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Grass Roots Research

July 11, 2013

With news that an iceberg the size of Chicago has peeled off from Antarctica, attention has focused once again—however briefly—on global warming and the primary driver behind the phenomenon: atmospheric carbon. In other words, emissions—mostly carbon dioxide—from cars, factories, power plants, landfills and cows.

For some time, scientists have researched the possibility of using woodlands as carbon “sinks.” Trees draw carbon from the atmosphere, sequestering the element in their leaves, trunks, and roots. Plant enough trees, the reasoning goes, and you could make the planet as cool as a cucumber.

That would take a whole lot of trees, of course—and that may involve more expense, effort, and political accommodation than can be mustered. Plus, carbon stored in trees is vulnerable to logging and wildfires. Once timber is cut or burned, much of its hoarded carbon is released back into the atmosphere.

Now researchers are considering another possibility for storing carbon: rangelands. And the preliminary research is promising, says Whendee Silver, a professor of ecosystem sciences at Berkeley’s Department of Environmental Science, Policy and Management.

“Rangeland plants store a lot of their carbon below ground, where there’s a better pathway for [sequestration],” said Silver in a phone interview. “There’s less oxygen, and the carbon isn’t degraded as quickly. It gets locked up in soil aggregates—it’s just more difficult for it to get into the atmosphere than carbon stored in plant tissues that are above ground.”

Healthy rangelands stocked with plants boasting deep, robust root systems could thus store megatons of carbon. But many rangelands are not healthy; they’ve been mismanaged, says Silver, and their carbon-carrying capacity is low.

“Plowing and overgrazing are the main problems,” she said. “Soil that is dark and friable is rich in carbon—you don’t see that on rangelands that have been improperly managed.”

To restore these lands to their full carbon-carrying potential, then, the soils have to be restored and managed appropriately. To that end, Silver and her associates have conducted experiments on plots at UC’s Sierra Foothill Research and Extension Center in Browns Valley and on land near Nicasio in Marin County. Central to their work is compost.

“Spreading compost is a good way to get carbon back into the soil, and ultimately increase the soil’s capacity to sequester even more carbon by improving [pasturage],” says Silver. “Pure uncomposted manure releases its carbon fairly quickly when it’s spread on soil. But compost made from manure and other materials—discarded food and yard waste—slows carbon release.”

Early results of their experiments have been impressive. The researchers applied a single thin layer of compost on their plots five years ago. Over the past four years, the treated land showed a 40 to 50 percent increase in both plant growth and carbon storage.

Extrapolating those findings to larger scales hint at big pay-offs. Our cities generate millions of tons of compostable garbage annually. Some municipalities—San Francisco foremost among them—compost their wastes, but most ends up in landfills. Far better, says Silver, to turn all those potato peels, orange rinds, and grass clippings into compost and spread them on our grasslands.

“We did an analysis based on 6 percent of the available rangeland in California—which amounts to about 8 million acres,” said Silver. “We only considered material that’s available right now—wastes that are not currently used, and end up in landfills. We figured that would give us about 7 million tons of compost that we could spread to a depth of a quarter-inch on those 8 million acres, once every ten years.”

Such an effort would do two things, observed Silver. First, it would annually sequester about 6 million metric tons of carbon—or CO2 equivalent, in official parlance—via the grass roots the nutrient-rich compost would generate.

“It would also divert 21 million metric tons annually of CO2 equivalent from the waste stream.” Silver says. “That carbon would go into the soil where it would be locked up rather than landfills where it would eventually disperse into the atmosphere. So altogether, you’re looking at about 27 million metric tons of stored carbon annually.”

That’s a lot of metric tons no matter how you parse them. But would it make any difference?

“The California livestock industry generates about 15 million metric tons per year of CO2 equivalent, the commercial and residential energy sector produces about 42 million metric tons, and electricity production generates about 112 million metric tons,” said Silver. “So when you get 21 million metric tons of storage using only 6 percent of the available rangeland—I’d say yes, it does make a difference.”

—Glen Martin

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