Turmoil over soil: a climate warming wildcard

We trample it, shovel it, bury things with it, and grow food in it, but do we truly understand the soil beneath our feet? Scientists don’t think so. Many are beginning to realize that soil represents a massive carbon gorilla poised to play a dangerous wildcard in the high sweepstakes game of climate warming.

In a recent Science News article (“Soil’s Hidden Secrets,” Jan. 28, 2012) Charles Petit said, “If the bank of carbon held in the world’s soils were to drop by just 0.3 percent, the release would equal a year’s worth of fossil fuel emissions.”

Soil hoards about three times the amount of carbon contained in the air and all above-ground vegetation combined, but it doesn’t just hold carbon like a laundry basket holds dirty socks. Soils form complex and varied ecosystems like the prairies, rain forests and coral reefs humans can more readily recognize. Soil scientists create color-coded soil type maps of the world that look like someone spilled a handful of confetti on an atlas. The problem is that these scientists sometimes have a hard time knowing just how and when different soils may play their carbon trump cards in a warming world.

In eastern Colorado, we walk on prairie soils. Such soils contain a layered mix of living and non-living components. Finely ground rocks, clay, sand and wind-blown debris (loess) form the bottom layer. The middle layers house a community of worms, mites, insects and microorganisms swimming in a sea of partially decayed organic matter invaded by networks of feathery roots and spider-web filaments of fungi. The remains of Ice Age mammoths and tigers mingle with those of the cattle and corn stalks of more recent grasslands. Billions of bacterial and fungal cells constantly work at breaking down a backlog of complex carbon molecules into climate-warming gases such as carbon dioxide.

In 2009, a gaggle of soil scientists put their heads together at a self-proclaimed “think tank” on soil dynamics at a monastery in Switzerland. They arrived at some conclusions that overturned or improved expert opinions regarding the role of soil in global carbon dynamics. The Oct. 6, 2011, issue of Nature published “Persistence of soil organic matter as an ecosystem property.”

Soil chemistry revised
Soil scientists once believed that organic molecules in soil tended to form long-lasting, tough-to-digest (by microorganisms) “humic” substances. New studies suggest that how and to what extent organics in the soil decompose depends on many factors including soil climate, texture, how the organics bunch together, and the particular suite of microbes available to act on them. Even highly digestible sugars, the primary products of photosynthesis, can persist far longer than once thought under certain conditions.

Hot spots in the soil for carbon recycling also occur at different layers than once thought. Many early soil studies concentrated on leaf litter and relatively shallow soil layers, not only because they are technically easier to reach but also because that’s where many soil organisms congregate. Some decade-long studies with trees point toward different soil dynamics. It might seem that a warming world would ramp up photosynthesis, extracting more carbon dioxide from the air, muting the greenhouse effects of the gas.

Increased root growth and sugar production does occur in trees exposed to high carbon dioxide atmospheres, but this also energizes or “primes” microbe metabolism — often at depths of two meters or more, where the roots reside. These energized microbes not only consume the new excess biomass, but also metabolize older organics, releasing more CO2 and/or methane into the atmosphere.

“Biochar,” a clever term for charcoal created during fires in forests and grasslands, represents another wildcard in soil chemistry. Under certain conditions, biochar can sequester carbon in the soil for decades or centuries, but sometimes it can degrade within years and cycle the carbon into the atmosphere. Soil scientists, like Prof. M. Francesca Cotrufo at Colorado State University, work on ways to produce biochar for different purposes.

“Depending on the feedstock, temperature, and other conditions of pyrolysis (burning),” Cotrufo said, “we can make a biochar which is relatively easy to decompose and works best for soil fertility but not for carbon sequestration, as well as build a very recalcitrant biochar.”

The latter type keeps carbon compounds out of the air longer.

The biggest carbon gorilla
The soil that may hold the fate of the world’s climate in its black, carbon-rich depths lies in the Arctic. Scientists estimate that once permanently frozen Arctic soils contain 1.5 billion tons of carbon — about half of all the carbon contained in soils worldwide. As these soils warm and microbes fire up their engines, a torrent of greenhouse gases could tip the planet from the relatively icehouse conditions of today to the hothouse conditions of eons past. This means not only hotter temperatures, but also higher sea levels that would inundate major coastal cities and perhaps erase entire countries.

Scientists are acquiring new and useful tools. Field researchers can now read organisms’ DNA like barcodes. If soil microbes can be identified, mapped, and tied to the colorful confetti of soil types in climate simulation programs, then improved climate models may give us critical clues to keeping the carbon gorilla from playing her wildcard too soon.