Rising levels of atmospheric carbon dioxide—a spur to policymakers worldwide to plan for climate change—are also prompting agricultural scientists to prepare for impacts on the food we grow and eat.
A growing body of scientific evidence suggests that elevated levels of carbon dioxide, or CO2, can contribute to greater yields and water-use efficiency in many crops. But it’s not all good news, as some crops lose their nutrient value as CO2 levels increase.
Carbon dioxide is an important component of photosynthesis, the process by which plants convert sunlight into chemical energy. The vast majority of carbon sequestration in plants of commercial interest occurs through one of two photosynthetic pathways, known as C3 and C4. Which of these pathways a plant uses will help determine how it is affected by greater concentrations of CO2 in the atmosphere. And even within the same species, carbon sequestration can vary by plant variety and geography.
C3 plants tend to thrive in places with moderate sunlight intensity and temperature and they are highly dependent on ambient CO2 concentrations for growth. In general, C3 plants, which include crops such as soybeans, wheat, rice, potatoes, barley, rye, and cotton, are relatively inefficient photosynthesizers. As a result, they have the most to gain in yield from higher levels of carbon dioxide, but at the same time can have lower nutrient content. About 85 percent of plant species perform photosynthesis using the C3 pathway.
C4 plants thrive in warmer environments, and are planted more frequently in hot and dry environments. Although these plants make up only 3 percent of flowering species, they account for a significant portion of global crop production. Corn, sorghum, millet, and sugar cane are among the C4 crops that are integral to global food security. These plants, which are more efficient at fixing carbon from the atmosphere in photosynthesis, are less affected by rising CO2 levels than their C3 counterparts.
Agricultural scientists at research centers around the world, including Wagenigen University in the Netherlands and The C4 Rice Project, funded by The Bill and Melinda Gates Foundation, are working to marry the greater efficiency of C4 plants with the higher yield potential of C3 plants to ensure sufficient and nutritious food supplies in anticipation of climatic changes.
In this Insight article, we will touch on some examples of major C3 and C4 plants that figure prominently in modern agriculture, and how they are expected to adapt to rising global CO2 levels. Gro Intelligence subscribers can keep abreast of changes in yield for major crops and geospatially derived environmental data, including precipitation patterns, evapotranspiration anomalies, and land-surface temperatures, to track commodities across the globe.
The boost in crop productivity at greater concentrations of CO2, dubbed the CO2 fertilization effect, comes with a caveat. Not all C3 plants respond to elevated CO2 in the same way, and food quality seems to be sacrificed as quantity increases. Studies suggest that, while yields increase, nutritional value generally decreases, and the magnitude of the effect varies by crop. For instance, soybeans and other leguminous crops fare better than other C3 plants due to their ability to sequester nitrogen. Under elevated CO2, legumes actually fix more nitrogen and therefore perform a self-fertilization that buffers against reductions in protein content.
For non-leguminous plants such as rice and wheat, the protein-reducing effect of elevated CO2 can be somewhat alleviated if there is adequate soil nitrogen. Greater investment in nitrogen-based fertilizers may be required in regions that struggle with soil fertility to prevent significant protein loss from non-leguminous C3 crops, which may prove financially problematic in places like Africa where fertilizer is already expensive.
Unfortunately, zinc and iron content in soybeans as well as other legumes and non-leguminous C3 plants tend to decrease when grown under elevated CO2, and the mechanism behind these observations isn’t currently understood. It’s possible that breeding or engineering soybeans and other important C3 crops for increased iron and zinc content can address this issue, but the trade-offs associated with cost and resistance to pests and disease are unclear and may jeopardize food security if introduced on a commercial scale prematurely.
Photosynthesis in C4 plants is fundamentally different than in C3 plants. The chemical reactions that drive photosynthesis in C4 plants are sectioned off in a separate cellular environment where CO2 levels are regulated. This physiological adaptation ensures CO2 levels are never low enough for photorespiration to occur. (In photorespiration, a plant will use up energy and emit CO2, essentially the opposite of photosynthesis.) Because of this, C4 plant growth is less affected by rising CO2 levels than is C3 plant growth.
In studies on corn, yields only noticeably increased under drought conditions because elevated CO2 increases water-use efficiency. This may come as welcome news for corn-growing regions that experience periods of low rainfall. In the future, corn and other C4 plants may require less water, which could further reduce cost of production by lessening the need for irrigation infrastructure.
C4 crop yields are not expected to increase as dramatically as C3 yields under elevated CO2 levels but they may suffer slight reductions in nutritional value. Interestingly, in a study on various C4 plants, corn was the only crop to show nutrient decline. On the other hand, C4 crops like sorghum were virtually unaffected. In the future, differences in crop vulnerability to nutrient decline may shift priorities in planting intentions across different geographies.
Current scientific evidence suggests that the nutrient-depleting effect of elevated CO2 varies by species, variety, and cultivar. Studies on rice show that iron and zinc content decrease in some cultivars but actually increase marginally in others. This nutrient-fluctuation trend also appears to hold true across most C3 plants.
Unclear effects on nutrient content is alarming from a public health standpoint, as 60 percent of the global population depends on rice and other C3 plants for dietary iron and zinc requirements.The UN’s Food and Agriculture Organization estimates that 2 billion people currently suffer from iron and zinc deficiencies, which translates to an annual loss of 63 million life-years. Scientists are seeking ways to combat crop-nutrient imbalance in future scenarios.
Rice is a staple food for nearly 3 billion people worldwide. The goal of The C4 Rice Project is to introduce C4 traits into rice, a C3 plant. By genetically altering certain types of rice, photosynthetic efficiency could increase by up to 50 percent and cultivation should require less nitrogen-based fertilizer. In theory, this development could reduce cost of production while increasing rice yields and nutrient content. The success of this program may set a precedent for engineering other crops (like soybeans, mentioned above) to meet yield demand and dietary requirements.
In the coming decades, some regions will experience greater growing pains than others, particularly those that depend primarily on C3 plants to feed their populations. Places like India, Southeast Asia, and the Middle East that rely on grains such as rice and wheat are in greater jeopardy of nutrient deficiency, for example. However, South America and Western Africa, regions that primarily consume crops such as corn, millet, and sorghum, will not feel the nutrient-reducing effects of elevated CO2 as strongly.
The magnitude of the effects of carbon fertilization on yield will also vary by geography and irrigation capacity. In places with longer growing seasons like North America and Europe, yields will increase more significantly. By the same token, arid locations like Africa with a greater reliance on rainfed irrigation will benefit from higher yields due to increased water-use efficiency that comes with elevated CO2 conditions.
Of course, the world of agriculture is subject to volatility in the form of socio-economic, logistical, and climatic factors. Elevated CO2 levels are expected to generate a range of environmental challenges, but this situation may also provide unique opportunities to capitalize on the potential of new and evolving agricultural technologies. The Gro Intelligence data platform and yield models help users stay ahead of the curve with the latest developments on crop conditions around the globe.
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