When settlers plowed the North American prairie, they uncovered some of the most fertile soil in the world. But tilling those deep-rooted grasslands released massive amounts of underground carbon into the atmosphere. More greenhouse gases wafted into the skies when wetlands were drained and forests cleared for fields. Land conversion continues today, and synthetic fertilizer, diesel-hungry farm machinery, and methane-belching livestock add to the climate effects; all told, farming generates 10% of climate-affecting emissions from the United States each year. Now, Congress would like to turn back the clock and return some of that carbon to the soil.
The Inflation Reduction Act, a broad bill signed into law today, has historic climate provisions, including massive subsidies for clean power and electric vehicles. But lawmakers also included more than $25 billion to expand and safeguard forests and promote farming practices thought to be climate friendly. Those include no-till agriculture and “cover crops,” plants cultivated simply to protect the soil. Researchers, environmental groups, and the farm industry agree that paying and training farmers to adopt those measures will improve soil health and water and air quality. “I think pretty much everyone across the board is pretty happy,” says Haley Leslie-Bole, a climate policy analyst with the World Resources Institute. But how much these practices will slow global warming is unclear.
“It’s probably going to be positive, but how positive we don’t really know yet,” says Jonathan Sanderman, a soil scientist at the Woodwell Climate Research Center. A major factor is whether the U.S. Department of Agriculture (USDA) spends the money on the practices most likely to have climate benefits. Another challenge is measuring and quantifying the reductions, a task complicated by the great diversity of U.S. land and farming practices and the complex biogeochemistry of the carbon cycle.
Sanderman and others think bigger climate gains could come from other changes in farming, such as lessening emissions from fertilizer and livestock. And climate and agriculture expert Tim Searchinger of Princeton University sees a need for more research on climate-beneficial farming practices. USDA “needs to come up with an ambitious, creative plan and include a really good system for tracking progress.”
U.S. farmers have long received payments for conserving soil and reducing the pollution that can run off their fields. The new bill expands funding for those programs, allowing more farmers to get a per-acre payment for a wide range of activities expected to reduce or sequester carbon emissions, including no-till and cover crops.
Yet how those practices will affect the soil’s ability to accumulate and hold carbon depends on a complex interplay between plants, which add organic material to the soil, and microbes and other tiny organisms that break down everything they can, sometimes releasing greenhouse gases. Those interactions vary enormously with soil type and environmental conditions, and it can take years for the impact of changes in crop types or farming techniques to emerge.
Planting cover crops such as cowpeas and vetch, for example, boosts soil carbon on average each year by one-third of a ton per hectare, according to a 2015 meta-analysis in Agriculture, Ecosystems & Environment. The cover crops also help control weeds and make the soil more porous, says Cristine Morgan, scientific director of the Soil Health Institute. “Grab a shovel and jump on it,” she says. “And the shovel goes deeper.” Yet the net climate benefits remain uncertain. On the one hand, leguminous cover crops such as cowpeas and vetch add nitrogen to the soil, which microbes can transform into nitrous oxide, a potent greenhouse gas. But farmers might then add less fertilizer and that would reduce emissions.
Another reputedly climate-friendly practice is no-till farming, which is growing in popularity. By not plowing fields, farmers protect the topsoil from erosion. They also save on diesel, which benefits their bottom line and reduces carbon dioxide emissions. Crop yields can increase as carbon accumulates in the upper root zone, enriching the soil and helping it retain moisture.
At the same time, studies have shown that carbon decreases in deeper layers of certain soils, making the net change small or zero in those places. And during the first decade, no-till fields release higher levels of nitrous oxide as microbes break down crop residues, although the levels eventually decline. A further complication is permanence: Few U.S. farmers continue the practice uninterrupted. Factors such as the growth of herbicide-resistant weeds and the need to rotate crops eventually convince them to plow again, which can allow nearly one-third of the accumulated carbon to escape into the atmosphere.
Still, Keith Paustian, a soil scientist at Colorado State University, and others argue that together, these regenerative soil practices are powerful climate tools. “We can regain potentially quite a fair amount of that lost carbon,” he says, by combining no-till and cover crops with other steps, such as planting trees on farms and improving grazing management to revitalize pastures. “We need to get agriculture to a new paradigm where these kinds of practices are the norm and not the exception,” he says.
In addition to fields and forests, the bill’s climate-related provisions target livestock, which emit methane that accounts for 27% of all greenhouse emissions from U.S. agriculture. As part of one USDA program awarded $8.5 billion, the bill instructs the agency to prioritize projects that will reduce these emissions from cows and other ruminants by modifying their feed. An additive called 3-NOP can cut methane burped by dairy cattle by about 30% and is used in the United Kingdom and Europe. But it hasn’t been greenlighted yet by the U.S. Food and Drug Administration, which considers it a drug, slowing the approval process.
Searchinger and others hope USDA will also emphasize the incentives the bill provides to cut fertilizer use. Nitrification inhibitors, compounds that slow microbial conversion of fertilizer into nitrous oxide, can help farmers apply less and also cut emissions. Farmers may be receptive because of fertilizer shortages and high prices. USDA announced yesterday it will streamline applications for fertilizer management.
Many climate advocates say the most cost-effective way to help the climate through agriculture is simply to farm less land and raise less livestock. That means persuading farmers not to convert grasslands or other carbon-rich lands to row crops such as corn and soybeans. But the bill includes no additional funding for USDA’s main program for protecting sensitive private land, the Conservation Reserve Program.
What might it all add up to? Paustian admits it’s hard to pin down the total climate benefit of the $25 billion. To help, the bill also includes a special pot of $300 million for USDA and partners to collect field data on carbon sequestered and emissions reduced—data could help target future climate efforts more effectively, he and others say. “That’s really significant,” says Alison Eagle, an agricultural scientist with the Environmental Defense Fund. “This investment can help direct the next set of funding to the right place.”
The new money will expand on grants USDA recently funded, such as a 5-year project Ellen Herbert, an ecologist with Ducks Unlimited, is leading to measure carbon in wetlands across the central United States. She hopes to learn whether restoring wetlands or protecting adjacent land boosts the carbon they store. To resolve that, they’re taking unusually deep soil cores, delving to 1 meter or more. “It’s sometimes like trying to pound an aluminum tube through a brick,” Herbert says.
Also in the Midwest, agro-ecosystem scientist Bruno Basso of Michigan State University and colleagues are tracking how improved fertilizer strategies can lessen nitrous oxide emissions. In addition, they’re measuring soil carbon tucked away by perennial grasses planted on less productive parts of fields from North Dakota to Mississippi. Those grasses can be harvested for biofuels or hay—as well as cash for the carbon left by the roots—which perks up farmers’ ears, Basso says. “I say listen, you know what, there is a new crop and it’s called carbon.”
In the southeast, a team led by Austin Himes, a forest ecologist at Mississippi State University, is taking stock of carbon sequestered when farmers are paid to plant trees on marginal land.
Data from the three projects will improve biogeochemical models that estimate daily fluxes of key greenhouse gases from agricultural land. One such model is DayCent, used by the Environmental Protection Agency and others for the national greenhouse gas inventory. “The accounting is not glamorous,” Himes says, “but if you don’t get that right, nothing else works.”
Erik is a reporter at Science, covering environmental issues.
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