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

Johanna Marketta Laakso, Risto Uusitalo, Markku Juhani Yli-Halla

Research output: Contribution to journalArticleScientificpeer-review


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.
Original languageEnglish
Pages (from-to)18-26
Number of pages9
Publication statusPublished - 2016
MoE publication typeA1 Journal article-refereed

Fields of Science

  • 1172 Environmental sciences
  • Soil science
  • Soil chemistry
  • Phosphorus
  • Erosion
  • Reducing conditions
  • Sediment
  • 4111 Agronomy
  • Soil amendment
  • soil analysis

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