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Plant Micronutrient Concentrations in a CO2-Enriched World
Volume 7, Number 31: 4 August 2004

Loladze (2002) speculates that the dilution effect of the extra plant biomass produced by the aerial fertilization effect of atmospheric CO2 enrichment may reduce the plant tissue concentrations of a number of micro-nutrients, many of which are important to human health and are currently present in common food plants in what are believed by some to be insufficient quantities.  Hence, he suggests that the increase in the air's CO2 content that has occurred over the industrial era may have caused an elemental imbalance in some of earth's plants, contributing to the problem of micro-nutrient malnutrition and harming the health and economy of over half the world's population, in what he describes as the problem of "hidden hunger."

Although this reasoning has a compelling ring to it, Lieffering et al. (2004) note that "the conclusions of Loladze (2002) were based on data obtained from plants grown under artificial growth conditions (notably small pots)," such that "the extent of dilution was probably exaggerated in these studies and the potential for elevated CO2 to exacerbate micro-nutrient deficiencies overestimated."  Hence, in an effort to rectify this situation, Lieffering and his co-workers analyzed the elemental concentrations of archived grain samples from temperate rice (Oryza sativa L. cv. Akitakomachi) crops they had grown previously under more realistic FACE conditions out-of-doors in a fertile agricultural field (Okada et al., 2001), where an approximate 200-ppm increase in the air's CO2 concentration increased rice grain yields by about 14% (Kim et al., 2003a,b).

Of the five macro-nutrients they measured (N, P, K, Mg, S), Lieffering et al. report that "only N showed a decrease in concentration with elevated CO2 in both years," while all six of the micro-nutrients studied (Zn Mn, Fe, Cu, B, Mo) exhibited concentration increases.  For Zn and Mn, in particular, they say "there was a strong tendency [for concentrations] to increase," while the same could also have been said of Fe, which in the second year of the study exhibited a CO2-induced concentration increase on the order of 68%, as best we can determine from Lieffering et al.'s bar graphs.

In concluding their paper, Lieffering et al. reiterate that their study of the effects of elevated CO2 on grain elemental concentrations under real-world field conditions is "the first such report for a staple food crop: all other previously reported data were obtained from plants growing in pots and in some kind of enclosure."  In contrast to the results obtained in most of these latter root-confining experiments, they note that, other than for N, "no dilution of [the] elements in the grain was observed, contrary to the general conclusions of Loladze (2002)."  Hence, they conclude that "as long as there is a readily available supply of nutrients and that the nutrient uptake capacity response to elevated CO2 is equal [to] or greater than the whole plant biomass response [which was the case in their experiment, except for N], then no dilution should be observed."

So, there has now been one field study (of two years' length) of the effects of elevated CO2 on plant stoichiometry; and the results appear to favor our view of the subject over that of Loladze (2002).  Nevertheless, this is still just one study; and we are in no way prepared to claim proof of superiority for our view until several more field experiments have been conducted.  We are, however, very pleased with this first result.

Sherwood, Keith and Craig Idso

Kim, H.-Y., Lieffering, M., Kobayashi, K., Okada, M. and Miura, S.  2003a.  Seasonal changes in the effects of elevated CO2 on rice at three levels of nitrogen supply: a free-air CO2 enrichment (FACE) experiment.  Global Change Biology 9: 826-837.

Kim, H.-Y., Lieffering, M., Kobayashi, K., Okada, M., Mitchell, M.W. and Gumpertz, M.  2003b.  Effects of free-air CO2 enrichment and nitrogen supply on the yield of temperate paddy rice crops.  Field Crops Research 83: 261-270.

Lieffering, M., Kim, H.-Y., Kobayashi, K. and Okada, M.  2004.  The impact of elevated CO2 on the elemental concentrations of field-grown rice grains.  Field Crops Research 88: 279-286.

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

Okada, M., Lieffering, M., Nakamura, H., Yoshimoto, M., Kim. H.-Y. and Kobayashi, K.  2001.  Free-air CO2 enrichment (FACE) using pure CO2 injection: system description.  New Phytologist 150: 251-260.