Strong mixture effects among four species in fertilized agricultural grassland led to persistent and consistent transgressive overyielding

Increasing plant species richness often increases biomass production in nutrient-poor semi-natural grasslands. If such positive diversity-productivity effects also apply to nutrient-rich agricultural grasslands, mixtures could improve resource-use efficiency in the vast area used for forage production. We therefore quantified the diversity-productivity effects in nutrient-rich agricultural grasslands using four-species grass-legume mixtures. The sown overall density and species proportions of Lolium perenne, Dactylis glomerata, Trifolium pratense and Trifolium repens were varied in a 3-year field experiment to investigate the effects of species richness (1, 2, 4 species) and species proportion (0, 3, 10, 25, 40, 50, 70, 90, 100% sowing proportion) on productivity under a nitrogen fertilization of 50, 150 or 450 kg N ha(-1) year(-1). The four-species mixtures reached up to twice the yield of the average of the four species' monocultures (overyielding up to 106%), predominantly due to combining grass and legume species. Mixtures were up to 57% more productive than the most productive monoculture (transgressive overyielding). Both these diversity-productivity effects appeared across a broad range of species proportions and persisted at the two lower levels of N fertilization for 3 years. Mixtures fertilized with 50 kg N ha(-1) year(-1) produced yields comparable to grass monocultures fertilized with 450 kg N ha(-1) year(-1), if the legume proportion was about 50 to 70%. Diversity-productivity effects were reduced at the highest level of N fertilization, where they virtually disappeared in the third year. Increased N fertilization also accelerated the observed general trend towards D. glomerata dominated and legume-poor swards. Synthesis and applications. Diversity-productivity effects led to consistent transgressive overyielding in intensively managed grasslands, suggesting a highly increased resource-use efficiency in mixtures. Performance better than monocultures can be achieved with grass-legume mixtures that have a low number of species, across a wide range of species proportions and in nutrient-rich conditions. Processes such as niche complementarity and positive interspecific interactions leading to diversity effects proved to be highly relevant and widely applicable for intensive forage production. Such diversity-productivity effects could allow reduced inputs of N fertilizer without loss of productivity in different grassland production systems.