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 similar to 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 root(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 similar to 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 (R(2) = 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.