Phosphate mobilization and immobilization in two soils incubated under simulated reducing conditions

Inorganic phosphate (P) associated with Fe(III) (hydr)oxides can be mobilized by reductive dissolution of the oxides. Bulk A(p) samples from two loamy soils differing by nearly 50% in total P were amended with 6 or 60 mg glucose-C per 100 g soil at a water tension of 0.2 m. During 29 days of anoxic incubation at room temperature, the soil solution pH, concentrations of Fe(II) and molybdate reactive P were measured. The concentrations of P were correlated to neither total soil-P nor Fe(II) concentrations. Lack of proportionality between Fe(II) and P in solution was attributed to microbial uptake, resorption of P and also, at high Fe(II)-concentrations, to precipitation of Fe(II)-P compounds (e.g. vivianite). The highest concentrations were observed in samples amended with the low-C dose, where the concentrations increased six-fold to approximately 0.3 mg PO,-P l(-1). This indicated that P leaching might increase from clayey soils subjected to moderately reducing conditions.