Paper Reviewed
Madhu, M. and Hatfield, J.L. 2013. Dynamics of plant root growth under increased atmospheric carbon dioxide. Agronomy Journal 105: 657-669.

In a review of the impacts of atmospheric CO2 enrichment on the root growth of agricultural crops, Madhu and Hatfield (2013) cite a total of 182 scientific journal articles from which they extracted the information upon which they based their various conclusions about the subject, which are - among other things - that "roots become more numerous, longer, thicker, and faster growing in crops exposed to high CO2." And they describe additional evidence that demonstrates that the increased branching and extension of roots under elevated CO2 may enable roots to acquire more water and nutrients from the soil profile, as they are able to explore significantly greater volumes of soil.

The two scientists also note that the root growth of crop plants is often stimulated to a greater extent than that of other plant parts in response to atmospheric CO2 enrichment; and they report that in two earlier comprehensive reviews of Free-Air Atmospheric CO2 Enrichment experiments on agricultural crops, Kimball et al. (2002) and Kimball (2011) determined that for a 300 ppm increase in atmospheric CO2 concentration, the root biomasses of wheat, ryegrass and rice each increased about 70% with ample water and nitrogen, by 58% at low nitrogen, and by 34% at low soil water content, while "changes were greatest in cotton with a 96% increase in root biomass at ample water and nitrogen." And they go on to report that "a 110% increase in root length of soybean was observed as CO2 concentration increased from 350 to 700 ppm," citing Rogers et al. (1992).

Madhu and Hatfield also emphasize that "the enhanced proliferation of roots grown under elevated atmospheric CO2 concentration may be a strategy which permits adequate nutrient acquisition in the absence of normal water absorption rates," noting that longer roots "could penetrate into deeper soils to create more resilience to droughts in natural plant communities," which deeper rooting "would be an advantage if climates became drier and altered belowground plant competition."

Another important point made by the two researchers is that "enhanced root growth and biomass production from exposure to increased CO2 concentrations could deliver more carbon to the soil profile potentially altering rhizosphere microbiology (population and dynamics) and inducing possible physicochemical changes in soils by increasing root activity, including rhizodeposition due to root turnover."

Clearly, there is much more of good produced by atmospheric CO2 enrichment than what meets the eye; and it's down there, just beneath our feet, providing a firm foundation for significantly enhanced food production in the years and decades to come.

References
Kimball, B.A. 2011. Lessons from FACE: CO2 effects and interactions with water, nitrogen and temperature. In: Hillel, D. and Rosenzweig, C. (Eds.). Handbook of Climate Change and Agroecosystems: Impacts, Adaptation, and Mitigation. Imperial College Press, Hackensack, New Jersey, USA, p. 87-107.

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

Rogers, H.H., Peterson, C.M., McCrimmon, J.M. and Cure, J.D. 1992. Response of soybean roots to elevated atmospheric carbon dioxide. Plant, Cell and Environment 15: 749-752.