When it comes to adding carbon to the soil, all cover crops don’t perform equally, according to a team of researchers whose new study revealed the disparity for the first time.
The study, led by Clemson University Assistant Professor Vidya Suseela in collaboration with researchers from Penn State University and Cornell University, shows cover crop mixtures with legumes, grasses and brassicas can increase long- and short-term soil carbon sequestration.
Carbon sequestration occurs when carbon is absorbed from the atmosphere and stored in the soil.
“Soil organic carbon is justifiably the most important component that increases soil’s physical, chemical and biological health, as well as improves agroecosystem profitability and sustainability,” said Suseela, a soil ecologist who investigates the mechanisms that promote the formation and endurance of soil carbon in ecosystems. “Although farmers use cover crops for various functions, including soil carbon formation, it is unknown whether cover crop mixtures can increase soil carbon compared to single cover crop species.”
Suseela points out it is important to understand how this newly formed carbon is stored as this determines the carbon’s fate – or if it will be persistent in soil – particularly in a changing climate. This study presents one of the first attempts to dig deeper into the effects of cover crops belonging to different plant species in increasing long- and short-term soil carbon.
The researchers sampled soils from under three cover crops — a legume, crimson clover; a grass, triticale (a cross between wheat and rye); and a brassica, canola — and a mixture of these three species, from a long-term cover-crops experiment at Penn State’s Russell E. Larson Agricultural Research Center in central Pennsylvania.
They measured two different types of soil carbon — particulate organic matter and mineral-associated organic matter.
Particulate organic matter is carbon that mainly comes from plant material that drops into the soil and breaks down into small particles. It is a relatively short-lived form of soil carbon, often decomposed by soil microbes in a matter of a few years. Although it does not represent a long-term form of carbon sequestration, the decomposition and cycling of particulate organic matter help support plant growth.
Mineral-associated organic matter is carbon that becomes chemically attached to soil minerals, like clay. This type of organic matter is a more persistent form of soil carbon that can remain in the soil for decades or even centuries. Increasing the amount of this type of soil carbon can help keep carbon out of the atmosphere over longer time periods.
Before this research, the relationship between the two types of soil carbon and cover crops was not well understood.
During the study, Clemson postdoctoral researcher Ziliang Zhang found cover crop mixtures contributed to a higher concentration of plant-derived compounds in particulate organic matter. The research findings, recently published in Global Change Biology, show soil organic carbon content was higher in all cover crop treatments than in fallow plots. Compared to the legume cover crop, soils under grass and brassica cover crops had higher proportions of plant-derived carbon in particulate organic matter. In contrast, soils under legumes had greater accumulations of microbial-derived carbon in mineral-associated organic matter.
“In terms of the global carbon cycle, understanding how plant carbon gets into soils and how long it stays there is a big deal,” Zhang said. “This study advances our fundamental understanding of how we might manage the global carbon cycle. Our discovery that different plant species tend to create different types of carbon — which we think have different lifespans in the soil — is significant.”
Jason Kaye, Penn State distinguished professor of soil biochemistry and study co-author, said the study is important because it may lead to ways of sequestering more carbon in agricultural soils, which comprise about one-third of the global land surface. In addition, soils are the largest reservoir of carbon on Earth.
“I think we’re at a crucial point nationally where we’re thinking about sequestering carbon in agricultural soils and wondering if cover crops can be a key tool to do that,” Kaye said.
Identifying ways to build soil carbon has become a major research priority for climate sustainability and food security, while simultaneously boosting soils’ ability to support agricultural production.
“This research provides farmers with an example of how they can build climate-friendly, fertile soils,” Kaye said. “And scientists need to identify strategies around this concept that strike a balance between helping plants grow in the short-term and locking away carbon for the long term.”
“Most of the cover crop species selected for our study are also used by farmers in South Carolina,” Suseela noted. “Thus, the results of the study could be applicable to farmers in South Carolina as we could design cover crops mixtures with plant species belonging to legume, grass, and brassica to enhance the long-and short-term carbon in South Carolina soils, which we are currently planning to pursue.”
Clemson researchers from various disciplines are conducting additional carbon research.