Submitted to: Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2001
Publication Date: 1/1/2002
Citation: Fiscus, E.L., Miller, J.E., Booker, F.L., Heagle, A.S., Reid, C.D. 2002. The impact of ozone and other limitations on the crop productivity response to carbon dioxide. Technology 8:181-192. Interpretive Summary: Global atmospheric CO2 levels are increasing and studies indicate that the productivity of current cropping systems would benefit from this increase (the "CO2 fertilization" effect). Evidence is also building that this potential productivity benefit may not be realized or that the benefit will be smaller than previously thought. In a series of experiments conducted over a period of 6 years using cotton, rice, soybean and wheat we have demonstrated that the crop yield response to doubled atmospheric CO2 concentrations was inconsistent. Responses, in the absence of damaging levels of the air pollutant O3 ranged from small yield losses to yield increases up to about 25%. Although we consistently observed increased shoot mass and leafiness in elevated CO2 treatments, this increase was not consistently turned into increased seed yield. Elevated CO2 did, however, moderate the yield loss caused by pollutant O3, mainly by reducing entry of O3 into leaves. Relief from O3 damage, and consequent higher yields, would appear to be a direct stimulation of productivity by CO2 if there was no recognition that O3 stress occurred during the experiment. The same misperception might arise in the case of unrecognized or unmonitored water or nutrient stress conditions. Experiments with rice and cotton suggest that productivity may be limited by resources other than CO2, most likely nutrients and/or light. Perhaps increased growth before flowering more rapidly depleted the available nutrients, leaving little to sustain increased seed production. Regardless of the cause of the seemingly erratic yield response to elevated CO2 in clean air, the effect of background levels of air pollutant and other stresses seem to exaggerate the yield response to elevated CO2 in the atmosphere.
Technical Abstract: Studies indicate that increasing atmospheric CO2 concentrations might enhance the productivity of cropping systems. It seems this benefit may not be realized or that its potential size will be smaller than previously thought. In a series of experiments conducted in open-top field chambers over a period of 6 years using cotton, rice, soybean and wheat we demonstrated that the crop yield response to doubled CO2 concentration was highly dependent on the co-occurring level of pollutant O3. Yield responses to elevated CO2 ranged from losses of 26% to increases of 39% in charcoal filtered air with averages of 5%, -5%, 9% and -1% for cotton, rice, soybean and wheat, respectively. However, plants grown under O3 stress showed yield increases in response to CO2 of up to 141%, with respective averages for the four crops of 90%, 14%, 59% and 38%. Although we usually observed increased vegetative biomass in elevated CO2 treatments in clean air, the additional biomass, for reasons not yet clear, did not always result in increased economic yield. Elevated CO2 concentrations did, however, ameliorate the average 27% yield suppression caused by pollutant O3, primarily through reducing leaf conductance and limiting the O3 flux into leaves. The amelioration of O3 damage and consequent higher yields would appear to be a direct stimulation of productivity by CO2 if there was no knowledge of extant O3 pollution during the experiment. The same misperception might arise in the case of unrecognized water or nutrient stress. We conclude that the effect of background levels of air pollutants and other environmental stresses may exaggerate the yield response to elevated CO2.