Redox-induced mobilization of phosphorus in groundwater affected arable soil profiles

Mobilization of phosphorus (P) in arable soils might be affected by groundwater fluctuations and the associated changes in redox potential (E_H). However, the impact of systematic changes of E_H on P mobilization in redoximorphic arable soils along a catena has not been studied so far. Therefore, we investigated P mobilization under different redox conditions in top- and sub-soil horizons of three groundwater affected arable soils along a slight slope (toe-, mid-, and upper-slope position) in Northern Germany using an automated biogeochemical microcosm system. The impact of pH, Al, Fe, Mn, and dissolved organic carbon (DOC) on P mobilization was also studied. The initial E_H (+351 to +431 mV) and pH (6.5-7.0) decreased in all soil samples (E_H = -280 mV; pH = 4.4) when creating a slurry. Thereafter, the pH increased to 7.1 and 6.4 with increasing E_H in the mid-and toe-slope soil, respectively. Concentrations of dissolved P ranged between 20.8 mg L^-1 under low E_H in the toe slope topsoil and 0.69 mg L^-1 under high E_H in the toe- and mid-slop subsoil. Concentrations (mg L^-1) of dissolved Fe (0.31-13.3) and DOC (92-2651) increased under low E_H and decreased under high E_H. The increase of P mobilization under low E_H and pH in the soils might be due to the release of P via the reductive and acidic dissolution of Fe-(oxhydr)oxides and/or due to soil organic matter mineralization. The high mobilization of P under reducing conditions may increase its bioavailability; however, it may increase its loss in the soils, particularly in the toe slope profile.