The response of terrestrial ecosystems to elevated atmospheric CO2 is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO2 Enrichment (FACE) for six years. A N-15 labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO2 on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a N-15 tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH4+ and from organic-N to NO3- and analysed the rate of dissimilatory NO3- reduction to NH4+ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO2. At the same time the gross rate of NH4+ immobilization increased by 20%, while NH4+ oxidation to NO3- was reduced by 25% under elevated CO2. The NO3- dynamics under elevated CO2 were characterized by a 52% increase in NO3- immobilization and a 141% increase in the DNRA rate, while NO3- production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH4+ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH4+ under elevated CO2. The advantage of such a shift is that NH4+ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO2. (C) 2009 Elsevier Ltd. All rights reserved.