FRACTIONATION OF REDUCTANT SOLUBLE PHOSPHORUS IN LAKE SEDIMENTS

Selective extraction methods have been applied to P in lake sediments in an effort to evaluate the fractionation of P and it’s cycling through the environment. Methods for P fractionation are considered operationally defined. While the selected extracting solutions favor P removal from the specified sediment phase, the process is not specific to that phase. Inasmuch, the comparison of data sets must be limited to equivalent methods. The method of Psenner (1984) is widely used to characterize the fractionation of P in lake sediments. This method defines 6 sediment phases: 1) labile and loosely bound P, 2) reductant soluble P, 3) Al oxyhydroxide associated P, 4) organic P, 5) Ca associated P, and 6) residual P.

In this study, we focused on better defining the reductant soluble P phase while attempting to maintain comparability. This phase is the most active in P cycling in seasonally stratified lakes that experience some degree of anoxia or very low dissolved oxygen levels in the hypolimnion. To determine the factors controlling P cycling, it is important to determine the P released from easily reducible phases such as Mn oxyhydroxides and amorphous Fe oxyhydroxides and from the dominantly crystalline Fe oxyhydroxides that become soluble under stronger reducing conditions. To do this, the Psenner method was modified. The single reductant soluble extraction using a buffered dithionite (BD) solution was substituted with two acid hydroxylamine (HA) extraction steps. Eight sediment samples were examined in a parallel study of the original and modified Psenner methods. Results show that the two HA steps are relatively selective for the define phases. The first HA step removes on average 81.5% and 22.7% of the reductant soluble Mn and Fe, while the second HA step removes 18.5% and 77.3% of the reductant soluble Mn and Fe, respectively. The first HA extraction, which reduces Mn oxyhydroxides, removed only 29.5% of the P removed by the DB step. In comparing the sum of the two HA steps to the single BD step, results show a much higher removal of P in the two HA extractions with the sum of these steps extracting 77% more P than the single DB extraction. This indicates that the first HA extraction provides good evidence for the amount of P associated with Mn oxyhydroxides, but that the total reductant soluble P varies substantially with the method selected.