A Carbon Source, Sink and Store: Explaining Soil Carbon

2 September 2021

A Carbon Source, Sink and Store:

There is a lot of confusion surrounding terms such as ‘carbon sink’, ‘carbon store’ and ‘carbon source’.
In a nutshell:

• A carbon source releases more carbon than it absorbs.

• A carbon sink is any reservoir that absorbs more carbon than it releases.

• A carbon store/stock maintains a constant amount of carbon.

These three terms are neatly illustrated during a woodland’s life cycle. Initially, a planting project is a carbon source, due to the ground disturbance, the use of tree guards, the fencing etc. Thereafter, as the trees grow, it becomes a (significant) carbon sink before, as chart 1 shows, maturing into a carbon store. Depending on the species choice, the soil type and other factors, the time between these stages varies considerably but, generally speaking, the woodland is a source in years 0-5, a sink in years 5-100, and beyond that they act as stores.

Chart 1: Chronology of Sequestration Volumes on a Standard Woodland Scheme

Explaining Soil Carbon

An important but often overlooked store of carbon is the soil. On average, the top 1m of soils in UK woodlands contain three quarters of the ‘in-forest’ carbon stock (see image 1). This ‘in-forest’ stock accounts for the timber of the tree itself, all biomass in the leaf litter, other vegetation and the soil.

Image 1: The Carbon Cycle of a Single Tree.

Even though the soil disturbance from ground preparation for tree planting will release carbon dioxide back into the atmosphere (primarily through microbial respiration), over the lifetime of the woodland the increase in biomass, improved soil stability and the mitigation of flooding means that the amount of carbon stored in the soil can increase dramatically.

Want to read more? Download the CarbonStore User Guide below:


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Pending Issuance Unit: A promise to deliver a Woodland Carbon Unit during a given period, based on the trees’ predicted growth Woodland Carbon Unit: A ton of carbon dioxide which has been sequestered in a scheme verified under the Woodland Carbon Code