On a bright October morning Dave Brandt tromps through the middle of his central Ohio wheat field. The grain was harvested months ago, but there isn't an inch of bare dirt anywhere. Instead, more than 10 varieties of plants, including crimson clover, pearl millet, and Austrian winter peas, form a "cover crop cocktail" that stretches all the way to the road bordering his property. "This will be here all winter," Brandt says. "And in the spring, we'll plant corn right into this."
Brandt hasn't tilled his soil since 1972, when he rented his first 600 acres of farmland to grow wheat, corn, and soybeans. And by keeping plants on his land in various stages of growth and decomposition, Brandt appears to have increased the amount of carbon in his soil over the years. One study estimated that total organic carbon in the top foot of Brandt's soil increased by 10% after six years of no-till, 35% after 20 years, and 61% after 35 years. (The data on which this estimate was based were not peer reviewed.) Overall, Brandt's soil stored, or sequestered, an estimated average 960 kg of carbon per hectare per year.
With figures like those, soil scientists and climate researchers believe that returning carbon to the soil on a large scale could help mitigate climate change. The world's terrestrial carbon stores - the combined amount of carbon in soil and in plant matter - are much greater than the amount of carbon in the atmosphere: 3.12 trillion metric tons in the top meter of soil versus 780 billion metric tons in the atmosphere, by one estimate. Before the dawn of agriculture, there was even more carbon in the global soil pool - an estimated 55–78 billion additional metric tons, and the world's plant biomass held still more, much of it lost to land use changes.
That's why Rattan Lal, one of world's preeminent soil scientists and director of the Carbon Management and Sequestration Center (C-MASC) at The Ohio State University, has called for recarbonizing the world's soils. Doing so, Lal says, "would be a truly win–win–win situation." In addition to carbon sequestration, increasing carbon in soil has many other co-benefits: increased water storage in soil, increased length of the growing season, cooling of the ground via evapotranspiration, recharging groundwater aquifers, keeping springs and rivers flowing in the dry season. Soil itself filters water, reduces flooding, and provides a water reserve for plants in times of drought.
Lal believes food security is an especially important co-benefit. By one estimate, about 24% of total global land area shows evidence of impaired productivity, and each year some 1–2.9 million hectares is degraded so badly that it becomes unsuitable for farming. Increasing population levels, predicted to reach 9.2 billion by 2050, will place more pressure on the world's farmlands to produce enough food.
Lal has calculated that increasing organic carbon in the soil surrounding plant roots by 1 ton per hectare per year can increase grain production by 32 million tons per year. "This is especially important for small landholders of sub-Saharan Africa, South Asia, and the Caribbean," he says.