Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO₂ and O₃

Increases in atmospheric CO 2 and tropospheric O 3 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 CO 2 –O 3 enrichment experiment, plant growth has been stimulated by elevated CO 2 resulting in greater substrate input to soil; elevated O 3 has counteracted this effect. We hypothesized that rates of soil N cycling would be enhanced by greater plant productivity under elevated CO 2 , and that CO 2 effects would be dampened by O 3 . We found that elevated CO 2 did not alter gross N transformation rates. Elevated O 3 significantly reduced gross N mineralization and microbial biomass N, and effects were consistent among species. We also observed significant interactions between CO 2 and O 3 : (i) gross N mineralization was greater under elevated CO 2 (1.0 mg N kg −1 day −1 ) than in the presence of both CO 2 and O 3 (0.5 mg N kg −1 day −1 ) and (ii) gross NH 4 + immobilization was also greater under elevated CO 2 (0.8 mg N kg −1 day −1 ) than under CO 2 plus O 3 (0.4 mg N kg −1 day −1 ). We used a laboratory 15 N tracer method to quantify transfer of inorganic N to organic pools. Elevated CO 2 led to greater recovery of NH 4 + ‐ 15 N in microbial biomass and corresponding lower recovery in the extractable NO 3 − pool. Elevated CO 2 resulted in a substantial increase in NO 3 − ‐ 15 N recovery in soil organic matter. We observed no O 3 main effect and no CO 2 by O 3 interaction effect on 15 N 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 CO 2 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.