Forecasting the decline of excess soil phosphorus in agricultural catchments

Measures within the European Union Nitrates Directive National Action Programmes in many member states aim to reduce soil phosphorus (P) levels from excessive to agronomically optimum. This is to reduce the risk of diffuse P losses to water and ultimately help achieve the Water Framework Directive targets of good water quality status. In this study, a Soil P Decline' model was used to evaluate this expectation for soils in four intensive agricultural catchments. Realistic annual P-balance scenario deficits (30, 15, 7kgP/ha) were used to estimate the average time required for soils to decline to optimum levels in two predominantly arable and two grassland catchments with excessive soil test P (STP). Depending on the STP concentration and total P reserves, for the smallest field P deficit scenario (7kgP/ha), the model predictions were for an average of between 5 and 20yr for agronomically optimum levels to be reached. Under the largest P deficit scenario (30kgP/ha), it was forecast to take between 2 and 10yr. These predictions highlight the likely time lag that exists between implementation of soil P mitigation regulations and the desired outcome of few or no fields with excessive soil P. Expectations for water quality improvement through diffuse P source mitigation must also factor in additional time for P decline model uncertainty, land management variability and time for P sources to transfer to and within river networks.