Biochar is a form of carbon that results from pyrolysis (heating in the absence of oxygen) of biomass – e.g. wood, bark, stalks, chaff and crop residue, nut shells, or virtually any organic material – leaving behind a durable matrix with a porous structure. This substance has a range of physical, chemical and biological properties that can provide multiple, cascading ecological services. The first, and most compelling, application of biochar comes from the nature of the carbon itself. If we pyrolyse biomass at the correct temperature range, the resulting product is a highly stable and long-lived form of carbon, and as such represents a low-tech means of sequestering atmospheric CO₂ captured by photosynthesis of the plants that provide the feedstock.

We could pyrolise large quantities of biomass and then simply store the end result in landfill, and by doing this we would have an effective tactic for global carbon drawdown over time. The fifteenth special report of the IPCC includes biochar as a promising mitigation technology and this development has acted as a catalyst for expanded funding of trials in the EU.

But there are so many uses of biochar aside from its value as a carbon sink that this direct storage route, although worthwhile, would represent a host of missed opportunities. The physical structure of biochar is a massively porous matrix, averaging over a hectare of surface area per 25 g of material. This property means that it can retain moisture and provide habitat for diverse and flourishing microbial populations. The carbon matrix also provides an array of chemical bonding sites where ions dissolved in solution can attach and be held loosely enough to be available for uptake by microbes or plants. The combination of these factors mean that biochar can effectively take out dissolved contaminants from water, for example. It also makes biochar an effective soil amendment for a range of settings, from arable, pastoral, and forestry lands to rehabilitated wetlands and marginal habitats. There is nothing novel or radical in adding carbon in this form to soils, either: it is a natural process of soil building that has been going on in nearly all parts of the world where fires occur, and on average pyrogenic (fire derived) carbon accounts for at least 13% of soil organic carbon globally.

The effects of biochar incorporation into soils over the long term can be seen in many settings, but the most striking examples would be the terra preta (black earth) of the Amazon, the famed black soils of Iowa and Ukraine, and, in Aotearoa, in māra kai where burnt matter was deliberately added to kūmara plots. In the Amazon, a flourishing and settled farming civilisation, numbering perhaps in the millions, was reported by the first Europeans to explore the interior in the 16th century. As the harsh and infertile nature of tropical rainforest soils was better understood, the presence of this culture (subsequently exterminated by introduced diseases) stood as a mystery that was not solved until the the late 1990s, as scientists worked with samples of the soil and found that it had high levels of carbon. As the carbon was dated, its effects understood, and a mechanism for its presence was formally described, a whole new area of research opened up into the qualities of biochar. Now we know that for a period of at least 1,500 years, and likely up to twice as long, the settled farming culture of the Amazon basin was routinely and purposefully incorporating charred organic matter into their severely depleted and leached rainforest soils and reaping the rewards of the increased fertility.

In temperate regions, the most productive agricultural areas are often atop the deep prairie soils formed by humid grasslands – places like the US Midwest, southern Russia and Ukraine, and the Argentine pampas. Soils with high amounts of durable carbon have formed in these locations by repeated low-intensity fires that charred the abundant aboveground vegetation and surface litter. The fire regime had natural beginnings, but would have been enhanced in frequency by the presence of humans, who deliberately set them in order to drive game and modify the landscape. Radiocarbon dating of these soils has yielded carbon fraction ages of over 12,000 years in Ukraine and 7,000 years in Iowa.

This is our evidence that incorporation of biochar in soil is one of the surest and safest methods of long-term sequestration available. Its corollary benefits to primary productivity are just the icing on the proverbial cake, but these serve as additional incentives to bring biochar into our climate response strategy.