Effecs of including forbs on N2-fixation and N yield in red clover-ryegrass mixtures

Summary

Background: Legume biological nitrogen (N2)-fixation is stimulated by neighboring non-fixing species, but studies of legume N2-fixation in temporary grasslands including non-leguminous forage herb (forb) species are rare.
Methodology: We investigated N2-fixation and N yield in a range of species mixtures consisting of three forb species  chicory (Cichorium intybus L.), ribwort plantain (Plantago lanceolata L.) and caraway (Carum carvi L.)  mixed into a traditional perennial ryegrass (Lolium perenne L.) and red clover(Trifolium pratense L.) mixture at two fertilisation levels.
Results: The percentage of red clover N derived from the atmosphere (%Ndfa) was higher in mixtures than in pure stand, but did not increase with inclusion of forbs. Red clover in all the mixtures derived over 90% of its N from fixation even when fertilised with 216 kg total N ha-1. Forbs affected the amount of N2-fixation by affecting the red clover proportion in the harvested biomass.
Conclusions: We conclude that forbs can be included in temporary grasslands with no negative effect on total herbage production and percentage of legume N2-fixation. However, mixtures should not include a high seeding proportion of chicory for a balance between legume and non-legume biomass proportions.