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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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The optimal allocation theory predicts that growth is allocated between the shoot and the roots so that the uptake of the most limiting resource is increased. Allocation is dynamic due to resource depletion, interaction with competitors, and the allometry of growth. We assessed the effects of intraand inter-specific competition on growth and resource allocation of the meadow species Ranunculus acris and Agrostis capillaris, grown in environments with high (?) or low (-) availability of light (L) and nutrients (N). We took samples twice a week over the 7 weeks experiment, to follow the changes in root-toshoot ratios in plants of different sizes, and carried out a larger scale harvest at the end of the experiment. Of all the tested factors, availability of nutrients had the largest effect on the growth rate and shoot-to-root allocation in both species, although both competition and light had significant effects as well. The highest root-to-shoot ratios were measured from the L?Ntreatment, and the lowest from the L-N? treatment, as predicted by the optimal allocation theory.Competition changed resource allocation, but not always toward acquiring the resource that is most limiting to growth. We thus conclude that the greatest variation in shoot-to-root allocation was due to the resource availability and the effects of competition were small, probably due to low density of plants in the experiment.