PHOSPHATE MOBILIZATION AND IMMOBILIZATION IN 2 SOILS INCUBATED UNDER SIMULATED REDUCING CONDITIONS

Inorganic phosphate (P) associated with Fe(III) (hydr)oxides can be mo bilized 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. Durin g 29 days of anoxic incubation at room temperature, the soil solution pH, concentrations of Fe(II) and molybdate reactive P were measured. T he concentrations of P were correlated to neither total soil-P nor Fe( II) concentrations. Lack of proportionality between Fe(II) and P in so lution was attributed to microbial uptake, resorption of P and also, a t high Fe(II)-concentrations, to precipitation of Fe(II)-P compounds ( e.g. vivianite). The highest concentrations were observed in samples a mended with the low-C dose, where the concentrations increased six-fol d to approximately 0.3 mg PO,-P l(-1). This indicated that P leaching might increase from clayey soils subjected to moderately reducing cond itions.