Phosphate Leaching from Intact Soil Column in Response to Reducing Conditions

Iron(III)(hydr)oxides can dissolve under reducing soil conditions. Simultaneously, oxide-associated inorganic phosphate is released to the soil solution. In this study, the effect of reducing soil conditions on phosphate leaching from transient waterlogging clayey soil is evaluated. We applied glucose solutions (either 100 or 1000 mg glucose-C L-1) at a steady flow rate of 0.63 mm h-1 to a saturated intact column of structured Alfisol (diam. 0.5 m, height 1.0 m). Effluent concentrations of iron(II) and reactive orthophosphate (Pi) increased slightly during 5 d of low glucose application, reaching values of 2.5 mg Fe L-1, and 0.02 mg PO4-P L-1, respectively. During 10 d of high glucose application, the iron(II) concentration increased to 14 mg Fe L-1 and fluctuations in the Pi-concentration between 0.002 and 0.1 mg PO4-P L-1 were observed. The fluctuations in Pi-concentration are ascribed to interactions between progression of the glucose front, and Pi-mobilization/resorption processes at the walls of macropores. The daily P-losses during low and high glucose applications averaged 0.3 mg PO4-P m-2 d-1, and 0.5 mg PO4-P m-2 d-1, respectively. Comparisons with a parallel topsoil study suggest that subsoil exerts a strong control on leaching – probably via resorption – of Pi mobilized in the topsoil.