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Nitrogen Use Efficiency -- Summary
Long-term exposure to elevated atmospheric CO2 concentrations often, but not always, elicits photosynthetic acclimation or down regulation in plants, which is typically accompanied by reduced amounts of rubisco and/or other photosynthetic proteins that are typically present in excess amounts in ambient air.  As a consequence of this latter phenomenon, foliar nitrogen concentrations often decrease with atmospheric CO2 enrichment, as nitrogen is mobilized out of leaves and into other areas of the plant to increase its availability for enhancing sink development or stimulating other nutrient-limited processes.

In reviewing the literature in this area, one quickly notices that in spite of the fact that photosynthetic acclimation has occurred, CO2-enriched plants nearly always display rates of photosynthesis that are greater than those of control plants exposed to ambient air.  Consequently, photosynthetic nitrogen-use efficiency, i.e., the amount of carbon converted into sugars during the photosynthetic process per unit of leaf nitrogen, often increases dramatically in CO2-enriched plants.

In the study of Davey et al. (1999), for example, CO2-induced reductions in foliar nitrogen contents and concomitant increases in photosynthetic rates led to photosynthetic nitrogen-use efficiencies in the CO2-enriched (to 700 ppm CO2) grass Agrostis capillaris that were 27 and 62% greater than those observed in control plants grown at 360 ppm CO2 under moderate and low soil nutrient conditions, respectively.  Similarly, elevated CO2 enhanced photosynthetic nitrogen-use efficiencies in Trifolium repens by 66 and 190% under moderate and low soil nutrient conditions, respectively, and in Lolium perenne by 50%, regardless of soil nutrient status.  Other researchers have found comparable CO2-induced enhancements of photosynthetic nitrogen-use efficiency in wheat (Osborne et al., 1998) and in Leucadendron species (Midgley et al., 1999).

In some cases, researchers report nitrogen-use efficiency in terms of the amount of biomass produced per unit of plant nitrogen.  Niklaus et al. (1998), for example, reported that intact swards of CO2-enriched calcareous grasslands grown at 600 ppm CO2 attained total biomass values that were 25% greater than those of control swards exposed to ambient air while extracting the same amount of nitrogen from the soil as ambiently-grown swards.  Similar results have been reported for strawberry by Deng and Woodward (1998), who noted that the growth nitrogen-use efficiencies of plants grown at 560 ppm CO2 were 23 and 17% greater than those of ambiently-grown plants simultaneously subjected to high and low soil nitrogen availability, respectively.

In conclusion, the scientific literature indicates that as the air's CO2 content continues to rise, earth's plants will likely respond by reducing the amount of nitrogen invested in rubisco and other photosynthetic proteins, while still maintaining enhanced rates of photosynthesis, which consequently should increase their photosynthetic nitrogen-use efficiencies.  In addition, enhanced rates of photosynthetic carbon uptake invariably lead to greater biomass production, which often occurs without increasing soil nitrogen uptake and thus enhances growth nitrogen-use efficiency as well.  Hence, as overall plant nitrogen-use efficiency increases with the ongoing rise in the atmosphere's CO2 concentration, it is likely that plants will grow ever better on soils containing less-than-optimal levels of this important soil nutrient.

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.

Deng, X. and Woodward, F.I.  1998.  The growth and yield responses of Fragaria ananassa to elevated CO2 and N supply.  Annals of Botany 81: 67-71.

Midgley, G.F., Wand, S.J.E. and Pammenter, N.W.  1999.  Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment.  Journal of Experimental Botany 50: 533-542.

Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C.  1998.  Nutrient relations in calcareous grassland under elevated CO2Oecologia 116: 67-75.

Osborne, C.P., LaRoche, J., Garcia, R.L., Kimball, B.A., Wall, G.W., Pinter, P.J., Jr., LaMorte, R.L., Hendrey, G.R. and Long, S.P.  1998.  Does leaf position within a canopy affect acclimation of photosynthesis to elevated CO2Plant Physiology 117: 1037-1045.