In 2000, there were about a dozen articles on biochar in the scientific literature. In 2012, 3,000-plus. In between occurred what some have called a “biochar revolution.” Biochar—the trendy, yet ancient soil amendment now available at Whole Foods Market—has been known to man for thousands of years. Today, it is discussed as a possible means to reduce dependence on nonrenewable energy reserves, enhance agricultural productivity, reduce environmental pollution, and simultaneously reverse climate change.
Sound too good to be true? Read on.
Basically, biochar is an end product of pyrolysis. It is what you get when you take organic matter—anything from wood chips to corn stover to poultry litter—and heat it at a relatively low temperature under relatively oxygen-starved conditions. The original biomass, termed feedstock in the trade, releases heat and gases during its conversion into liquids (bio-oils) and various solid black carbon products, such as charcoal or ash.
According to ASA and SSSA member Kurt Spokas, a USDA-ARS soil scientist based in St. Paul, MN, that black carbon residue can be called biochar when it is created specifically for carbon sequestration. However, some other researchers define the term to include only black carbons created for soil application.
Semantics aside, the scientists who work in this field seem to agree on two things. First, biochar does indeed have potential to store carbon, boost soil fertility, generate energy, and mitigate pollution. But, second, there are a hundred caveats and unanswered questions. Biochar technology, it turns out, is both incredibly simple and, as yet, somewhat enigmatic.
In the end, says ASA and SSSA member Johannes Lehmann, a Cornell University soil scientist, biochar may not “save the world.” But he says, “That’s okay . . . even if it turns out that it’s appropriate only for a certain segment of farmers, it still justifies the effort that we are putting in.”
The promise of biochar comes from a confluence of amazing attributes. With the right feedstock and optimal pyrolysis, biochar retains half or more of the carbon in the original biomass. And, in the soil, that carbon is remarkably stable.
Lehmann was among the first modern researchers to rediscover ancient biochar deposits. Working in the Brazilian rainforest in the 1990s, he saw swaths of ebony soil—termed terra preta or “black earth”—standing out against the surrounding brown dirt and red clay soils. Terra preta, he says, “turned out to be full of this char material and that char material turned out to be responsible for the enduring high fertility of these soils over several hundred or several thousands of years.” According to radiocarbon dating, the biochar was at least 1,500 years old and up to 8,000 years old.
In addition to long-term carbon sequestration—and associated reduction of carbon dioxide emissions—Spokas says biochar has demonstrated “marvelous suppression” of other greenhouse gas emissions from soil, especially nitrous oxide.
Moreover, Lehmann calculates that the initial pyrolysis used to create biochar produces at least two to four times more energy than is used to make it, including the energy costs associated with biomass production, transport to the manufacturing site, the pyrolysis itself, and subsequent biochar soil application.
And there is more good news.
Biochar can have an unusually high cation exchange capacity (here’s an On Pasture article on the importance to you of cation exchange capacity), but also appears able to adsorb phosphate, an anion (no one knows why). As Lehmann writes in Frontiers in Ecology and the Environment, “These properties make biochar a unique substance, retaining exchangeable and therefore plant-available nutrients in the soil, and offering the possibility of improving crop yields while decreasing environmental pollution by nutrients.”
Like all organic matter, biochar also increases the soil’s water-holding capacity, sometimes dramatically so.
And, last, but not least, biochar alters the dynamics of soil microbial communities. For example, volatile organic compounds released by the material (or sorbed from the environment into the biochar) may stimulate microbial breakdown of soil minerals or alter plant–microbial interactions. And the physical structure of biochar, with millions of micropores, makes it excellent habitat for soil inoculants and for beneficial organisms already in the soil. Paul Wever, one of a handful of commercial biochar manufacturers, calls the substance “a hotel for microbes.”