What is Soil Carbon?

Soil Carbon is one of the many resting places of Carbon as it cycles throughout the biosphere (the liveable area on the planet). Carbon is the basic chemical building block of all life on Earth. It also resides in mineralform in rock formations and in fossil fuels, coal and oil as well as in the ocean. The amount of Carbon on Earth is fixed. So the many processes that use it need to access a supply of it and have somewhere to let it go. The result is a cycle as Carbon moves between the oceans, rocks, soil,
and atmosphere. There are 38,000 Gigatonnes (Gt) of carbon stored in the oceans, 2500 Gt/C in soil, 750gt/C in the atmosphere, and 650 Gt/C in forests, grasslands, and other vegetation. (Footnote 3)

(The “Greenhouse” effect is caused by the cycle getting out of balance, resulting in the atmosphere housing more on a rolling basis than it was designed to hold in order to manage stable weather patterns.)

Photosynthesis is a process that cycles Carbon out of the air and into plants, to be eaten by animals and humans as well as being deposited in soils. Photosynthesis is the only process that can take CO2 out of the atmosphere. It separates the C atom from the O atoms, releasing the Oxygen and incorporates the C in the plant, or transfers it to the soil where it becomes humus or other forms of Carbon. Some of it is released into the air if plants die and oxidize or dry out, or rot, releasing C in the form of methane.

Soil Carbon takes two main forms:

1. All the decomposed bodies of microbes such as bacteria, fungus, nematodes and root systems that die when plants are grazed as well as other decomposed plant residues. These forms of Carbon can be cycled quickly, within weeks.

2. The Carbon which is incorporated into the soil itself, such as humus. In these forms it can remain stable for thousands of years.

Total Organic Carbon is the amount of C stored in the soil of whatever type, source, or location. It can be measured very accurately. While soil carbon is subject to “flux” – different amounts can be measured according to time of day, time of year, and weather conditions – averaging techniques make assessing the amount of increase or decrease in soil C percentage
possible. (Footnote 4)



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Footnotes

3. Lal, R., “Soil Carbon Sequestration in Latin America”, in Carbon Sequestration in Soils of Latin America, Haworth, 2006
4. Kimble, B, “Advances in Models to Measure Soil Carbon”, in Carbon Sequestration in Sils of Latin America, Haworth, 2006