The leaching of soil particles and surface applied 14C-labeled glyphosate and pendimethalin from intact soil columns (height: 50 cm; diameter: 30 cm) were investigated, and the relative significance of particle-facilitated pesticide transport was quantified. Investigations were performed with a recently plowed (four columns) and an untilled (five columns) sandy loam soil. Leaching was driven by three irrigation events (15 mm h(-1); 2 h each). Samples of the leachate were filtered immediately (within 1.5 minutes) using 20 nm filters, and the 14C-pesticide content was determined for filtered and unfiltered samples. Pesticide leaching was driven by preferential water flow in macropores. For the plowed structure, 68+/-10% of the leached glyphosate (average of 6 events+/-std.) was bound to particles whereas significantly less glyphosate was bound to particles in leachate from minimally disturbed columns (17+/-12%). Thus, the results suggest that soil structure affected the mode of transport of glyphosate. It is likely that glyphosate sorbed strongly when applied on recently plowed soil (Kd=503 L kg(-1) for the soil), and that it could be mobilized and transported independently of soil particles more easily when applied on the minimally disturbed soil covered in part with crop residues (Kd<1 L kg(-1) for straw). Significantly less amounts of soil particles were leached from minimally disturbed (119-247 mg) than from recently plowed (441-731 mg) columns. The significance of particle-facilitated pendimethalin leaching could not be accurately quantified due to disagreement between control measurements based on both 14C-activity and chemical analyses.
Because dissolved organic matter (DOM) plays an important role is terrestrial C-, N-and P-balances and transport of these three components to aquatic environments, there is a need to include it in models. This paper presents the concept of the newly developed DOM modules implemented in the DAISY model with focus on the quantification of DOM sorption/desorption and microbial-driven DOM turnover. The kinetics of DOM sorption/desorption is described by the deviation of the actual DOM concentration in solution from the equilibrium concentration, C eq . The C eq is soil specific and estimated from pedotransfer functions taking into account the soil content of organic matter, Al and Fe oxides. The turnover of several organic matter pools including one DOM pool are described by first-order kinetics.The DOM module was tested at field scale for three soil treatments applied after cultivating grass-clover swards. Suction cups were installed at depths 30, 60 and 90 cm and soil solution was sampled for quantification of dissolved organic C (DOC) and dissolved organic N (DON). In the topsoil, the observed fluctuations in DOC were successfully simulated when the sorption/desorption rate coefficient k was low. In the subsoil, the observed concentrations of DOC were steadier and the best simulations were obtained using a high k. The model shows that DOC and DON concentrations are levelled out in the subsoils due to soil buffering. The steady concentration levels were based on the C eq for each horizon and the kinetic concept for sorption/desorption of DOC appeared a viable approach. If C eq was successfully estimated by the pedotransfer function it was possible to simulate the DOC concentration in the subsoil. In spite of difficulties in describing the DOC dynamics of the topsoil, the DOM module simulates the subsoil concentration level of DOC well, and also-but with more uncertainty-the DON concentration level. r
Desorption kinetics of chemical compounds can be important both for their mobility in soil and for the significance of particle‐facilitated transport. We studied desorption of glyphosate [N‐(phosphonomethyl)glycine] on mobilized particles from two soil columns (50‐cm height, 30‐cm diameter), i.e., particles leached by free drainage from the bottom and particles mobilized by splash erosion and collected next to the top of the column. Leaching and splash erosion were driven by three, 30‐mm irrigation events following surface application of 14C‐labeled glyphosate. Fresh leachate samples were investigated within 30 min of sampling, and desorption from splash‐eroded particles in suspension (100 mg solid L−1) was followed for 48 h (starting 2.0 min after immersion). Glyphosate concentrations were determined by measuring the 14C activity using liquid scintillation counting. Similar fractional amounts of glyphosate (on average, 10–20% in 20 min) desorbed from leached and from splash‐eroded particles (>20 nm) shortly after leaching or immersion, respectively, indicating that the processes of desorption from the different sources of particles were similar. In leachate, about 45 to 79% remained particle bound after 20 min, while calculated values at equilibrium were 20% or less. Equilibrium was established after about 5 to 10 h in suspensions with splash‐eroded particles, except for one sample. These direct observations, supported by estimated values of the Damköhler number, lead to the conclusion that desorption kinetics are important for evaluating the significance of dissolved and particle‐facilitated transport of glyphosate. To quantify particle‐facilitated glyphosate transport, the water and solid phases in the leachate should consequently be separated within a few minutes after leaching.
Mobility of dissolved organic matter (DOM) strongly affects the export of nitrogen (N) and phosphorus (P) from soils to surface waters. To study the sorption and mobility of dissolved organic C and P (DOC, DOP) in soil, the pH-dependent sorption of DOM to samples from Ap, EB, and Bt horizons from a Danish agricultural Humic Hapludult was investigated and a kinetic model applicable in fieldscale models tested. Sorption experiments of 1 to 72 h duration were conducted at two pH levels (pH 5.0 and 7.0) and six initial DOC concentrations (0-4.7 mmol L 21 ). Most sorption/desorption occurred during the first few hours. Dissolved organic carbon and DOP sorption decreased strongly with increased pH and desorption dominated at pH 7, especially for DOC. Due to fractionation during DOM sorption/ desorption at DOC concentrations up to 2 mmol L 21 , the solution fraction of DOM was enriched in P indicating preferred leaching of DOP. The kinetics of sorption was expressed as a function of how far the solution DOC or DOP concentrations deviate from "equilibrium."The model was able to simulate the kinetics of DOC and DOP sorption/ desorption at all concentrations investigated and at both pH levels making it useful for incorporation in field-scale models for quantifying DOC and DOP dynamics.
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