Reducing P Mobilization from Agricultural Soils to Floodwater Under Anoxic Conditions.

Seasonally flooded conditions that change soil's oxidation-reduction status may increase phosphorus (P) released from agricultural soils to overlying floodwater, and consequently increase P loadings to surface water bodies. To help in developing beneficial management practices to reduce P mobilization from agricultural soils to waterways, we examined the magnitude and temporal pattern of P release from soils to flood water under simulated flooded conditions using different soil types from Manitoba. Surface soils collected from different locations were packed into ponding basins to a depth of 15 cm, with soil solution samplers installed during packing at 10 cm depth to extract pore water. During a simulated flooding period of eight weeks under field conditions, dissolved reactive phosphorus (DRP) and total dissolved phosphorus (TDP) released to ponded surface water and soil pore water were monitored using four replicates for each soil. Concentrations of DRP and TDP in floodwater and/or pore water increased in 10  soils over the flooded period but were stable in two clay soils. Generally, in soils that showed an increase in DRP concentrations in pore water and flood water, the release of increasing quantities of P started after 21-28 days of flooding, which corresponded with the decrease in redox potential to negative values. The relative increase in DRP concentration in floodwater (ratio of maximum DRP concentration to the initial DRP concentration in floodwater) showed a significant positive relationship with degree of P saturation (r=-0.73, P<0.01), and a significant negative correlation with single point P adsorption capacity (r=-0.61, P<0.05). Based on these observations, draining water from fields within three weeks after flooding would minimize redox-induced P loadings to flood water, especially in sols with high soil test P and low P adsoprtion capacities that are susceptible to large releases of P when anaerobic conditions develop.