The time it takes to reduce soil legacy phosphorus to a tolerable level for surface waters: What we learn from a case study in the catchment of Lake Baldegg, Switzerland

Abstract

The role of diffuse phosphorus (P) losses from agricultural land gained importance since technical and constructional measures in urban areas and industry have reached their potential in temperate regions. Phytoremediation strategies are a new suggestion to effectively reduce soil legacy P but until now, there is a lack of estimates on the time such strategies should take. With a rainfall-runoff model, spatial information on the hydrological risk of the catchment of Lake Baldegg (Switzerland) was generated and combined with a soil test P (CO2-saturated water extraction) map. Based on these results, two potential soil target test P values (2.0 mg P (kg soil)−1 target-P 1 and 1.6 mg P (kg soil)−1 target-P 2) were set. A simple nonlinear mixed effects model was used to compare different balance and cropping scenarios to decrease soil test P. The relationship between P-balance (input - output) and soil test P was found to be exponential. The confidence interval for the predicted time necessary to reach target-P 2 after a cessation of P-fertilization on intensively managed grassland was 2 to 9 years depending on the initial soil test P. If fertilization is completely ceased, the predicted P-decline times were longer (8 to 32 years). The decline-time for the balance that is recommended for farmers in the catchment of a P-fertilization that covers 80% of the demand was predicted to be 11 to 47 years. The study emphasizes that P phytoextraction can be an effective and time and resource efficient mitigation strategy for catchments with high legacy P.