Sequestering Carbon in the Soil

The conversation around climate change mitigation has largely centered on reducing emissions -- hugely important and certainly the largest single factor in mitigation.  However, more and more, the conversation is starting to focus on carbon sinks.  It is, after all, largely the extraction of fossil fuel (a major carbon sink) that has gotten us into this mess, so where then does this excess carbon go?

The Carbon Cycle

Most of the carbon on earth is in relatively permanent storage in the planet's core, mantle, marine sediments, sedimentary rocks, unextracted fossil fuels or dissolved in the deep sea. The bulk of carbon released into the atmosphere happens through respiration or decomposition at those places where the land or sea meets the air -- the ocean surface, the biosphere and the soil. These are also the places with the greatest potential for drawing down carbon from the atmosphere.

philpot carbon cycle.jpg

diagram from Philpot Education (.com)

The ocean is a huge carbon sink and has so far been absorbing about a third of our man-made CO2 emissions, but there are indications that its capacity for absorption is reaching its limit. The biosphere also helps absorb carbon dioxide, and is the primary mechanism for introducing carbon into the soil, but any one plant has a relatively short lifetime compared to what can be stored in the deeper layers of the soil.  Still, increasing the total amount of biomass on the planet will increase the carbon sequestered, and adding carbon to the soil will help to grow more biomass. 

Soils hold four times the amount of carbon stored in the atmosphere.  About half of this is found deep within soils.  About 90% of this deep soil C is stabilized by mineral-organic associations

so, the soil is the place.

Scientists say that more carbon resides in soil than in the atmosphere and all plant life combined; there are 2,500 billion tons of carbon in soil, compared with 800 billion tons in the atmosphere and 560 billion tons in plant and animal life. And compared to many proposed geoengineering fixes, storing

Desertification

To make matters worse, the way we have grown our food and raised our livestock has, over thousands of years, resulted in the desertification of much of our planet.  Desertification is the process by which a soil loses its carbon and becomes dirt.  According to some experts, the world's cultivated soils have lost between 50 and 70 percent of their original carbon stock, much of which has oxidized upon exposure to air to become CO2. 
 

So:... carbon into the soil.  Win, win.

Anti-Desertification

So the soil certainly seems to be the place to sink carbon.  Biogeochemist Thomas Goreau says we need to seek opportunities to increase soil carbon in all ecosystems — from tropical forests to pasture to wetlands — by the replanting of degraded areas, increased mulching of biomass instead of burning, large-scale use of biochar, improved pasture management, effective erosion control, and restoration of mangroves, salt marshes, and sea grasses.

One huge advantage of sequestering carbon in planted soils is that it increases the growth and resiliency of those plants, helping to fuel an engine that goes on extracting carbon from the air and adding it to the soil.  Another is that those planted areas are less prone to damage from flood or drought, thus helping adapt to climate change, not just helping to slow it down.

loess valley

loess valley.jpg

The Loess Plateau in China is one of the most striking illustrations of what can be done to reverse the process of desertification.  The transformation shown in the above photos took place in less than twenty years.  This project and others have been documented in a film by John D. Liu. "Hope in a Changing Climate" (30 min).

3 Main Types of Soil Carbon

Organic Soil Carbon

Organic soil carbon is derived from living tissue: plant leaves and roots, sap and exudates, microbes, fungi, and animals. It takes a bewildering variety of complex chemical forms, many of which remain unclassified. Much of it is a result of decay processes and microbial metabolisms. Soil organic matter is a generic common name. It contains 50–58 percent carbon by dry weight.

 

Soil organic matter holds many times its weight in water. Its critical sticky components (such as glomalin) play a critical role in the formation of soil aggregates which give soil its stability against weathering and erosion, and its ability to hold water and air for plants and microbes.

 

The number one recommendation of the USDA-NRCS Soil Quality Team is to enhance soil organic matter (http://soils.usda.gov/sqi/).

 

Soil organic matter may be the most valuable form of soil carbon, but is generally the least stable, though some forms may persist for a thousand years or so.  Many forms can be readily oxidized (turned into carbon dioxide) by common bacteria in the presence of oxygen. But it is also the form of soil carbon that can readily increase as a result of plant growth, the root shedding of perennial grasses, the incorporation of manure or compost, the liquid carbohydrate exudates of plant roots, all processed by microbial metabolisms.

Soil organic matter is the most abundant form of soil carbon.

Charcoal

Charcoal also derives from living tissue, so it is considered organic. It is often called biochar. It can range from 50 to 95 percent carbon by weight. It is more stable and more resistant to bacterial oxidation than most other forms of organic carbon, which is one reason why there is considerable interest in incorporating biochar into soil as a carbon sequestration strategy.

Inorganic Soil Carbon

Inorganic soil carbon is mineralized forms of carbon, such as calcium carbonate (CaCO3) or caliche. It is more stable than most organic carbon because it is not food or fuel for microorganisms. Because acid dissolves calcium carbonate, it is not usually

abundant in soils of pH 7 or lower, or in humid regions. Carbonates are common in more arid regions and alkali soils, and are a significant soil carbon pool worldwide, derived mostly from organic carbon fixed by photosynthesis.

This section has been largely lifted from a paper by Peter Donovon on Measuring Soil Carbon Change

Sequester strategy:  Increase the organic matter in the soil (SOM) by adding plantings, compost, mulch or biochar, and try to keep it in the soil as long as possible.  The deeper the organic matter is buried, the less will be lost to the atmosphere.  The more fully the ground is covered with growing plants, the less carbon will be lost to the atmosphere.  The more stable the form of the SOM, the less will be lost to the atmosphere.

Types of Soil Organic Matter

with typical % of total SOM and approximate lifetime in the soil

Soluble root exudates, simple sugars and decomposition
by-products

DISSOLVED OM

<5%

minutes to hours

Fresh or decomposing plant and animal matter with identifiable cell structure

PARTICULATE OM

2-25%

2-50 years

Older decayed organic compounds that have resisted decomposition

HUMUS

can be >50%

10s to 100s of years

Relatively inert material e.g. chemically resistant or organic remnants such as biochar

RESISTANT OM

up to 10%

100s to 1000s yrs