Not a Miracle Cure
With so much to commend it, biochar may seem like a miracle cure for some of the serious problems plaguing the environment and challenging farmers. But, says Lehmann, “There is no one-stop shop.”
The demythification of biochar begins with terra preta. Although pottery shards mixed with the char material in Amazonia are evidence of human intervention, Spokas says, “We do not know what the soils were like when the biochar was added. A very romantic image has grown up around terra preta as an intentional cultivation management practice. We don’t know truly what the purpose was. There is no documentation, and there are a lot of other hypotheses you can come up with to explain how these soils could have been formed.”
Lehmann adds that it is “too simplistic to say you could just add biochar to create terra preta.” Nor, he says “is it essential to recreate the exact conditions found in terra preta; not that you could because one terra preta site is completely different from another terra preta site.”
Biochar itself also turns out to be not one material, but a seemingly infinite variety, whose exact properties vary depending on the feedstock, pyrolysis conditions, and postproduction handling. Wood-based biochar, for example, has a lot of carbon, but not much nitrogen, phosphorus, potassium, or calcium, while manure-based char has less carbon and more nutrients.
Slow pyrolysis at temperatures between 400 and 700°C maximizes solid biochar yield and produces a highly recalcitrant, decay-resistant product—a good choice if the primary goal is long-term soil carbon sequestration. However, there may be a reason to choose other thermal conversion processes. For example, microwave-assisted pyrolysis can handle wetter feedstocks, even though it yields more gas and less solid material.
The problem is that similar manufacturing methods can still produce a variable product. Spokas compares biochar production to baking: cakes made by two cooks following the same recipe may not have the exact same taste and texture.
Spokas and colleagues reviewed more than four dozen biochar and black carbon studies dating from 1850 to 2011 to assess “agronomic impact beyond carbon sequestration.” Findings were inconclusive. Half of the studies reported positive effects on yield, 20% no effect, and 30% negative effects. The overall impact on yield ranged anywhere from +200% to –87%.
However, without standardized ways to characterize biochar and full reporting of manufacturing processes (including postproduction handling and storage), Spokas says researchers are comparing “apples to oranges.”
Lehmann is not surprised at the findings. “We could probably do a study and irrigate soils everywhere across the world and find that, in half the cases, the irrigation had no effect while 25% had a positive effect and 25% had a negative effect,” he says. “Where water is not limiting, additional water will not improve growth. Clearly we would not conclude that water is bad or that there is no role for irrigation.”
Lehmann says the conclusion from the meta-analysis “is not (a) that we have no clue what’s going on—partly true, but we’re getting there—or (b) that we won’t be able to optimize the biochar method to address certain constraints effectively.”
Already the International Biochar Initiative (IBI)—a member-based organization promoting the development of sustainable biochar systems—has published a product definition and testing guidelines for biochar intended for use as a soil amendment. The organization has also begun a voluntary program to certify that biochar products meet quality standards.
IBI-certified biochar must include a label reporting feedstock composition and the results of all required tests, covering everything from moisture and ash content to mineral composition to pH to the presence of toxicants, such as polycyclic aromatic hydrocarbons. Lehmann, who chairs the IBI board, says the guidelines result from a rigorous, multi-step process and are akin to similar standards for fertilizers or compost, which must meet state-of-the-art regulatory thresholds.
Yet, given the many uncertainties surrounding biochar, Spokas says certification standards may be premature. “They’re ahead of where the science is,” he says. “We’re not just amending soil, we’re potentially changing its future development, and that’s the big unknown.”
One of the big hopes for biochar has been large-scale, agricultural carbon sequestration and soil greenhouse gas suppression to mitigate—or reverse—global climate change. Even here there are hurdles.
“A big challenge is understanding how and why these greenhouse gas suppressions are occurring and how long they will last,” Spokas says.
Lehmann notes that making biochar does not automatically result in net carbon sequestration. As an example, he says, taking down a forest that would otherwise stand to put biochar in the soil would not constitute net carbon sequestration. However, making biochar from trees killed by pine bark beetles—trees that would otherwise be burned—“would indeed net sequester them.”
“The take-home message,” Lehmann says, “is that one can find plenty of opportunities and types of interventions that would dramatically decrease emissions. The question is whether this is cost-effective compared with doing nothing or compared with doing something else, such as afforestation, no-till [farming], or solar power generation.”