CO2 Fertilization May Be a Mixed Blessing

They say “plants love CO2,” but we may not love what CO2 can do to plants.

In addition to being a greenhouse gas, carbon dioxide (CO2) plays a central role in one of our planet’s most important chemical cycles: the one involving photosynthesis and respiration. Green plants take in CO2 and emit oxygen (O2) while making food. We respirers — aka breathers — eat the food, inhale O2, burn the food for energy, and exhale CO2.

So, CO2 in the atmosphere is essential to green plants — in fact, as atmospheric CO2 concentrations increase, green plants become more productive. We call this the fertilization effect, and generally it’s a good thing because:

  • it helps dampen the effects of CO2 emissions we humans generate (because the more CO2 in the atmosphere, the more productive green plants are and the more CO2 they draw down); and
  • it helps crops grows faster because they are among the myriad plants fertilized by CO2.

Of course, whether increased CO2 will mean an overall increase in crop yields is an open question since we must also factor in the confounding effects of climate change (e.g., heat stress, changing precipitation patterns, increasing weather variability, rising sea levels). Our best guess is that the overall impact of crop yields will be a mixed bag, with only a few places like Canada and Northern Europe likely seeing increases over the long term.

And now new research suggests a thorn in the fertilization effect‘s otherwise rosy effect on crops. Sure, increased CO2 will make crops more productive, but the quality of the crops may be degraded.

Studying the Fertilization Effect: Artificial Versus Real-World Conditions

Much of the data on the effects of enhanced CO2 on plants come from so-called chamber experiments, where a plant is grown in a greenhouse or closed container with varying levels of CO2.

The advantage of such experiments is the ability to carefully control the plant’s growing conditions. The disadvantage is that the growing conditions are artificial and we can’t be sure if the plant’s behavior resembles what would occur in the real world.

To address this issue, scientists have developed an open-air approach called the Free-Air Carbon Dioxide Enrichment (FACE) experiment, in which field crops (or forest stands) are exposed to continuous high levels of CO2. These free-air, real-world experiments provide a bridge between the more controlled chamber experiments and what the future might hold. I tend to favor the results from FACE experiments since they use real-world conditions. (Full disclosure: Duke has one of 10 U.S. FACE sites.)

CO2 and Wheat: How We Get Less Protein, More Sugar

Without question, wheat is a staple crop. With the help of corn and rice, wheat provides about 60 percent of the world’s food. Therefore, how wheat responds to enhanced CO2 is a big deal, and FACE study results (for example here and here) are not encouraging: while wheat yields increase, its protein content drops.

Take a recent German study published in Plant Biology for example. Petra Hogy of the University of Hohenheim and colleagues compared wheat grown under today’s conditions (with CO2 levels at about 387 parts per million) with those projected for the year 2050, with CO2 levels around 550 ppm. Hogy’s team found about a seven percent drop in protein content, noting several other changes in nutritional value:

  • amino acid concentrations decreased, with greater reductions in non-essential rather than essential amino acids;
  • minerals such as potassium, molybdenum, and lead increased;
  • minerals such as manganese, iron, cadmium, and silicon decreased; and
  • sugars including fructose and fructan increased.

It’s too early to tell how these changes will affect our health and whether the changing chemical properties of wheat will prompt new ways of processing it (e.g., will a changed gluten profile impact bread-making?). However, we can easily speculate that less protein and higher lead levels in wheat will require some adjustments. Perhaps people will have to consume more sugars to get the same amount of wheat protein. Alternatively, maybe new cultivars of wheat will need to be engineered that are better adapted to high CO2 concentrations.

More CO2 Means More Toxicity in Some Plants

A number of other (mostly chamber) studies suggest that the toxicity of some crops will increase with rising CO2 levels. Of particular concern is cyanide.

About 60 percent of today’s crop plants are cyanogenic, meaning that when their leaves are chewed or crushed, cyanide is released as a defense mechanism. A series of papers by Roslyn Gleadow of Monash University and her colleagues find that many cyanogenic plants will have greater cyanide toxicity when CO2 concentrations are higher.

CO2, Clover, and Cyanide

While we humans don’t usually eat clover, foraging animals like cows and sheep do, so how clover responds to elevated CO2 is of interest. 

A recent study led by Gleadow, published in the Journal of Chemical Ecology, found that while cyanide levels in clover grown under ambient (360 ppm) and elevated (700 ppm) CO2 levels did not rise, protein content dropped some 25 percent — in other words, the cyanide-to-protein ratio increased.

This is important because the ability of livestock to tolerate cyanogenic compounds depends on their protein intake. The more protein they take in, the more cyanide the animals can handle and vice versa. In controlled greenhouse experiments, Gleadow’s group found that the relative ratio of protein to cyanide dropped by 30 percent or more, suggesting that livestock will be exposed to greater amounts of cyanide in a CO2-enhanced world. This could also mean that livestock may need to forage more to get the same amount of protein to combat the cyanide, which could lead to other downsides like more waste and methane production.

CO2, Cassava, and Cyanide

Cassava, also called manioc, is a cyanogenic staple that supports hundreds of millions of people. The starchy tuber’s leaves and roots are critical to tropical populations in Latin America, Africa, and Asia, despite its harmful amounts of cyanide. (We Westerners tend to eat it as tapioca.) A Gleadow study published in Plant Biology found that while growing cassava at CO2 levels of 710 ppm had no effect on the cyanide in its root, it doubled the plant’s cyanide levels in its leaves and shrank the tuber’s size.

To remove the cyanide and make it safe for eating, cassava is generally rasped, fermented and dried, but residual amounts remain. In a typical year, for instance, flour in Mozambique markets has been found to have cyan
ide levels ranging from 20 to 40 ppm but in a drought year (cassava is a very drought-resistant crop), cyanide levels have been found to be as high as 100–200 ppm. The World Health Organization recommends a limit of 10 ppm cyanide in food.

Cyanide poisoning, which can be fatal, can disrupt normal endocrine function and impair neurological function. Konzo, a form of cyanide poisoning that causes leg paralysis, already affects nine percent of Nigerians. Not a good situation, and one that could grow unless alternate cassava varieties are developed with lower concentrations of cyanogenic compounds. (And strangely at present, according to the Encyclopedia of Food and Culture, ”many farmers prefer to cultivate the high-cyanide varieties for reasons that are not entirely clear.”)

All these studies at the intersection of crop nutrition and enriched CO2 levels illustrate the complex ways that the natural system can respond to changes and perturbations which in turn can have serious ramifications for humans. It’s another geo-engineering quandary: stop CO2 increases and avoid the crop changes, or allow CO2 to increase and find a workaround like developing new cultivars that compensate for the nutritional changes.

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