Particle leaching and particle-facilitated transport of phosphorus at field scale

Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle-facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle-facilitated transport of P at the field scale, and investigate which soil-physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of -20 cm and irrigated at a rate of 10 mm h-1 with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle-facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.