Nitrogen (N) availability is a major factor limiting plant production in many terrestrial ecosystems and is a key regulator of plant response to elevated CO₂. Plant N status is a function of both soil N availability and plant N uptake and assimilation capacity. As a rate-limiting step in nitrate assimilation, the reduction of nitrate is an important component of plant physiological response to elevated CO₂ and terrestrial carbon sequestration. We examined the effects of elevated CO₂ and N availability on the activity of nitrate reductase, the enzyme catalyzing the reduction of nitrate to nitrite, in two temperate forests —a closed canopy sweetgum (Liquidambar styraciflua) plantation in Tennessee (ORNL) and a loblolly pine (Pinus taeda) stand in North Carolina (Duke).

Results/Conclusions
Both CO₂ and N had species specific impacts on nitrate reductase activity (NaR). Elevated CO₂ and N-fertilization decreased mass-based foliar NaR in P. taeda (p<0.05), but there were no treatment effects on L. styraciflua NaR at ORNL or Duke. NaR in 1-yr P. taeda needles was significantly greater than in 0-yr year needles in all treatments (p<0.05). P. taeda NaR was negatively correlated with bio-available molybdenum concentrations in soils, suggesting that CO₂ and N-mediated changes in soil nutrient status are altering soil-plant N-dynamics. The variation in response between species may reflect different N assimilation strategies and suggests that potential changes in plant N status under elevated CO₂ may alter plant community N dynamics.