The multiplication factor

Tom Miles

The multiplication factor
Richard Douthwaite,, July 5, 2008

Dear All:

Everyone on this list subscribes because he or she believes that biochar has the potential to ease the climate crisis. But how great is that potential? It seems pretty clear that we will never be able to bury enough biochar each year to match the amount of fossil carbon being taken out of the earth. In any case, what would be the point? Instead of digging up coal and burning it we could burn the biochar instead.

So biochar's potential usefulness depends on what I call the multiplication factor - the amount by which the incorporation of biochar in the soil either leads to more carbon being sequestered or prevents the release of greenhouse gases that would have been emitted had it not been put in the soil. Let's look at these in turn.

1. Additional carbon sequestration.

We know that biochar increases the fungal and microbial content of the soil. Both contain carbon, and therefore represent an increase in the soil's carbon content. The fungi develop because the plants growing in the soil send sugars down their roots on which the fungi feed and, in return, the fungi release the nutrients locked up by the biochar and make it available to the plant. So the plants themselves sequester carbon. But how much? What we need to know is the weight of the carbon in the fungi and microbes in comparison with the weight of the biochar. Does anyone have a figure? And, as plants grow better on biochar soil, we could also include the carbon held by their bigger roots and, if they are semipermanent, locked up in their greater above-ground stems and foliage.

2. Reduced greenhouse emissions

The use of biochar increases the amount of nitrogen-fixing bacteria in the soil. This makes it necessary for the farmer to apply less. So does the fact that the char will hold a lot of whatever external nitrogen is applied, whether in dung/slurry or in artificial fertiliser, preventing it being leached away and also being broken down into nitrous oxide, a powerful greenhouse gas. Nitrous oxide emissions currently account for approximately one third of GHG emissions from agriculture in Ireland. Biochar also reduces the release of methane from the soil and, because it holds phosphate ions, reduces the need for applications, thus saving the energy required by the mining, preparation and delivery process. In short, biochar both reduces greenhouse emissions from the land itself and from the agricultural chemical supply chain. There may also be other greenhouse emissions savings I've not considered.

The two effects are different analytically. The additional carbon sequestration seems likely to be a once-off effect - the amount of fungi, roots etc in the soil will increase for a few years and then level off. However, the lower annual greenhouse emissions from soil treated with biochar, and from the agricultural chemical supply chain that feeds it, will persist for many years. The total tonnage of emissions saved over, say, a century, by one tonne of biochar could be huge.

We really need to be able to put figures on the tonnage of carbon likely to be taken out of the air by putting biochar into different soils in different parts of the world, and with different types of plants being grown on them. We need figures too for the emissions savings that biochar can achieve in different types of farming so that we can work out where, from a climate perspective, the limited amount of biochar likely to be available should best be applied.

Is anyone working in this area? It would be great if this list was able to come up with some rough figures for the two effects as, if no academics are active already, it might spur one of them to become so.

Best wishes,


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