Highly productive forage legume stands show no positive biodiversity effect on yield and N2-fixation

Summary

Background and aims While N2-fixation in diversified grasslands including forage legumes and non-legumes has been widely studied, N2-fixation in swards containing only forage legumes remains unclear. In this study, we investigated N2-fixation in pure stands and mixtures of three forage legumes. Methodology N2-fixation, dry matter (DM) and nitrogen (N) yields were quantified in a field experiment for red clover (Trifolium pratense L.), white clover (Trifolium repens L.) and lucerne (Medicago sativa L.) pure stands and mixtures using the isotope dilution method. Results All three forage legume species derived most (around 85%) of their N from atmospheric N2-fixation (%Ndfa). However, no positive effect of species diversity was found in any of the mixtures. Species composition of the forage legume mixtures affected the amount of N from N2-fixation by affecting DM production and N accumulation of the species, where the seasonal amount of N2-fixation ranged from 370 to 500 kg N ha−1; which was highest in the presence of red clover. Conclusions We found thatmixturesof thethree forage legumes were highly productive, but did not show positive advantages compared to the red clover pure stands in terms of DM, N yield and %Ndfa.