An understanding of root system capacity to acquire nitrogen (N) is critical in assessing the long-term growth impact of rising atmospheric CO2 concentration ([CO2]) on trees and forest ecosystems. We examined the effects of mycorrhizal inoculation and elevated [CO2] on root ammonium (NH4+) and nitrate (NO3−) uptake capacity in sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus taeda L.). Mycorrhizal treatments included inoculation of seedlings with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith in sweetgum and the ectomycorrhizal (EM) fungus Laccaria bicolor (Maire) Orton in loblolly pine. These plants were then equally divided between ambient and elevated [CO2] treatments. After 6 months of treatment, root systems of both species exhibited a greater uptake capacity for NH4+ than for NO3−. In both species, mycorrhizal inoculation significantly increased uptake capacity for NO3−, but not for NH4+. In sweetgum, the mycorrhizal effect on NO3− and NH4+ uptake capacity depended on growth [CO2]. Similarly, in loblolly pine, the mycorrhizal effect on uptake capacity depended on growth [CO2], but the effect on uptake capacity did not. Mycorrhizal inoculation significantly enhanced root nitrate reductase activity (NRA) in both species, but elevated [CO2] increased root NRA only in sweetgum. Leaf NRA in sweetgum did not change significantly with mycorrhizal inoculation, but increased in response to [CO2]. Leaf NRA in loblolly pine was unaffected by either treatment. The results indicate that the mycorrhizal effect on specific root N uptake in these species depends on both the form of inorganic N and the mycorrhizal type. However, our data show that in addressing N status of plants under high [CO2], reliable prediction is possible only when information about other root system adjustments (e.g., biomass allocation to fine roots) is simultaneously considered.