Does vivianite control phosphate solubility in anoxic meadow soils?

Vivianite (Fe3(PO4)2·8H2O) may precipitate in anoxic wetland soils where it may control orthophosphate (Pi) equilibrium solution concentrations at micromolar levels, and thus be of key importance in reducing excessive P from agricultural sources and eutrophication. However, vivianite equilibria and kinetics under in situ conditions are not fully understood and the occurrence of vivianite in wetland soils is rarely documented. In the present investigation we have monitored the temporal (November to June) variation in the pore water chemistry of a wet meadow soil (Sapric Medihemist) including a vivianite-containing gyttja layer. Pore water concentrations of Ca, FeII, HCO3, and NH4 in the gyttja layer were higher than in adjacent horizons. In contrast, dissolved Pi concentrations were the lowest observed in the profile and showed only minor fluctuations (between 0.1 and 6 μM). Pore water composition in the gyttja layer was close to equilibrium with vivianite (saturation index, SIviv, 2.01 ± 0.53) at constant pH (~ 6.8). Dissolution and precipitation experiments in the laboratory with soil suspensions from the gyttja layer demonstrated that vivianite solubility equilibria were only slowly restored. Even after 120 days following perturbation the supersaturation was still high (SIviv ~ 6). It seems that vivianite does contribute to Pi immobilization in anoxic soil horizons, but due to slow precipitation kinetics such soils cannot maintain Pi concentrations at levels below critical thresholds for eutrophication (~ 1 μM), except if pore water geochemistry is kept stable.

► Vivianite is formed along pores and cracks in a gyttja layer.
► Vivianite formation via reduction of iron oxides.
► Solubility equilibrium of vivianite is slowly established (> 120 days).
► Vivianite keeps dissolved phosphate < 6 μM when pore water composition is stable.