Simulated effects of gypsum amendment on phosphorus losses from agricultural soils

Our objective was to incorporate the effects of gypsum on phosphorus (P) losses into the field-scale ICECREAM model and to simulate the treatment of 93 ha of clayey soil with gypsum in a catchment located in southern Finland. In addition to the gypsum effects, a macropore flow description was added to ICECREAM. First, a sensitivity analysis was performed for the new macropore parameters, retrieved from a literature survey. After this, the model was calibrated for a reference period by setting of the macropore parameters to correspond to the P losses observed in the catchment experiment. Next, the effect of gypsum was added to the model in line with laboratory and field experiments that suggested decreased P losses and changed hydraulic properties of the soil. Finally, the modified model was verified for four seasons after the gypsum amendment in the catchment experiment. The model was able to simulate the P losses in three out of the four seasons. According to the simulations, gypsum reduced total P losses by 44%. Although the effect of gypsum on P and erosion is a complex process, our relatively simple modifications to the ICECREAM model described the impact with reasonable accuracy. However, to increase confidence in the performance of the model, it should be tested under other environmental conditions.