Abstract
A sequential five-step extraction scheme for phosphorus pools in freshwater 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 water depth in Aarhus Bay, DK and used in two other marine sediments: Kattegat at 56 m and Skagerrak at 695 m depth. By comparing the BD-extractable P-pool with both the pool of iron in the BD-fraction and the pool of oxidized, amorphous or poorly crystalline iron (am.FeOOH), highly significant correlations (p < 0.001) were observed in all three sediments. Thus, we conclude that the BD-reagent was very specific for iron-bound P. Further evidence for this came from two experiments: 1) Enhanced BD treatment did not result in additional phosphate extraction and 2) by sequential extraction of phosphorus pools in pure cultures of diatoms and cyanobacteria no phosphate was recovered in the BD-fraction. The pool of am.FeOOH was very important for controlling porewater phosphate concentration which was inferred from the significant inverse relationships between the two parameters (p < 0.001) in all sediments studied. Further, an isotopic exchange experiment with 32POf4/p3− revealed that BD-extractable P was by far the most exchangeable P-pool 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 between iron-bound phosphate and am.FeOOH was 8–11 in Aarhus Bay and Kattegat. In Skagerrak the ratio was 17, which may indicate that the iron mineral extracted from this sediment is less capable of adsorbing phosphate or less saturated with phosphate.
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Jensen, H.S., Thamdrup, B. Iron-bound phosphorus in marine sediments as measured by bicarbonate-dithionite extraction. Hydrobiologia 253, 47–59 (1993). https://doi.org/10.1007/BF00050721
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DOI: https://doi.org/10.1007/BF00050721