Biogas Digestate Application Modifies Solute Transport Conditions in Soils and Increases the Release of Phosphorus

Abstract

The application of fertilizers to soils may impair the quality of both surface and subsurface waters. Severe rainfall events that follow fertilizer application can increase P release from soils. In this study, suction plates were installed in a loamy soil with the aim of determining the P transport patterns and processes in the soil before and after the application of biogas digestate. Prior to the application of biogas digestate, the pore water concentrations of dissolved reactive P (DRP) were significantly lower than the total P (TP) concentrations, with DRP/TP ratios increasing from 0.03 to 0.89. After fertilizer application, the TP and DRP pore water concentrations increased 400-fold. The DRP/TP ratio remained almost constant at 0.83. After 220 mm of precipitation, 4.5% of the P mass applied was leached to below the 50-cm soil depth, indicating the general mobility of P in soils during high-intensity rainfall. Brilliant Blue dye tracing experiments revealed that organic-matter-rich biogas digestate obviously changes the flow and transport patterns in soils from being homogenous to moderately heterogeneous, including preferential flow. Although the P loading across the flux plane had been equilibrated prior to treatment, after the application it was found to be concentrated in preferred transport regions. The emerging conclusion is that the risk of ground and subsequent surface water pollution with P after fertilization with biogas digestate originates not only from the applied P mass but also from the shift in the flow and transport regime caused by the organic-matter-rich slurry.