0 Müller, C Rütting, T Abbasi, M K Laughlin, R J Kammann, C Clough, T J Sherlock, R R Kattge, J Jäger, H J Watson, C J Stevens, R J 2009 Soil Biology and Biochemistry 41 1996-2001 Elevated CO2, 15N tracing, Model, Progressive N limitation, Gross N transformation, Temperate grassland 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 15N 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 15N 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 NH4þ 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 NH4þ 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.