Cover crops can have implications for carbon sequestration

When it comes to adding carbon to the soil, all of them cover crops they don’t perform equally, according to a team of Penn State University researchers whose new study revealed the disparity for the first time.

The research – a collaboration between Penn State, Clemson and Cornell universities – may lead to ways to capture more carbon in agricultural soilsaccording to the study’s co-author Jason KayeDistinguished Professor of Soil Biogeochemistry at Penn State College of Agricultural Sciences. Agricultural soils comprise approximately one-third of the global land area, and soils are the largest carbon reservoir on Earth.

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With the help of plants and microbes, soils can pull carbon out of the atmosphere and lock it into the ground, helping to mitigate climate change, explained Kaye, whose research group was experimenting with cover crops for two decades. At the same time, he added, soil carbon is crucial to sustaining plant growth because it helps create healthier, more fertile soils.

Image by maxbelchenko, Shutterstock

“I think we’re at a crucial point nationally where we’re thinking about carbon capture agricultural soils and wondering if cover crops can be a key tool to do that,” he said. “There are a lot of questions about how much carbon could be stored and what types of carbon. So this research has really remarkable implications.”

To reach their conclusions, the researchers took soil samples under monocultures (a single crop in a given area) of cover crops belonging to three functional plant types – a legume (purple clover), a grass (triticales, a cross between wheat and rye), and a brassica (canola), as well as a mixture of these three species. The plots were part of a long-term cover crop experiment at Penn State Russell E. Larson Agricultural Research Center in central Pennsylvania.

The researchers measured two types of soil carbon – particulate organic matter and organic matter associated with minerals. The first is carbon that comes mainly from plant material that falls into the soil and breaks down into small particles; the latter is carbon that becomes physically or chemically bound to soil minerals such as clay. Both processes involve soil bacteria and fungi.

Before this research, the relationship between the two types of soil carbon and cover crops was not well understood, the researchers said. Particulate organic matter is a relatively short-lived form of soil carbon, often broken down by soil microbes within a few years. Although this means it is not a long-term form of carbon sequestration, the decomposition and cycling of particulate organic matter helps support plant growth.

On the other hand, organic matter associated with minerals 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 carbon in the soil can help keep carbon in the atmosphere for longer periods of time.

The researchers recently published their discoveries in the Biology of Global Changereporting that organic soil carbon content was greater in all cover crop treatments than in untreated plots. Compared to legumes, soils under grass and brassica monocultures had a higher proportion of plant-derived carbon in particulate organic matter. In contrast, soils under legumes had a greater accumulation of microbially derived carbon in mineral-associated organic matter.

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Cropping the cover (Image by USDA)

The study results revealed, for the first time, that the mix of cover crops contributed to a higher concentration of plant-derived compounds in organic particles, explained Ziliang Zhang, who led the study. As a postdoctoral researcher in Clemson University’s Department of Plant and Environmental Sciences when the research was conducted, he performed the vast amount of soil and data analysis involved in the study.

“In terms of the global carbon cycle, understanding how plant carbon gets into the soil and how long it stays there is a big problem,” he said. “This study advances our fundamental understanding of how we might manage the global carbon cycle. Our finding that different plant species tend to create different types of carbon – which we think have different lifetimes in the soil – is significant.”

Identifying ways to build soil carbon has become a major research priority for climate sustainability and food security, Kaye noted, while boosting the ability of soils to support agricultural production.

“Our findings demonstrate that including a diversity of crop types on farms is critical to increasing soil carbon in both the short and long term,” he said. “This research gives farmers an example of how they can build fertile and ecological soils. And scientists need to identify strategies around this concept that strike a balance between helping plants grow in the short term and sequestering carbon in the long term.”

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