Impact of Swiss agricultural policies on nitrate leaching from arable land

Abstract - From 1993 onwards, various policy measures aiming at reducing nitrogen (N) losses were introduced in Swiss agriculture. We studied the impact of these measures on nitrate leaching under arable land in the Fehraltorf catchment near Zurich. Because of the well-drained soils, the shallow water table and the intensive agricultural activity, the groundwater in this catchment is highly vulnerable to nitrate pollution. N fluxes and transformations on the field scale were simulated using the deterministic model LEACHMN. Following the calibration of the model with a dataset from a long-term lysimeter experiment, simulations were run for the period 1998-2003. As far as possible, field-specific management data from farmers' control sheets were used as input. Average nitrate leaching in the period 1998-2003 was estimated to amount to 39 kg N ha^-1 year^-1. The conditions before the introduction of the policy measures were simulated in three scenarios with a higher use of both mineral N fertiliser and animal manure and 50% less or no catch crops. We found that lowering mineral N fertiliser and manure application by about 25% led to an average decrease in nitrate leaching of 10 kg N ha^-1 year^-1. Doubling the catch crop area yielded a further decrease of 6 kg N ha^-1 year^-1 on average. Altogether, the implementation of reduced N fertiliser inputs and increased use of catch crops proved to be effective and led to a mean reduction in N leaching of 29% (16 kg ha^-1 year^-1) for the arable land in the Fehraltorf catchment.