Soils as Carbon Sinks
The carbon cycle is a key process for Earth’s ability to support life. The carbon cycle can be defined as a biogeochemical cycle that passes carbon through the biosphere, and the atmosphere. It has become increasingly important as humans have altered it, primarily through the removal and burning of fossil (fuels) carbon sinks, land use changes, and deforestation. A carbon sink is any area or reservoir that absorbs more carbon than it releases. Conversely, carbon sources are areas or reservoirs that release more carbon than they absorb. Large sinks of carbon include forests, fossil sinks (natural gas, oil), oceans, the atmosphere, and soils. Fossil sinks are not naturally included in the carbon cycle, as they are geologically trapped (at least temporarily), however human extraction and burning has released that carbon into the atmosphere.
The world’s soils contain twice as much carbon as the atmosphere. Carbon can exist as organic and inorganic forms within the soil. Decomposing remnants of flora and fauna build up within the soil and form detritus. While some of the carbon-rich detritus will remain in the soil for thousands of years, some of it will also be eaten and respired by soil organisms (decomposers), often bacteria, fungi, or insects within hours or days. The detritus also adds fertility to the soil, allowing more vegetative productivity to thrive within it, which in turn, sequesters more carbon.
One of the critical ways humans have been altering soil carbon cycles for millions of years is land use change. By clearing a forest or field for agriculture, one is not only removing the flora carbon stores, but also degrading the soil carbon as well. With exception of some sustainable farming initiatives, most farms degrade soil quality by causing severe erosion of topsoil, and nutrient depletion over a relatively short period of geologic time. In severe cases 100% of the topsoil is washed away, depleting fertility; scientists estimate 1 inch of topsoil forms every 100-500 years. Understanding soils as carbon sinks is integral as we continue to change land, and face climatic change.