Mediterranean forage legumes grown alone or in mixture with annual ryegrass: biomass production, N2 fixation, and indices of intercrop efficiency

Aims: To evaluate the productivity and N2 fixation of a range of Mediterranean forage legume species as well as their ability to be grown in mixture with a forage grass, and to verify whether N transfer occurs from the legume to the non-legume component of the mixtures and, if so, to what extent this process is affected by legume species. Methods: Seven legume species (Hedysarum coronarium L., Medicago scutellata L., Trifolium resupinatum L., Trifolium squarrosum L., Trigonella foenum-graecum L., Vicia sativa L., Vicia villosa Roth) were grown alone or in mixture with annual ryegrass (Lolium multiflorum L.). Biomass and N yields and biological N2 fixation (15N dilution technique) were measured. N transfer from legume to the non-legume component was also assessed. The efficiency of the intercrops was evaluated using the land equivalent ratio (LER), aggressivity index, and competitive ratio. Results: Differences were observed among the monocropped legumes for biomass yield, N2 fixation, and ability to utilize inorganic soil N. Moreover, the proportion of legume species to the total biomass yield of the intercrop varied from 30 % (T. resupinatum) to 69 % (T. foenum-graecum). All intercrops showed an advantage over monocrops in terms of biomass and N yields (LER and NLER values always >1). No N transfer occurred from legume to ryegrass in any of the mixtures. Conclusions: The large differences observed among the studied legumes must be taken into account when trying to develop cropping systems with more efficient N use. Moreover, as all legume\u2013ryegrass intercrops used natural resources more efficiently than pure crops, intercropping is a relevant cropping strategy for sustainable agricultural systems in Mediterranean environments