Lis Wollesen de Jonge

Influence of pH and solution composition on the sorption of glyphosate and prochloraz to a sandy loam soil

Standard protocols for batch sorption experiments prescribe the use of 0.01 M CaCl2 as the aqueous solution, but sorption can strongly depend on the solution composition. The present work quantifies the variation in sorption behavior of the herbicide glyphosate and the fungicide prochloraz to a sandy loam soil, arising from differences in pH, ionic strength, ortho-phosphate concentration, and dominant cation in solution (Ca2+, K+, NH4+). Using batch experiments, we measured the amount sorbed to the bulk fraction and clay-sized particles. From the adsorption and desorption isotherms, we estimated the Freundlich parameters, Kƒ and N. Sorption isotherms were mostly non-linear and manifested adsorption-desorption non-singularity. Adsorption Kƒ values were in the range 0.6–78.5 L kg−1 for glyphosate and 31.2–155 for prochloraz. The pH and ortho-phosphate affected the sorption of both glyphosate and prochloraz, whereas ionic strength and dominant cation only affected sorption of glyphosate.

Water-Dispersible Colloids: Effects of Measurement Method, Clay Content, Initial Soil Matric Potential, and Wetting Rate

The fraction of clay that disperses in water, water-dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; ψ = −2.5, −100, and −15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low-energy input measurement of WDC (LE-WDC). These results were referenced by WDC obtained by a conventional, high-energy input measurement based on air-dried soil (HE-WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE-WDC increased with clay content, while the amount of LE-WDC decreased with increasing clay content. The decrease in LE-WDC was explained by an increase in cohesive strength, reflected by the increase in water-stable aggregates (≥4 mm). A strong dependency of IMP on LE-WDC was observed, with maximum release of LE-WDC from soils that were at −2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE-WDC and also reduced the dependency of soil clay content, with soils incubated at −15500 hPa releasing a low amount of LE-WDC independent of clay content. The content of particulate organic C (POC) in the LE-WDC decreased with increasing clay content, and increased after drying to −15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE-WDC released from the initially dry soil (−15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.

Diffusion-limited mobilization and transport of natural colloids in macroporous soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time-dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time-dependent and possibly diffusion-limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.

[14C]Glyphosate transport in undisturbed topsoil columns

Although glyphosate (N-(phosphonomethyl)glycine) is one of the most frequently used herbicides, few controlled transport experiments in undisturbed soils have been carried out to date. The aim of this work was to study the influence of the sorption coefficient, soil-glyphosate contact time, pH, phosphorus concentration and colloid-facilitated transport on the transport of [14C]glyphosate in undisturbed top-soil columns (20 cm height × 20 cm diameter) of a sandy loam soil and a sandy soil. Batch sorption experiments showed strong Freundlich-type sorption to both soil materials. The mobility of glyphosate in the soil columns was strongly governed by macropore flow. Consequently, amounts of glyphosate leached from the macroporous sandy loam soil were 50–150 times larger than from the sandy soil. Leaching rates from the sandy soil were not affected by soil-glyphosate contact time, whereas a contact time of 96 h strongly reduced the leaching rates from the sandy loam soil. The role of pH and phosphorus concentration in solution was relatively unimportant with respect to total glyphosate leaching. The contribution of colloid-facilitated transport was <1 to 27% for the sandy loam and <1 to 52% for the sandy soil, depending on soil treatment. The risk for glyphosate leaching from the top-soils seems to be limited to conditions where pronounced macropore flow occurs shortly after application. © 2000 Society of Chemical Industry

Colloid Mobilization and Transport in Undisturbed Soil Columns. I. Pore Structure Characterization and Tritium Transport

While it is recognized that preferential flow may increase the transport of colloids, less is known about the actual influence of preferential flow on colloid mobilization in situ. Changes in pore structure upon soil exposure to drying and rewetting may additionally affect colloid mobilization. Information about the pore structure and the active flow volume, as well as the changes in these properties, are therefore important when investigating colloid mobilization. We investigate the pore structure characteristics and the transport of tritium ( 3 H 2 O) during steady unsaturated flow conditions. A total of 54 soil columns sampled along a natural clay gradient representing six clay contents (12, 18, 24, 28, 37, and 43% clay) were equilibrated to three different initial matric potentials (IMP), ψ = −2.5, −100, and −15500 hPa. Pore structure characteristics were deduced from water retention characteristics and measurements of air-filled porosity and air permeability. Tracer experiments were conducted at 1 mm h −1 and with a suction of 5 hPa. A mobile–immobile region model (MIM) and a three-region model (2MIM) with two mobile and one immobile region were used for describing the breakthrough curves (BTCs). The 2MIM model was able to fit the data well and predicted the existence of two mobile flow regions, most pronounced at higher clay content. The 12% clay soil exhibited matrix-dominated flow behavior, which is probably attributable to a large fraction of drained pores disconnecting the rapidly conducting flow system. Soils with ≥18% clay exhibited asymmetrical BTCs with early breakthrough and tailing and an increasing amount of immobile water, indicating preferential flow. Drying and rewetting, because of associated changes in the pore structure, significantly reduced the degree of preferential flow.

Transport modes and pathways of the strongly sorbing pesticides glyphosate and pendimethalin through structured drained soils

Leaching of the strongly sorbing pesticides glyphosate and pendimethalin was evaluated in an 8-month field study focussing on preferential flow and particle-facilitated transport, both of which may enhance the leaching of such pesticides in structured soils. Glyphosate mainly sorbs to mineral sorption sites, while pendimethalin mainly sorbs to organic sorption sites. The two pesticides were applied in equal dosage to a structured, tile-drained soil, and the concentration of the pesticides was then measured in drainage water sampled flow-proportionally. The leaching pattern of glyphosate resembled that of pendimethalin, suggesting that the leaching potential of pesticides sorbed to either the inorganic or organic soil fractions is high in structured soils. Both glyphosate and pendimethalin leached from the root zone, with the average concentration in the drainage water being 3.5 and 2.7 μg L(-1), respectively. Particle-facilitated transport (particles >0.24 μm) accounted for only a small proportion of the observed leaching (13-16% for glyphosate and 16-31% for pendimethalin). Drain-connected macropores located above or in the vicinity of the drains facilitated very rapid transport of pesticide to the drains. That the concentration of glyphosate and pendimethalin in the drainage water remained high (>0.1 μg L(-1)) for up to 7d after a precipitation event indicates that macropores between the drains connected to underlying fractures were able to transport strongly sorbing pesticides in the dissolved phase. Lateral transport of dissolved pesticide via such discontinuities implies that strongly sorbing pesticides such as glyphosate and pendimethalin could potentially be present in high concentrations (>0.1 μg L(-1)) in both water originating from the drainage system and the shallow groundwater located at the depth of the drainage system.

Measuring binding and speciation of hydrophobic organic chemicals at controlled freely dissolved concentrations and without phase separation.

The binding and speciation of hydrophobic organic chemicals (HOCs) in aqueous solutions were determined by controlling chemical activity and measuring total concentrations. Passive dosing was applied to control chemical activities of HOCs in aqueous solutions by equilibrium partitioning from a poly(dimethylsiloxane) polymer preloaded with the chemicals. The HOC concentrations in the equilibrated solutions [C(solution(eq))] and water [C(water(eq))] were then measured. Free fractions of the HOCs were determined as C(water(eq))/C(solution(eq)), whereas enhanced capacities (E) of the solutions for HOCs were determined as C(solution(eq))/C(water(eq)). A mixture of polycyclic aromatic hydrocarbons served as model analytes, while humic acid, sodium dodecyl sulfate, hydroxypropyl-β-cyclodextrin, and NaCl served as model medium constituents. The enhanced capacities were plotted versus the concentrations of medium constituents, and simple linear regression provided precise partition ratios, salting out constants, and critical micelle concentrations. These parameters were generally in good agreement with published values obtained by solid phase microextraction and fluorescence quenching. The very good precision was indicated by the low relative standard errors for the partition ratios of 0.5-8%, equivalent to 0.002-0.03 log unit. This passive dosing approach allows binding and speciation of HOCs to be studied without any phase separation steps or mass balance assumptions.

Comparative Mapping of Soil Physical–Chemical and Structural Parameters at Field Scale to Identify Zones of Enhanced Leaching Risk

Preferential flow and particle-facilitated transport through macropores contributes significantly to the transport of strongly sorbing substances such as pesticides and phosphorus. The aim of this study was to perform a field-scale characterization of basic soil physical properties like clay and organic carbon content and investigate whether it was possible to relate these to derived structural parameters such as bulk density and conservative tracer parameters and to actual particle and phosphorus leaching patterns obtained from laboratory leaching experiments. Sixty-five cylindrical soil columns of 20-cm height and 20-cm diameter and bulk soil were sampled from the topsoil in a 15-m × 15-m grid in an agricultural loamy field. Highest clay contents and highest bulk densities were found in the northern part of the field. Leaching experiments with a conservative tracer showed fast 5% tracer arrival times and high tracer recovery percentages from columns sampled from the northern part of the field, and the leached mass of particles and particulate phosphorus was also largest from this area. Strong correlations were obtained between 5% tracer arrival time, tracer recovery, and bulk density, indicating that a few well-aligned and better connected macropores might change the hydraulic conductivity between the macropores and the soil matrix, triggering an onset of preferential flow at lower rain intensities compared with less compacted soil. Overall, a comparison mapping of basic and structural characteristics including soil texture, bulk density, dissolved tracer, particle and phosphorus transport parameters identified the northern one-third of the field as a zone with higher leaching risk. This risk assessment based on parameter mapping from measurements on intact samples was in good agreement with 9 yr of pesticide detections in two horizontal wells and with particle and phosphorus leaching patterns from a distributed, shallow drainage pipe system across the field.

Direct and Indirect Short-term Effects of Biochar on Physical Characteristics of an Arable Sandy Loam

Abstract Biochar addition to agricultural soil is reported in several studies to reduce climate gas emissions, boost carbon storage, and improve soil fertility and crop productivity. These effects may be partly related to soil physical changes resulting from biochar amendment, but knowledge of how biochar application mechanistically affects soil physical characteristics is limited. This study investigated the effect of biochar application on soil structural and functional properties, including specific surface area, water retention, and gas transport parameters. Intact soil cores were taken from a field experiment on an arable sandy loam that included four reference plots without biochar and four plots with 20 tons ha−1 biochar incorporated into the upper 20 cm 7 months before sampling. Water retention was measured at matric potentials ranging from wet (pF 1.0) to extremely dry conditions (pF ∼6.8), whereas gas transport parameters (air permeability, ka, and gas diffusivity, Dp/Do, where Dp is the gas diffusion coefficient in soil and Do is the gas diffusion coefficient in free air) were measured between pF 2.0 and 3.0. Water retention under dry conditions and measured specific surface area were not significantly greater in the biochar-amended soil than the reference soil probably because of the relatively low biochar application rate. Yet, the biochar-amended soil showed a significant decrease in soil bulk density and an accompanying increase in total porosity. Water retention and air-filled porosity (&egr;) were both markedly greater in the biochar-amended soil than in the reference soil between pF 1.0 and 3.0. Soil macroporosity (equivalent to >0.1 mm pore diameter) and the ratio of macroporosity to total porosity were also significantly greater in the biochar-amended soil. As a result, the level of the pore organization (PO, ka/&egr;) was greater in the biochar-amended soil. Across the tested matric potentials, biochar amendment caused average increases of 28 to 34% in &egr;, 53 to 161% in Dp/Do, and 69 to 223% in ka, with the most significant increases occurring around natural field capacity (pF 2.0). Overall, the results suggest that biochar application even at a relatively low rate can alter soil functional characteristics, especially under normal field moisture conditions.

Colloid and Phosphorus Leaching From Undisturbed Soil Cores Sampled Along a Natural Clay Gradient

The presence of strongly sorbing compounds in groundwater and tile drains can be a result of colloid-facilitated transport. Colloid and phosphorus leaching from macropores in undisturbed soil cores sampled across a natural clay gradient at Aarup, Denmark, were studied. The aim of the study was to correlate easily measurable soil properties, such as clay content and water-dispersible colloids, to colloid and phosphorus leaching. The clay contents across the gradient ranged from 0.11 to 0.23 kg kg−1. Irrigating with artificial rainwater, all samples showed a high first flush of colloids and phosphorus followed by lower and stable colloid and phosphorus concentrations. The mass of particles leached at first flush was independent of clay content and was attributed to the instant release of particles associated with the macropore walls and released upon contact with flowing water. Below a clay content of ∼0.15 kg kg−1, the later leaching (after the first flush) of particles was independent of the clay content. Above this threshold, there was a positive relationship between the mass of leached particles after the first flush and the clay content. Particle release after the first flush was linearly correlated to the accumulated outflow and was described as a diffusion controlled process, using √(accumulated outflow). The mass of leached particles was positively correlated to the clay content as well as to water-dispersible colloids. Particulate phosphorus (P) was linearly correlated to concentration of leached particles and accounted for ∼70% of the total mass of leached P. Approximately 50% of particulate P was associated with the first flush. The P concentration on leached particles was negatively correlated to clay content (R2 = 0.89) and followed the same trend as the P concentration on soil clay and the so-called degree of P saturation (oxalate-extractable P on iron and aluminum minerals). Because higher colloidal P concentration was countered by a lower colloidal leaching, the total amount of leached P stayed remarkably constant along the natural clay gradient.