Light availability is improved for legume species grown in moderately N-fertilized mixtures with non-legume species

Empirical evidence indicates a positive relationship between grassland phytodiversity and yield. One cause may be species’complementary use of available resources. The aim of this study was to investigate possible complementarities between grassland species with differing spatial arrangements of leaves. Mixtures of Trifolium pratense L., Phleum pratense L., Lolium perenne L., and Cichorium intybus L. or Medicago sativa L. and pure stands of all species were established in 2007 at Svalöv, Sweden, in a field experiment receiving a total input of 100 kg N ha−1. Community height, light transmission, yield, and species composition as well as species’ 813C signatures and N concentrations were measured on four mowing occasions in 2009. Species’ 813C signatures are directly affected by carbon assimilation and stomatal conductivity for water, and indirectly by light, nitrogen and water availability as well as community composition. Light transmission through the sward was greatest in pure stand non-legumes; mixed communities intercepted more light than these, albeit not generally more than pure legumes. Non-legume species had more depleted 813C signatures when grown in mixtures than in pure stands, but the opposite was true for legumes. The 813C signatures generally became enriched with increases in light transmission (grasses and legumes), but not with increases in N concentration (grasses). Community composition affected the 813C signatures of all species except C. intybus. Our results suggest that mixing species of contrasting leaf morphologies and biomass distribution contributed to (i) increased light captureby mixtures over pure stand non-legumes, and (ii) better light availability in mixed than in pure stand legumes