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The importance of facilitative processes due to the presence of nitrogen-fixing legumes in temperate grasslands is a contentious issue in biodiversity experiments. Despite a multitude of studies of fertilization effects of legumes on associated nonfixers in agricultural systems, we know little about the dynamics in more diverse systems. We hypothesised that the identity of target plant species (phytometers) and the diversity of neighbouring plant species would affect the magnitude of such positive species interactions. We therefore sampled aboveground tissues of phytometers planted into all plots of a grassland biodiversity-ecosystem functioning experiment and analysed their N concentrations, delta15N values and biomasses. The four phytometer species (Festuca pratensis, Plantago lanceolata, Knautia arvensis and Trifolium pratensis) each belonged to one of the four plant functional groups used in the experiment and allowed the effects of diversity on N dynamics in individual species to be assessed. We found significantly lower delta15N values and higher N concentrations and N contents (amount of N per plant) in phytometer species growing with legumes, indicating a facilitative role for legumes in these grassland ecosystems. Our data suggest that the main driving force behind these facilitative interactions in plots containing legumes was reduced competition for soil nitrate ("nitrate sparing"), with apparent N transfer playing a secondary role. Interestingly, species richness (and to a lesser extent functional group number) significantly decreased delta15N values, N concentrations and N content irrespective of any legume effect. Possible mechanisms behind this effect, such as increased N mineralisation and nitrate uptake in more diverse plots, now need further investigation. The magnitude of the positive interactions depended on the identity of the phytometer species. Evidence for increased N uptake in communities containing legumes was found in all three nonlegume phytometer species, with a subsequent strong increase in biomass in the grass F. pratensis across all diversity levels, and a lesser biomass gain in P. lanceolata and K. arvensis. In contrast, the legume phytometer species T. pratense was negatively affected when other legumes were present in their host communities across all diversity levels.

In plant communities, invasion resistance may increase with diversity because empty niche space decreases simultaneously. However, it is not clear if this only applies to exotic species or also to native species arriving at a site with few other native species during community assembly. We tested the latter by transplanting four native species into experimental grassland communities varying in species richness form 1–16 (−60) species. In addition, we tested the hypothesis that invasion is less successful if the invading species belongs to a functional group that is already present in the community. The test invaders included a grass species ( Festuca pratensis, FP), a short ( Plantago lanceolata, PL) and a tall herb species ( Knautia arvensis, KA), and a legume species ( Trifolium pratense, TP). The same four functional groups also occurred alone or in all possible combinations in the different experimental communities. The overall performance of the transplants was negatively related to the logarithm of the species richness of host communities. Plant biomass declined by 58%, 90%, 84% and 62% in FP, PL, KA and TP, respectively, from monocultures to 16-species mixtures, indicating lower invasiveness of the two herbs than of the grass and the legume. Resident grasses showed a strong negative effect on the performance of all test invaders, whereas resident small and tall herbs had neutral, and resident legumes had positive effects. The case of the legumes indicates that contributions to invasion resistance need not parallel invasiveness. Communities containing resident species of only one functional group were most inhibitive to transplants of the same functional group. These results indicate that invasion resistance of experimental plant communities is related to the degree of niche overlap between resident species and invaders. This niche overlap can be high due to generally low amounts of empty niche space in species-rich resident communities or due to the occurrence of the same functional group as the one of the invader in the resident community. Stronger within- than between-functional-group invasion resistance may be the key mechanism underlying diversity effects on invasion resistance in grassland and other ecosystems at large. [ABSTRACT FROM AUTHOR]