Rapid nutrient leaching to groundwater and surface water in clay soil areas

Nitrate teaching from agricultural soils has lead frequently to nitrate concentrations above the European Union drinking water standard of 11.3 mg l(-1) NO3-N. Most data originate from sandy soil areas. The objective of the present experiment was to investigate the mechanism and magnitude of nitrate leaching from grassland on a heavy clay soil. On an experimental field, nitrogen input and nitrate concentrations in groundwater and drain discharge were measured for two consecutive years. A bromide tracer was applied to the same experimental field to study solute transport mechanisms. It appeared that nitrate transport in the heavy clay soil studied occurred mainly via preferential flow through mesopores and macropores. This resulted in small mobile water volumes and high nitrate leaching rates. As a result, nitrate concentrations in the drain discharge were high, on the average some 70 mgl(-1)NO(3)-N, with maximum values of 136 mg l(-1)NO(3)-N. With an average N-input from fertilizer and slurry of 480 kg ha(-1) yr(-1) N, the average nitrate leaching was equal to 124 kg ha(-1) yr(-1) NO3-N. Computer simulation models for nitrogen transport and behaviour in soils, which compute water and solute transport on basis of the Darcy equation, cannot adequately predict nitrate leaching in heavy clay soils.