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Influence of elevated CO₂ and mycorrhizae on nitrogen acquisition: contrasting responses in Pinus taeda and Liquidambar styraciflua

John V. H. Constable (1), Hormoz Bassirirad (1, 2), John Lussenhop (1) and Ayalsew Zerihun (1)

1. Department of Biological Sciences, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA / 2. Author to whom correspondence should be addressed (hormoz@uic.edu) / Received December 22, 1999

Summary

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.

Keywords: ammonium uptake, loblolly pine, nitrate uptake, sweetgum, tree.

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ISSN 0829-318X Copyright © 2002–2008 Heron Publishing