PHOSPHATE MOBILIZATION IN IRON-RICH ANAEROBIC SEDIMENTS - MICROBIAL FE(III) OXIDE REDUCTION VERSUS IRON-SULFIDE FORMATION

Mechanisms of phosphate (PO43- mobilization and retention were examine d in iron-rich anaerobic freshwater wetland, lake, and coastal marine sediments. Direct microbial Fe(III) oxide reduction solubilized only 3 -25% of initial solid-phase PO43- during sulfate-free sediment incubat ion experiments. Experiments with reduced, non-sulfidic solid-phase Fe (II)-rich sediment demonstrated PO43- sorption by the solid-phase, and chemical equilibrium calculations indicated that conditions were favo rable for precipitation of Fe(II)-PO4 minerals [e.g. Fe-3(PO4)(2)] in such sediments. These results suggested that much of the PO43- release d from Fe(III) oxides during microbial Fe(III) reduction was captured by solid-phase reduced iron compounds (Fe(II) hydroxide-PO4 complexes and/or Fe(II)-PO4 minerals). Enhanced liberation of PO43- to sediment porewaters (33-100% of initial solid-phase PO43- occurred during anaer obic incubation in the presence of abundant sulfate and was directly c orrelated with sulfate reduction and iron-sulfide mineral formation. I ncubation of PO43--amended sediment with different amounts of sulfate demonstrated a linear correlation between PO43- release and sulfate re duction. Release of Po-4(3-) to sediment porewaters during decompositi on of fresh organic matter (freeze-dried cyanobacteria) was more exten sive in sulfate-amended (67% of added organic P) than in sulfate-free sediment (17% of added organic P), and the ratio of dissolved PO43- re leased to organic carbon oxidized was seven-fold higher in sulfate-ame nded sediment despite a common level of overall organic C and P minera lization in the two treatments. Our results demonstrate that iron-rich anaerobic sediments can immobilize substantial amounts of PO43- under Fe(III) oxide-reducing conditions, but that extensive PO43- release w ill take place if sediment Fe compounds are converted to iron-sulfides via bacterial sulfate reduction.