This study examines the effect of elevated CO2 on short-term partitioning of inorganic N between a grass and soil micro-organisms. 15N-labelled NH4 + was injected in the soil of mesocosms of Holcus lanatus (L.) that had been grown for more than 15 months at ambient or elevated CO2 in reconstituted grassland soil. After 48 h, the percentage recovery of added 15N was increased in soil microbial biomass N at elevated CO2, was unchanged in total plant N and was decreased in soil extractable N. However, plant N content and microbial biomass N were not significantly affected by elevated CO2. These results and literature data from plant–microbial 15N partitioning experiments at elevated CO2 suggest that the mechanisms controlling the effects of CO2 on short- vs. long-term N uptake and turnover differ. In particular, short-term immobilisation of added N by soil micro-organisms at elevated CO2 does not appear to lead to long-term increases in N in soil microbial biomass. In addition, the increased soil microbial C:N ratios that we observed at elevated CO2 suggest that long-term exposure to CO2 alters either the functioning or structure of these microbial communities.