I wanted to share an interesting article I read recently, and some thoughts that I’ve had about its implications for carbon markets. Damien Beillouin, from the University of Montpellier, along with many colleagues, recently completed an expansive meta-analysis of over 25,000 primary studies, all looking at soil organic carbon. I know that it’s tempting to look at something like a forest and think that the story about carbon is in the trees. But for many forests, and many other terrestrial ecosystems, most of the carbon in the forest is belowground, in the soil. So considering the dynamics of soil carbon is crucial to understanding climate change, and evaluating the potential effectiveness of many of the commercial applications that seek to curb climate change and its catastrophic impacts.
The authors had three main findings. First, that converting land to agriculture leads to a lot of carbon moving from the soil to the atmosphere. That has been a longstanding finding, so I’m glad to see that it has held up over time. Second, land management application in forests generally lead to carbon moving from the soil to the atmosphere, but this is highly variable. More on this in a moment. And third, wildland fire can lead to huge and sudden movement of carbon from the soil to the atmosphere. Again, this isn’t surprising to me. Sure, warmer temperatures may lead to increased decomposition, but if a fire burns through an area, a lot of carbon can get burned away suddenly, and without as much vegetation, a great deal of carbon can leave the soil over the next year or two.
How about those land management applications? Interestingly, these application may include deliberately putting biochar into the soil, which is becoming more common as a carbon mitigation techniques, and more popular in carbon markets. The study finds that biochar application can work very well. On average, it can lead to a gain in soil carbon of 67%, which is remarkable. It can be most effective when there is abundant biomass available nearby for conversion to biochar, and where that biomass isn’t destined for other long-lived sequestration.
On the other hand, one common carbon mitigation tactic is agroforestry, where trees are planted, or planted in croplands. And the authors find that this is much less effective at storing carbon in the soil for the long-term. Likewise, crop rotation and diversification were not especially effective at storing carbon in the soil. But diversifying tree species in forestry made a small and desirable difference in soil carbon.
Some treatments proved to be absolutely misguided, such as removing dead leaves, stems, or branches from forests or grasslands. Their Figure 3 should be mandatory reading for any policy-maker, anyone buying or validating carbon credits, and anyone who works on greenhouse gas emissions in ESG. But the short version is this: in forestry, plant a mixture of species; in grasslands, apply biochar.