Summary 1.In plant communities, a better use of nitrogen for growth and access to additional nitrogen by N 2 fixing legumes have been suggested as mechanisms that contribute to the positive relationship between species richness and above-ground biomass production. We studied above-ground biomass production, and plant and soil nitrogen in a biodiversity experiment using a pool of nine potentially dominant grassland species (five grasses: Alopecurus pratensis , Arrhenatherum elatius , Dactylis glomerata , Phleum pratense , Poa trivialis ; two legumes: Trifolium pratense , T. repens ; two non-legume herbs: Anthriscus sylvestris , Geranium pratense ) in the second and third year after establishment. 2. Total above-ground biomass and nitrogen pools increased with species richness in the second year. In the third year, the positive effect of species richness on above-ground biomass was less pronounced but still significant, while nitrogen pools did not increase with species richness. Above-ground nitrogen pools and biomass production were significantly lower in the third year. 3. Above-ground nitrogen pools of mixtures were higher than expected from monocultures in 94% of cases and increased with the species richness of mixtures in the second year. In the third year, only 55% of the mixtures had higher nitrogen yields than expected. A significant increase in the amount of biomass produced per gram nitrogen, a decrease of soil nitrate pools and the inability of mixtures to use soil nitrate more completely than expected from monocultures indicated lower nitrogen availability in the third year. Complementary resource use was not solely dependent on the occurrence of N 2 fixing legumes in mixtures, but also played a role in mixtures without legumes. 4. Synthesis . Our study shows that biodiversity effects through complementary nitrogen use can occur among potentially dominant grassland species. The degree of resource partitioning depends on growing conditions, with stronger complementarity effects under nutrient-rich conditions.

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 15 N 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.

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