Soil is a hugely complex matrix of living organisms, inert minerals and organic matter. There are more organisms in a teaspoon of healthy soil than there are humans on the planet. In this mix, carbon, in various forms, plays a vital role in soil health and productivity. Soil carbon is in constant flux, cycling between more and less stable forms in processes that fuel much of the web of life in the soil.
A common and informative, but simplistic, description of soil carbon is to classify it into 3 pools relating to how carbon is cycled and stored in soil.
- The labile pool which includes living and readily decay-able plant matter and other organic substances.
- Humus pool which is assumed to include a range of relatively stable forms of carbon that contribute most to soils structure and health.
- Recalcitrant carbon which includes very stable substance such as charcoal.
The listed carbon pools are a very useful way of describing and understanding soil carbon and have been successfully applied in models to predict and understand carbon dynamics in soil.
As useful as the soil carbon pool description is there is a growing recognition that soil carbon is much more complex and dynamic than the simplified pool model suggests. The emerging view is that soil carbon is a function of the overall soil matrix ecosystem. In particular the limitations of the concept of humus as a class of stable chemical compounds is being replaced by a greater understanding of the importance of roots and stable aggregates within the soil matrix.
In the context of carbon trading, soil carbon is a quantity that can be measured by a set of agreed methods. Typically this is done by sieving out particles >2mm and then measuring the remaining carbon by chemical or combustion based methods. You don’t need to understand the complexity of soil carbon in order to trade soil carbon. A deeper understanding of soil carbon is however very useful to understanding how it can be improved over time.