Impact of Swiss agricultural policies on nitrate leaching from arable land

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.