, Volume 15, Issue 4, pp 365–373

Sequential extraction of phosphorus in freshwater wetland and lake sediment: Significance of humic acids

  • Claus Paludan
  • Henning S. Jensen


We modified a five-step sequential extraction scheme originally developed for phosphorus (P) in lake and marine sediments for use in a Danish freshwater wetland rich in organic matter. In contrast to the original scheme, the modification suggests isolation of P associated with aluminum (Al) and iron (Fe) in humic acid complexes. As in the original scheme, discrimination also was obtained between inorganic P associated with reducible forms of Fe and inorganic P associated with Al oxides. This is important because only iron-bound P is likely to be mobilized when wetland sediments become anoxic. Extraction of ironbound P was performed by use of bicarbonate-buffered dithionite and was followed by a NaOH treatment that extracted most organic bound P together with P adsorbed onto clays and Al oxides. The NaOH extract was dark brown, but upon acidification (pH≈1), we produced a precipitate of presumeably humic acids (HA) and a clear supernatant. The precipitate contained up to 30% of the total sediment P and significant fractions of sediment Al and Fe. Ratios of ∼ 12 between HA-Al and HA-P and of ∼ 2.5 between HA-Fe and HA-P suggest that most HA-P was associated with Al in the humic acid complexes. Comparison with sediment from an acid-bog lake suggests that the HA-P fraction might also be important in other freshwater sediments. A similar Al:P and Fe:P ratio was found in the humic acid precipitate from the lake sediment, indicating that the same mechanism was responsible for binding inorganic P in the humic acid complex as in the wetland sediment. When wet sediments were sieved through a 2-mm mesh and handled under N2, the method was highly reproducible, and no further precision could be obtained by grinding the wet sediment prior to extraction.

Key Words

method phosphorus fractionation organic sediments wetlands 


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Copyright information

© Society of Wetland Scientists 1995

Authors and Affiliations

  • Claus Paludan
    • 1
  • Henning S. Jensen
    • 1
  1. 1.Department of Freshwater EcologyNational Environmental Research InstituteSilkeborgDenmark

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