Soil Storage: Sequestering Carbon Dioxide in Agricultural Land

California’s agricultural industry ships a staggering amount of prized products all over the world: almonds, grapes, and strawberries, to name a few. But in the future, our farms may become even better known for something they’ll keep to themselves: carbon dioxide.

Seven years ago, three farms in Marin County became testing grounds for the Marin Carbon Project. On this acreage, the non-profit coalition of land stewards study management practices designed to pull more carbon dioxide out of the atmosphere and store it in soil. If these “carbon capture” methods gain traction, then Golden State farmers could help slow the growth of greenhouse gas emissions that contribute to global warming — and reap benefits from sowing a local carbon economy.

“The idea of soil as a source for storing carbon is just catching fire,” said Jeff Creque, a rangelands management expert and co-founder of the Marin Carbon Project. “It’s been a gradual awakening for people to see the environmental potential across the agricultural landscape.” If enough “carbon farms” can sequester carbon dioxide from the atmosphere, it could make a positive impact in the fight against climate change.

In the natural carbon cycle on land, plants pull carbon dioxide out of the atmosphere for photosynthesis, a process that creates energy for plants to grow and provides storage space in roots and soil. Eventually, plants decompose, soil erodes, and other processes release carbon dioxide back into the atmosphere. But rising carbon dioxide emissions from burning fossil fuel — coal plants and automobile engines — have unbalanced that cycle. As those additional emissions accumulate in the atmosphere, each molecule of carbon dioxide absorbs radiation and adds more heat to the Earth’s surface. It adds up.

Just a half-inch “dusting” of compost could sequester almost 1,000 pounds of carbon dioxide, per acre, per year, according to research conducted by Dr. Whendee Silver, a biogeochemist at UC Berkeley. Even better, she found that soil moisture improved and plants thrived more, both of which helped draw additional carbon dioxide into the soil. Further studies demonstrated those effects could last for decades. With approximately 38 million acres of grasslands in California, even if only five percent of that land was managed to store more carbon dioxide, it could make an impact, Silver said.

Although some conservationists are concerned that composting grasslands will have adverse effects on native habitats and grasses, Creque noted the idea isn’t to spread compost on all the land available, but to use the method as one of many carbon-storing tools available. The project’s planning protocol also requires consulting with rangeland professionals and regional conservation districts. “There’s a whole team of professionals involved,” Creque said.

The concept of carbon farming isn’t new. For decades, organic farmers have used crop rotation, planting trees, and other methods to move carbon dioxide out of the atmosphere and into plants and soil. The Marin Carbon Project partnered with the USDA’s Natural Resources Conservation Service and Colorado State University to develop a suite of tools that individual farms can use to create customized plans for capturing carbon dioxide. One key part of the project was accounting for the amount of carbon dioxide kept out of the atmosphere. With the ability to calculate carbon “offsets,” the landholders have a potentially bankable way to “sell” the carbon they sequester to industries creating too much carbon to satisfy regulations.

The trio of demonstration farms — Stemple Creek Ranch, Straus Dairy, and the Corda Ranch — just completed their first carbon farming plans. Now, there will be a process of comment and analysis, and new projects are in the works.

In the future, Creque said it would be nice to see a revolving conservation fund, created with carbon farm offsets, to help move new carbon farming projects forward. Although a greenhouse gas exchange is only in the exploratory stage, Creque is optimistic. He noted the governor’s proposed 2015-16 budget includes a commitment to establish long-term goals for carbon levels in all California’s agricultural soils.

“I don’t know of another government in the world that’s set a positive goal for soil carbon increases,” Creque said.

Elizabeth Devitt is a freelance science writer based in Santa Cruz.