Grass-legume leys combine multiple agronomic benefits, several of which are associated with symbiotic di-nitrogen (N2) fixation. However, whether significant symbiotic N inputs could lead to increased nitrate leaching is still debated. In a field experiment, we compared pure grass swards (G), grass-legume mixtures (M) and pure legume swards (L) at a fertiliser level of either 50, 150 or 450 kg N ha–1 year–1 (N50, N150 and N450). The leys were frequently mown for four years before being tilled to cultivate winter wheat. The risk of nitrate leaching was determined from monitoring soil mineral N and nitrate concentration in the soil solution. Furthermore, the soil surface N balance was calculated by summing up N applied as fertiliser and N derived from symbiosis (total N input) and N in the harvested biomass (N output). During the period of intact plant cover, an increased risk for nitrate leaching was only observed for G- and M-swards fertilised at N450, and L swards at all three N levels. Despite their large N input from symbiosis, no nitrate leaching risk was revealed for M swards. After tilling, the nitrate leaching risk strongly increased until December; importantly, it was not elevated for M- compared to G-swards.
