In a huge pit on the fringes of Vashon’s largest forest, an effort to address what many consider the world’s most pressing ecological challenge is about to unfold.

Scientists from King County and the University of Washington are teaming up to see if large quantities of nutrient-rich compost along the shoulders of a five-acre pit of degraded land can transform it into what good soil for centuries has provided: a repository for carbon.

The goal is to offset the predicted rise in greenhouse gas by storing or sequestering carbon — in other words, taking it out of the atmosphere and putting it back into the soil column.

Bob Fuerstenberg, a senior ecologist in the county’s Department of Natural Resources and Parks, said such carbon-sequestration projects represent one of the best hopes for slowing atmospheric processes that are already wreaking havoc in the Arctic and Australia and that are beginning to melt glaciers, change migration patterns and affect ecologic processes in our region, as well.

“If this works,” Fuerstenberg said of the demonstration plots he hopes to have in place by August, “then we just need to start doing it wherever we can, because we don’t have a lot of time left. … There’s a small window of opportunity here.”

The project, part of the county’s much-touted Climate Change Initiative, will unfold at the so-called “borrow pit” next to the Vashon Transfer Station — a crater of barren land where the soil was “borrowed” over several years as a daily cover to the adjacent landfill. The landfill closed in 2001.

That kind of soil disturbance is just one of the reasons the world is now facing potentially cataclysmic alterations to its climate, Fuerstenberg said. In their natural condition, soils are the third largest repository of carbon, after oceans and fossil carbon. But in King County, as in much of the world, the soils have been so depleted and disturbed that they’ve lost 60 to 70 percent of their historic carbon, gasses that have been released into the atmosphere, he said.

Others, particularly in Europe, are also sounding the alarm about carbon depletion in soils; a March report by the European Commission noted that the conversion of grasslands and forestlands into agriculture, where the soil is tilled and disturbed, is “turning what were carbon sinks into carbon sources.”

Fuerstenberg and others involved in the project are trying to demonstrate a way to dramatically and quickly rebuild soil, so that it once again acts as a carbon sink.

He plans to do that by layering compost over the thin rocky soil that now covers at the site. The nutrient-rich compost will then kick-start one of the foundational processes in the biochemistry of the planet: Microbes will grow quickly, breaking down carbon in the soil that plants need to grow; and plants will take off, “mining CO2 from the atmosphere,” Fuerstenberg said.

As the plants decay, the microbes will take the carbon they leave behind deeper and deeper into the soil, turning the soil, once again, into a carbon sink.

“Once the process starts, it becomes a positive feedback loop,” he said.

Fuerstenberg, an Islander, called it “a fortunate accident” that the project is unfolding in what he considers his backyard. It also represents collaboration at its best, he said.

When he decided to try to undertake such a project, one of his first dilemmas was finding the right site — a swath of public land with depleted soils and big enough to have an impact. He learned of the borrow pit in that process of discovery that sometimes happens on Vashon — because of a chance conversation in Vashon Thriftway, where he ran into David Warren, who heads Vashon Forest Stewards.

Warren, coincidentally, had been thinking along similar lines — bringing large quantities of compost to the site — but for a different reason. He wanted to get some trees growing at the borrow pit as part of a larger effort at ecological restoration at Island Center Forest. The borrow pit is on the western rim of the 363-acre forest, which is owned and managed, conveniently, by the county’s Department of Natural Resources and Parks, Fuerstenberg’s employer.

With a potential site, Fuerstenberg started talking to others in the county, including the Wastewater Treatment Division, which has been developing a “waste-to-resource” project for several years. The division takes waste from the county’s sewer treatment plants, including the one on Vashon, turns them into bio-solids and ships them across the state, where they’re used as compost for farming and forestry projects.

The division had been working with Sally Brown, a University of Washington soil scientist, to find a demonstration site closer to home for a composted version of the nutrient-rich organic matter — with a goal of building greater receptivity for compost made from bio-solids, Fuerstenberg said. His project struck a chord.

“It was serendipity in its finest sense,” he said.

What’s more, others, too, were captivated by both the potential for the project and the need to act quickly to address climate change. The county had been exploring ways to address its carbon footprint, but all of the projects had focused on reducing output — changing the county’s fleet of cars to hybrids, for instance — rather than pulling CO2 out of the atmosphere.

“Suddenly the room was filled with four or five people who said, ‘Let’s go,’” Fuerstenberg recalled.

Now, Fuerstenberg, Brown and some of her graduate students are in the process of establishing 16 75-square-foot demonstration plots along the shoulders of the pit (the bottom of the pit is too wet to create the kind of soil Fuerstenberg says is needed for the project). Each plot will be amended with varying amounts of compost to determine “which kinds of mixes do the best job,” Fuerstenberg said.

Last week, Fuerstenberg and Warren walked along the rim of the pit, discussing the project. The crater, covered with grass but little else, is ringed by alders and Scotch broom. Because of the lack of soil, nothing — not even the highly opportunistic Scotch broom — has begun the ecological process of reclaiming the site and rebuilding its soil, the two men noted.

“If this works, then the next step would be to try it in a more environmentally complex area, and if that works, then bingo,” Fuerstenberg said.

The use of composts made from bio-solids at the pit could raise concerns, both men noted. The substance is sometimes controversial because of its smell and high levels of nitrates, harmful if they get into drinking water sources.

But Fuerstenberg said neither should be an issue at the borrow pit. The material they plan to use will be mixed with wood chips and highly composted, making it similar or even identical to compost commercially available for gardeners.

“It’ll smell like compost,” he said.

And because the nitrogen fixes to the carbon in the compost and the material will be placed on a hard-pan surface, he added, there’s virtually no chance nitrates will leach into the aquifer.

Warren, who wants to see the borrow pit become forestland again, said the project is exciting to him. It works on so many levels, he noted, providing not only a demonstration site for carbon-sequestration but also a jump-start to much-needed ecological restoration at the pit.

“It seems world-class to me,” he said. “And it seems so appropriate that it would be in our backyard.”

As for Fuerstenberg, a scientist with a doctorate in the philosophy of science and a Midwesterner who grew up hearing stories of the Dust Bowl and its devastating impact, he says he’s gripped with a sense of urgency.

“Climate change scares me,” he said. “You can read a lot about it, as I do, and there’s just no good news.”

But if this project effectively rebuilds soil, sequesters carbon and can be easily replicated, “I think you have a chance,” he said.

“If we can show this can work and get people to think about it a little more broadly, … then maybe we can apply it at a scale that will actually be meaningful,” he said.