Effect of Soil Organic Matter on Adsorption and Insecticidal Activity of Toxins from Bacillus thuringiensis

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.     

Ionic-strength and pH effects on the sorption of cadmium and the surface charge of soils

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp^?3 Ca(NO₃)₂, and various concentrations of NaNO₃ (0.01–1.5 mol dm^?3) as background solutions at a range of pH values (3–8). In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaNO₃ increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH>PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.     

An improved method for determining the specific surface areas of topsoils with varied organic matter content, texture and clay mineral composition

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.     

Adsorption von Wasser, Äthylenglycolmonoethyläther sowie Stickstoff und ihre Beziehung zu Eigenschaften deutscher und israelischer Bodenproben

Adsorption of Water, Ethylene Glycol Monoethyl Ether as well as Nitrogen and its Relation to Properties of German and Israeli Soil Samples To determine the specific surface area, samples taken from 140 soil horizons were selected and the significant physical and chemical properties were investigated. The specific surface area of the samples was determined by adsorption of H₂O, ethylene glycol monoethyl ether (EGME) and N₂. A comparison of the three methods shows that the specific surface area determined by the adsorption of H₂O and EGME does not differ significantly. However, the specific surface determined by N₂-adsorption differs markedly from the results obtained using the polar substances. As a dependent variable the specific surface area is mainly influenced by the clay content and clay mineral type. Further, the organic substance content as well as the iron- and manganoxide content have a more significant influence on the specific surface area the lower the clay content is, or the content of three layer minerals. As an independent variable, the specific surface area determined by adsorption of EGME, proved to be the characteristic soil factor which reflects the effective cation exchange capacity as well as the hygroscopicity with the highest degree of accuracy.     

Influence of organic matter on selenite sorption by Andosols

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.     

Adsorption of the Bacillus thuringiensis toxin (Cry1Ab) on Na‐montmorillonite and on the clay fractions of different soils

The adsorption of the toxin from Bacillus thuringiensis (Bt‐toxin), which is synthesized in genetically modified maize, on sterilized Na‐montmorillonite and on H₂O₂‐treated and untreated clay fractions of three soils from different sites were studied. All adsorption isotherms can be described by a linear isotherm. Although all clay fractions from the different soils show nearly the same mineralogical composition, we found different affinities ranging from k = 47.7 to k = 366.7 of the adsorbates for the Bt‐toxin. The H₂O₂‐treated clay fractions show no correlation between the adsorption affinity and the amount of soil organic matter. On the other hand, there is a correlation between the content of organic carbon and the adsorption affinity of the untreated clay fractions. This can be explained by the fact that due to the coatings of soil organic matter on aggregates, the Bt‐toxin polymers are not able to adsorb within the clay aggregates.     

Production and consumption of ethylene in temperate volcanic forest surface soils

To date our knowledge is limited with regard to the cycling of ethylene (C₂H₄) in temperate forest soils containing volcanic ash, and the effect of forest‐to‐orchard conversion on its cycling. We studied ethylene accumulation in such forest soils by oxic and anoxic incubations, along with the stimulatory effect of glucose addition on soil C₂H₄ accumulation. We also studied the effect of antibiotics and autoclaving on C₂H₄ production and consumption by volcanic forest soils, and the cycling of C₂H₄ and CH₄ in surface soils after conversion of a Japanese cedar forest to an orchard. Ethylene production and consumption by forest surface soils results from a microbial process, and soil streptomycin‐sensitive bacteria make a minor contribution. Soil C₂H₄ accumulation was much larger during anoxic than during oxic incubation, which indicates that anoxic conditions can induce C₂H₄ accumulation in forest soils. Glucose addition as a carbon source can sharply increase C₂H₄ accumulation rates in the anoxic and oxic forest soils during the first week of incubation. However, there was no difference in total C₂H₄ accumulation in the amended and non‐treated soils after 35 days of anoxic incubation. Ethylene production of the 0–5 cm and 5–10 cm soils beneath forest and orchard showed the greatest rate after 2 weeks of anoxic incubation when soil CH₄ production started to increase sharply, and later it was strongly suppressed. The forest‐to‐orchard conversion showed little influence on the CH₄ production of surface soils during short‐term anoxic incubation, but significantly reduced soil C₂H₄ production. The conversion also significantly decreased the consumption of soil CH₄ and C₂H₄, the former more than the latter. Soil properties such as total C, water‐soluble organic C and pH contribute to the consumption and production of C₂H₄ in the 0–5 cm and 5–10 cm soils, and there are the parallels between CH₄ and C₂H₄ consumption in soils, which suggests the presence of similar microorganisms. Long‐term anoxic conditions of in situ surface upland soils are normally not prevalent, so it can be reasonably concluded that there is a larger C₂H₄ accumulation rather than CH₄ accumulation in surface soils beneath forest and orchard after heavy rainfall, especially beneath forest.     

A RAPID METHOD FOR DETERMINING THE SURFACE AREA OF ALUMINOSILICATES FROM THE ADSORPTION DYNAMICS OF ETHYLENE GLYCOL VAPOUR

A new method is described for determining the surface area of soils and clays by adsorbing ethylene glycol vapour under continuous evacuation at c. 10^-2 cm of mercury. The adsorption of a monolayer of ethylene glycol is inferred from the dynamics of pressure changes in the adsorption chamber. The time at which monolayer adsorption is evaluated is proportional to the total weight of ethylene glycol required to form a monolayer on all samples in the adsorption chamber. This period ranged from 30 to 75 minutes when from 0·39 to 1·56 g of glycol was adsorbed in the experiments. A single 0·75 g sample of Wyoming bentonite required 0·195 & glycol.     

The specific surface of soils determined by water sorption

The hypothesis that the specific surface of soil can be measured by water sorption is tested with data for 62 subsoils of widely differing origins. Ethylene glycol and water sorption at p/p₀=0.47 are found to be very closely related measurements and both are highly correlated with CEC. Both methods give a satisfactory measure of total specific surface for soils classed as smectitic and having a large CEC. However, the application of the multilayer theory to the sorption of water on external surfaces of clayey soils with small CEC suggests that both sorbates overestimate the specific surface of such soils. A better estimate of the errors would be obtained from isotherm measurements with water, which is more suitable for this purpose than ethylene glycol.     

土壤对镉的吸附与解吸——Ⅰ.土壤组份对镉的吸附和解吸的影响

采用选择溶解法研究了有机质、游离铁、无定型硅、铝等土壤组份对青黑土、黄棕壤、红壤和砖红壤胶体吸附和解吸Cd的影响。结果表明,去除有机质后胶体吸附Cd减少,这可能是由于交换吸附的减少所致;游离铁的去除使得黄棕壤、红壤和砖红壤的吸附量显著减少,显示了在这些土壤中游离氧化铁专性吸附的重要性;随着无定形铝含量的上升,吸附量下降,这是因为铝离子占据了高能量的吸附位。经不同处理后的土壤胶体,其Cd的解吸顺序(解吸%)大致为:去无定型硅、铝者>去游离铁者>去有机质者>原胶体,但在不同土壤和不同pH条件下该顺序略有差别。研究结果为控制和改造土壤Cd污染提供了理论依据。     

Ion adsorption mechanisms in oxidic soils; implications for point of zero charge determinations

In order to clarify the adsorption mechanisms for various ions (e.g., P, K, Ca, Mg), ion adsorption studies were carried out on a range of soils of widely different mineralogical compositions. The soils were classified as: Hydrandept, Gibbsihumox, Eutrustox and Haplustoll and belonged, respectively, to the Honokaa, Halii, Wahiawa and Waialua soil series in Hawaii.Soils containing mostly hydrous Fe and Al oxides, thus colloids of the variable-charge type, retain ions through various types of adsorption mechanisms. As a result, certain interactions between cation adsorption and anion adsorption occur which affect the ion exchange processes and thereby the determination of the point of zero charge (PZC).In this study adsorbed phosphate increased the adsorption of cations and lowered the PZC and adsorbed Ca interfered with PZC determinations unless these were carried out with CaCl₂ as supporting electrolyte. It is likely that this latter interference will have influenced the results of many earlier studies because of the very strong adsorption of Ca by oxidic colloids. In this study only part of the adsorbed Ca could be recovered from the two most highly weathered soils, by repeated extractions with NH₄-acetate at pH 7.The results indicate that many soils of the tropics can be characterized effectively by their PZC's. One should be aware, however, of the effects of strongly adsorbed ions commonly present in these soils and thus use those supporting electrolytes for the PZC determinations which counteract these effects.     

Short communication Estimation of the specific surface area of soil particles by adsorption of polyvinylalcohol in aqueous suspension

Adsorption of polyvinylalcohol (PVA) in aqueous suspension has been used to measure the specific surface area (SSA) of a silicon dioxide, a goethite, a clay, and a sample of a topsoil, the latter before and after treatment with hydrogen peroxide. Surface areas were calculated from each of the plateaus of the isotherms derived from the Langmuir equation fitted to the data, using a value of 0.04268 nm² for the molecular area of a monomer of PVA. We compared these SSA values with those measured by N₂ adsorption. The SSA values of the silicon dioxide, the goethite and the clay are in excellent agreement with the corresponding N₂‐BET areas. The removal of organic matter by H₂O₂ from the topsoil sample led to a marked increase in the SSA measured by the BET‐method. For this sample, the SSA measured by PVA was considerably larger than the one that was obtained by the BET‐method and showed only a little change after removal of SOM.     

Properties of sub soils in relation to various measures of surface area and water content

Neither the specific surface area values (from N₂ sorption) nor the ethylene glycol monoethyl ether (EGME) retention values of 21 soil samples from New Zealand and Fiji could all be accounted for by the sum of the contributions from their component minerals. Much EGME is probably retained by internal surfaces of inter layered and interstratified clay minerals. EGME retention correlated well with cation exchange capacity (CEC) and a number of measures of water content of these soils. The water contents of air-dried soils (measured as ‘moisture factors’ or the ratios of air-dry and oven-dry weights) showed almost as close a relationship to CEC as EGME retention for this set of 21 subsoils. Moisture factors and CEC were closely related, within groups defined by dominant clay mineralogies, for a much wider selection of 1318 New Zealand soil horizons with low carbon contents. The relationships between surface area and a number of other soil properties including dispersibility of soils were also examined.     

Ionic strength effects on surface charge and adsorption of phosphate and sulphate by soils

Two surface soils (Patua and Tokomaru) of contrasting mineralogy were incubated with several levels of either CaCO₃ or HC1. The effects of ionic strength on pH, on surface charge, and on the adsorption of phosphate and sulphate were measured in three concentrations of NaCl. The pH at which the net surface charge was zero (point of net zero charge—PZC) was 1.8 for the Tokomaru soil and 4.6 for the Patua soil: differences that can be related to mineralogical composition. There was an analogous point of zero salt effect (PZSE) that occurred at pH 2.8 for the Tokomaru soil and at 4.6 for the Patua soil. The presence of permanent negative charge in the Tokomaru soil resulted in an increase in PZSE over PZC. The effect of ionic strength on adsorption varied greatly between phosphate and sulphate. For phosphate, there was a characteristic pH above which increasing ionic strength increased adsorption and below which the reverse occurred. This pH (PZSE for adsorption) was higher than the PZC of the soil and was 4.1 for the Tokomaru soil and 5.3 for the Patua soil. In contrast, increasing ionic strength always decreased sulphate adsorption and the adsorption curves obtained in solutions of different ionic strengths converged above pH 7.0. If increasing ionic strength decreases adsorption, the potential in the plane of adsorption must be positive. Also, if increasing ionic strength increases adsorption, the potential must be negative. This suggests that, depending upon pH, phosphate is adsorbed when the potential in the plane of adsorption is either positive or negative, whereas sulphate is absorbed only when the potential is positive.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.     

Physicochemical properties of the soils associated with shifting cultivation in Northern Thailand with special reference to factors determining soil fertility

Abstract

Recently agricultural activity in the mountainous area of northern Thailand has increased and problems relating to soil fertility have arisen. In order to gain basic information about the soil properties associated with shifting cultivation, physicochemical properties of the surface soils (0–10 cm) and subsoils (30–40 cm) were investigated in selected villages in the area. The physicochemical properties of the soils studied are summarized as follows: 1) The soils were rich in organic matter, content of which ranged from 11.4 to 63.3 g C kg^?1 in the surface soil. 2) The pH(H₂O) of the soils mostly ranged from 5 to 7 and soil acidity was more pronounced in the deeper horizons. In the surface soils, exchangeable Ca and Mg were generally dominant, whereas exchangeable Al was often predominant in the subsoils. 3) Most of the soils showed a medium to fine texture with more than 30% clay. The clay mineral composition was characterized by various degrees of mixture of kaolin minerals and clay mica with, in some cases, a certain amount of 2:1-2:1:1 intergrades. 4) According to the ion adsorption curves, most of the B horizon soils were characterized by the predominance of permanent negative charges. On the other hand, organic matter contributed to the increase of variable negative charges in the surface soils. The content of organic matter and the percentage of the clay fraction were essential for determining the CEC of the soils of the surface 10 and 30–40 cm depths, respectively. Under the field conditions, the composition of exchangeable cations largely reflected the soil acidity. In addition, the content of organic matter also showed a significant correlation with that of available N in the surface soils. Thus, soil acidity both in the surface soils and subsoils, organic matter content in the surface soils, and clay content in the subsoils were considered to be the main factors that affected soil chemical fertility in the area.     

Adsorption density,mobility and limit values of Cd,Zn, Cu and Pb in acid forest soils

To investigate Cd, Zn, Cu and Pb adsorption in acidified forest soils, six soil samples of the aluminium buffer range were selected and analyzed for their physical and chemical properties. Determination of the specific surface area using ethylene glycol monoethyl ether (EGME) adsorption yielded a characteristic value of the solid phases, which can parameterize the major properties of the various soil constituents with sufficient accuracy.

Traditional adsorption isotherms reveal the relation between the amount of a heavy metal adsorbed and the heavy metal concentration in the soil solution only for the soil under study and can therefore not be applied to other soils. To meet the aim of modelling heavy metal adsorption and mobility also for soils differing greatly in their properties, it was attempted to establish a generalizing adsorption isotherm for soils of entirely different composition of the solid phase. The generalizing adsorption density isotherms introduced in the following provide a useful mathematical model for the quantity/intensity relation of heavy metals in soils that differ greatly in their specific surface area and their composition.

It is also shown that limit values which take into account the major quantities influencing heavy metal adsorption and mobility in acid soils can be established from the regression equation between the adsorption density of a heavy metal (ions/m² specific surface area) and its concentration in the soil solution. In particular in view of the groundwater contamination to be expected if acid rain and, as a result, soil acidification continues, these limit values seem to provide considerably more information than the European limit values, given in mg heavy metal /kg soil, which are presently valid for any soil condition and property.     

Copper and lead adsorption as influenced by organic matter in soils from a tropical toposequence with different chemical and mineralogical attributes

The aim of this work was to investigate the influence of the organic matter on copper and lead adsorption in soils with different physiochemical and mineralogical attributes. Suspensions (pH 6.0) of a Latosol, a Neosol and a Vertisol containing increasing amounts of copper or lead were used to obtain sorption isotherms while identical experiments were carried out with the soils previously treated with H₂O₂ to remove organic matter (OM). For the undisturbed soils, L-type and H-type isotherms were predominant for copper and lead respectively, showing that lead interacts more strongly with adsorption sites. For both metals, the non-linear Freundlich adsorption model revealed higher concentration of adsorption sites for Vertisol due to 2:1 clays. For the OM-removed soils, C-type isotherms were observed for copper with the permanence of less stable and more homogeneous sites. For this metal, a high correlation (R² = 0.997) was observed between the decrease of adsorbent sites and the loss of organic carbon, evidencing the central role of organic matter on copper complexation, while lead may be able to interact efficiently with both organic matter and soil minerals.     

Effect of organic matter on the parameters of the selective sorption of cobalt and zinc by soils and their clay fractions

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.     

Association of the Fungicide Propiconazole with Size Fractionated Material from a Silty Clay Soil – S.E. Norway

Eroded soil material may be an important transporting agent for pesticides that are strongly sorbed to soil. The abilityof the fungicide propiconazole to interact with colloidal andparticulate materials has been studied by means of sorptionand desorption experiments. Size separation of silty clay soilfrom Mørdre, Norway and subsequent characterization showedthat different size fractions of soil possessed different physical and chemical properties and, therefore, different capacity to associate with propiconazole. A large part of the soil organic carbon was associated with coarser material (2–0.02 mm), which also showed higher affinity towards propiconazole than for smaller size fractions (<20 and <2 μm). Similar K _oc values (2306 and 2244) for the size fractions <2 and <20 μm indicate that organic carbon played a dominant role in the sorption of propiconazole. Furthermore, organic carbon associated with these size fractions seemed to have similar properties withrespect to binding of propiconazole. Although, poor in organiccarbon (0.4%), the smallest size fraction (<2 μm) had higher sorption capacity for propiconazole compared to the medium size fraction (<20 μm). Higher sorption for the smallest size fraction (<2 μm) is probably due to higherspecific surface area, cation exchange capacity and content of Fe/Al oxides (free, organically bound and amorphous oxides) than the other size fractions. Results from the desorption experiments indicate that a part of propiconazole associates with sites in the soil material that resist desorption. Fluvialsediments originating from propiconazole treated fields may, therefore, represent potential reservoirs of propiconazole.Treatment with H₂O₂ modified the sorption/desorptioncharacteristics of the soil beyond that which could be expectedsimply by the removal of organic material. The pH values for all the size fractions decreased, and the specific surface areaof the medium sized fraction (<20 μm) increased from 14 to 19 m² g^-1 after the treatment with H₂O₂,probably due to disruption of the aggregate structure. Carrying out fractionation and separation procedures, it is important to be aware of physical and chemical changes that areintroduced during the different steps. An effort should be made to develop fractionation methods that keep the original characteristics of the soil material as intact as possible.