The large number of experiments in controlled environments has allowed yield dose response curves to be calculated for the major C3 food crops, soybean, wheat and rice. In Fig. 7.2, the ratio of yield at elevated [CO2] relative to ambient [CO2] was calculated from all available studies of soybean, wheat and rice grown to maturity at elevated [CO2] in controlled environments and open-top chambers (for original references, see Ainsworth et al. 2002 for soybean, Amthor 2001 for wheat, Ainsworth 2008 for rice). After averaging all studies within 100 ppm intervals, a non-rectangular hyperbola was fit to the data independently for each crop (as in Long et al. 2006; Fig. 7.2). The response of C3 crop yield to [CO2] is approximately hyperbolic, increasing linearly at sub-ambient, and saturating at approximately 1,000 ppm (Fig. 7.2). This theoretical response of crop yield to [CO2] is expected based on the response of photosynthetic carbon gain to elevated [CO2] (Allen et al. 1987).
Although an increase in C3 crop yield to elevated [CO2] is supported by controlled environment, OTC and FACE studies, the magnitude of the change in yield has been the subject of ongoing debate in the literature (Long et al. 2006; Tubiello et al. 2007a, b; Ainsworth et al. 2008b). Superimposed upon each yield response curve in Fig. 7.2 is the average yield response to elevated [CO2] from the FACE experiments (open symbols). The stimulation of yield at elevated [CO2] (~550 ppm) observed in FACE experiments is approximately half the stimulation predicted from the controlled environment studies (Fig. 7.2). Rice, wheat and soybean yields were increased by 12, 13 and 14% respectively by growth at elevated [CO2] in FACE (Long et al. 2006), compared to an approximate 30% increase for those crops at 550 ppm predicted by the hyperbolic yield response curves.
A limitation in this comparison is that FACE experiments have primarily used a single elevated [CO2], close to 550 ppm, the concentration anticipated for the middle of this century. FACE experiments have also been conducted over a narrower range of ambient [CO2] compared to controlled environment studies. Therefore, a more direct comparison of yield results from controlled environments and FACE was taken by limiting the comparison of FACE experiments and chamber studies to those with similar ambient [CO2] and similar elevated [CO2] (Ainsworth 2008; Ainsworth et al. 2008b). This more direct comparison showed a wider range of responses of C3 crops to elevated [CO2] in controlled environment studies, but confirmed the result that the stimulation in harvestable yield at elevated [CO2] in FACE experiments is approximately half of the stimulation in controlled environments (Ainsworth et al. 2008b).
A number of C3 crops other than staple cereals have been grown at elevated [CO2] in FACE experiments, including potato (Solanum tuberosum), barley (Hordeum vulgare), sugar beet (Beta vulgaris) and oilseed rape (Brassica napus).
600 800 Growth [CO2]
Fig. 7.2 The effects of elevated [CO2] on soybean, wheat and rice yield (adapted from Long et al. 2006; Ainsworth 2008). Data are yields at elevated [CO2] relative to yield at ambient [CO2] for crops grown in enclosures (solid symbols) and FACE (open symbols). Error bars represent 90% confidence intervals around the means for the FACE studies. The solid lines are the least squares fit for the nonrectangular hyperbolic response of yield to growth [CO2] from the enclosure studies of soybean (r2=0.98), wheat (r2 = 0.88) and rice (r2 = 0.96)
Two cultivars of potato were grown at elevated [CO2] in Central Italy (Miglietta et al. 1998; Bindi et al. 1999). In the first experiment, the Primura cultivar was exposed to three elevated [CO2]: 460, 560 and 660 ppm. Tuber dry mass was stimulated by 13.8, 27.7 and 41.5% at each respective [CO2] (Bindi et al. 2006). In the second experiment with potato, the Bintje cultivar was grown at 550 ppm for two growing seasons and showed a 36-50% increase in tuber dry mass (Bindi et al. 2006). These results suggest that other tuber crops, such as cassava and sweet potato, also have the potential to benefit from elevated [CO2]; however, when these crops are grown with limited fertilization and under water stress, gains may not be realized.
Sugar beet and barley were grown at elevated [CO2] (550 ppm) in a FACE experiment in Braunschweig, Germany (Weigel et al. 2006). Sugar beet showed a 6-8% stimulation in tuber production at elevated [CO2], while barley showed an 8-14% stimulation in grain production (Weigel et al. 2006). Oilseed rape yield was ~18% higher when grown at elevated [CO2] of 500 ppm (Franzaring et al. 2008).
The response of two C4 crops to elevated [CO2] has been studied in FACE experiments (Ottman et al. 2001; Leakey et al. 2004, 2006). Sorghum (Sorgum bicolor) was grown in Maricopa, Arizona with and without ample water supply, and maize (Zea mays) was grown in Champaign, Illinois with ambient precipitation. Elevated [CO2] had no effect on seed yield when averaged across growth conditions and two growing seasons for each crop (Ottman et al. 2001; Leakey et al. 2004, 2006). There was a trend towards an increase in sorghum yield when the crop was grown without ample water supply (Ottman et al. 2001). While millets and sugarcane (Saccharum sp.) have not been grown at elevated [CO2] in FACE experiments, a recent OTC study of sugarcane revealed that elevated [CO2] (720 ppm) increased photosynthesis by 30%, height by 17%, biomass by 40% and sucrose content by 29% (De Souza et al. 2008). These data suggest that sugarcane productivity might increase in the future; however, the OTCs also may have overestimated the effects of elevated [CO2] and caused transient water stress (De Souza et al. 2008).
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.