I think it's important to recognize that the increase in fertilizer use has been commensurate with the increase in crop production, particularly on the Prairies, and in that shift in production, canola now comprises a greater proportion of Canada's crop production.
Also, the total production from the Prairies has increased more than it has in eastern Canada. The nitrous oxide emission coefficient currently used in the national inventory is lower in the Prairies than it is in eastern Canada and, for this reason, nitrous oxide emissions aren't shown to have increased as much as the nitrogen fertilizer use.