IRON-BOUND PHOSPHORUS IN MARINE-SEDIMENTS AS MEASURED BY BICARBONATE-DITHIONITE EXTRACTION

A sequential five-step extraction scheme for phosphorus pools in fresh water sediment was modified for use in marine sediments. In the second step phosphate bound to reducible forms of iron and manganese ('iron- bound P') is extracted by a bicarbonate buffered dithionite solution ( BD-reagent). The extraction scheme was tested on sediment from 16 m wa ter depth in Aarhus Bay, DK and used in two other marine sediments: Ka ttegat at 56 m and Skagerrak at 695 m depth. By comparing the BD-extra ctable P-pool with both the pool of iron in the BD-fraction and the po ol of oxidized, amorphous or poorly crystalline iron (am. FeOOH), high ly significant correlations (p < 0.001) were observed in all three sed iments. Thus, we conclude that the BD-reagent was very specific for ir on-bound P. Further evidence for this came from two experiments: 1) En hanced BD treatment did not result in additional phosphate extraction and 2) by sequential extraction of phosphorus pools in pure cultures o f diatoms and cyanobacteria no phosphate was recovered in the BD-fract ion. The pool of am.FeOOH was very important for controlling porewater phosphate concentration which was inferred from the significant inver se relationships between the two parameters (p < 0.001) in all sedimen ts studied. Further, an isotopic exchange experiment with (PO43-)-P-32 revealed that BD-extractable P was by far the most exchangeable P-poo l even deep in the sediment where the pool size was small. Iron-bound P made up 33-45% of total P in the surface sediments. The ratio betwee n iron-bound phosphate and am.FeOOH was 8-11 in Aarhus Bay and Kattega t. In Skagerrak the ratio was 17, which may indicate that the iron min eral extracted from this sediment is less capable of adsorbing phospha te or less saturated with phosphate.