Ohio State University soil scientist Rattan Lal says the agricultural soils of the world have the potential to soak up 13 percent of the carbon dioxide in the atmosphere today—the equivalent of scrubbing every ounce of CO2 released into the atmosphere since 1980. The claim is a bold one, but researchers around the globe are digging up evidence that even modest changes to farming and ranching can have a major impact on carbon sequestration.
Some growers have already embraced an approach known as regenerative agriculture, which aims to boost soil fertility and moisture retention through established practices such as composting, keeping fields planted year-round, reducing tillage, and increasing plant diversity. Since these strategies can also significantly increase the amount of carbon stored in the soil, some agricultural researchers are now building a case for their use in combating climate change. This year seven international conferences will examine soil’s potential to sequester greenhouse gases.
Lal first came to the idea of soil as a powerful carbon sink (pdf) not through an interest in climate change, but rather out of concern for the land itself and the people who depend on its productivity. While carbon-depleted soils tend to be dry and prone to erosion, carbon-rich soil is dark, crumbly, fertile, and moist. In the 1970s and 1980s, Lal was studying soils in Africa so devoid of organic matter that the ground had become like hardened cement. There he met Roger Ravelle, a pioneer in the study of global warming. When Lal made a despairing remark about the impoverished soil, Ravelle suggested that the carbon had moved into the atmosphere. “I told Roger I didn’t know where it had gone; I just wanted to put it back,” Lal recalls.
Ravelle was right. For millions of years, a natural partnership between plants and soil microbes has helped regulate carbon dioxide levels in the atmosphere. During photosynthesis, plants absorb carbon dioxide from the air and transform it into sugars and other carbon-based molecules. Some of those carbon products transfer from the roots to symbiotic fungi and soil microbes, which store the carbon in the soil as humus.
The invention of agriculture some 10,000 years ago disrupted these ancient soil-building processes. When humans started draining and plowing up the natural topsoil for planting, they exposed the buried carbon to oxygen, creating carbon dioxide and releasing it into the air. Animal husbandry made things worse, as domesticated animals began grazing grasslands down to the earth. In places where the ground is bare—from overgrazing or from the common practice of leaving fields unplanted for part of the year—photosynthesis stops, and so does the storage of carbon in the soil. Lal calculates that land-use changes such as these have stripped 70 billion to 100 billion tons of carbon from the world’s soils and pumped it into the earth’s atmosphere, oceans, and lakes since the dawn of agriculture. Today agriculture and other land-use changes account for about a third of global greenhouse gas emissions.
To quantify soil’s carbon sequestration potential on agricultural lands, soil scientist Whendee Silver of the University of California, Berkeley, is conducting a first-of-its-kind study on a 539-acre cattle ranch near Nicasio, California. In a collaboration with ranchers and local and state land management organizations called the Marin Carbon Project, she and her students are testing the effects of compost created from city yard waste (such as leaves, branches, and lawn trimmings) and agricultural waste (including manure and cornstalks) on carbon storage.
Although previous experiments have shown that compost increases soil carbon, Silver is among the first to examine whether real-world ranchers can use it effectively to enrich the soil on their rangeland. She has already found a large increase in soil carbon two years after a single application of compost, probably due to enhanced vegetation growth. On the basis of her results, Silver projects that 28 million acres of grazing land in California could absorb 42 million tons of carbon dioxide—nearly 40 percent of what the state’s electrical power plants produce in a year. To accomplish that, each acre of land must absorb just 1.5 additional tons of carbon dioxide. “Given what we’ve seen in our experiments,” Silver says, “one and a half tons is doable.”
In Australia, Christine Jones, soil ecologist emerita of the New South Wales Department of Land and Water Conservation, is testing another promising soil- enrichment strategy, one that relies on perennial grasses. Since carbon sequestration stops in the absence of living plants, Jones and 12 ranchers in Western Australia are working to build up soil carbon by cultivating grasses that stay green year-round. Like composting, the approach has already been proved experimentally; Jones now hopes to show that it can be applied on working ranches and that the resulting carbon capture can be accurately measured. Over the course of four years, she has charted the carbon content of the grasslands, and when the first phase of the project concludes this August, philanthropist Rhonda Willson will pay the ranchers for every additional ton of carbon tucked away in their soils. “The changes we’ve registered over the past few years will surprise the world,” Jones says.
Silver and Jones hope that projects such as theirs will demonstrate the role that farmers, ranchers, and other land managers can play in mitigating the effects of heat-trapping greenhouse gases. Lal says that the greatest opportunities lie in the world’s most depleted and eroded soils, in sub-Saharan Africa, south and central Asia, and Central America. Success there will rely on providing farmers the tools and knowledge to improve their land, as well as financial compensation for their carbon enrichment of the soil.
The same is true in wealthier societies like the United States, where most farming operations chase productivity through large applications of fertilizer. Changing long-standing habits will require a system that rewards land managers not just for the corn or beef they produce, but also for the carbon they can build into their property. “Farmers should get compensated for protecting the ecosystem,” Lal says. “This is something worth paying for.”
Regenerative Agriculture An approach that aims to protect natural resources such as soil through techniques including crop diversity and rotation.
Carbon Sink A reservoir that can hold carbon and prevent it from escaping into the atmosphere. Proper management could turn agricultural soils into a powerful sink.
Marin Carbon Project A joint effort by scientists and ranchers in California to study rangeland’s potential to soak up carbon.
Compost Fertilizer made of decaying organic matter. By boosting plant growth, compost helps to increase soil carbon storage.
Perennial Grass According to an ongoing study in Australia, planting ranch lands with grasses that remain alive year-round also increases the amount of carbon trapped in the land.
