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.
William Albrecht, who was soils professor at the University of Missouri during the 1920s and 1930s, wrote in the 1938 Yearbook of Agriculture:
"But with the removal of water through furrows, ditches, and tiles, and the aeration of the soil by cultivation, what the pioneers did in effect was to fan the former simmering fires of acidification and preservation into a blaze of bacterial oxidation and more complete combustion. The combustion of the accumulated organic matter began to take place at a rate far greater than its annual accumulation. Along with the increased rate of destruction of the supply accumulated from the past, the removal of crops lessened the chance for annual additions. The age-old process was reversed and the supply of organic matter in the soil began to decrease instead of accumulating."
"Organic matter may well be considered as fuel for bacterial fires in the soil . . . "
For the source of this quotation see Steve Solomon's Soil Health Library, http://www.soilandhealth.org/01aglibrary/010120albrecht.usdayrbk/lsom.html
See also Paul Sachs's article at http://www.nofa.org/tnf/humus.php
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, or fire. 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
Elaine Ingham's Soil Biology Primer. There are some additional good resources on soil health at http://soils.usda.gov.sqi
To summarize, growing soil carbon usually involves:
1. Soils covered at all seasons with living or dead plant material (no bare exposed ground).
2. Healthy, productive, diverse plants, which require animals to function as a whole system.
3. Perennials, because of their greater investment in root mass, have advantages in growing soil carbon. But annuals, particularly with diversity and long seasons, can do well.
How much of the biosphere's carbon is in the soil?
Estimates vary. Rocks, such as limestone and chalk, contain enormous tonnages of carbon. Much 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. Fossil fuels are significant, and so are soils, 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) Fossil fuels: 4,000 gigatons C (estimate) Soils: 1600 - 2,400 gigatons C (average turnover about 35 years); recent new estimates of soil organic matter in peat and in polar regions have increased these estimates Atmosphere: 800 gigatons C (average turnover 5 years) Biomass: 600 gigatons C (average turnover 10 years)
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