Adsorption and desorption of nitrous oxide by raw and thermally air-oxidized chars

Addition of biomass chars (biochar) to soil reportedly suppresses emissions of nitrous oxide (N₂O), a potent greenhouse and ozone-depleting gas, but the causes and endurance of the effect are unclear. To determine whether adsorption may play a role, adsorption isotherms of N₂O were constructed at 273 K on outgassed samples of anoxically-prepared wood-derived chars (300-700 °C) and on a subset briefly reheated in air at 400 °C. Sorption by the chars was greater and more reversible than sorption by soils or soil mineral phases. Adsorption by chars increased with pyrolysis temperature and upon post-pyrolysis air oxidation. The Langmuir maximum capacity correlates well with the CO₂-determined (but not N₂-B.E.T.-determined) surface area. At environmentally realistic partial pressures in soil, N₂O adsorption correlates with CO₂ adsorption, and is found to predominate in the micropores (<1.5 nm), especially ultramicropores (<0.7 nm). Neither adsorption nor adsorption reversibility was affected by coating the char with soil organic matter extract. It is concluded that char added at levels above 1% in soil would act as a strong and reversible sink for N₂O, and could be responsible for the temporary nature of N₂O emission suppression observed in some cases.