The Carbon Cycle
Carbon and the Soil Food Web
Understanding the carbon cycle and the soil food web gives context to how organic matter and nutrients such as nitrogen, phosphorus, potassium and sulfur are recycled. Below is a simplified diagram representing both the general carbon cycle and the soil food web. If one tracks who is consuming what in the soil, one can also track the flow of carbon (energy) and nutrients through the soil, as they are closely linked (see below). Each “block” or grouping of like consumers is called a trophic level. Conversely, throughout the levels of the diagram, as a group of organisms dies, they are then consumed by other organisms which, in turn, die and become dead organic material. This process continues to recycle nutrients (such as nitrogen, phosphorus, potassium and sulfur) and thus continues the carbon cycle.
How the Soil Food Web Impacts Carbon Sequestration
The addition of dead organic matter to the soil (OM) acts as a driver of metabolism and growth for soil organisms. Consumption of this matter by soil organisms results in respiration of carbon back into the atmosphere as gaseous carbon dioxide. Thus, organic matter can be a source of atmospheric CO2. Furthermore, biomass decomposition also releases nutrients that are used by plants and microbes.
Soil organic matter accumulation occurs over long stretches of time with a small proportion of organic matter making its way into the long-term, stable pool. Moreover, much of the chemical makeup of organic matter is carbon. Thus, when stable organic matter pools are formed, we can consider this sequestered carbon. In order for net sequestration of carbon in the soil to occur, more carbon must be added to the soil than is released back into the atmosphere by the decomposition processes described above. Good root growth of healthy crops is a key contributor to increased carbon storage in soil, as this is a good source of bulk organic matter in the soil. The diagram below illustrates this process over time.
The stabilization of organic matter is important to overall soil health and its physical properties. Aggregation is the process by which mineral and organic particles bind together, forming aggregates. As aggregates form, small organic matter particles become ‘trapped’ inside the aggregates, a process called occlusion, which then acts as a protective barrier from further degradation by decomposers. Aggregation and occlusion may result in the net sequestration of carbon from the atmosphere.
