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

An understanding of root system capacity to acquire nitrogen (N) is critica l in assessing the long-term growth impact of rising atmospheric CO2 concen tration ([CO2]) on trees and forest ecosystems. We examined the effects of mycorrhizal inoculation and elevated [CO2] on root ammonium (NH4+) and nitr ate (NO3-) uptake capacity in sweetgum (Liquidambar styraciflua L.) and lob lolly 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 bi color (Maire) Orton in loblolly pine. These plants were then equally divide d between ambient and elevated [CO2] treatments. After 6 months of treatmen t, root systems of both species exhibited a greater uptake capacity for NH4 + than for NO3- In both species, mycorrhizal inoculation significantly incr eased uptake capacity for NO3-, but not for NH4+. In sweetgum, the mycorrhi zal effect on NO3- and NH4+ uptake capacity depended on growth [CO2]. Simil arly, in loblolly pine, the mycorrhizal effect on NO3- uptake capacity depe nded on growth [CO2], but the effect on NK: uptake capacity did not. Mycorr hizal inoculation significantly enhanced root nitrate reductase activity (N RA) in both species, but elevated [CO2] increased root NRA only in sweetgum . Leaf NRA in sweetgum did not change significantly with mycorrhizal inocul ation, but increased in response to [CO2]. Leaf NRA in loblolly pine was un affected by either treatment. The results indicate that the mycorrhizal eff ect 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 addres sing N status of plants under high [CO2], reliable prediction is possible o nly when information about other root system adjustments (e.g., biomass all ocation to fine roots) is simultaneously considered.