Biochar is a diverse material with two distinct carbon pools, each varying in durability: the labile (less stable) and the recalcitrant (stable, aromatic) fraction. A higher recalcitrant fraction indicates better durability, with stable polycyclic aromatic carbon persisting for over 1000 years in soils.
The ratio of labile to recalcitrant fractions is influenced by pyrolysis conditions, such as pressure and temperature, and the chemical composition of the feedstock. The molar ratio between hydrogen and carbon (H:C) serves as a proxy for aromatization, with a ratio below 0.4 indicating a high percentage of stable, aromatic carbon. All current suppliers adhere to a H:C ratio below 0.4, established as a vetting requirement in autumn 2022.
Biochar's durability is affected by soil type and temperature, with colder soils leading to slower degradation. Credits for biochar are currently classified as medium durability (100-1000 years), storing 1 tonne of CO₂ per credit over 100 years. The two carbon pools exhibit distinct degradation dynamics, with the labile pool degrading over decades to centuries, while the aromatic pool may persist over 1000 years. Durability estimates will be updated as scientific consensus on biochar evolves.
Biochar's sponge-like structure and slight electrical charge contribute to its ability to bind water and nutrients, enhancing soil retention. This property leads to improved water retention, reducing crop vulnerability to drought, and a diminished need for nitrogen fertilizers, thereby decreasing nitrogen pollution and nitrous oxide emissions. Biochar also aids in building soil organic carbon content, contributing to additional carbon removal and fertility improvement. In tropical regions, biochar has demonstrated significant improvements in crop yields. While these co-benefits, including removal and emission reduction, accompany biochar application, they are not factored into the calculation of carbon removal credits, representing additional benefits beyond the core biochar carbon removal.
