Experimental Approaches for Investigating Crop Production in Elevated [CO2

Several technologies have been used to study the effects of elevated [CO2] on crop productivity, including controlled environmental chambers, greenhouses, open-top chambers (OTC), and Free-Air CO2 Enrichment (FACE; Long et al. 2004). In controlled environmental chambers and greenhouses, plants are typically grown in pots, with lighting, nutrients and water supplied by the researcher in specified amounts. There are practical advantages to using controlled environments, including precise control of precipitation, humidity and light, as well as ready availability of such facilities at academic and government research laboratories. Controlled environments have also been used to conduct dose response curves for crops grown at a range of elevated [CO2] (e.g., Allen et al. 1987; Long et al. 2006). However, there are several drawbacks to use of these technologies in the context of investigating crop yield responses to climate change (Long et al. 2004). Pots limit root growth, which can negatively feed back on photosynthetic capacity, shoot growth and harvestable yield potential, and thus reduce the magnitude of CO2 stimulation (Arp 1991). Growth in pots can also alter nutrient availability, thereby changing the CO2 response (McConnaughay et al. 1993). The size, light levels and forced air circulation in controlled environments also alter plant growth, which compromises the ability to accurately measure crop yield responses to [CO2] (McLeod and Long 1999; Long et al. 2004, 2006).

In the field, crops can be grown in OTCs, where plants are rooted in the ground and exposed to natural light and precipitation through the top of the chamber (Heagle et al. 1989; Leadley and Drake 1993; Whitehead et al. 1995). OTC walls are typically clear plastic, allowing light penetration, and air enriched with CO2 is introduced to the chamber by a blower system. Although OTCs eliminate some of the problems associated with greenhouses and growth chambers, OTCs alter the environmental conditions, such that temperatures and relative humidity are higher, wind velocity and light intensity are lower, and light quality is changed (Leadley and Drake 1993). Another problem with OTCs is their small plot size. Typically, agronomic trials also use buffer rows, with a width approximately twice the height of the crop. However, with OTCs, most of the treated crop is within the buffer zone, which causes "edge effects" and could exaggerate the response to elevated [CO2] (McLeod and Long 1999).

In response to the limitations of controlled environments and OTCs and as the need to test hypotheses under open-air field conditions arose, FACE technology was developed (Hendrey and Miglietta 2006; Fig. 7.1). FACE allows elevated [CO2] to be maintained without significantly altering the micrometeorological conditions around a plot of vegetation (Hendrey et al. 1993). FACE plots encompass up to hundreds of square meters of vegetation, allowing for use of a buffer zone, which eliminates problems of edge effects experienced in chambers (Long et al. 2006). The size of FACE plots also enables investigation of plant responses to

Fig. 7.1 A FACE plot at the University of Illinois SoyFACE facility where soybean is exposed to elevated [CO2] (550 ppm). CO2 is released from small holes in the green pipe into the wind, on the upwind side of the plot. The release rate is determined by the wind speed and [CO2], measured at the center of the ring (photo credit: Andrew D.B. Leakey)

Fig. 7.1 A FACE plot at the University of Illinois SoyFACE facility where soybean is exposed to elevated [CO2] (550 ppm). CO2 is released from small holes in the green pipe into the wind, on the upwind side of the plot. The release rate is determined by the wind speed and [CO2], measured at the center of the ring (photo credit: Andrew D.B. Leakey)

elevated [CO2] from the genomic to ecological scale (Leakey et al. 2009b). FACE systems release CO2-enriched air through vertical vent pipes (e.g., Lewin et al. 1992) or pure CO2 through horizontal pipes (Miglietta et al. 2001). The gas is released just above the canopy surface on the upwind side of the plot. Fast-feedback computer control adjusts the position and amount of CO2 released at different points around the plot, based on measurements of wind speed, direction and [CO2] in the center of the plot (Long et al. 2004). In FACE plots, the natural environment is essentially unperturbed, as there are no barriers to light, precipitation, wind or pests. A major limitation to widespread use of FACE experimentation is the financial investment in the infrastructure, land and personnel needed to successfully run the experiments (Ainsworth et al. 2008a). Therefore, far fewer FACE experiments have been conducted than controlled environment studies. Still, because plants are grown in soil without significant alteration of the microenvironment, FACE experiments likely offer the most realistic estimates of crop yield responses to elevated [CO2] (Long et al. 2004, 2006; Ainsworth et al. 2008b).

Renewable Energy 101

Renewable Energy 101

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.

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