We investigated effects of plant species richness in experimental grassland plots on annual above- and belowground biomass production estimated from repeated harvests and ingrowth cores, respectively. Aboveground and total biomass production increased with increasing plant species richness while belowground production remained constant. Root to shoot biomass production ratios (R/S) in mixtures were lower than expected from monoculture performance of the species present in the mixtures, showing that interactions among species led to reduced biomass partitioning to belowground organs. This change in partitioning to belowground organs was not confined to mixtures with legumes, but also measured in mixtures without legumes, and correlated with aboveground overyielding in mixtures. It is suggested that species-rich communities invest less in belowground biomass than do monocultures to extract soil resources, thus leading to increased investment into aboveground organs and overyielding.