|Mayeux Jr, Herman|
Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 21, 1994
Publication Date: N/A
Interpretive Summary: Production of natural vegetation and extensively-managed agricultural crops often is limited by the availability of water and nitrogen. Plant growth then is regulated by uptake of these essential resources from soil and the amount of growth that can be realized per unit of water and nitrogen acquired (plant requirement). Plant requirements for water and nitrogen sometimes decrease and the production of plants limited by these resources increase when the concentration of carbon dioxide (CO2) in air is raised above the current atmospheric level. Little is known, however, of how of past changes in atmospheric CO2 concentration, including the 30% increase in the last two centuries and near 100% increase since the last ice age, affected plant requirements for water and nitrogen. We determined the amounts of water and nitrogen required by the annual grasses cheatgrass and wheat and the woody perennial mesquite at CO2 concentrations representative of atmospheric levels since the last ice age. Plants consistently needed less water as the CO2 concentration rose, but often required the same amount of nitrogen. Increasing CO2 thus reduced the amount of water relative to nitrogen that some plants need. These changes caused by rising CO2 may have altered water and nitrogen cycles in ecosystems and increased production of vegetation that was water- limited
Technical Abstract: Nitrogen and water use efficiencies in biomass production were determined for three C3 plant species, annual grasses Bromus tectorum L. and Triticum aestivum L. (two cultivars) and a woody perennial Prosopis glandulosa Torr. (alone and in mixtures with the C4 grass, Schizachyrium scoparium), grown across daytime gradients in carbon dioxide concentrations ([CO2]) that spanned glacial to present atmospheric levels (200 to 350 umol CO2 (mol air)** -1). 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. 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. Intrinsic WUE (net assimilation/stomatal conductance to water), calculated from stable carbon isotopes in plants, increased by about the same fraction as [CO2] in all species. Nitrogen use efficiency (biomass produced/plant N; NUE) rose at higher [CO2] only in well-watered B. tectorum and in P. glandulosa grown alone. 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.