Researchers say a soil revitalization technique used by a lost civilization in the Amazon jungle could help reverse the damage done by global warming.
What’s more, the process of creating “biochar,” similar to charcoal making, also offers the promise of an alternative source of energy and fertilizer derived from renewable sources.
Cornell University biogeochemist Johannes Lehmann, who wrote Amazonian Dark Earths, is one of the leading authorities studying terra preta.
Scientists theorize that inhabitants of the sites – some of which were abandoned for unknown reasons thousands of years ago -– deliberately charred wood and crop residues for use as an amendment to preserve fertility in the Amazon basin’s impoverished soil, which is heavily subject to leaching from 21/2 metres of annual rainfall.
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The soil at these archeological sites is said to be self-regenerating at a rate of about one centimetre per year. How it works is not fully understood, Lehmann said, but it has long been known that adding charcoal to soil boosts its capacity to store and gradually release nutrients.
Instead of using compost, which quickly disappears under the region’s hot, humid conditions, the jungle dwellers may have made “biochar” through a crude process of pyrolysis and added it to their fields.
The charred wood found in the archeological sites can resist decomposition for centuries. Compost, on the other hand, is almost entirely consumed by soil bacteria within a year or two.
Once charred, “crop residue or your compost that had a half-life of a few months to a few years now has a staying power of several decades to several hundred, maybe several thousand years,” Lehmann said.
It is still unknown whether terra preta soil was created intentionally or as a byproduct of charcoal production or slash and burn agriculture.
One thing is certain, Lehmann said – the enriched soil at the sites remains fertile amid some of the toughest conditions on Earth.
Potential feedstocks for modern terra preta experiments have included woody materials, grasses, crop residues, yard wastes and poultry litter.
Nutrient values vary, he said. Poultry manure is higher in phosphorus and calcium, while charred wood leaves mainly pure carbon.
“Its major benefit is not its nutrient or fertilization effect, but rather its soil conditioning effect. It transforms the soil and makes it better,” he said.
Because of biochar’s capacity to gradually meter out nutrients to plants, mixing it with nitrogen, phosphorus and potassium-based fertilizers might be one way to prevent wasting precious nutrients in runoff water or seeing it volatilize into the atmosphere.
“You still need to apply nutrients,” Lehmann said.
“Any nutrients that are taken up by a grain crop, for example, need to be returned. Biochar makes the nutrient cycles much more efficient.”
Unlike cellulosic ethanol, which some critics have suggested is tantamount to burning the world’s topsoil in fuel tanks, biochar could provide an alternative energy source while at the same time restoring fertility to depleted soil and sequestering carbon.
“With all bio-energy options, first you take the biomass and remove it from the field,” he said.
“But a combination of pyrolysis and biochar production has some clear advantages in that the strategy would be to return the biochar to the field where you extract the biomass. You’re returning most of the nutrients to the field that you have extracted them from …. And biochar is a bit smarter because you’re not only returning the nutrients, but also the organic matter, the carbon.”
With this goal in mind, dozens of studies are underway at universities around the world on how to use biochar to restore fertility to eroded and depleted soil.
As concern about global warming grows, Lehmann said converting all U.S. cropland to native grasses under the Conservation Reserve Program would offset only 3.6 percent of the country’s emissions. Harvesting those grasses using a carbon-negative biochar process that produces hydrogen fuel could conceivably triple that figure, he said, and lock excess carbon into the soil for centuries.
Research into biochar’s potential for large-scale use is hampered by a lack of infrastructure for producing it. In an article for Nature, Lehmann argued that biochar carbon sequestration as a byproduct of biomass energy production could become viable if the value of carbon offsets were to reach $37 per tonne, up from $6 now.
While scientists speculate that uninterrupted no-till cropping may eventually cease sequestering carbon after 15 to 20 years, biochar production is a way to lock carbon into the soil in easily measurable amounts in exchange for carbon credits.
Lehmann said universities are exploring the possibility of taking biochar research to the next level by setting up large-scale plants for supplying field trials.
Prototypes could include farm-scale units that produce heat for livestock housing as well as biochar or district heating operations for neighborhoods, towns or industrial parks.
Dynamotive Corp. in Vancouver operates two industrial scale plants in Ontario that produce a bunker oil replacement called bio-oil and char as a byproduct.
The waste wood and sawdust is heated in a chamber without oxygen, and the gases are condensed to form bio-oil.
“It’s almost like liquefied wood,” said company spokesperson Nathan Neumer.
The oil is too thick and corrosive to be used as a transport fuel, but it exceeds the BTU rating of No. 2 diesel when used in boilers. Unlike wood pellets, the liquid fuel is 10 times as dense as the original biomass, which facilitates shipping.
“It can sit in a barrel like petroleum, and be shipped or stored,” he said.
Neumer said the char can be used for charcoal briquettes or pellets, but his company is using it for agricultural applications.
Two 10 tonne shipments of char were recently sent to an Ontario soybean farmer who plans to try it as a soil amendment.
Both facilities are still in the experimental stages, but have recently been upgraded to turn 130 to 200 tonnes of sawdust and wood waste per day into bio-oil and char. The company has tested more than 100 feedstocks, from straw to cornstalks, and all produced similar results.
“As long as it’s organic, fairly clean, and free of stones, you can use them all,” he said.
Eprida Inc. in Athens, Georgia, is commercializing a one tonne per hour, farm-scale biomass processor that converts agricultural biomass such as peanut shells and corn stover into charcoal and bio-oil or flammable gas.
“You could run your poultry litter through the pyrolyser and create heat for your poultry house in the winter,” said Eprida researcher Rebecca Oglesby.
She said the company is working on creating a charcoal-based fertilizer by turning hydrogen produced in the process into ammonia.
Oglesby said interest in biochar has exploded since she joined Eprida four years ago.
“I think it has a lot of potential for making improvements in a lot of people’s lives,” she said.
“It’s going to do a lot for crop production, improving the soil and cleaning the air.”
