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Gas exchange and carbon metabolism in two Prosopis species (Fabaceae) from semiarid habitats: effects of elevated CO₂, N supply, and N source¹

²Departamento de Biodiversidad y Biología Experimental, F.C.E. N., Universidad de Buenos Aires, Ciudad Universitaria, 1428 Capital Federal, Argentina; ³Department of Crop Science, P.O. Box 7620, North Carolina State University, Raleigh, North Carolina 27695-7620 USA; ⁴Division of Environmental Science and Policy, Nicholas School of the Environment & Earth Science, and Department of Biology, Phytotron Building, Box 90340, Duke University, Durham, North Carolina 27708-0340 USA

ABSTRACT

Predicting future plant and ecosystem responses to elevated CO₂ also requires an understanding of the role of other factors, especially soil nitrogen. This is particularly challenging for global aridlands where total N and the relative amounts of nitrate and ammonia vary both spatially and seasonally. We measured gas exchange and primary and secondary C metabolites in seedlings of two dominant aridland shrub species (Prosopis flexuosa [S America] and P. glandulosa [N America]) grown at ambient (350 ppm) or elevated (650 ppm) CO₂ and nitrogen at two levels (low [0.8 mM] and high [8.0 mM]) and at either 1 : 1 or 3 : 1 nitrate to ammonia. Whereas elevated CO₂ increased assimilation rate, water use efficiency, and primary carbon metabolites in both species, these increases were strongly contingent upon nitrogen availability. Elevated CO₂ did not increase secondary metabolites (i.e., phenolics). For these important aridland species, the effects of elevated CO₂ are strongly influenced by nitrogen availability and to a lesser extent by the relative amounts of nitrate and ammonia supplied, which underscores the importance of both the amount and chemical composition of soil nitrogen in mediating the potential responses of seedling growth and establishment of aridland plants under future CO₂-enriched atmospheres.

Key Words: ammonia-N • global change • nitrate-N • phenolic compounds • Prosopis flexuosa • Prosopis glandulosa • rangeland