Decomposition is also important because it is part of the global carbon cycle. The carbon cycle is the cyclical movement of carbon atoms from the atmosphere to the biosphere/lithosphere and back to the atmosphere (Figure 1). In the atmosphere, carbon is in the form of carbon dioxide gas. Through the process of photosynthesis, some of that carbon is converted into organic carbon which makes up organic matter or biomass. Plants and animals perform cellular respiration and convert a small percentage of that organic carbon back to CO2.
A larger portion of that organic carbon in plants is transferred to the soil when plants shed their leaves or when they die. Decomposers then begin their work of breaking down the organic matter. Some of the organic carbon in the organic matter is converted into CO2 which is released into the soil pore spaces leading to relatively high concentrations of CO2 compared to the atmosphere. This difference in concentration causes CO2 to diffuse from the soil to the atmosphere. This movement or flux of CO2 is known as CO2 emission (Figure 1).
Decomposition is not the only source of CO2 in soil. In a forest or grassland ecosystem, plant roots are abundant in the soil and root cells perform cellular respiration, metabolizing carbohydrates that are sent down from the leaves. This CO2 is released to the soil and can be responsible for anywhere between 0 and 60% of a soil’s CO2 emission. Note that CO2 emission is the movement of CO2 from soil to the atmosphere, whereas decomposition and root respiration are processes that produce CO2 in the soil (Figure 2).