FIELD MONITORING OF NITRATE LEACHING AND WATER-FLOW IN A STRUCTURED CLAY SOIL

Bypass flow and nitrate leaching from a field at the experimental farm The Kandelaar farm in Eastern Flevoland in the Netherlands was measured in drain outflow for a period of 4 years. Automated monitoring equipment registered precipitation, groundwater level and drain discharge continuously and was complemented with manually collected data on soil water potentials and soil mineral nitrogen. Bypass flow occurred although the air-filled porosity in the soil was more than adequate to accept the rain. In a soil without macropores this would, therefore, not have led to any bypass flow. During bypass flow events in summer, 30% of the precipitation was directly discharged via the drains. In winter periods this amount corresponded to 84% of the precipitation. Bypass flow induced nitrate leaching from the fine aggregated top soil towards the groundwater. In periods with an active growing crop, nitrate leaching was negligible. A catch crop grown directly after slurry application in late summer seemed to have a reducing effect on nitrate leaching during the entire winter season. In the subsoil, large amounts of inaccessible nitrogen are present in the soil, within large soil peds. Soil structure strongly influences bypass flow and can be used as a management tool for farmers to decrease nitrate leaching.