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Acclimation to Elevated CO2 Increases Photosynthetic Nitrogen-Use Efficiency in Grassland Species
Davey, P.A., Parsons, A.J., Atkinson, L., Wadge, K. and Long, S.P.  1999.  Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers.  Functional Ecology 13: 21-28.

What was done
Three economically important and common grassland species of the United Kingdom were grown in open-top chambers fumigated with atmospheric CO2 concentrations of 360 and 700 ppm for two full years.  In addition, half of the plants received monthly fertilization to establish a "moderate" nutrient treatment, while the other half received no fertilization to establish a "low" nutrient regime.  Thus, the objective of this study was to determine whether or not long-term CO2 enrichment affects photosynthesis under nutrient-limiting conditions.

What was learned
After two years of atmospheric CO2 enrichment, photosynthetic acclimation had occurred in Agrostis capillaris, as indicated by an average decrease in rubisco activity of 26%, regardless of nutrient regime.  However, CO2-enriched Agrostis capillaris still maintained photosynthetic rates that were 12 and 38% greater than those measured in ambiently-growing plants subjected to moderate and low nutrient conditions, respectively.  In addition, because elevated CO2 decreased leaf nitrogen content on an area basis, the photosynthetic nitrogen-use efficiency -- the amount of carbon fixed per unit of leaf nitrogen -- increased by 27 and 62% under moderate and low nutrient conditions, respectively.

Similarly, photosynthetic acclimation was also apparent in CO2-enriched Trifolium repens (a nitrogen-fixing species), which displayed an average decrease in rubisco activity of 28% across both soil fertility treatments.  Likewise, elevated CO2 increased photosynthetic rates in this species by 25 and 74% at moderate and low nutrient treatments, respectively.  However, because Trifolium repens did not exhibit any reductions in leaf nitrogen in response to atmospheric CO2 enrichment, its photosynthetic nitrogen-use efficiency was relatively large.  In fact, it was 66 and 190% greater under moderate and low nutrient conditions, respectively, when CO2-enriched than it was under ambient CO2 conditions.

Elevated CO2 also decreased rubisco activity in Lolium perenne (which is highly responsive to fertilization) by approximately 28% in both nutrient treatments.  However, elevated CO2 only stimulated photosynthesis in the moderate, but not the low, nutrient regime (30% increase).  Nonetheless, because elevated CO2 decreased leaf nitrogen by 17 and 29% at moderate and low soil fertility, respectively, photosynthetic nitrogen-use efficiency was enhanced under both nutrient treatments by nearly 50%.

What it means
As the CO2 content of the air rises, it is likely that these economically important grassland species of the United Kingdom will exhibit increased photosynthetic rates, regardless of soil fertility and photosynthetic acclimation.  Such increases in photosynthesis and redistribution of important resources --like nitrogen-- within tissues can lead to increased biomass production and help maintain the presence of these plants in nutrient-limited ecosystems.  Thus, this paper suggests that rising atmospheric CO2 levels will indeed stimulate photosynthesis under low soil nutrient conditions.  As eloquently stated by the authors, "rather than limiting the photosynthetic response to elevated CO2, limiting nutrient conditions could lead to the most significant stimulation in carbon acquisition."

Reviewed 15 October 1999