Plants: A Curious Form of Air

Photographer: David Lynch
Summary Author: David Lynch

Every year, countless tons of plant material is burned, hauled to the dump or simply left to rot where it died. Virtually all of this mass comes “out of thin air.” Plants are made primarily of cellulose (C₆H₁₀O₅) with a little nitrogen.  The carbon comes from atmospheric CO₂, as does the oxygen.  Hydrogen comes from rainwater. As plants grow, they’re actually converting air into a solid – the plants themselves. The reverse also happens.  When wood is burned or decays, it “evaporates” and becomes air once again through the process of oxidation. Carbon oxidizes to become CO₂, hydrogen and oxygen combine into H₂O and and nitrogen forms N2 or oxidizes to become  NO_2. All of these products are gases.

Much of the mass of plants is in the form of water that’s derived primarily from the ground through root uptake.  When plants die, they quickly dry out, giving up their moisture to the air as water vapor. The fact that plant mass is derived from air has an interesting aspect: burning wood (as opposed to fossil fuels) does not influence the CO₂ budget of the modern atmosphere.  CO₂ released by burning and natural decay recently came from the atmosphere itself.  If less wood is burned or rots each year than is grown, CO₂ is removed from the atmosphere, and vice versa.  To be sure, the chemistry of growing is quite different than that of decomposition or combustion.  But in general, the combustion of biofuels is considered “carbon neutral.”