Claim: Grazed grasslands trump cover crops on long-term carbon sequestration
In the debate over how to use agricultural lands to sequester carbon and help mitigate climate change, no-till and cover cropping get most of the attention. But studies are starting to show that grazed perennial pastures, where the soil is rarely disturbed and continuously covered, may be the best strategy for locking carbon in the soil long-term, according to experts on a recent Environmental Working Group webinar.
“Our best and maybe only opportunity to really help mitigate climate change in our soils is with grazed grasslands,” said Randy Jackson, agronomy professor at the University of Wisconsin and one of three featured speakers on the webinar. “But it has to be done well. It’s really important that we understand what good grazing management is,” he said.
Jackson is part of Grasslands 2.0, which advocates for the restoration of “grasslands and other forms of perennial agriculture to the agricultural system.” As part of that mission, Grasslands 2.0 helps define best grazing practices, including a strategy called intensive rotational grazing in which the cows are moved every few days among different pastures.
Jackson shared insights from a long-term field study that he manages that compares soil carbon in three grain crop systems, including one that was minimally tilled, with three grassland systems that support foraging dairy cows. His researchers take core samples every 10 years; the most recent available results, 1989-2009, show that all of the plots with grain crops lost soil carbon while the grasslands, where cows were rotated frequently, allowing the soil to rest, retained it.
The grassland plots also had more of a type of carbon that binds to minerals in deeper soil layers, facilitating its storage long-term.
Jackson noted, though, that there may be a limit to how much carbon can be stored in those deeper layers because there are only so many minerals in the soil. But he said this is something that soil scientists are still debating. “This is the frontier of soil carbon research,” he said.
The speakers were not suggesting the use of cover crops had no environmental benefits. Anna Cates, an assistant extension professor of soil health at the University of Minnesota, said they prevent runoff and soil erosion and use existing soil nitrogen, holding it in place. Some cover crops also act as natural fertilizer, allowing farmers to apply less synthetic nitrogen. But the roots of cover crops stimulate microbial activity deep in the soil and this leads to carbon cycling: the microbes eat the carbon provided by the plants and then breathe it out, sending it back into the atmosphere.
Cates also discussed the importance of soil structure. Soil with air pockets and unbroken clumps of sand, silt, and clay is healthier than soil lacking those structures because they allow water to filter below the surface and help the soil to retain water longer.
“When we have crops growing, there’s a really striking tale told by the water behavior,” she said. “We want soil to let water in when it rains and store water when it’s dry. If your soil is leaving the field with wind or water, it’s not healthy.”
Another speaker, Mike Castellano, a professor of agronomy at Iowa State University, described a large study he’s leading that collects data on synthetic fertilizer use from hundreds of farmers in the Corn Belt.
The goal is to use those data sets to figure out the precise nitrogen input levels needed for different field types, which would help farmers use less fertilizer and avoid over-application. That would help reduce greenhouse gas emissions and the nitrogen runoff that pollutes waterways. “This will be transformational,” he said.