Growth Response of Radish to Super Atmospheric CO₂ Enrichment

Schubert abd Jahren (2011) write that "plant biomass is known to increase in response to elevated atmospheric CO₂ concentration," but they state that "no experiments have quantified the trajectory of crop fertilization across the full range of CO₂ levels estimated for the next 300 years," which Zachos et al. (2008) have suggested could extend all the way to a concentration of 1800 ppm if fossil fuel usage continues unabated.

In an attempt to obtain this information for a common food plant, Schubert and Jahren grew radishes (Raphanus sativus) from seed to maturity (a period of four months) in standard potting soil within eight growth chambers maintained at optimum temperature, humidity and soil water and fertility conditions in air of eight different CO₂ concentrations (348, 388, 413, 426, 760, 1090, 1425 and 1791 ppm), after which they harvested the plants and determined their above- and below-ground biomass, both of which they found to be well described by a two-parameter rectangular hyperbola, "employing the method used by Hunt et al. (1991, 1993) for assessing the trajectory of the biomass response for 36 herbaceous species grown under CO₂ levels ranging from 365 to 812 ppm."

Going from the lowest to the highest CO₂ concentration employed in their study, the two U.S. researchers found that above-ground biomass rose by a modest 58%, but that below-ground biomass rose by a phenomenal 279%, which trajectory "greatly exceeded a trajectory based on extrapolation of previous experiments for plants grown at CO₂ < 800 ppm."

Considering such findings, Schubert and Jahren state "if the below-ground biomass enhancement that we have quantified for R. sativus represents a generalized root-crop response that can be extrapolated to agricultural systems, below-ground fertilization under very high CO₂ levels could dramatically augment crop production in some of the poorest nations of the world." And "needless to say," as they continue, "a doubling or tripling of below-ground crop tissue due to CO₂ fertilization would be welcome on both a nutritional and economic basis."

Additional References
Hunt, R., Hand, D.W., Hannah, M.A. and Neal, A.M. 1991. Response to CO₂ enrichment in 27 herbaceous species. Functional Ecology 5: 410-421.

Hunt, R., Hand, D.W., Hannah, M.A. and Neal, A.M. 1993. Further responses to CO₂ enrichment in British herbaceous species. Functional Ecology 7: 661-668.

Zachos, J.C., Dickens, G.R. and Zeebe, R.E. 2008. An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature 451: 279-283.