Effect of CO₂ enrichment and nitrogen availability on resource acquisition and resource allocation in a grass, Bromus...

The effects of CO₂ enrichment on the growth, biomass partitioning, photosynthetic rates, and leaf nitrogen concentration of a grass, Bromus mollis (C₃), were investigated at a favorable and a low level of nitrogen availability. Despite increases in root: shoot ratios, leaf nitrogen concentrations were decreased under CO₂ enrichment at both nitrogen levels. For the low-nitrogen treatment, this resulted in lower photosynthetic rates measured at 650 μl/l for the CO₂-enriched plants, compared to photosynthetic rates measured at 350 μl/l for the non-enriched plants. At higher nitrogen availability, photosynthetic rates of plants grown and measured at 650 μl/l were greater than photosynthetic rates of the non-enriched plants measured at 350 μl/l. Water use efficiency, however, was increased in enriched plants at both nitrogen levels. CO₂ enrichment stimulated vegetative growth at both high and low nitrogen during most of the vegetative growth phase but, at the end of the experiment, total biomass of the high and low CO₂ treatments did not differ for plants grown at low nitrogen availability. While not statistically significant, CO₂ tended to stimulate seed production at high nitrogen and to decrease it at low nitrogen.