The legume family is great at fixing nitrogen, a biochemical process that moves nitrogen from the air into the soil, where it is an essential nutrient for plant growth.

That is one reason peas were the plant of choice for a Colorado State University study that suggests adding legumes to crop rotations can help rebuild healthy soils. Led by Laura K. van der Pol, a Ph.D. candidate in Francesca Cotrufo’s lab in the Department of Soil and Crop Sciences, the study and findings were recently published in the journal Agriculture, Ecosystems, and the Environment.

Healthy soils for climate change resilience

Farming in semi-arid regions like the Mountain West can be challenging in part due to limited rainfall, and climate change increasingly threatens growers with higher drought risk and hotter temperatures.

“One way farmers have responded to this is by switching to a continuous cropping system. In this type of system, the fallow period is eliminated, and crops are planted each year,” said van der Pol. “This protects against erosion and, when considered over a full rotation period, produces higher yields.”

However, continuous cropping does not necessarily help rebuild soil organic matter, which is critical to soil health. Organic matter – animal and plant tissue in various stages of decay – improves soil structure to resist compaction and helps it hold more moisture. Increasing soil organic matter makes farms more resilient to the effects of climate change and stores carbon away from the atmosphere. Also, healthier soils require less fertilizer and other inputs, which saves money in the long run.

The soil carbon dilemma

When seeking to increase soil organic matter, farmers face what has been termed the “soil carbon dilemma.”

“The dilemma with soil carbon is analogous to the cake cliché – we want to have carbon in the soil but have the microbes eat it, too,” said van der Pol. Farmers want plenty of organic matter in their fields, explains van der Pol, but they also want that organic matter to decompose (e.g., be consumed by microbes) so that those nutrients can be made available to crops.

Adding legumes makes a difference

The CSU research team partnered with farmers and long-term agricultural research sites in Colorado and Nebraska to assess soil organic matter under three different no-till rotations – wheat-fallow, continuous crop (without legume), and continuous crop with legume as one of the crops in rotation.

Results of this study suggest that having a legume in a continuous rotation may help solve the soil carbon dilemma. When farmers had the leguminous pea crop in rotation, soil organic matter was higher than in the conventional wheat-fallow rotation common to the region, while the yields across the full rotation were maintained.

Microbes feeding on nitrogen-rich legume residue can grow more efficiently, spending less energy (carbon) to meet their metabolic needs, meaning less CO2 is released into the atmosphere.

And, as the microbial community grows in the soil, so does its capacity for breaking down plant residue.

“A key thing to note here is that microbial activity releases carbon and nitrogen in a form of soil organic matter that resists further microbial consumption and thus can persist in soil for centuries,” said van der Pol.

Van der Pol said these results are important because it shows one way that farmers can improve their soil health in this water-scarce region.

“Having more organic matter in the soil as well as greater diversity of crops are important strategies for farm resilience. Planting peas can help achieve both of these goals.”