Interspecific and spatial differences in nitrogen uptake in monocultures and two-species mixtures in north European grasslands

1. To study the potential for complementarity in nitrogen acquisition from different soil depths, we injected an isotope tracer ((15) NH4 Cl) at 5 and 20 cm depths in plant communities containing Achillea millefolium L. and Festuca ovina L. or Phleum pratense L. and Trifolium pratense L. in monocultures and two-species mixtures. 2. In monoculture, Festuca and Phleum took up tracer at 5 and 20 cm depths. In contrast, Achillea and Trifolium monocultures acquired the tracer mainly from 5 cm depth. In two-species mixtures, all four species took up tracer at 5 cm depth. 3. Achillea N acquisition from 20 cm depth increased in mixture with Festuca in comparison to that in monoculture; Festuca N acquisition from 20 cm depth decreased, although not significantly. Trifolium N acquisition remained unchanged when grown in mixture with Phleum . Phleum behaved like Festuca : its N acquisition from 20 cm depth in mixture was reduced in comparison to monoculture. 4. Our data on Festuca and Achillea support spatial partitioning in resource acquisition. This was not evident in Phleum and Trifolium mixture, potentially because Trifolium relied on N-2 fixation as N source. 5. These results demonstrate spatial variation among plant species and plant communities in their N acquisition in the field.