Determination of the effects of elevated CO2 and interacting environmental variables on agronomic crops.

Numerous CO2-enrichment studies in greenhouses and growth chambers have suggested that growth of most plants should increase about 30% on the average with a projected doubling of the atmospheric CO2 concentration. However, the applicability of such work to the growth of plants outdoors under less ideal conditions has been seriously questioned. The only approach that can produce an environment as representative of future fields as possible today is the free-air CO2-enrichment (FACE) approach.

Working collaboratively with personnel from Brookhaven National Laboratory and others, we were the first to conduct such FACE experiments in an open field (Hendrey, 1993). Here is a picture of a FACE ring. It consists of a 25-m-diameter toroidal plenum ring constructed from 12-in.-dia. pipe. The ring has 2.5-m-high vertical pipes with individual valves spaced about every 2 m around the periphery. Air enriched with CO₂ is blown into the rings, and it exits through tri-directional jets in the vertical pipes at elevations near the top of the crop canopy. Wind direction, wind speed, and CO₂ concentration are measured at the center of each ring.

A computer-control system uses the wind speed and CO₂ concentration information to adjust the CO₂flow rates to maintain the desired CO₂ concentration at the center of the FACE ring. The system uses the wind direction information to turn on only those pipes upwind of the plots, so that CO₂-enriched air flows across the plots, no matter which way the wind blows. When wind speeds are low (< 0.4 m/s) and direction is difficult to detect, the CO₂-enriched air is released from every other vertical pipe around the ring. The CO₂ flow rate is updated every second, and the choice of which vertical pipes to release from is updated every 4 seconds.

Here is an aerial photograph of a FACE experiment on sorghum. There are four replicate FACE rings, and there are also four Control rings at ambient CO₂ concentration. The center-to-center spacing of the rings is 90 m. This experiment was Arizona Maricopa Agricultural Center (MAC), Maricopa, . In this particular experiment, the left and right halves of the rings received either an ample or a limited supply of irrigation water.

The third picture shows a FACE ring with a sorghum crop. The relatively large size of the ring (compared to plant growth chambers) means that a large amount of high-CO₂-grown plant material can be produced, enough to support the research of many cooperating scientists. About 50 scientists from 25 research organizations in eight countries participated in FACE by water stress experiments on wheat.

The following FACE experiments were conducted at Maricopa, AZ.

1989 cotton at ample water

1990 and 1991 cotton at ample and limited supplies of water

1993 and 1994 wheat at ample and limited supplies of water

1996 and 1997 wheat at ample and limited supplies of nitrogen

1998 and 1999 sorghum at ample and limited supplies of water

More than a hundred papers have been published on various aspects of these experiments, including: leaf area, plant height, above- and below-ground biomass, apical and morphological development, canopy temperature, reflectance, chlorophyll, light use efficiency, energy balance, evapotranspiration, soil and plant elemental analyses, soil water content, sap flow, root biomass from soil cores, leaf and canopy photosynthesis, respiration, stomatal conductance, leaf water potential, carbohydrates, photosynthetic proteins, antioxidants, stomatal density and anatomy, digestibility, decomposition, grain quality, soil CO₂ fluxes, and changes in soil C storage from soil and plant C isotopes. In addition, more than a dozen papers have been written by wheat growth modelers who utilized our data for validation of their models.

It is beyond the scope of this report to present all results presented in the above-mentioned numerous papers. Moreover, FACE experiments have also been conducted on ryegrass and white clover in Switzerland, on grape and potato in , and on rice in Japan. Data from these plus our Maricopa FACE experiments were assembled and analyzed by Kimball et al. (2002). They looked at many physiological, growth, water relations, and soil processes aspects. The results for agricultural yield are shown in the figure to the right. Focusing on our cotton, its yields were increased about 40% with CO₂concentrations elevated to 550 ppm (or about 200 ppm above today’s ambient) at both limited and ample supplies of water. Wheat was less responsive, averaging a 13% yield increase at ample water and nitrogen, whereas the increase was 23% when water was limited and about 8% when nitrogen was limited ( Kimball et al., 2002). As expected for a C4 crop, sorghum had very little growth response to elevated CO₂ at ample water, whereas when water was limited, grain yields increased an average 25% due to FACE (Ottman et al., 2001; Kimball et al., 2002).

There was no significant increase in water use of our cotton, as shown in the graph to the right (Kimball et al., 2002). Water use of wheat at ample nitrogen declined about 5%, whereas the decrease was about 20% at limited nitrogen based on energy balance measurements ( Kimball et al., 1999, 2002). Water use at ample supply declined about 13% based on energy balance measurements (Triggs et al., 2004).
Currently, we are also collaborating on a SoyFACE experiment on soybean and corn at Urbana, IL (http://www.soyface.uiuc.edu/research.htm). Our measurements are of canopy microclimate, energy balance, evapotranspiration, and plant water releations.

References:

Hendrey, G.R. (Ed.) 1993. Free-air Carbon Dioxide Enrichment for Plant Research in the Field. C.K. Smoley, Boca Raton, FL, USA.

IPCC. 2001. [Houghton. J.T., Ding. Y/, Griggs. D.J., Noguer, M., Van der Linden, P.J., Dai, X., Maskell, K., Johnson, C.A. (Eds)] Climate Change 2001: The Scientific Basis, Contribution from Working Group I to the Third Assessment Report, Inter-governmental Panel for Climate Change. Cambridge University Press, Cambridge, UK.

Kimball, B.A., R.L. LaMorte, P.J. Pinter Jr., G.W. Wall, D.J. Hunsaker, F.J. Adamsen, S.W. Leavitt, T.L. Thompson, A.D. Matthias, and T.J. Brooks. 1999. Free-air CO₂ enrichment (FACE) and soil nitrogen effects on energy balance and evapotranspiration of wheat. Water Resources Research 35(4): 1179-1190.

Kimball, B.A., K. Kobayashi, and M. Bindi. 2002. Responses of agricultural crops to free-air CO₂ enrichment. Advances in Agronomy 77:293-368.

Ottman, M.J., B.A. Kimball, P.J. Pinter Jr., G.W. Wall, R.L. Vanderlip, S.W. Leavitt, R.L. LaMorte, A.D. Matthias, and T.J. Brooks. 2001. Elevated CO₂ increases sorghum biomass under drought conditions. New Phytologist 150(2):261-273.

Triggs, J.M., B.A. Kimball, P.J. Pinter Jr., G.W. Wall, M.M. Conley, T.J. Brooks, R.L. LaMorte, N.R. Adam, M.J. Ottman, A.D. Matthias, S.W. Leavitt, and R.S. Cerveny. 2004. Free-air carbon dioxide enrichment effects on energy balance and evapotranspiration of sorghum. Agricultural and Forest Meteorology 124:63-79.