NITROGEN AND WATER REQUIREMENTS OF C3 PLANTS GROWN AT GLACIAL TO PRESENT CARBON-DIOXIDE CONCENTRATIONS

1. Nitrogen- and water-use efficiencies in biomass production were determined for three C3 plant species at carbon dioxide concentrations ([CO2]) that spanned glacial to present atmospheric levels [200-350 mumol CO2 (mol air)-1]. The species were annual grasses Bromus tectorum and Triticum aestivum (two cultivars) and a woody perennial Prosopis glandulosa (alone and in mixtures with the C4 grass, Schizachyrium scoparium). 2. Changes in nitrogen- and water-use efficiencies were used to investigate effects of increasing [CO2] on the relative requirements of C3 plants for these frequently limiting resources. 3. Water-use efficiency (biomass produced/evapotranspiration; WUE) increased at higher [CO2] in all species but relative responses to [CO2] varied among species, cultivars and watering regimes. 4. Intrinsic WUE (net assimilation/stomatal conductance to water), calculated from stable carbon isotopes in plants, increased by about the same relative amount as did [CO2] in all species. 5. Nitrogen-use efficiency (biomass produced/plant N; NUE) rose at higher [CO2] only in well-watered B. tectorum and in P. glandulosa grown alone. 6. The more consistent increase in WUE than NUE in these species at higher [CO2] implies that rising [CO2] may have reduced the amount of water relative to nitrogen that some C3 plants require and thereby altered the composition and function of terrestrial ecosystems.