N₂ fixation, N transfer, and yield in grassland communities including a deep-rooted legume or non-legume species

Background and aims Legumes are important components of grassland mixtures due to their ability to sustain high yields with moderate nitrogen inputs. This study investigates nitrogen relationships in mixtures of Trifolium pratense and grasses into which a deep-rooted forb was included, and particularly whether these realtionships differ when the forb is a legume or a non-legume species. Methods A field experiment in which mixtures of T. pratense, Phleum pratense, Lolium perenne, and Medicago sativa or Cichorium intybus, and monocropped stands of all species was established in 2007 and harvested in 2008 and 2009. The experiment received a total input of 100 kgha⁻¹ N yearly. Yield and botanical composition were determined in seven harvests. Species were analysed for ¹⁵N abundance, and N₂ fixation and N transfer were calculated. Soil samples were analysed twice for inorganic N. Results Non-legumes benefitted from the presence of legumes in terms of N concentration, and the yield of mixtures exceeded that of monocropped non-legumes but not monocropped legumes. The mixture containing M sativa did not yield more DM or N than did the mixture containing C. intybus. A total of 17.08 kgN ha⁻¹ was transferred from T. pratense to the non-legumes in the mixture in which it was the sole legume species. Conclusions It is concluded that there was a synergy effect in species mixtures, but the effect did not differ between the two deep-rooted species.