Elevated CO₂ and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?

Area of Science:

• Plant Physiology
• Environmental Science
• Ecology

Background:

• Elevated atmospheric CO₂ often leads to reduced plant tissue nitrogen concentrations when compared at a common time.
• Plant growth rates accelerate under elevated CO₂, and nitrogen concentration naturally decreases with plant age.
• This age-related decline can confound interpretations of CO₂ effects on nitrogen concentration.

Purpose of the Study:

• To investigate if elevated CO₂ reduces plant nitrogen concentration independently of growth rate.
• To differentiate between CO₂-induced physiological changes and size-dependent dilution effects on plant nitrogen.

Main Methods:

• Two annual species, Abutilon theophrasti (C₃) and Amaranthus retroflexus (C₄), were grown in chambers at ambient (350 μmol·mol^-1) and elevated (700 μmol·mol^-1) CO₂.
• High and low fertilizer levels were applied.
• Plants were harvested every two days from germination to measure biomass and nitrogen concentration.

Main Results:

• Plants grown at elevated CO₂ had lower nitrogen concentrations and higher biomass when compared at the same age.
• When compared at the same size (biomass), tissue nitrogen concentrations did not differ between CO₂ levels.
• The rate of biomass accumulation relative to nitrogen increase was unaffected by CO₂ but influenced by nutrient availability.

Conclusions:

• Elevated CO₂ does not inherently reduce plant nitrogen concentration through altered nitrogen use efficiency.
• The observed decrease in nitrogen concentration under elevated CO₂ is a consequence of accelerated plant growth and size-dependent dilution.
• Nutrient availability, not CO₂, is a key factor in the efficiency of biomass accumulation per unit of nitrogen uptake.