Soil Organic Carbon

The Earth’s terrestrial ecosystems store over 2,000 gross tons (Gt; 1 Gt = 1,015 g) of soil organic carbon (SOC), which is about four times more carbon than is stored in the atmosphere. Annually, soils release over 60 Gt carbon to the atmosphere, which is about 10 times that amount released by fossil fuel combustion. Warming can increase the rate at which fresh organic matter (e.g., recently senesced leaves, or fruits) decomposes, with the highest rates found where it is wet and warm. Less is known about how more thoroughly decomposed material (e.g., SOC) responds to changes in temperature, but it is assumed that global warming will increase SOC decomposition rates, with the sensitivity of SOC decomposition to warming determining the extent to which SOC storage will be altered by climate change. Because the sensitivity of SOC decomposition to temperature remains poorly quantified, it cannot be accurately predicted whether global soils will change from a net sink to a net source of CO2 as the planet warms.

SOC serves many important ecosystem roles. Because the supply of organic carbon exerts a dominant control on the activity of soil heterotrophic organisms—from bacteria to insects—SOC is critical to regulating the structure and functioning of soil communities. Further, during SOC decomposition, large quantities of nutrients are released from organic to mineral forms, and so SOC provides a critical source of nutrients to growing vegetation.

The amount of SOC can also affect the water-holding capacity of a soil, as well as water movement through soils. Despite these important roles, the tremendous complexity of SOC in natural and agricultural systems presents important challenges to quantifying SOC formation and decomposition, including the sensitivity of these processes to climate change. This complexity results from the fact that very large quantities of organic matter are cycled through soils annually, but only a very small fraction remains in soils, typically in a highly transformed state that can persist in soils for millennia as a result of chemical recalcitrance or protection by clay minerals.

Given the important effect that climate may have on SOC decomposition, it is critical that tools be developed to accurately predict how SOC formation, decomposition, and storage respond to climate change. Of particular importance is quantifying interactions among driving variables, as these interactions will influence responses in difficult-to-predict ways. For example, warming in cold and wet climates may result in the loss of SOC, as warming can dry out often anaerobic soils in which oxygen supply limits decomposition rates. In contrast, warming in temperate or tropical climates may have little effect on or even slow SOC decomposition rates, especially if moisture is limiting for the soil microbes responsible for decomposing SOC. Reducing uncertainty is important for accurately predicting how the terrestrial carbon cycle, and hence the climate, will respond to global warming.