Grass-legume leys combine multiple agronomic benefits, several of which are associated with symbiotic nitrogen (N) fixation. However, the best combinations of legume abundance and N fertilisation to achieve high productivity at low nitrate leaching are not yet established. Nitrate leaching risk of pure grass swards (Grs), grass-legume mixtures (Mix) and pure legume swards (Leg) at a fertiliser level of either 50, 150 or 450 kg N ha−1 yr−1 (N50, N150 and N450) was studied during two key periods: the intact ley for forage production and after tilling for subsequent winter wheat in crop rotation. The risk of nitrate leaching was determined from (i) monitoring of the nitrate concentration in the soil solution (NCSS) and (ii) the soil mineral N (SMN) content. Furthermore, the soil surface N balance was assessed by measuring N input from fertiliser and symbiosis, and N output with harvested biomass. During the period of intact plant cover, soil surface N balance of Grs swards was strongly negative at N50 and N150 with negligible NCSS and SMN. At N450, the positive N balance of Grs swards resulted in a high leaching risk after three years of cultivation (NCSS ≤ 13 mg NO3-N L−1 and SMN ≤ 20 kg N ha−1). For Mix swards, the N balance was close to zero at N50 and N150, which led to similarly low NCSS and SMN. At N450, the N balance of Mix swards was beyond zero, resulting in significantly increased NCSS and SMN (≤ 30 mg NO3-N L−1 and ≤ 35 kg N ha−1). For Leg swards, the N balance was far beyond zero at all fertilisation levels, resulting in a leaching risk even at N50 (≤ 8 mg NO3-N L−1 and ≤ 23 kg N ha−1). During the period following tilling the leys, SMN was similar or lower for Mix compared to Grs swards (≤ 43 vs. ≤ 62 kg N ha−1 at N50 and N150, 72 vs. 68 kg N ha−1 at N450), while it was significantly higher for Leg swards (SMN ≤ 95 kg N ha−1). We conclude that grass-legume mixtures combine high yields, low fertiliser requirements, and low nitrate leaching better than either pure grass or pure legume swards. This holds true both during the period with intact plant cover and after tillage for the subsequent crop.
