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Responses of Agricultural Crops to Free-Air CO2 Enrichment
Volume 5, Number 31: 31 July 2002

There are a variety of experimental techniques that have been used to create higher-than-normal concentrations of atmospheric CO2 in order to study the impacts of CO2-enriched air on plant growth and development.  It is generally agreed that the most realistic of these techniques - in that it provides the least disturbance to the natural environment - is the Free-Air CO2 Enrichment or FACE approach.  This technique for maintaining an elevated atmospheric CO2 concentration utilizes computer-controlled vertical vent pipes arranged in circular arrays out-of-doors in the natural environment that are programmed to respond to real-time changes in wind speed and direction in such a way that they continuously release just the right amount of CO2-enriched air from the appropriate (upwind) pipes, so that the plants growing within the circular arrays are always supplied with air of the desired CO2 concentration.

The FACE technique, as currently employed, was developed primarily by engineers from Brookhaven National Laboratory in conjunction with scientists from the USDA's Agricultural Research Service and Tuskegee University (Kimball, 1992; Hendrey, 1993).  It was first used successfully on a cotton crop in 1989 at Maricopa, Arizona, in a study directed by Dr. Bruce Kimball of the U.S. Water Conservation Laboratory in Phoenix.  Since that time, a number of similar studies have been conducted at a variety of locations around the world; and there are currently about 30 active or planned sites where future FACE work will be conducted.  At this juncture of time, therefore, it is only fitting that a summary of all FACE work conducted to date on agricultural crops be presented; and it is both appropriate and gratifying that the senior author of that study is the scientist who was there at the beginning of it all: Dr. Bruce Kimball.

In most of the FACE experiments that have been conducted over the past dozen or so years, the degree of atmospheric CO2 enrichment above the background level has been something on the order of 190-200 ppm.  In the great bulk of all other CO2 enrichment studies that have been conducted, however, a 300-ppm enrichment of the air's CO2 content has typically been used.  Hence, to express the summary FACE results presented by Kimball et al. (2002) on a more readily comparable basis, we have multiplied them by the factor 300/200 = 1.5, since other reviews of the literature indicate that between these two enrichment levels, plant physiological responses to atmospheric CO2 enrichment are approximately linear (Idso and Idso, 1994).

So what did Kimball et al. learn from their massive review of the FACE literature?  To what degree were different plant physiological processes and properties altered by a 300 ppm increase in the air's CO2 concentration?

With respect to net photosynthesis, the rates of this process in upper-canopy leaves of the several C3 grasses studied were enhanced by an average of 46% under ample water and nitrogen, and by 44% when nitrogen was limiting to growth.  In the case of the C4 crop sorghum, however, the net photosynthetic enhancement at ample water and nitrogen was only 14%; but when water was limiting to growth, the CO2-induced stimulation rose to 34%.  And when net photosynthesis was measured throughout the entire canopy of a C3 wheat crop, the mean enhancement was 28%.

With respect to aboveground biomass, three C3 grasses (wheat, ryegrass and rice) experienced an average increase of 18% at ample water and nitrogen, 4% at low nitrogen, and 21% at low water.  C4 sorghum, however, experienced a mere 4% increase at ample water and nitrogen, but a 24% increase at low water.  Most surprising of all, perhaps, was potato (a C3 forb), which experienced a 32% decrease at ample water and nitrogen.  Clover (a C3 legume), on the other hand, experienced a 36% increase at ample water and nitrogen, as well as a 38% increase at low nitrogen.  Last of all, woody cotton and grape plants experienced an average 48% increase at ample water and nitrogen, and an average 39% increase at low water.

With respect to belowground biomass, wheat, ryegrass and rice experienced an average increase of 70% at ample water and nitrogen, 58% at low nitrogen, and 34% at low water.  Clover experienced a 38% increase at ample water and nitrogen, plus a 32% increase at low nitrogen.  Outdoing them all, however, was cotton, with a 96% increase at ample water and nitrogen.

With respect to agricultural yield - which represents the bottom line in terms of food and fiber production - ryegrass and wheat experienced an average increase of 18% at ample water and nitrogen, while wheat also experienced an increase of 10% at low nitrogen and 34% at low water.  Sorghum yield was unchanged at ample water and nitrogen; but at low water it rose by 38%.  And potato, in spite of its 32% decrease in aboveground growth, experienced a yield increase of 42% at ample water and nitrogen.

Continuing, clover yields increased by 36% at ample water and nitrogen, and by 38% at low nitrogen; while grape experienced a yield increase of 42% at ample water and nitrogen.  Last of all, cotton experienced an increase of 57% in boll (seed + lint) yield at ample water and nitrogen, plus 64% at low water; while it experienced an increase of 84% in lint yield alone at ample water and nitrogen, plus 78% at low water.

With respect to stomatal conductance - which is a measure of the ease with which water can escape from plant leaves and be lost to the air - wheat experienced a 51% decrease at ample water and nitrogen, plus a 66% decrease at low nitrogen.  Sorghum experienced a 56% decrease at ample water and nitrogen; while cotton and grapes experienced decreases of 22% at ample water and nitrogen.

In discussing these several observations, Kimball et al. note that "growth stimulations were as large or larger under water-stress compared to well-watered conditions."  They also note that "roots were generally stimulated more than shoots," and that "woody perennials had larger growth responses to elevated CO2, while at the same time their reductions in stomatal conductance were smaller."  Also, although "growth stimulations of non-legumes were reduced at low-soil nitrogen," they note that "elevated CO2 strongly stimulated the growth of the clover legume both at ample and under low nitrogen conditions."

All of the above observations are consistent with what has been observed in other types of CO2 enrichment experiments over the years, with one significant exception.  The CO2-induced decreases in stomatal conductance observed in the FACE studies are about 50% greater than those observed in prior non-FACE experiments, which suggests that the water use efficiency of these particular crops - and perhaps other plants as well - may be increased considerably more by the ongoing rise in the air's CO2 content (perhaps by as much as 50% more) than what had previously been thought likely.

In conclusion, we can safely say that the wealth of FACE data that has been obtained since 1989 has only served to strengthen our positive view of the historical and still-ongoing rise in the air's CO2 content.  Earth's biosphere, of which we are an integral part, has already benefited immensely from the 100-ppm increase in atmospheric CO2 concentration brought to us as an unanticipated consequence of the Industrial Revolution; and we and all of nature will benefit still more from increases yet to come.

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

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

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Kimball, B.A.  1992.  Cost comparisons among free-air CO2 enrichment, open-top chamber, and sunlit controlled-environment chamber methods of CO2 exposure.  Critical Reviews in Plant Sciences 11: 265-270.

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