Supercharging farm-soil to hoover up atmospheric carbon

Here's some promising eco-news: A simple scientific experiment tweaked the ecosystem of a California farm, and the soil began capturing tons of carbon from the atmosphere.

The experiment began in 2007, when some Californian farmers collaborated with some ecologists. They wanted to enrichen the soil so that it'd grow grasses and plants that trap more carbon, particularly "occluded" carbon, which is usually dead microbes. That stuff gets in the earth and stays there for a long time.

To try and encourage new forms of growth that trap that type of carbon, they did a simple treatment: They covered three acres of the farm with a half-inch of compost, bought from a nearby composting plant.

When they checked back three years later, the soil in the treated acres was indeed trapping far more carbon than untreated soil in nearby farms (which, interestingly, was mostly losing carbon, on balance). How much capture had taken place? About 1.5 tons of carbon dioxide per acre per year, which is "roughly equal to your car's emissions if you drove from Miami to Seattle."

The scientists' research suggests that the soil will continue to capture carbon at that rate. That's particularly remarkable considering it was a one-time treatment!

This suggests something quite enticing: Treat farms all over America just one time with a half-inch of compost, and they could become sponges that soak up tons of carbon. Moses Velasquez-Manoff has written a fascinating long feature about the work in the New York Times Magazine:

In the years that followed, Silver's analyses of soil cores indicated that the treated land kept taking in carbon. Computer simulations suggest that it will continue to do so for decades. It also retained more moisture and grew about 50 percent more grass. One dose of compost ignited what Silver calls a state change: The plants and the soil - and everything that inhabited it - moved toward a new equilibrium in which the soil ecosystem pulled in and retained greater amounts of carbon.

Silver began publishing her findings in scientific journals in 2010. Her second paper, written with her postdoc Marcia DeLonge and the graduate student Rebecca Ryals, offered a remarkable bit of extrapolation. California has about 56 million acres of rangeland, the single largest type of land use in the state. If compost made with manure was applied to just 5 percent of that area, they calculated, it would offset emissions from about 80 percent of the state's agricultural sector - all the cows raised, crops grown, fertilizer applied and tractors driven in California. Much of that offset came from diverting manure from festering lagoons - where it releases methane and nitrous oxide into the atmosphere - into compost, a one-time benefit. But the ongoing drawdown of carbon dioxide from enhanced grass growth could be important, too. If you treated 41 percent of the state's rangeland, Silver told me, carbon pumped into the earth by photosynthesis might render the entire agricultural sector of the world's sixth-largest economy carbon-neutral for years to come.

As it turns out, small farmers have themselves been latching onto the idea of increasing the level of carbon in their dirt, because it means they need less fertilizer, which is expensive and erodes their soil. So the idea of mass-treating farms with compost could have huge buy-in from farmers and rural areas: It'd be a good-old-fashioned win-win, improving farm economics while fighting climate change.

Mind you, there plenty of caveats and unsettled questions about this carbon-capture strategy, as Velasquez-Manoff notes.

One is that the study here hasn't been replicated. Would this compost-treatment work in other regions outside California? Nobody knows. What's more, compost is expensive and requires energy to produce, so a full-cost accounting would have to make sure this technique isn't actually increasing our carbon burn. And man, you'd need a lot of compost: The scientists used ten times the amount a farmer normally would on an acre of land.

So I'd love to see this work replicated. If it holds up, and the cost-accounting works out, it could be a fantastic, sustainable front on mitigating climate change.

(CC-licensed photo via John-Thomas Nagel)