Leaching of nitrogen from a 3-yr grain crop rotation on a clay soil

Crop N uptake, soil mineral N and N leaching were measured during 5 yr in a field experiment with separately tile-drained plots on a clay soil in southwest Sweden. The main objective was to quantify the potential risks of N leaching associated with a 3-yr grain cropping sequence, which consisted of spring oat, winter wheat and winter wheat. In treatment A the soil was left fallow after winter wheat harvest, mouldboard ploughed in mid-November and sown with spring oats in April. Spring oat was followed by treatment B, with tine cultivation as the primary tillage operation before winter wheat was sown in September. In treatment C, the soil was mouldboard ploughed in September before sowing of winter wheat. Drainage water was sampled on a flow-proportional basis and analyzed for total and NO(3)-N. Soil samples were taken for the 0-0.9 m depth four times per year for determination of the mineral N content of the profile. Harvested crops were measured and analyzed for N. Above-ground plant material of winter wheat and weeds was sampled in late autumn. Mean annual N leaching amounted to 8.5 kg total N/ha, which corresponded to 7% of the measured N exports associated with harvest and leaching. Not all leachable N was lost during the main drainage period from September to May and there was an accumulation of mineral N in the soil during winter in all years. There were also indications of gaseous N losses from the soil during autumn. The results showed that moderate deviations (-15 to +50 kg/ha) from the optimum N application for winter wheat affected N leaching. It was therefore concluded that development of methods for appropriate adjustment of fertiliser levels to predicted crop yields and potential N mineralization is crucial to reduce the risk of N leaching from this type of clay soils. Timing of autumn ploughing proved to be of minor importance for the risk of N leaching in this cropping sequence.