Deep N acquisition, overyielding and vertical niche differentiation in hemiboreal cultivated grass-clover mixtures

Abstract Aims To explore the contribution of vertical root niche differentiation to recovery of deep NH 4 + by species in cultivated grass-clover mixtures, and to mixtures’ overyielding. Methods The uptake of a deep-placed (42 cm) slow-release 15 NH 4 + label was tracked in the herbage of two grass-clover mixtures during one growing season and the subsequent spring (3 + 1 harvests) and compared with uptake in monocultures. Results Mixtures overyielded in biomass (+ 11%), total N (+ 24%), and deep 15 NH 4 + recovery (+ 17%). The latter was most pronounced in the autumn (+ 47% to + 62%). Diversity effects on deep 15 N recovery were a combined result of changes in growth vigor and 15 N recovery per biomass. We found evidence of vertical niche differentiation: Schedonorus arundinaceus , Schedonorus pratensis and Lolium perenne displayed positive diversity effects on deep 15 N uptake per biomass combined with increased total N concentration; not so in Phleum pratense and Poa pratensis . Though the diversity effect on deep 15 N per biomass was small in N-fixing Trifolium pratense , due to its high yields this translated into a large negative diversity effect on deep N recovery. Enhanced deep N uptake was contingent on enhanced growth vigor. Conclusions Improved N nutrition in grasses due to clovers did not prevent overall positive diversity effects on deep N recovery. This is potentially beneficial for reducing N leaching. The grass-clover mixtures better combined recovery of deep-placed 15 NH 4 + with yield performance and high protein content than any single species. Diversity effects on deep N uptake were species-specific and could not be predicted by performance in monoculture.