Biochar is made by heating biomass, like crop residue or wood, with very little oxygen so it chars instead of burning away completely. The question asks which element this charred material ends up rich in.
- Nitrogen: Biomass contains some nitrogen, but a lot of it volatilizes and escapes as gas during heating, so nitrogen is not the dominant leftover element.
- Sulphur: Sulphur is present only in small quantities in most plant material, so it is not what biochar is known for.
- Phosphorus: Phosphorus is present in biomass in modest amounts and is not the defining component of biochar.
- Carbon: Plant tissue is built mostly from carbon-based compounds like cellulose and lignin. When combustion is incomplete, this carbon framework does not fully oxidize to \(CO_2\), instead it survives as a stable, blackened, carbon-rich solid, which is exactly what biochar is.
Because incomplete combustion stops the carbon in biomass from fully escaping as gas, the leftover charred material is dominated by carbon, which is also why biochar is used to lock carbon into soil for long periods.
Let's summarize:
- Incomplete combustion (pyrolysis) leaves behind a stable, carbon-heavy solid.
- This carbon-rich residue is called biochar and is added to soil to boost soil carbon and structure.
So the correct answer is option 4, carbon.