N2 fixation and performance of 12 legume species in a 6-year grassland biodiversity experiment

Highly variable effects of legumes have been observed in biodiversity experiments, but little is known about plant diversity effects on N-2 fixation of legume species. We used the N-15 natural abundance method in a non-fertilized regularly mown 6-year biodiversity experiment (Jena Experiment) to quantify N-2 fixation of 12 legume species. The proportion of legume N derived from the atmosphere (%N-dfa) differed significantly among legume species. %N-dfa values were lower in 2004 after setting-up the experiment (73 +/- 20) than in the later years (2006: 80 +/- 16; 2008: 78 +/- 12). Increasing species richness had positive effects on %N-dfa in 2004 and 2006, but not in 2008. High biomass production of legumes in 2004 and 2006 declined to lower levels in 2008. In 2006, legume positioning within the canopy best explained variation in %N-dfa values indicating a lower reliance of tall legumes on N-2 fixation. In 2008, larger %N-dfa values of legumes were related to lower leaf P concentrations suggesting that the availability of phosphorus limited growth of legumes. In summary, diversity effects on N-2 fixation depend on legume species identity, their ability to compete for soil nutrients and light and may vary temporally in response to changing resource availability.