What makes amazing soil amazing? High organic matter, comprised largely of carbon.

The most agriculturally productive soils in the world, Mollisols, (found, for example, in Iowa and Ukraine) contain up to 3.3% carbon. Carbon is black gold.

Yet, agricultural soils have lost, on average, half of their carbon due to intensive cultivation and other degradations of human origin, says Bruno Glaser, soil physics department, University of Bayreuth, Germany.

Scientists are studying ancient Amazonian soils to learn how to restore carbon to soils in the form of biomass charcoal — biochar. This charred waste can do double duty as a soil amendment, concentrated carbon sequesterer, and a critical link between biofuels and sustainable farming practices.

This is not an entirely new idea. Thousands of years ago, Amazonian Indians enriched their infertile tropical soils with charcoal composted with manure, fish bones and plant residues. Even today, the resulting dark miracle soil retains high levels of nitrogen (N), phosphorus (P), organic matter (13-14%), calcium, zinc and manganese. And it anchors soil nutrients amazingly well. This Terra Preta (Portuguese for “black soil”) enabled large Amazon Indian societies to thrive on poor tropical soils.

Today, scientists recognize adding biochar to the soil as an efficient way to sequester carbon, and the resulting fertility boost could make the practice a win-win. Greenhouse gases (carbon dioxide) could be removed from the atmosphere and sequestered underground as solid carbon via the cycle of photosynthesis, plant decomposition and pyrolysis.

WHAT IS PYROLYSIS? It makes fuel from the thermal decomposition of biomass in the absence of air to produce charcoal, flammable gas and liquid fuel. The fuel, known as bio-oil, can be burned to generate heat or refined into transportation fuels. Pyrolysis’ charcoal byproduct, biochar, can be a long-term soil amendment and sequester carbon for millennia, says David Laird, author of “The Charcoal Vision” and soil scientist at USDA-ARS National Soil Tilth Laboratory (see sidebar on page 31 and http://agron.scijournals.org/cgi/reprint/100/1/178).

“Most companies that generate bioenergy using pyrolysis view biochar as merely a byproduct burned to offset fossil fuel use,” he says. But if biochar is incorporated into the soil instead, “this sequestration offers the chance to turn bioenergy into a carbon-negative industry.”

Locking carbon in soil “makes more sense than storing it in plants that eventually decompose,” adds Johannes Lehmann, a Cornell University soil scientist who researches biochar.

“The advantage with biochar is that it is a very stable form of carbon. After five years, more than 95% of the carbon that is in corn residue will be back in the atmosphere as CO₂, whereas after five years, more than 95% of the carbon that is in biochar will still be in the soil regardless of the form of tillage,” he points out.

RICH IN NUTRIENTS and humus, biochar-enriched soil is a powerful magnet for soil nutrients and moisture. Its ultra-high carbon content attracts (adsorbs) soil nutrients and crop chemicals, preventing them from leaching through the soil profile and reducing fertilizer needs. Biochar retains high soil carbon and moisture levels for a long time, thanks to Mother Nature’s lock: carbon bonds.

Substantial crop yield increases have been reported for the few trials where biochar has been added to agricultural soils.

“Biochar, or charcoal, contains most of the plant nutrients removed when the biomass was harvested and can slowly release them to growing plants,” Laird says. He lists additional agronomic benefits of adding biochar to soils:

• Lowers the density of clay soils, increasing drainage, aeration and root penetration.
• Increases sandy soils’ retention of water and nutrients.
• Partially offsets the acidity of N fertilizers (liming agent).

“Compared to the 15-20 years’ carbon sequestration represented by a plant or tree, biochar is orders of magnitudes more stable,” Lehmann says. No-till cropland may cease to capture additional carbon after 15-20 years, and even forests eventually start to release as much carbon dioxide as they take up, he says.

“Even switching from conventional tillage to no-till will only add carbon to soils for just a few years,” Laird says. “After a few years the soil will reach a new equilibrium so the annual loss of carbon from the soil will equal the amount of new carbon added to the soil. Also, plowing up no-till ground releases most of the carbon stored in the soil from no-till. And if you harvest most of your corn stover for bioenergy, the amount of carbon in your soils will decrease even if you do use no-till.”

Society has to figure a new paradigm for agriculture “where we put more carbon in the ground than what we remove,” says Robert Brown, director of the Bioeconomy Institute at Iowa State University. “Instead of simply minimizing the carbon loss, a new model for good land stewardship would gradually increase the amount of carbon in the soil. This is not impossible to do.

“It may not be enough to merely shut down coal plants to reduce greenhouse gases; we may need to take CO₂ out of the atmosphere and sequester it. That’s a big challenge,” says Brown.

The answer to that challenge may be a modern-day biomass charcoal that restores nutrients to the soil, reduces greenhouse gases and yields low-cost energy. This answer began in part as an ancient fertilizer from Amazonia, which is now also turning up in Ecuador, Peru, Liberia and the savannahs of South Africa.