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 50 to 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. Darwin called it vegetable mould (though he recognized the important role of animals such as earthworms in its formation), and it is also called humus.
Some soil carbon is inorganic, such as calcium carbonate or caliche. Carbonates are typically more prevalent in arid environments, where soil pH is above 7.5. They do not have the water-holding properties of organic soil carbon, but are a significant sink for atmospheric carbon.
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 fairly resistant to decay and oxidation, although there are some losses through leaching into water.
Soil organic matter, by contrast, is the product of biological decay processes. These processes are often slow, and require the participation of millions of self-motivated 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.