What is soil carbon?
Living organisms contain a fair proportion of the element carbon. So do the remains of living organisms. Some of these remains end up in the soil, processed and decomposed in various ways by fungi, microorganisms, insects, and worms. This soil organic matter can be 58% carbon by dry weight, and some of it can remain stable in the soil for generations or centuries. The vast majority of carbon in the top layers of soil is in soil organic matter.
What is the difference between soil organic matter and biochar, which helps form terra preta?
Biochar is a product of fire. It is plant matter that has been burned in a low-oxygen environment. Another word for it is charcoal. It is mostly carbon, and resistant to decay and oxidation.
Soil organic matter, by contrast, is the product of biological decay processes. These processes are often slow, and require the participation of millions of microorganisms.
What removes soil carbon from the soil?
Microorganisms can combine the carbon in soil organic matter with oxygen, creating carbon dioxide. In the soil, oxygen is often limited, especially deep down. When soil is plowed or turned over and exposed to air, these microbes can turn much of the carbon into atmospheric carbon dioxide.
How does carbon get stored in the soil?
For atmospheric carbon dioxide to become soil carbon, it first needs to be captured by green plants in photosynthesis. Much of this carbon is released right back into the air by respiration or decay of plant material. But some of it can become soil organic matter. Perennial grasses, for example, periodically shed their roots into the soil. These dead roots feed complex soil foodwebs, and soil organic matter and humus can be the stable result. Also, these grasses exude carbohydrates into the rooting zone, typically at night, which feed complex foodwebs. For some lively explanations from Dr. Christine Jones, see http://managingwholes.com/new-topsoil.htm and http://www.carbonfarmersofamerica.com/Jones1.htm
Elaine Ingham's Soil Biology Primer
How much of the biosphere's carbon is in the soil?
Estimates vary. Most of the biosphere's carbon is in the ocean, and most of that is in the deep layers that may take thousands of years to be exposed to the atmosphere. The next largest pool is the soil, followed by the atmosphere, and then biomass (vegetation, bacteria, fungi, animals). Usually, living bacteria in the soil are considered part of soil organic carbon.
Oceans: 38,000 gigatons C (stable, average turnover of a C atom is about 100 years)
Soils: 1600 - 2400 gigatons C (average turnover about 35 years)
Atmosphere: 800 gigatons C (average turnover 5 years)
Biomass: 600 gigatons C (average turnover 10 years)