Phosphorus leaching through intact soil columns before and after poultry manure application

Recent application of manure can increase phosphorus (P) loss from soil in subsurface flow (e.g., drainage water). This study investigated vertical leaching of P through two soils. Eight 30- and 50-cm-deep, intact soil columns (30-cm diameter) were collected. Columns were irrigated periodically (2.4 cm day(-1)) before and after surface application of poultry manure (85 kg total P ha(-1)), continuing for 11 weeks after the application. A dye tracer (FD&C blue No. 1) was used to identify the presence of active macropores at the bottom of each column, and to compare properties of undyed soil matrix material with dyed soil bordering active macropores. Before manure application, concentrations of total P (TP) in leachate did not exceed 0.57 mg L-1, with dissolved reactive P (DRP) a minor fraction of leachate TP (averaging 7%). Manure application resulted in significant increases in leachate P concentrations, with DRP averaging 72% of leachate TP. No significant differences in leachate DRP and TP concentrations were observed between 30- and 50-cm-deep columns or between soils, either before or after manure was applied, reflecting considerable variability in leachate P trends. In many columns, P concentrations in leachate peaked soon after manure application, with maximum DRP concentrations ranging from 1.1 to 11.2 mg L-1. In other columns, concentrations increased slowly over time, but maximum DRP concentrations were only 0.19 to 0.55 mg L-1. Different temporal trends in leachate P concentrations were unrelated to trends in flow. Increased P sorption saturation of soil bordering macropores in subsurface horizons, due to elevated Mehlich-3 P and depleted Mehlich-3 Al, points to the importance of macropores as preferential flow pathways for P. Results of this study highlight the significant, but temporally and spatially variable, nature of P leaching in manured soils.