SEQUENTIAL EXTRACTION OF PHOSPHORUS IN FRESH-WATER WETLAND AND LAKE SEDIMENT - SIGNIFICANCE OF HUMIC ACIDS

We modified a five-step sequential extraction scheme originally develo ped for phosphorus (P) in lake and marine sediments for use in a Danis h freshwater wetland rich in organic matter. In contrast to the origin al scheme, the modification suggests isolation of P associated with al uminum (Al) and iron (Fe) in humic acid complexes. As in the original scheme, discrimination also was obtained between inorganic P associate d with reducible forms of Fe and inorganic P associated with Al oxides . This is important because only iron-bound P is likely to be mobilize d 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 w as associated with Al in the humic acid complexes. Comparison with sed iment 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 r atio was found in the humic acid precipitate from the lake sediment, i ndicating that the same mechanism was responsible for binding inorgani c P in the humic acid complex as in the wetland sediment. When wet sed iments were sieved through a 2-mm mesh and handled under N-2, the meth od was highly reproducible, and no further precision could be obtained by grinding the wet sediment prior to extraction.