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Protein -- Summary
In a review of the scientific literature related to effects of atmospheric CO2 enrichment on plant constituents of significance to human health, Idso and Idso (2001) cited a number of studies where elevated levels of atmospheric CO2 either increased, decreased or had no effect on the protein concentrations of various agricultural crops, the first two of which consequences have subsequently also been observed by Kaddour and Fuller (2004) and Veisz et al. (2005), respectively, in wheat.

In the case of this particular crop - which according to Wittwer (1995) is "the most widely grown plant in the world today," contributing "more calories and protein to the human diet than any other food" - Pleijel et al. (1999) were able to bring some semblance of order to this confusing situation by analyzing the results of 16 open-top chamber experiments that had been conducted on spring wheat in Denmark, Finland, Sweden and Switzerland between 1986 and 1996.  In addition to CO2 enrichment of the air, these experiments included increases and decreases in atmospheric ozone (O3); and Pleijel et al. found that when increasing O3 pollution reduced wheat grain yield, it simultaneously increased the protein concentration of the grain.  They also found that when O3 was scrubbed from the air and grain yield was thereby increased, the protein concentration of the grain was decreased.  Moreover, this same relationship described the degree to which grain protein concentration dropped when atmospheric CO2 enrichment increased grain yield.  Hence, it became clear that whenever the grain yield of the wheat was changed -- by CO2, O3 or even water stress, which was also a variable in one of the experiments -- grain protein concentrations either moved up or down along a common linear relationship in the opposite direction to the change in grain yield elicited by the CO2, O3 or water stress treatment.

In an earlier study of CO2 and O3 effects on wheat grain yield and quality, Rudorff et al. (1996) obtained essentially the same result.  They observed, for example, that "flour protein contents were increased by enhanced O3 exposure and reduced by elevated CO2" but that "the combined effect of these gases was minor."  Hence, they concluded that "the concomitant increase of CO2 and O3 in the troposphere will have no significant impact on wheat grain quality."

Earlier still, Evans (1993) had found similar relationships to exist for several other crops, further observing them to be greatly affected by soil nitrogen availability.  It is highly likely, therefore, that the differing availability of soil nitrogen could have been responsible for some of the differing results observed in the many other studies reviewed by Idso and Idso (2001); and, in fact, that is precisely what the study of Rogers et al. (1996) suggests.  Although the latter investigators observed CO2-induced reductions in the protein concentration of flour derived from wheat plants growing at low soil nitrogen concentrations, no such reductions were evident when the soil nitrogen supply was increased to a higher rate of application.  Hence, Pleijel et al. concluded that the oft-observed negative impact of atmospheric CO2 enrichment on grain protein concentration would probably be alleviated by higher applications of nitrogen fertilizers; and the study of Kimball et al. (2001) confirmed their hypothesis.

Kimball et al. studied the effects of a 50% increase in atmospheric CO2 concentration on wheat grain nitrogen concentration and the baking properties of the flour derived from that grain throughout four years of free-air CO2 enrichment experiments.  In the first two years of their study, soil water content was an additional variable; and in the last two years, soil nitrogen content was a variable.  The most influential factor in reducing grain nitrogen concentration was determined to be low soil nitrogen; and under this condition, atmospheric CO2 enrichment further reduced grain nitrogen and protein concentrations, although the change was much less than that caused by low soil nitrogen.  When soil nitrogen was not limiting, however, increases in the air's CO2 concentration did not affect grain nitrogen and protein concentrations; neither did they reduce the baking properties of the flour derived from the grain.  Hence, it would appear that given sufficient water and nitrogen, atmospheric CO2 enrichment can significantly increase wheat grain yield without sacrificing grain protein concentration in the process.

There are some situations, however, where atmospheric CO2 enrichment has actually been found to increase the protein concentration of wheat.  Agrawal and Deepak (2003), for example, grew two cultivars of wheat (Triticum aestivum L. cv. Malviya 234 and HP1209) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm alone and in combination with 60 ppb SO2 to study the interactive effects of elevated CO2 and this major air pollutant on crop growth.  They found that exposure to the elevated SO2 caused a 13% decrease in foliar protein concentrations in both cultivars; but when the plants were concomitantly exposed to an atmospheric CO2 concentration of 600 ppm, leaf protein levels only decreased by 3% in HP1209, while they actually increased by 4% in Malviya 234.

In the case of rice - which according to Wittwer (1995) is "the basic food for more than half the world's population," supplying "more dietary energy than any other single food" - Jablonski et al. (2002) conducted a wide-ranging review of the scientific literature, finding that it too appeared to suffer no reduction in grain nitrogen (protein) concentration in response to atmospheric CO2 enrichment.  Likewise, they found no CO2-induced decrease in seed nitrogen concentration in the studies of legumes they reviewed.  This finding is also encouraging, since according to Wittwer (1995) legumes "are a direct food resource providing 20% of the world's protein for human consumption," as well as "about two thirds of the world's protein concentrate for livestock feeding."  What is more, the biomass of the CO2-enriched wheat, rice and legumes was found by Jablonski et al. to be significantly increased above that of the same crops grown in normal air.  Hence, there will likely be a vast increase in the total amount of protein that can be made available to humanity in a future CO2-enriched world, both directly via food crops and indirectly via livestock.

With respect to the leguminous soybean, Thomas et al. (2003) additionally note that "oil and protein comprise ~20 and 40%, respectively, of the dry weight of soybean seed," which "unique chemical composition," in their words, "has made it one of the most valuable agronomic crops worldwide."  In addition, they say that "the intrinsic value of soybean seed is in its supply of essential fatty acids and amino acids in the oil and protein, respectively;" and in this regard they report that Heagle et al. (1998) "observed a positive significant effect of CO2 enrichment on soybean seed oil and oleic acid concentration."

Legumes and their responses to atmospheric CO2 enrichment also figure prominently in a number of studies of mixed forage crops.  In a study of nitrogen cycling in grazed pastures on the North Island of New Zealand, for example, Allard et al. (2003) report that under elevated CO2, leaves of the individual species exhibited lower nitrogen concentrations but higher water-soluble carbohydrate (WSC) concentrations.  They also say "there was a significantly greater proportion of legume in the diet at elevated CO2," and that this "shift in the botanical composition towards a higher proportion of legumes counterbalanced the nitrogen decrease observed at the single species scale, resulting in a nitrogen concentration of the overall diet that was unaffected by elevated CO2."  What is more, they report that "changes at the species level and at the sward level appeared to combine additively in relation to WSC."  Hence, they note that "as there was a significant correlation between WSC and digestibility (as previously observed by Dent and Aldrich, 1963 and Humphreys, 1989), there was also an increase in digestibility of the high CO2 forage," which result, in their words, "matches that found in a Mini-FACE experiment under cutting (Teyssonneyre, 2002; Picon-Cochard et al., 2004)," where "digestibility also increased in response to CO2 despite reduced crude protein concentration."  These data, plus the strong relationship between soluble sugars (rather than nitrogen) and digestibility, led them to suggest that "the widespread response to CO2 of increased soluble sugars might lead to an increase in forage digestibility."

Luscher et al. (2004) found much the same thing in their review of the subject, which was based primarily on studies conducted at the Swiss FACE facility that hosts what has become the world's longest continuous atmospheric CO2 enrichment study of a naturally-occurring grassland.  In response to an approximate two-thirds increase in the air's CO2 concentration, leaf nitrogen (N) concentrations of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were reduced by 7% and 18%, respectively, when they were grown separately in pure stands.  However, as Luscher et al. report, "the considerably lower concentration of N under elevated CO2, observed for L. perenne leaves in pure stands, was found to a much lesser extent for L. perenne leaves in the bi-species mixture with T. repens (Zanetti et al., 1997; Hartwig et al., 2000)."  Furthermore, as they continue, "under elevated CO2 the proportion of N-rich T. repens (40 mg N g-1 dry matter) increased in the mixture at the expense of the N-poor L. perenne (24 mg N g-1 dry matter when grown in monoculture)," the end result being that "the concentration of N in the harvested biomass of the mixture showed no significant reduction."

That this phenomenon is likely ubiquitous is suggested by the still more comprehensive review of the subject produced by Campbell et al. 2000), who analyzed research conducted between 1994 and 1999 by a worldwide network of 83 scientists associated with the Global Change and Terrestrial Ecosystems (GCTE) Pastures and Rangelands Core Research Project 1 (CRP1).  This program had resulted in the publication of more than 165 peer-reviewed scientific journal articles; and Campbell et al. determined from this massive collection of data that the legume content of grass-legume swards was typically increased by approximately 10% in response to a doubling of the air's CO2 content.

Also of interest within this context, Luscher et al. (2004) state that "the nutritive value of herbage from intensively managed grassland dominated by L. perenne and T. repens ... is well above the minimum range of the concentration of crude protein necessary for efficient digestion by ruminants (Barney et al. 1981)."  Hence, they conclude that "a small decrease in the concentration of crude protein in intensively managed forage production systems [which may never occur, as noted above] is not likely to have a negative effect on the nutritive value or on the intake of forage."  In addition, in a CO2-enriched world of the future there would be much more such forage produced per unit of land and water devoted to the enterprise, clearly making the ongoing rise in the air's CO2 content a big plus for animal husbandry.

One final forage study we have reviewed on our website is that of Newman et al. (2003), who investigated the effects of two levels of nitrogen fertilization and an approximate doubling of the air's CO2 content on the growth and chemical composition of tall fescue (Festuca arundinacea Schreber cv. KY-31), both when infected and uninfected with a mutualistic fungal endophyte (Neotyphodium coenophialum Morgan-Jones and Gams).  They found that the elevated CO2 reduced the crude protein content of the forage by an average of 21% in three of the four situations studied: non-endophyte-infected plants in both the low and high nitrogen treatments, and endophyte-infected plants in the high nitrogen treatment.  However, there was no protein reduction for endophyte-infected plants in the low nitrogen treatment.

This latter point is very important; for as noted by Newman et al., "the endophyte is present in many native and naturalized populations and the most widely sown cultivars of F. arundinacea," so that the first two situations in which the CO2-induced protein reduction occurred (those involving non-endophyte-infected plants) are not typical of the real world.  In addition, since the dry-weight biomass yield of the forage was increased by fully 53% under the low nitrogen regime, and since the ten-times-greater high nitrogen regime only boosted yields by an additional 8%, there would appear to be no need to apply any extra nitrogen to F. arundinacea in a CO2-enriched environment.  Consequently, under best management practices in a doubled-CO2 world of the future, little to no nitrogen would be added to the soil and there would be little to no reduction in the crude protein content of F. arundinacea, but there would be more than 50% more of it produced on the same amount of land.

With respect to the final plant quality studied by Newman et al., i.e., forage digestibility, increasing soil nitrogen lowered in vitro neutral detergent fiber digestibility in both ambient and CO2-enriched air; and this phenomenon was most pronounced in the elevated CO2 treatment.  Again, however, under low nitrogen conditions there was no decline in plant digestibility.  Hence, there is a second good reason to not apply extra nitrogen to F. arundinacea in a high CO2 world of the future and, of course, little to no need to do so.  Under best management practices in a future CO2-enriched atmosphere, therefore, the results of this study suggest that much greater quantities of good quality forage should be able to be produced without the addition of any -- or very little -- extra nitrogen to the soil.

But what about the unmanaged world of nature?  Increases in the air's CO2 content often - but not always (Goverde et al., 1999) - lead to greater decreases in the concentrations of nitrogen and protein in the foliage of C3 as compared to C4 grasses (Wand et al., 1999); and as a result, in the words of Barbehenn et al. (2004a), "it has been predicted that insect herbivores will increase their feeding damage on C3 plants to a greater extent than on C4 plants (Lincoln et al., 1984, 1986; Lambers, 1993).

To test this hypothesis, Barbehenn et al. (2004a) grew Lolium multiflorum Lam. (Italian ryegrass, a common C3 pasture grass) and Bouteloua curtipendula (Michx.) Torr. (sideoats gramma, a native C4 rangeland grass) in chambers maintained at either the ambient atmospheric CO2 concentration of 370 ppm or the doubled CO2 concentration of 740 ppm for two months, after which newly-molted sixth-instar larvae of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid) were allowed to feed upon the grasses.  As expected, foliage protein concentration decreased by 20% in the C3 grass, but by only 1% in the C4 grass, when grown in the CO2-enriched air.  However, and "contrary to our expectations," according to Barbehenn et al., "neither caterpillar species significantly increased its consumption rate to compensate for the lower concentration of protein in [the] C3 grass," noting that "this result does not support the hypothesis that C3 plants will be subject to greater rates of herbivory relative to C4 plants in future [high-CO2] atmospheric conditions (Lincoln et al., 1984)."  In addition, and "despite significant changes in the nutritional quality of L. multiflorum under elevated CO2," they report that "no effect on the relative growth rate of either caterpillar species on either grass species resulted," and that there were "no significant differences in insect performance between CO2 levels."

In a similar study with the same two plants, Barbehenn et al. (2004b) allowed grasshopper (Melanoplus sanguinipes) nymphs that had been reared to the fourth instar stage to feed upon the grasses; and once again, "contrary to the hypothesis that insect herbivores will increase their feeding rates disproportionately in C3 plants under elevated atmospheric CO2," they found that "M. sanguinipes did not significantly increase its consumption rate when feeding on the C3 grass grown under elevated CO2," suggesting that this observation implies that "post-ingestive mechanisms enable these grasshoppers to compensate for variable nutritional quality in their host plants," and noting that some of these post-ingestive responses may include "changes in gut size, food residence time, digestive enzyme levels, and nutrient metabolism (Simpson and Simpson, 1990; Bernays and Simpson, 1990; Hinks et al., 1991; Zanotto et al., 1993; Yang and Joern, 1994a,b)."  In fact, their data indicated that M. sanguinipes growth rates may have actually increased, perhaps by as much as 12%, when feeding upon the C3 foliage that had been produced in the CO2-enriched air.

In conclusion, with respect to both managed agricultural crops and the wild plants of earth's natural ecosystems, it would appear that the ongoing rise of the air's CO2 concentration will have few negative impacts of any consequence on the nutritive value of their grains and foliage in terms of protein concentration.  In fact, in tree crops such as citrus, CO2-induced changes in the activities of certain foliar proteins could well lead to vast increases in yield potential, as elucidated by the work of Idso et al. (2001).

References
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Last updated 3 August 2005