Of all the many forms that carbon can take in our world, biochar is the most stable - except for diamonds. Research has shown it can last for thousands of years in topsoils, even in tropical regions where it is exposed to heat, moisture and a wide variety of microorganisms and fungi. The underlying reason is because of its molecular structure.
Plants absorb carbon dioxide from the atmosphere and use it it create carbohydrates and the molecular building blocks of their structure - cellulose, hemicellulose and lignin - which are all infused with carbon atoms. As the pyrolysis reaction proceeds, stable aromatic rings of carbon, composed of 6 tightly bound carbon atoms, condense into an interconnected three dimensional structure that is highly resistant to decomposition.
Biochar's ability to enhance soil fertility and sequester carbon build on that backbone of stability.
Naturally present carbon in soils in the form of humus isn't very stable in agricultural settings where the soil is repeatedly disturbed. Soils all over the world have been rapidly losing carbon from intensive agricultural practices. As the carbon is depleted, productivity falls off dramatically.
As a source of carbon in soils, biochar is much more stable in agricultural contexts than humus. There are some 20 billion tonnes of biomass waste produced in agricultural and forestry operations worldwide every year. Stabilizing the carbon from these waste streams in biochar would have a profound effect, both on stabilizing our climate, and on the sustainability of agricultural and renewable energy systems.