title: Elevated CO2and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?
abstract: Plants often respond to elevated atmospheric CO2levels with reduced tissue nitrogen concentrations relative to ambient CO2-grown plants when comparisons are made at a common time. Another common response to enriched CO2atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO2-grown plants made at a common time may not demonstrate CO2-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO2environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C3photosynthetic pathway) and Amaranthus retroflexus (C4photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO2levels of 350 μmol·mol-1or 700 μmol·mol-1and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO2-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO2-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO2level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO2availability, but was altered by nutrient availability. These results indicate that a CO2-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.
authors.affiliation: Biological Research Laboratories, Syracuse University, 13244, Syracuse, NY, USA.|The Biological Laboratories, Harvard University, 16 Divinity Avenue, 02138, Cambridge, MA, USA.
authors.forename: J S
authors.initials: JS
authors.lastname: Coleman
authors.affiliation: The Biological Laboratories, Harvard University, 16 Divinity Avenue, 02138, Cambridge, MA, USA.
authors.forename: K D M
authors.initials: KD
authors.lastname: McConnaughay
authors.affiliation: The Biological Laboratories, Harvard University, 16 Divinity Avenue, 02138, Cambridge, MA, USA.
authors.forename: F A
authors.initials: FA
authors.lastname: Bazzaz
keywords: Allometry; C/N Balance; Global change; Nitrogen-use efficiency; Physiological adjustments