concept

Pyrogenic carbon

The black carbon left behind when organic material pyrolyses with too little oxygen to burn completely - charcoal and biochar are both this material. Its structure is tightly bonded aromatic rings that soil microbes struggle to break down, so it persists on timescales of 10² to 10⁴ years rather than the few years typical of ordinary humus. In the soil, these porous fragments raise cation exchange capacity and water-holding capacity, retaining exactly the nutrients that the first rain after a fire would otherwise wash away. Every fire therefore both vents CO₂ to the atmosphere and files part of its carbon into a very long-term store.