Phosphorus speciation in agricultural catchment soils and in fresh and dried sediments of five constructed wetlands

Constructed wetlands and ponds (CWs) are installed to trap suspended material and particulate phosphorus (P) in agricultural runoff. This study investigated whether the P speciation and P sorption capacity of source soils differ from those of CW sediments and whether drying of dredged sediment changes its characteristics. Samples collected from five agricultural CW sites in south-west Finland, two with chemical-aided (aluminium chloride and ferric sulphate) P precipitation and all representing fine-textured mineral soils, were analysed for various P plant availability indices. Clay contents of the CW sediments were much higher than in catchment soils, likely because of selective erosion. All CW sediments were characterised by similar total P content but clearly higher content of anion exchange resin-extractable P in fresh sediments than the source soils. In general, sediment content of NH₄F-extractable (aluminium (Al)-associated) P was significantly lower and NaOH-extractable (iron (Fe)-associated) significantly higher than in source soils. Reduced conditions, conducive to mobilisation of Fe-associated P, were observed in all CWs. Accumulation of sulphur (S) in sediments and a pH decline of up to two units upon drying suggested presence of Fe sulphides. Drying also increased oxalate-extractable Al and Fe (hydr)oxide content by 9–47%, resulting in lower degree of P saturation. These results indicate that dredged CW sediments differ greatly in their P retention characteristics from their parent soils. Returning CW sediments to fields is likely to decrease the amount of readily available P for crop uptake.