Recently on Quora I was challenged to assess the likely capacity of soil carbon sequestration approaches (sometimes referred to as biological carbon capture and sequestration or BCCS) by a researcher in the space. The premise was that two thirds of the carbon which had been sequestered in the soil had been lost into the atmosphere as grasslands were converted to large-scale agriculture, and that changing agricultural practices would be sufficient to act as a sink for the majority of excess CO2 emitted.
What exactly is the mechanism? How much potential does BCCS offer? How much effort would be required to implement a large scale fix? Reasonable questions, so I went hunting for answers.
How does BCCS work?
There have been some interesting findings in plant biology in the past two decades, specifically concerning something called glomalin.
Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal (AM) fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service. The name comes from Glomales, an order of fungi.
To summarize the premise behind modern BCCS:
- glomalin binds carbon better than previously understood
- good soil management practices allow the fungi which produces glomalin to thrive
- which allows more binding of carbon
Carbon has been lost from native soils as they became agriculturally productive. The concept of BCCS is shift to agricultural approaches which support glomalin from approaches which reduce it, increasing the carbon uptake of soil. The various sources provided supported this theory (any sources not linked in the body of this article are provided below as additional reading).
How much carbon might be sequestered?
The part that leapt out at me in the Executive Summary of the material on p. iv:
Globally, this loss of SOC has resulted in the emission of at least 150 Petagrams (Pg) of carbon dioxide to the atmosphere (1 Petagram = 1 Gigatonne = 10^15 grams). Recapturing even a small fraction of these legacy emissions through improved land management would represent a significant greenhouse gas emissions reduction.
As CO2 has risen from 150 to 400 ppm, this represents an increase of about 1,170 gigatonnes of excess CO2 in total, and annually we are contributing about 10 gigatonnes.
Let’s make the assumption that all agricultural land globally could be returned to a baseline of the same sequestration as native land over the course of the next 50 years. That means that we’d be at about 1,222 gigatonnes of extra CO2 and the soil would sequester about 150 gigatonnes out of that total, or about 12%.
However, this 12% is dominantly a temporary biological sink.
That study showed that glomalin accounts for 27 percent of the carbon in soil and is a major component of soil organic matter. Nichols, Wright, and E. Kudjo Dzantor, a soil scientist at the University of Maryland-College Park, found that glomalin weighs 2 to 24 times more than humic acid, a product of decaying plants that up to now was thought to be the main contributor to soil carbon. But humic acid contributes only about 8 percent of the carbon. Another team recently used carbon dating to estimate that glomalin lasts 7 to 42 years, depending on conditions.
Glomalin, as with all biological sinks, is temporary. Movement of CO2 into permanent sinks occurs, but also movement back into the atmosphere. Biological sinks become saturated and then atmospheric levels of CO2 remain in balance with the sink.
Would it be sequestered fast enough?
There has been recent bad news for soil sequestration via a radiocarbon dating of soil carbon study published in Science by a UCal team in September 2016.
A gloss on the study in the Guardian is good and in more accessible terms.
Scientists from the University of California, Irvine (UCI) found that models used by the UN’s Intergovernmental Panel on Climate Change (IPCC) assume a much faster cycling of carbon through soils than is actually the case. Data taken from 157 soil samples taken from around the world show the average age of soil carbon is more than six times older than previously thought.
This means it will take hundreds or even thousands of years for soils to soak up large amounts of the extra CO2 pumped into the atmosphere by human activity – far too long to be relied upon as a way to help the world avoid dangerous global warming this century.
So the answer of 12% by 2050 is actually much slower, centuries slower in fact. That’s too slow to be of use in any near term attempt to deal with warming.
Does this mean BCCS will go away?
The science says it’s not a short-term climate solution, but that better tillage practices are a very good choice regardless. While sequestration might be a bit of a red herring, reduced soil erosion and better soil biology are strong net benefits regardless.
Is there good news out of this?
Global green biomass has been increasing for the past 15 years or so as the rural poor move to cities and leave semi-arable land to go wild. In a tightly related story, we’re producing more food from less land under agriculture globally.
The combination means that a great deal of land is returning to being a better carbon sink and that the area of land amenable to better practices is both smaller and under organizations more amenable to seeing it as a long term asset, agricorporations who are more likely to follow the science of better land management.
To be clear, improving land management practices to make the soil healthier and sustainable is something that’s excellent to do. It will help with long term ecosystem health and biosequestration. But it’s not a fix for our climate change problem this century or next. That will take electrifying everything, decarbonizing electricity and then cleaning up around the edges.
- Cenozoic Expansion of Grasslands and Climatic Cooling
- Mollic Epipedon – Springer
- Farming Claims Almost Half Earth’s Land, New Maps Show
- Liquid carbon pathway unrecognised
- The System of Rice Intensification (SRI)… … is climate-smart rice production
- Grassland Birds: Fostering Habitats Using Rotational Grazing
Many thanks to Scott Strough whose Quora answers on soil carbon sequestration and persistence in forcing me to look through the material led me to make this assessment.
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