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Effects of Elevated CO2 on Nitrogen Balance in Wheat
Smart, D.R., Ritchie, K., Bloom, A.J. and Bugbee, B.B.  1998.  Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations.  Plant, Cell and Environment 21: 753-763.

What was done
Wheat was grown from seed for 23 days in controlled environments subjected to ambient (360 ppm) and elevated (1000 ppm) concentrations of atmospheric CO2 and two concentrations of soil NO3- (nitrate) to study the effects of elevated CO2 on nitrate absorption and assimilation.

What was learned
Wheat plants grown under atmospheric CO2 enrichment lost 30% less transpirational water than control plants grown at ambient CO2.  In addition, atmospheric CO2 enrichment increased the average biomass of plants by approximately 15%, regardless of soil nitrogen content.  Moreover, elevated CO2 preferentially shifted carbon allocation belowground, as indicated by an 11% increase in the root:shoot ratio of CO2-enriched plants, suggesting that root growth in wheat may be carbon-limited at ambient CO2 concentration.

Although soil nitrogen content had no effect on daily root consumption rates of nitrate, the larger CO2-enriched plants displayed significantly greater rates of soil nitrate extraction than did the ambiently-grown plants.  However, on a per unit biomass basis, the total amount of nitrogen absorbed was not significantly affected by atmospheric CO2 enrichment.  Once the nitrate was absorbed, elevated CO2 impacted its partitioning, with CO2-enriched plants displaying significantly greater amounts of nitrate nitrogen and significantly reduced amounts of organic nitrogen relative to control plants, irrespective of soil nitrogen treatment.  Thus, it is possible that atmospheric CO2 enrichment reduced the ability of CO2-enriched plants to assimilate nitrate nitrogen, thereby accounting for their lower organic nitrogen concentrations, which are commonly observed under conditions of photosynthetic acclimation to elevated CO2.

What it means
As the CO2 content of the air continues to rise, it is likely that wheat plants will reduce their stomatal conductance, thus reducing the amount of transpirational water lost to the atmosphere.  In addition, plants growing in soils of varying nitrogen fertility should significantly increase their biomass, both above- and belowground, with preferentially greater growth occurring belowground.  With larger root systems, wheat plants will likely be able to consume greater amounts of nitrogen from soils to support increased development and growth, even if photosynthetic acclimation occurs to some degree.

Reviewed 15 August 1999