Grassland-based livestock systems play an important role as a provider of food in the more marginal areas in Nordic regions. Such production systems are commonly dominated by grass monocultures receiving relatively high levels of fertilizer. Here, we investigated whether grass-legume mixtures can improve the productivity and robustness of cultivated grassland under extreme growing conditions over a period of ive years. Monocultures and mixtures of Phleum pratense L., Festuca pratensis Huds., Trifolium pratense L., and Trifolium repens L., difering in species’ relative abundances, were established in Iceland in spring 2008 and maintained under three N levels (20, 70 and 220 kg N ha-1 year-1) for ive consecutive harvest years. We observed signiicant positive diversity efects in all individual years and averaged across the ive years. Across years, the four-species equi-proportional mixture was 71% (N20) and 51% (N70) more productive than the average of monocultures, and the highest yielding mixture was 36% (N20) and 39% (N70) more productive than the highest yielding monoculture. Importantly, diversity efects were evident also at low relative abundances of either species group grasses or legumes in the mixture. Our results demonstrate that northern grassland production can beneit from grass-legume systems, that these beneits can sustain ive years and that they were robust over years, N fertilizer treatments and a wide range of relative abundances of legumes in the mixture.