Slow Burn

Scientists tap ancient soils to fight climate change   If Professor Johannes Lehmann has his math right, Amazonian soil created long before the birth of Columbus may hold the key to the modern-day fight against climate change. Carbon-rich terra preta de Indio ("Indian dark earth" in Portuguese) owes its inky hue to charcoal spread by […]

Scientists tap ancient soils to fight climate change

hazelnut  shells 

If Professor Johannes Lehmann has his math right, Amazonian soil created long before the birth of Columbus may hold the key to the modern-day fight against climate change. Carbon-rich terra preta de Indio ("Indian dark earth" in Portuguese) owes its inky hue to charcoal spread by river basin natives almost 10,000 years ago. More than a meter deep and as much as three-and-a-half times more productive than nearby soils, terra preta combines charcoal, pottery shards, plant residue, animal waste, and fish remains to yield a substance that looks, smells, and even feels distinct from the sterile red clay and sand native to the region. Mid-nineteenth century explorers gave the soil its name, but archaeologists now speculate that local populations established robust civilizations fueled by its rich agricultural yields—and sparred over its control—some 1,500 years ago.

Even after 150 years of investigation, contemporary scientists have yet to fully understand the soil's agricultural effects. Think of it as compost on steroids: the carbon in terra preta stores a range of nutrients, improves water availability, and fosters microorganisms that boost soil fertility and aeration. But unlike compost, its effect persists for hundreds, if not thousands of years—all the more remarkable given the rainfall that pummels the nutrients from nearby, unamended soils. "Terra preta does all the great things organic matter does," says Lehmann, "but more effectively and over a longer period."

The stuff first caught Lehmann's eye during a three-year assignment in Brazil during the late Nineties. "Almost anywhere in the central Amazon, you can ask a local, 'Is there any terra preta around?' and they'll show you," he says. "It's everywhere." At the time, the Bavarian-born soil biogeochemist was on a team, funded by the German government, investigating mixed cropping systems. The group finished its work ahead of schedule and started looking around for ancillary projects; terra preta was the obvious next step.

Johannes Lehmann 

Now a Cornell associate professor of crop and soil sciences and editor of the 523-page compendium Amazonian Dark Earths, Lehmann also investigates biochar, a modern-day approximation of the carbon source in terra preta. Unlike ash (the white, powdery residue in wood stoves), biochar owes its genesis to a lack of ventilation. In the fireplace, flames licking across a log's surface liberate greenhouse gases embedded by sunlight in the tree's cells. Oxygen fuels the process, converting the wood into—among other things— heat, water vapor, and carbon dioxide, leaving an ashy residue. Eliminate the oxygen by combining heat with pressure, and combustion becomes pyrolysis; instead of ash, carbon-rich charcoal remains.

Lehmann's current effort has two prongs—optimizing biochar production for agricultural applications and evaluating its potential for carbon sequestration. He currently oversees field sites in Brazil, Colombia, Kenya, and Zambia, as well as greenhouse and laboratory experiments in Ithaca. At the moment, he's evaluating the horticultural effect of 100 biochar variations comprising everything from hazelnut shells, switchgrass, and poultry litter to food waste discarded in University dining halls. Biochar has the potential to reduce fertilizer demand, protect waterways, and maintain crop yields worldwide—but, he says, "We need to do it the right way with the right stuff."

low-tech stove 

Lehmann has a growing hunch that biochar will also feature prominently in the fight against global warming. Given its persistence in soil, the material offers a critical mechanism for extended sequestration of atmospheric carbon. Global stores of the element—released by fossil fuel combustion—are increasing exponentially, with a direct correlation to temperature. Pull some of that carbon from the atmosphere, the thinking goes, and the rate of climate change will slow. That's why we plant trees: photosynthesis traps carbon dioxide. But when they die and decompose—or humans fell and burn them—they release that CO2 along with other greenhouse gases. Enter pyrolysis, which chemically locks away carbon as well as the nitrous oxide and methane released in aerobic decomposition.

In January 2008, Environmental Science & Technology published Lehmann's calculation that unlike corn-based ethanol, the net benefit of heat generated in making biochar—combined with the carbon sequestration and emissions reductions from using the end product as a soil amendment—exceeds the energy consumed in its production. In December, Lehmann traveled to Poland for the United Nations Framework Convention on Climate Change; in January, another member of his team heads to Kenya to refine low-emission cookstoves that rely on pyrolysis instead of combustion, decreasing indoor air pollution. "We claim we can use the same amount of fuel as a traditional stove, cook the same amount of food, and retain half of the carbon as biochar to return to the soil," Lehmann says. "It makes better use of resources, with a positive effect on health."

— Sharon Tregaskis '95


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