Where's the carbon?
In the terrestrial carbon cycle, carbon moves from the atmosphere, to vegetation via photosynthesis in the form of complex carbon compounds (plain C in the animation), to litter and soil when the plants or leaves die, and back to the atmosphere as carbon dioxide via decay, oxidation, or burning.
The approximate pools of carbon are indicated in gigatons (billion metric tons). These carbon pools are broad averages, and are proportional to the average residence time of carbon in them. The terrestrial carbon cycle is an emergent phenomenon, the sum of the metabolisms of countless self-motivated organisms, most of them microscopic.
Human management influences both the biomass pool and the soil pool, either shortening or lengthening the time that carbon spends in each. Human management of soil and vegetation thus has great leverage on the atmospheric pool.
Fossil fuel burning (not shown) also contributes carbon dioxide to the atmosphere, about a tenth of the total terrestrial circulation.
From a forthcoming animation on the carbon cycle. Figures are from Rattan Lal, Sequestration of atmospheric CO2 in global carbon pools, Energy and Environmental Science 1:86-100 (2008).