OC pools. Depending on the amendment, the C signal visibility was weakened by a factor of 2–4 for the clay‐associated OC. This estimation was rendered possible by combining mineral specific surface area (SSA) measurements with the N2 gas adsorption method (BET method) and determination of TOC and iron concentrations. This approach contributes to the quantitative evaluation of the CPMAS 13C‐NMR detection.  相似文献   

19.
The 0 to 20-cm surface layer of a sandy loam soil was sampled in early autumn from plots where straw had either been removed or incorporated annually for 22 years. Denitrification in whole soils, 1–2-mm wet-stable aggregates, clay and silt size fractions was determined by acetylene blocking during anaerobic incubation with excess nitrate. Thus available organic matter was the limiting factor. Samples were exposed to one or two freeze/thaw cycles, or used unfrozen. K2SO4-extractable carbon (C) was determined before and after CHCI, fumigation. Freeze/thaw increased denitrification in whole soils and in aggregates. In aggregates and in whole soil without straw the increase in denitrification was similar following two freeze/thaw cycles, and well above the amount that could be fed by extractable soil C. In whole soils with straw addition, an extra denitrification increase occurred at first thaw only. This straw-induced denitrification surplus was matched by a decline in soil microbial biomass. For other samples and treatments, the freeze/thaw released C from additional organic matter sources. The availability of C in clay for denitrification was twice that of silt-associated C. Straw disposal generally had no effect on the bioavailability of particle-bound C. In contrast to whole soils and aggregates, the availability of organic matter in clay and silt after one freeze/thaw cycle was only half that observed from unfrozen samples. The effect of freeze/thaw on whole soils and aggregates may be to release organic matter available for denitrification by killing the microbial biomass and by disintegrating aggregates. However, the impact of freeze/thaw on completely dispersed samples such as clay and silt may be to promote the formation of granular structures (micro-aggregation) in which organic matter may become less accessible to denitrifiers.  相似文献   

20.
The bioavailability and stability of organic phosphorus (P) in the soil may be affected by exogenous phytase (EPase) activity and distribution, but remain poorly understood. The distribution of EPase activity and hydrolysis ability of EPase on organic P in soil solid‐liquid phases was investigated. The EPase addition to soil suspension (1:20, w/v) from three soil types (red soil, brown soil, and cinnamon soil) under three treatments (untreated soil, removing clay from soil, and removing organic matter from soil) with different characters in the solution and solid phases was assayed. The results showed that the disappearance pattern of EPase activity from solution was similar for all soils, whereas the enzyme activity on the solid phase was dependent on soil types and treatments with the greatest in red soil and untreated soil. When EPase was added to soils, the adsorption ratio of organic matter and clay was 10 to 25% and 3 to 7%, respectively, with sorption capacity of organic matter being significantly (p < 0.05) stronger than that of clay. Additionally, soil dehydrogenase activity, which is the indicator of overall soil microbial activities, increased after EPase addition and the two enzymes showed significant negative relation in the soil suspension and solution. At the same time, the organic P decreased significantly (p < 0.05) after the addition of EPase in the soil solid, which had a varied rate under –40% after incubating 192 h, whereas organic P in the solution phase increased significantly (p < 0.05). This study demonstrated that organic matter had a strong protective and adsorptive effect on EPase effectiveness and microbes mightbe directly affect EPase longevity and decay. This finding suggests that EPase activity in the solid phase played a more important role in organic P hydrolysis.  相似文献   

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