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 degrees C) and on a subset briefly reheated in air at 400 degrees 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 N-2-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. (C) 2018 Elsevier B.V. All rights reserved.