Increases in atmospheric CO2 and tropospheric O3 may affect forest N cycling by altering plant litter production and the availability of substrates for microbial metabolism. Three years following the establishment of our free-air CO2–O3 enrichment experiment, plant growth has been stimulated by elevated CO2 resulting in greater substrate input to soil; elevated O3 has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO2, and that CO2 effects would be dampened by O3. We found that elevated CO2 did not alter gross N transformation rates. Elevated O3 significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO2 and O3: (i) gross N mineralization was greater under elevated CO2 (1.0 mg N kg−1 day−1) than in the presence of both CO2 and O3 (0.5 mg N kg−1 day−1) and (ii) gross NH4+ immobilization was also greater under elevated CO2 (0.8 mg N kg−1 day−1) than under CO2 plus O3 (0.4 mg N kg−1 day−1). We used a laboratory 15N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO2 led to greater recovery of NH4+-15N in microbial biomass and corresponding lower recovery in the extractable NO3− pool. Elevated CO2 resulted in a substantial increase in NO3−-15N recovery in soil organic matter. We observed no O3 main effect and no CO2 by O3 interaction effect on 15N recovery in any soil pool. All of the above responses were most pronounced beneath Betula papyrifera and Populus tremuloides, which have grown more rapidly than Acer saccharum. Although elevated CO2 has increased plant productivity, the resulting increase in plant litter production has yet to overcome the influence of the pre-existing pool of soil organic matter on soil microbial activity and rates of N cycling. Ozone reduces plant litter inputs and also appears to affect the composition of plant litter in a way that reduces microbial biomass and activity.
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