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

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 iron bound 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 similar to 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 similar to 12 between HA-Al and HA-P and of similar to 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 AI: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 N-2, the method was highly reproducible, and no further precision could be obtained by grinding the wet sediment prior to extraction.