The role of colloidal particles in the speciation and analysis of "dissolved" phosphorus.
Colloidal-sized particles (1-1,000 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-microm 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-microm 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 microm, 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-microm filtered HMMP-containing solutions. In 0.025-microm filtrates, free orthophosphate was the only MRP species present.
Hens
Maarten
M
Laboratory of Soil Fertility and Soil Biology, Katholieke Universiteit Leuven, Belgium. maarten.hens@pi.csiro.au
Merckx
Roel
R
D002850:Chromatography, Gel; D003102:Colloids / Q000737:chemistry*; D004777:Environment; D005374:Filtration; D008970:Molecular Weight; D010710:Phosphates / Q000032:analysis; D010758:Phosphorus / Q000032:analysis* / Q000737:chemistry*; D015203:Reproducibility of Results; D012989:Soil Pollutants / Q000032:analysis; D012996:Solutions / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000032:analysis
D003102:Colloids; D010710:Phosphates; D012989:Soil Pollutants; D012996:Solutions; D014874:Water Pollutants, Chemical; D010758:Phosphorus