The effects of oxidation-reduction potential on the solubility of phosphorus in agricultural water management systems

Previous field studies have shown that dissolved phosphorus (P) concentrations are higher in subsurface drainage water from agricultural fields under water table management (WTM) than under conventional tile drainage. Anaerobic conditions have been shown to alter the Oxidation-Reduction Potential (Eh) of soils. The reducing conditions caused by anaerobic conditions result in the reduction of Fe (III), Mn (III), Mn(V) and other metal phosphates. These reductions cause dissolved P once held in metal phosphates to become mobile. The objectives of this research were to confirm the relationship between Eh and dissolved P and to determine whether Eh could be responsible for higher concentrations of dissolved P in drainage water from water table management (WTM) compared to conventional free outlet tile drainage (FD).;A field experiment determined that the Eh of drainage water from WTM differs from that of FD. In the field experiment, concentrations of dissolved P in water samples from WTM and ID were compared, and pH, Eh, dissolved Fe and Mn measurements were taken. The relationship between the Eh and dissolved P was further confirmed in two laboratory experiments. In the laboratory experiments, the effects of Eh on the solubility of P was determined by manipulating Eh, using glucose in one experiment and incubation periods in another, and measuring resulting Eh, concentrations of dissolved P, and dissolved Fe and Mn.;The field experiment showed that the concentration of dissolved P is much higher and the Eh is much lower in WTM than in FD. In the two laboratory experiments, soils from three depths (0-25 cm, 25-50 cm and 50-75 cm) were examined. The laboratory results for the top soil (0-25 cm), and the field experiment results all demonstrate a strong correlation between changes in Eh and changes in dissolved P, with concentrations of dissolved P increasing as Eh decreases. The increase in dissolved Fe and Mn and dissolved P confirms that the reduction of Fe in Fe-P compounds and of Mn in Mn-P compounds, by changed Eh conditions, causes a subsequent release of P from the soil into the equilibrium solution. Overall, these research results suggest that lower Eh of WTM is most likely responsible for higher concentrations of dissolved P in drainage water from WTM compared to FD.;It is also important to note that in the deeper soil depths (25-50 cm and 50-75 cm), no correlation between changes in Eh and changes in dissolved P were observed in the laboratory experiments.