Biochar is a heterogenous material that consists of two distinct carbon pools with different degrees of durability: labile and recalcitrant (stable, aromatic) fraction; larger recalcitrant fraction means better durability, as the stable polycyclic aromatic carbon has been shown to persist over 1000 years in soils.
The ratio between labile and recalcitrant fraction mostly depends on pyrolysis conditions such as pressure and temperature and somewhat on the chemical composition of the feedstock. Molar ratio between hydrogen and carbon (H:C) is used as a proxy for the degree of aromatization, as it can be easily and precisely measured. A ratio below 0.4 indicates a high portion (~75%) of the stable, aromatic carbon. All our current suppliers produce biochar with H:C below 0.4, as this has been introduced as a vetting requirement in autumn 2022.
Durability of biochar carbon also depends on soil type and temperature. Colder soils means slower degradation.
We currently classify biochar credits as medium durability (100-1000 years). At 100 years the amount of carbon stored by 1 credit will be 1 tonne of CO₂; this may mean a slightly higher amount (buffering) needs to be sequestered in the first place. From there, the two respective carbon pools show distinct degradation dynamics, each following a degradation curve. Labile pool degrades over decades to centuries, but aromatic pool may persist over 1000 years. The science of biochar durability is still evolving, and we will update durability estimates when firmer scientific consensus forms.
Biochar has a large, sponge-like surface area and slight electrical charge, which means it binds water and nutrients and holds them better in the soil.
The two main beneficial consequences of this property are improved water retention and therefore reducing crop exposure to droughts, and reduced need for nitrogen fertilisers (as less nitrogen is lost from field). The latter also means reduced nitrogen pollution and reduced emissions of nitrous oxide, another potent green-house gas. Furthermore it helps build-up soil organic carbon content, meaning there is additional carbon removal and fertility improvement. Particularly in tropical regions, biochar was shown to significantly improve crop yields. There is also evidence from traditional use of charcoal in soils, the most well-known example being the fertile Terra Pretasoils in Brazil.
While all of these co-benefits, including additional removal and emission avoidance, come with biochar application, none of them are used in the calculation of the carbon removal credit - meaning the co benefits come as ‘extra’ on top of the core biochar carbon removal.
