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An understanding of root system capacity to acquire nitrogen (N) is critical in assessing the long-term growth impact of rising atmospheric CO 2  concentration ([CO 2 ]) on trees and forest ecosystems. We examined the effects of mycorrhizal inoculation and elevated [CO 2 ] on root ammonium (NH 4  + ) and nitrate (NO 3  − ) 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 &amp; 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 [CO 2 ] treatments. After 6 months of treatment, root systems of both species exhibited a greater uptake capacity for NH 4  +  than for NO 3  − . In both species, mycorrhizal inoculation significantly increased uptake capacity for NO 3  − , but not for NH 4  + . In sweetgum, the mycorrhizal effect on NO 3  −  and NH 4  +  uptake capacity depended on growth [CO 2 ]. Similarly, in loblolly pine, the mycorrhizal effect on uptake capacity depended on growth [CO 2 ], but the effect on uptake capacity did not. Mycorrhizal inoculation significantly enhanced root nitrate reductase activity (NRA) in both species, but elevated [CO 2 ] increased root NRA only in sweetgum. Leaf NRA in sweetgum did not change significantly with mycorrhizal inoculation, but increased in response to [CO 2 ]. 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 [CO 2 ], reliable prediction is possible only when information about other root system adjustments (e.g., biomass allocation to fine roots) is simultaneously considered.

Influence of elevated CO 2 and mycorrhizae on nitrogen acquisition: contrasting responses in Pinus taeda and Liquidambar styraciflua