Enhancing Biochar Adsorption of Nitrous Oxide.

Nitrous oxide (N2O) is a powerful greenhouse gas and the dominant source of ozone depleting nitrogen oxides. A major source of N2O is microbial metabolism of fertilizers applied to soil. Biochar, a soil amendment made by anaerobic pyrolysis of biomass wastes, has also created excitement because of its ability to reduce N2O emissions in soil. One possible explanation for this is adsorption of N2O by the added biochar. In this study, we measured the adsorption and desorption of N2O on biochar and soil components (clays, sand, soil organic matter), and estimated the contribution of biochar to the adsorption of N2O by biochar-amended soil under various conditions. It was found that the adsorption of N2O on biochars showed modest irreversibility, while the adsorption on clay, sand, peat and a soil showed much greater irreversibility. The effect of biochar pyrolysis temperature was investigated. From the adsorption and desorption profiles determined, it appears that the amendment of raw biochar could, at least temporarily, reduce N2O emissions due to its much greater adsorption of N2O than the normal soil components, but the contribution of raw biochar amendment to the overall adsorption hysteresis of N2O is modest. We found a simple and effective method by post-pyrolysis air oxidation (PPAO) of biochar can further increases adsorption by a factor of ~2, and also significantly increases the adsorption hysteresis. The amendment of PPAO-tailored biochar is expected to contribute to a large extent of the reduction of N2O emission from agricultural soils.