To investigate how the level of microbial activity in grassland soils affects plant-microbial competition for different nitrogen (N) forms, we established microcosms consisting of a natural soil community and a seedling of one of two co-existing grass species, Anthoxanthum odoratum or Festuca rubra. We then stimulated the soil microbial community with glucose in half of the microcosms and followed the transfer of added inorganic (15NH 4 15 NO3) and organic (glycine-2- 13C-15N) N into microbial and plant biomass. We found that microbes captured significantly more 15N in organic than in inorganic form and that glucose addition increased microbial 15N capture from the inorganic source. Shoot and root biomass, total shoot N content and shoot and root 15N contents were significantly greater for A. odoratum than F. rubra, whereas F. rubra had higher shoot and root N concentrations. Where glucose was not added, A. odoratum had higher shoot 15N content with organic than with inorganic 15N addition, whereas where glucose was added, both species had higher shoot 15N content with inorganic than with organic 15N. Glucose addition had equally negative effects on shoot growth, total shoot N content, shoot and root N concentrations and shoot and root 15N content for both species. Both N forms produced significantly more shoot biomass and higher shoot N content than the water control, but the chemical form of N had no significant effect. Our findings suggest that plant species that are better in capturing nutrients from soil are not necessarily better in tolerating increasing microbial competition for nutrients. It also appears that intense microbial competition has more adverse effects on the uptake of organic than inorganic N by plants, which may potentially have significant implications for interspecific plant-plant competition for N in ecosystems where the importance of organic N is high and some of the plant species specialize in use of organic N. © 2006 Springer Science+Business Media B.V.