Sequestering Carbon in the Soil

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. 

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