Nitrate leaching after winter oilseed rape and peas has not been studied at the most northern limits of oilseed rape cultivation where winters vary between being mild, with continuous drainage, and cold, with periods of frozen soil. Here, we studied the effect of N fertilisation to oilseed rape, catch crops after oilseed rape and peas and dired drilling of winter wheat after oilseed rape on N leaching in south-west Sweden. Nitrate leaching was determined in two field experiments, dated 2004-2006 and 2005-2007, respectively, on a sandy loam. Our results show that under oilseed rape nitrate leaching was low, at 16-23 kg N ha(-1), in a mild winter with drainage from October to March. In the subsequent mild winter nitrate leaching under wheat was higher, amounting to 35-94 kg N ha(-1). Nitrate leaching levels were similar, 32-58 kg N ha(-1), for all crops in a cold winter with a long-lasting snow cover and main drainage occurring after snowmelt in March and April. Application of fertiliser N to oilseed rape at the optimum N rate, rather than 50 kg N ha(-1) above optimum, reduced leaching in a following winter wheat crop by 25 and 27 kg N ha(-1) in a cold and a mild winter, respectively. Spring undersowing of perennial ryegrass as a catch crop reduced leaching by 12 kg N ha(-1) after optimally fertilised oilseed rape in a mild winter, despite only growing until mid-September when winter wheat was sown. An undersown catch crop of peas, then grown until November, reduced leaching by 15 kg N ha(-1). Direct drilling of winter wheat after oilseed rape had no effect. These findings show that there are risks of enhanced leaching in early spring after a cold winter with a snow cover and superficially frozen soil. Optimising the spring N rate for oilseed rape was the most effective measure to decrease leaching in both mild and cold winters, and this effect was improved by an undersown catch crop in a mild winter.
