Nitrate leaching in a crop rotation from 1981 to 2000

Using 12 lysimeters nitrate leaching was determined for the following crop rotation: Potato – winter wheat – sugar beet – spring wheat – corn – spring wheat – rape – winter wheat – gras-clover mixture (green manure before potato, corn and sugar beet). Under the climatic conditions of Zurich-Reckenholz (1000 mm annual precipitation, 400 mm from October to March) the crop sequence was the most important factor for nitrate concentration in the leaching water at 2.5 m depth. Nitrate peaks, which probably originated from the mineralisation period between the two crops (July to September), were recorded regularly between the months of November and May depending on soil type and precipitation. Differences between two soil types, a well-drained stony cambisol (FAO) developed from a stony alluvium («Schotter») and a poorly drained fine textured cambisol from moraine deposits («Moräne»), are as follows: On «Schotter» the peaks of nitrate occurred two to four months earlier, were generally higher and lasted for less time than on «Moräne». We suppose that on «Schotter» preferential flow paths on the surfaces of big stones (estimated 25 % of volume) cause higher flow velocity than on «Moräne» where matrix flow should be predominant. When fertilization was reduced by 30% a reduction of 5 ppm in nitrate concentration resulted (mean of 1997 to 2000) while only potato showed a significant yield reduction.