CO2 EFFECTS ON MASS AND ENERGY EXCHANGE OF WHEAT UNDER DIFFERENT N FERTILIZATION: MODEL THEORY AND TESTING WITH A FREE AIR CO2 ENRICHMENT (FACE) EXPERIMENT

Authors: GRANT, R - UNIV OF ALBERTA CANADA; Kimball, Bruce; BROOKS, T - US WATER CONSERVATION LAB; Wall, Gerard - Gary; Pinter Jr, Paul; Hunsaker, Douglas - Doug; Adamsen, Floyd; La Morte, Robert; LEAVITT, S - UNIV OF ARIZONA; THOMPSON, T - UNIV OF ARIZONA

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2000
Publication Date: 5/15/2001
Citation: Grant, R.F., Kimball, B.A., Brooks, T.J., Wall, G.W., Pinter Jr, P.J., Hunsaker, D.J., Adamsen, F.J., La Morte, R.L., Leavitt, S.W., Thompson, T.L. 2001. Co2 effects on mass and energy exchange of wheat under different n fertilization: model theory and testing with a free air co2 enrichment (face) experiment. Plant Cell and Environment. 93(3):630-649.

Interpretive Summary: The CO2 concentration in the atmosphere is increasing and expected to double near the end of the next century. The elevated levels of CO2 affect plant photosynthesis and also cause a partial closure of the stomata in plant leaves through which the plant exchanges CO2 and water vapor with the atmosphere. The magnitude of both effects and the extent to which they change growth, yield, and water requirements of crops are likely to be influenced by other environmental factors such as soil nitrogen supply. In order to predict the effect of the elevated CO2 on future crop production and to aid in developing improved management strategies, crop growth simulation models are being developed. This paper reports a successful test of one such model called ecosys, comparing model predictions against the results of an experiment where open-field-grown wheat was exposed to elevated levels of CO2 using free- air CO2-enrichment (FACE) technology at ample and limited levels of soil nitrogen. Most model results were acceptably close to observed values, predicting for example, that at CO2 concentrations of about 550 ppm, such as expected near the middle of the next century, should cause wheat grain yield to increase about 18% at ample nitrogen but only 11% at low soil fertility. This work will benefit both future growers and consumers of wheat and wheat products.

Technical Abstract: Mass and energy exchange between terrestrial ecosystems and the atmosphere influence atmospheric thermodynamics and, hence, climate. Changes in mass and energy exchange under rising atmospheric CO2 concentration (CA) may be affected by the availability of N in many ecosystems. The effects of CA and N on exchange may be explained by hypotheses for coupling CO2 fixation to N uptake based on an assumed constant ratio of intercellular CO2 concentration to CA. These hypotheses were used in the ecosystem simulation model ecosys to simulate CA effects on mass and energy exchange under different N levels. Simulation results were tested with data from a Free Air CO2 Enrichment (FACE) experiment in which wheat was grown under 548 vs. 363 micro mol per mol CA and fertilized with 7 vs. 35 g N per m square. Both model and experimental results indicated that raising CA to 548 from 363 micro mol per mol reduced midday latent heat fluxes by ca. 25 W m square more, and raised canopy CO2 fluxes by ca. 5 micro mol per square m per second less, when wheat was fertilized with 7 than with 35 g N m square. At a seasonal time scale, raising CA to 548 from 363 micro mol per mol reduced simulated (measured) evapotranspiration of wheat by 4% (7%) when fertilized with 35 g N m square, and by 15% (19%) with 7 g N m square. Raising CA to 548 from 363 micro mol per mol increased simulated (measured) grain yield by 18% (15%) when fertilized with 35 g N m square, and by 11% (12%) with 7 g N m square. Changes with CA in mass and energy exchange used in climate change studies should, therefore, reflect the site-specific availability of N and other nutrients.