Release of phosphorus under reducing and simulated open drainage conditions from overfertilised soils.

Does removal of cations from soil solution during soil reduction stimulate phosphorus (P) release? An ion-exchange resin system was employed to provide a sink during the incubation of twelve soils under fully reducing conditions. This experimental design was considered to better simulate the loss of ions likely to occur under field conditions than more routine batch type closed extraction systems where solutes build-up in the extract solution. The small solute concentrations that remain in the equilibrating solution suggest the mixed resin system acted as an effective sink over the whole experimental period. By maintaining a small P concentration the resin system mimics soil drainage conditions and encourages P release from soil. Measurement of soil P forms by sequential extraction after the incubation period indicated an increase in the amorphous forms present. Here we show that even if the P-containing solution is retained by the soil, the potential for a subsequent P loss is increased under aerobic conditions. The management of drainage systems should try and avoid the onset of anaerobic conditions. Eventually, magnesium- and calcium-based precipitation products could recapture P from drains recycling it in topsoils as fertilizer.