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CO2 Effects on Plant Nutrient Concentrations
Reference
Duval, B.D., Blankinship, J.C., Dijkstra, P. and Hungate, B.A. 2012. CO2 effects on plant nutrient concentration depend on plant functional group and available nitrogen: a meta-analysis. Plant Ecology 213: 505-521.

Background
The authors note that atmospheric CO2 enrichment might logically be expected to lower plant nutrient concentrations (i.e., dilute them), due to the greater rate of carbohydrate production elevated CO2 concentrations induce in plants via their stimulation of the photosynthetic process, the end result of which in the case of agricultural crops has been hypothesized by Loladze (2002) to result in "hidden hunger," i.e., more - but less nutritious - food. But is that expectation borne out by experimental studies?

What was done
In a herculean effort to find the answer to this question, Duval et al. conducted a meta-analysis of the subject that was designed to include the results of all CO2 enrichment studies of all plant nutrients that had been conducted and published in the peer-reviewed scientific literature up to the time of the commencement of their analysis. More specifically, they say that they quantified "elevated CO2 effects on leaf, stem, root, and seed concentrations of B, Ca, Cu, Fe, K, Mg, Mn, P,S, and Zn among four plant functional groups and two levels of N fertilization," based on 90 individual experimental analyses involving a total of 478 independent replications.

What was learned
The four U.S. researchers did indeed find that in some cases "elevated CO2 tends to lower the concentration of nutrients in plants," but they say that in other cases they observed increased nutrient concentrations. In addition, they discovered that "the effects of elevated CO2 on mineral nutrition depend on the specific element, plant functional group, plant organ, N availability, and, in some cases, the level of CO2 enrichment." And these complexities, in their words, "preclude a universal hypothesis strictly related to carbohydrate dilution regarding plant nutrient response to elevated CO2." Also of great importance was their finding that when elevated CO2 did lower the nutrient concentrations of plants, most nutrients exhibited dilution that was "less than expected."

What it means
In discussing the results of their comprehensive analysis, Duval et al. conclude that their findings "challenge the assumption that plant nutrient concentrations are generally lowered by elevated CO2 strictly on the basis of carbohydrate dilution," thereby largely contradicting the "hidden hunger hypothesis" put forward a decade earlier by Loladze (2002).

Reference
Loladze, I. 2002. Rising atmospheric CO2 and human nutrition: toward globally imbalanced plant stoichiometry? Trends in Ecology and Evolution 17: 457-461.

Reviewed 25 July 2012