New framework reveals full ecological value of cover crops
Cover crops, such as red clover and rye, are proven tools for curbing fertilizer runoff and erosion from farms. “But we also know they do more,” says Colorado State University crop scientist, Meagan Schipanski. To demonstrate the full value of cover cropping, she and a diverse team of collaborators have produced a framework for evaluating cover crops’ many possible benefits simultaneously, and how those benefits change through time.
In a study published in March in Agricultural Systems, the team used the analytical framework to estimate the impacts of cover crops on 11 ecological benefits in a three-year, soybean-wheat-corn rotation.
Cover crops, they found, increased eight of the 11 benefits without hurting crop yields. These included higher carbon and nitrogen levels in soils, weed suppression, and greater colonization by beneficial fungi that help plants take up nutrients.
Moreover, the scientists pinpointed the key periods during the rotation when different ecosystem benefits accrue—something that’s missed when researchers only measure the cumulative effect at a study’s end, Schipanski says. For instance, retention of nutrients by cover crops happens mainly in spring and fall, weed suppression benefits occur in summer, while soil carbon benefits amass slowly over decades.
“So, [the time points] when we look at and measure these things matters,” Schipanski says.
Part of what’s driving the research is growing public demand that agriculture deliver not just food or fiber goods, but also a larger suite of environmental benefits that scientists call “ecosystem services,” explains Schipanski’s co-author, Jason Kaye, of Penn State University. In the Chesapeake Bay region where Kaye works, for example, cover crops are widely planted in farmlands to protect the bay’s water quality. The Natural Resources Conservation Service (NRCS), meanwhile, has made cover cropping a central practice in its national initiative to store more carbon in soils.
And yet, the most common metrics for assessing agricultural systems remain crop yield and short-term profitability, Kaye says. “In other words, cover crops are treated as a tool to be used only if they don’t interfere with cash crop production.”
By illustrating the many diverse benefits and services that cover crops can offer, the team hopes this will now begin to change. “We need a new way to evaluate ‘success’ in agriculture,” Kaye says.
The team’s framework for evaluating multiple ecosystem services emerged from a 2011 special topics course at Penn State led by Schipanski (then a postdoc working with Kaye and Penn State weed ecologist, David Mortensen) and others. Several Penn State scientists from different disciplines were already hypothesizing that agricultural systems could provide many more ecological benefits than people thought, Schipanski says. But no one had yet done the necessary synthesis.
“So we used the course to get everybody together once a week,” she says. “And through a semester, we hammered this out.”
What this meant is that each research group chose an ecosystem service to focus on, such as weed suppression, nitrogen retention, or natural pest control. Each team then used existing cropping systems models or data from the literature to estimate the impact of cover crops on the service in a typical soybean-wheat-corn rotation. The entire group also decided ahead of time how to put each estimate on a relative scale for easy comparison. They then graphed these “normalized” values in what are called spider plots to see how cover crops affected all 11 ecosystem services at different times during the rotation.
The framework isn’t restricted to the 11 services selected by the scientists, however. Instead, it provides a general approach to visualizing the complexity of agricultural systems and the decisions involved in their management—something that farmers deal with every day, Schipanski says.
“One of the strengths of this approach is that different groups can define the ecosystem services or benefits that are most important in their context or operation,” she says, and then use the framework to guide decision-making about them.
Penn State Extension has already done this with Pennsylvania farmers and conservation groups, she adds. And she herself is now applying a similar framework to the dryland (non-irrigated) farming systems of the Great Plains.
Cover crops are recognized as beneficial in the semi-arid West, too, she explains, but they also have a major drawback not found in Pennsylvania. They remove water from the soil during growth—so much water, in some cases, that yields of the following crop are reduced.
At the same time, cover crops can help build organic matter in the soil, giving it the capacity eventually to store more water. “So, there are some really tricky tradeoffs,” Schipanski says. “Can we integrate cover crops or forage crops into certain portions of the [rotation] to provide some of these multiple ecological benefits while maintaining yields and profitability?”
It’s a complicated question well suited to the new approach, and Schipanski and her colleagues are hoping now that the concept will spread. “We need more research on management approaches that look at system complexity,” she says. “We hope our framework will inspire that.”
The study was funded by the USDA National Institute for Food and Agriculture (NIFA).