Soybeans are growing bigger — but are they better?

Higher CO₂ means bigger plant but lower nutritional value

We all learned in elementary school that plants take in carbon dioxide and release oxygen during photosynthesis. But what happens when the air contains more carbon dioxide than it has in millennia? Crops grow faster and larger, but they also become less nutritious.

Researchers at the Alabama Agricultural Experiment Station were awarded $635,000 by the National Institute of Food and Agriculture to explore the physiological mechanisms that cause soybeans to become less nutritious as CO₂ levels rise. The team’s foundational research will help build a statistical model they and other researchers can use to better study the phenomenon.

“We want the model to be able to predict the reaction of the plant to elevated CO₂ levels for growth, but also for nutrient concentration and gene expression,” said Alvaro Sanz-Saez, associate professor in the Department of Crop, Soil & Environmental Sciences.

Crop scientists have known for some time that elevated CO₂ levels lead to larger, more productive plants. However, exactly how they become less nutritious is largely unclear. The researchers have several hypotheses related to dilution of minerals in larger foliage, and how a plant’s roots grow, take up nutrients and transport those nutrients. Central to these hypotheses is the process of transpiration.

When a plant “breaths in” carbon dioxide, it also “breathes out” water vapor through openings in its leaves called stomata. This is known as transpiration. When the air is richer in CO₂, the plant’s stomata need not be open as long, resulting in less water loss. This makes the plant more water efficient, but it also means that less water is traveling from roots to shoots—and it is on this flow of water through the xylem that a plant transports nutrients to leaves and seeds.

“The physiological mechanisms that link increased biomass response to elevated CO₂ with alterations to mineral nutritional content has not been tested and is not well understood,” said Sanz-Saez.

The researchers chose soybeans for their experiments, in part, because of the wide variety of cultivars that respond in different ways to varying CO₂ levels and other factors under study, said Sanz-Saez. Soybeans are the second most valuable U.S. crop after corn; in 2024, the farm value of soybeans topped $44 billion, according to the USDA’s National Agricultural Statistics Service.

Soybeans are also critical to Alabama’s economy in several ways. Alabama farmers harvested 350,000 acres of soybeans in 2024, valued at $110 million, according to the USDA. Alabama’s number one agricultural product—poultry—also runs on soybeans; about two-thirds of the U.S. domestic soybean meal is for chicken feed, according to the Iowa Soybean Association.

The group’s research also has humanitarian impact. People in developing countries tend to have plant-based diets. If carbon levels continue to rise as projected, soybeans, in particular, will experience an 8% drop in essential minerals, like zinc and iron, by the end of the century. This is important because an estimated 2 billion people currently suffer deficiencies in zinc and iron, said Sanz-Saez.

To conduct their experiments, the researchers partnered with a one-of-a-kind facility at the University of Illinois. The research farm has the capacity to, essentially, fumigate a field of soybeans with CO₂ at specified levels. This allows the researchers to explore the growth of soybeans at sub-ambient, ambient, and elevated CO₂ levels in a natural environment.

“They are able to increase the CO₂ in completely natural conditions. Without any plastic, without a greenhouse. Just plants in the ground. It’s super neat—and super expensive,” said Sanz-Saez.

The project collected data on plant growth rates and nutrient accumulation, but it also explored gene expression through a process called transcriptomics. Transcriptomics is the study of a complete set of RNA transcripts produced by an organism under specific conditions—in this case, varying CO₂ levels.

A plant’s DNA contains instructions for all the proteins it can make. When a plant needs to respond to the environment (e.g., varying CO₂ levels), specific genes in the DNA are “transcribed” into RNA. But not all DNA is expressed. Plants have a repertoire of responses. By studying the genes that control stomatal regulation, photosynthesis pathways, carbon storage and mineral transport, the researchers can better understand these physiological mechanisms and their responses to changing CO₂ levels.

And by collecting mass amounts of data through field physiological measurements and transcriptomics, the researchers can build a statistical model that allows them and other researchers to “experiment” simply by running various scenarios on a computer. The model will integrate data on physiology, growth, transcriptome and nutrient accumulation to predict which mechanisms cause soybeans to become less nutritious in higher CO₂ environments.

To date, the results have mostly prompted additional questions.

“The differences between cultivars were not as big as we expected. We think the environment affects more how the cultivars interact with the minerals than just the CO₂,” said Sanz-Saez. “All of the hypotheses related to mineral dilution played a role, but there was not a silver bullet. Roots play a bigger role than expected. We are intrigued because they’re understudied. It might not perform as we thought.”

Sanz-Saez and the research team will share their results at the annual Plant Breeders Conference, as well as through journal articles and Extension outlets. However, the three-year project is ongoing.

In addition to Sanz-Saez, Courtney Leisner of Virginia Tech is also on the research team.