The objective of this study was to better identify the mechanisms by which elevated CO2 (c. 665 mumol mol(-1)) alters soil nitrifying and denitrifying enzyme activity (NEA and DEA), and the dynamics of plant and microbial N pools. We measured the effects of elevated CO2 on plant biomass and N, soil microbial 2 biomass N, soil,ammonium and nitrate concentrations, NEA and IDEA in monospecific grassland mesocosms (Holcus lanatus and Festuca rubra) grown for 15 months in reconstituted grassland soil. NEA strongly decreased at elevated CO2 in the Holcus mesocosms, but was not affected in Festuca systems. DEA was less sensitive to elevated CO2 than NEA. In Holcus mesocosms, microbial N showed an initial growth phase that appeared to limit plant N acquisition, but was only transient. CO2-induced changes in NEA and DEA were best explained by factors that could reduce soil [O-2] (increased soil water content and organic C). Elevated CO2 may increase microbial immobilisation in-highly disturbed systems, this effect weakening as the system gets closer to equilibrium.