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

Legume biological nitrogen (N-2)-fixation is stimulated by neighbouring non-fixing species, but studies of legume N-2-fixation in temporary grasslands including non-leguminous forage herb (forb) species are rare. We investigated N-2-fixation, N yield, and total herbage production 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 red clover (Trifolium pratense L.) and perennial ryegrass (Lolium perenne L.) mixture at two fertilisation levels. The percentage of red clover N derived from the atmosphere (%Ndfa) was higher in mixtures containing non-legumes than in pure stand but, did not increase with inclusion of forbs. On a whole-seasonal basis, red clover in mixtures derived 90% or more of its N from fixation even when fertilised with 216 kg total N ha(-1). Forbs, in particular chicory, reduced the amount of N-2-fixation and total N yields by affecting the red clover proportion in the harvested biomass. Generally, inclusion of forbs in red clover-ryegrass mixtures had no negative effect on total herbage production and percentage of legume N-2-fixation. However, to maintain a high total N and N-2-fixation yields, mixtures should not include a high seeding proportion of chicory.