Organic Matter

"More carbon is stored in soils than in all plants, all animals, and
the atmosphere combined. Soil organic matter contains
an estimated four times as much carbon as living plants.
In fact, carbon stored in all the world’s soils is over three
times the amount in the atmosphere. As soil organic
matter is depleted, it becomes a source of carbon dioxide
for the atmosphere. Also, when forests are cleared and
burned, a large amount of carbon dioxide is released. A
secondary, often larger, flush of carbon dioxide is emitted
from soil from the rapid depletion of soil organic matter
following conversion of forests to agricultural practices.
There is as much carbon in six inches of soil with 1%
organic matter as there is in the atmosphere above a field.
If organic matter decreases from 3% to 2%, the amount
of carbon dioxide in the atmosphere could double."

From Building Soils for Better Crops