What is Biochar?

Biochar has become a hot topic lately, and there is some confusion about just what people mean when they say, “biochar.” I’d like to tell you how I think of biochar and answer a few frequently asked questions – hopefully that helps clear things up a bit.

My Definition of Biochar

“Biochar” is simply a term used to describe black carbon that is produced intentionally for (1) carbon management (i.e., trying to slow down climate change) or (2) agricultural or environmental management applications (e.g., trying to improve crop yields). However, this definition raises a new issue – now we need to define black carbon.

Unfortunately, as Carrie Masiello describes in her 2004 paper, scientists do not have an agreed-upon definition for black carbon. She refers to John Hedges’ conceptual model of black carbon as a continuum: “BC is a continuum of combustion products, ranging from slightly charred, degradable biomass to highly condensed, refractory soot.” This continuum concept is appealing because it can include a wide diversity of materials. (Others, like Bird and Ascough, use the term “pyrogenic carbon” to refer to the same types of materials.) Both of these terms explicitly refer to the carbon, since the materials are characterized by their carbon, which is enriched compared to the original materials, and forms stable rings (“aromatic rings”), which may join together, further increasing their stability. However, when people say “black carbon” or “pyrogenic carbon”, they often are actually referring to the entire material, not just the carbon within it. Thus, others prefer the term “pyrogenic organic matter” in these sorts of cases (the term I currently prefer). Still, we are left with a bit of a conundrum: fire, and even highly controlled charcoal production, is a heterogeneous process on some level, and so while some parts of the organic matter or carbon have changed substantially from their original forms, others remain essentially the same, especially at lower temperatures. Does this mean that some of it is pyrogenic, while some of it is not? How changed would a material have to be in order to be classified as black/pyrogenic carbon/organic matter? Some scientists would certainly argue that any less stable, unchanged compounds should not be included.

So, as you can see, it can get pretty complicated (great topic to discuss over a beer!), and most papers need to start off with a paragraph defining exactly what it is they are talking about, whether they use the terms “biochar”, “black carbon”, or “pyrogenic organic matter”. Still, we haven’t even gotten into what you probably really meant when you asked, “What is biochar?” if what you were really wondering is why people are so excited about it.

My advisor’s website has a great overview of biochar here, along with much more on its history. I won’t get into substantial detail here, but just explain briefly the aspects of biochar I find the most interesting. First, much of the carbon in biochar is very stable, compared to the carbon in the biomass it was produced from. If the carbon decomposes more slowly, then there is the potential to “slow down the carbon cycle”, keeping carbon out of the atmosphere for longer, helping to fight climate change. That said, it is important to note that the biochar will decompose eventually – it is still part of the global carbon cycle – it just decomposes much more slowly. Second, soils are fascinatingly complex mixtures of solids, liquids, gases, and (most exciting to me!) living organisms, and adding biochar to them can change their physical, chemical, and biological properties in many different ways. These changes can be beneficial, and research is beginning to show which soil and biochar properties are responsible for these changes and how we can use them to improve crop growth or for environmental management. Many people are hopeful about the potential for biochar to have positive agronomic and climate impacts, but there is a lot we still don’t understand, which makes it an exciting subject to study!