Differentiating between physical and chemical constraints on pesticide and water movement into and out of soil aggregates

A laboratory experiment comparing the movement of 3 H 2 O and [ 14 C]isoproturon into and release from soil aggregates is described. Small aggregates (2.0-2.4mm) were prepared from a clay topsoil and maintained at three different initial moisture conditions. A small volume of the radioisotope solution was introduced prior to bathing the aggregates in a 2mM CaCl 2 solution to represent new rainwater. Whilst the 3 H 2 O was imbibed by the air-dry aggregates, the pesticide did not follow the water but remained on the surface of the aggregates. This may be related to its sorptive properties and an excess of sorption sites on the sorbent with respect to the sorbate. Increasing the length of exposure of the moist aggregates to [ 14 C]isoproturon reduced the initial release of the compound into the bathing solution, probably due to diffusion (retarded by sorption) into the aggregates. The diffusion model described by Crank and a non-equilibrium desorption model were used to analyse the 3 H 2 O and [ 14 C]isoproturon release curves. This showed that the release of 3 H 2 O from the dry aggregates was controlled by diffusion. The release of isoproturon was probably controlled by non-equilibrium sorption/desorption from air-dry aggregates and by a combination of non-equilibrium sorption/desorption and diffusion from wet aggregates.