A friend recently emailed me the following question:
Is there a limit to how much carbon soil can hold which then prevents the kind of sequestration that would be needed to reverse climate change?
Or, in other words, does soil get saturated and therefore become unable to sequester more carbon and enough carbon to draw down enough atmospheric carbon to repair the carbon cycle?
The answer to this question is both no and yes. Most people who claim that soil becomes saturated are not accounting for the newer soil science, as explained below, which demonstrates that new soil- that can capture more carbon- can be built relatively quickly with proper land management (see video below). Soil carbon capture via better land management has numerous other benefits besides mitigating climate change concerns, but that’s a discussion for another blog entry.
Most carbon measurements are (or were in old papers) made to approx 13 inches deep. This is what’s known as “shallow” carbon. It’s formed by decomposition of organic matter largely by saprophytic fungi. This carbon “saturates” and then the portion that rapidly turns over, the labile carbon, is respired by soil microbes. Most people wrongly believe that this respired carbon oxidizes (becomes CO2) and escapes into the atmosphere. Much of the CO2 doesn’t escape as long as there is plant cover, since plants with their stomata on the underside of their leaves, actually get most of their CO2 from this respired carbon (1).
With soil building something you always hear, and I’ve heard repeated over and over, is that it take “500 years to build one inch of topsoil” or some variation of these numbers. This what was taught in geology courses and many people still believe. This is the way soil is built via weatherization, that is the very slow grinding of rock to form soil over a very long period of time. This is the weathering or the breaking down of rock in the C horizon of the soil profile.
Now look at what Gabe Brown has done on his farm in North Dakota. Gabe has built 29 inches of soil in what’s called the A/B horizon, that is topsoil. Most of this has actually occurred since he reintegrated livestock in his pastures, after meeting Neil Dennis at a conference around eight or so years ago. That’s a lot shorter period than five hundred or how so many years.
Well, plants also exude carbon (glucose made via photosynthesis which includes the Calvin cycle-see the 5:16 minute second mark of the video below) from the tips of their roots for as deep as their roots go. This carbon is exchanged with bacteria via fungi networks for minerals that the plants need. Fungi and bacteria continually die and turnover. Fungi are largely made of carbon in the form of chiton. So, it’s this necromass (2) of bacteria and fungi that are what actually build soil via what’s known as the liquid carbon pathway or microbial carbon pump (3).
So soil and soil organic matter (containing soil carbon) are built three ways….from the top via decomposition, and from the bottom via the LCPW or MCP and very slowly via weatherization. Weatherization is slow, but the other paths work faster as long as there’s less disturbance. When you add livestock , you add more microbes from their poo, pee and saliva. More microbes mean more necromass and thus more soil. In nature the soil’s gut , the rhizosphere, and the guts of animals are a virtuous cycle (4).
Thus shallow carbon saturates, but is continuously recycled, while deep carbon from roots builds more soil that hold more and more carbon. Thus healthy soils never become truly saturated
New research also notes:
“…We show that microbial necromass can make up more than half of soil organic carbon. Hence, we suggest next-generation field management requires promoting microbial biomass formation and necromass preservation to maintain healthy soils, ecosystems, and climate. Our analyses have important implications for improving current climate and carbon models, and helping develop management practices and policies.” (5)
Brinton, Will Farming the CO2 Factor, Eco-Farming Daily