Distribution of phosphorus in soil aggregate fractions and its significance with regard to phosphorus transport in agricultural runoff

Abstract

Surface runoff is the major way of P transport from agricultural land to surface waters. To assess the potential of P loss in runoff in relation to soil P status, the chemical nature and distribution of soil P in different size classes of water-stable aggregates were quantified for two distinctive soil types. For both soils unfertilized areas under pasture and well-fertilized arable soils were sampled. The content of total P, organic P and microbial biomass P (Pmic) decreased in the aggregate size order <0.1, 1–2, and 0.1–1.0 mm respectively. In contrast available P (extracted by Bray I reagent) was lowest in the <0.1 mm aggregate size. Cultivation decreased the percentage of 1–2 mm aggregates but increased that of the <0.1 mm aggregates. Fertilization increased markedly both total P and organic P in the <0.1 mm fraction of arable soils compared to the corresponding samples from unfertilized grassland soils. During aggregate separation, most of P loss was in the form of particulate P and less than 1% in solution. More organic P and Pmic were lost from the grassland soils than from the arable soils.