Phosphate mobilization in iron-rich anaerobic sediments : microbial Fe(III) oxide reduction versus iron-sulfide formation

Mechanisms of phosphate (PO4 3- ) mobilization and retention were examined 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 PO 4 3- during sulfate-free sediment incubation experiments. Experiments with reduced, non-sulfidic solid-phase Fe(II)-rich sediment demonstrated PO4 3- sorption by the solid-phase, and chemical equilibrium calculations indicated that conditions were favorable for precipitation of Fe(II)-P04 minerals [e.g. Fe 3 (PO 4 ) 2 ] in such sediments. These results suggested that much of the PO 4 3- released 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 PO 4 3- to sediment porewaters (33-100% of initial solid-phase PO 4 3- ) occurred during anaerobic incubation in the presence of abundant sulfate and was directly correlated with sulfate reduction and iron-sulfide mineral formation. Incubation of PO 4 3- -amended sediment with different amounts of sulfate demonstrated a linear correlation between PO 4 3- release and sulfate reduction. Release of PO 4 3- to sediment porewaters during decomposition of fresh organic matter (freeze-dried cyanobacteria) was more extensive in sulfate-amended (67% of added organic P) than in sulfate-free sediment (17% of added organic P), and the ratio of dissolved PO 4 3- released to organic carbon oxidized was seven-fold higher in sulfate-amended sediment despite a common level of overall organic C and P mineralization in the two treatments. Our results demonstrate that iron-rich anaerobic sediments can immobilize substantial amounts of PO 4 3- under Fe(III) oxide-reducing conditions, but that extensive PO 4 3- release will take place if sediment Fe compounds are converted to iron-sulfides via bacterial sulfate reduction.