The role of colloidal particles in the speciation and analysis of “dissolved” phosphorus

Colloidal-sized particles (1–1000 nm) and high molecular mass material play an important, yet poorly understood role in the aqueous speciation of P. This study assessed the size distribution of P in 0.45-μm filtered soil solutions and soil–water extracts from three sandy soils (grassland, arable field and forest) using gel filtration chromatography (GFC) and membrane filtration (0.22- and 0.025-μm pore-size) and evaluated the impact of P speciation on colorimetric and ion-chromatographic methods for orthophosphate analysis. Between 40% and 58% of molybdate reactive P (MRP) and >85% of molybdate unreactive P in the soil solution from the agricultural soils (pH 5.9–6.3) were associated with high molecular mass material (apparent size >0.025 μm, or >600 kDa on Superdex). In solutions from the forest soil (pH 3.2), high molecular mass P (HMMP) compounds were of minor importance (<8% of TP). The GFC elution profiles, composition and spectral characteristics of HMMP-containing solutions as well as the small relevance of HMMP at low pH were all indicative for associations between humic substances, Fe and/or Al, and P. Both MRP and ion chromatographic P measurements overestimated the free orthophosphate concentration (up to 2.3- and 1.4-fold, respectively) in 0.45-μm filtered HMMP-containing solutions. In 0.025-μm filtrates, free orthophosphate was the only MRP species present.