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We investigated the individual and interactive effects of moderately elevated CO 2 (ambient air + 100 ppm) and/or O 3 (40-50 ppb) on soil N cycling and microbial biomass N in a 3-year open-top chamber experiment conducted in meadow mesocosms. The results show that elevated O 3 decreased the concentrations of mineral N and NH 4 + -N in the mesocosm soil in the last growing season (2004). Total N, NO 3 --N, microbial biomass N, decomposition rate, potential nitrification and denitrification were not affected by elevated O 3 and/or CO 2 . It is thus concluded that the proposed future ambient O 3 and CO 2 levels, such as used in this experiment, may not induce major changes in the belowground N processes in N-poor northern European hay meadow ecosystems.

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