Provincial nitrogen footprints highlight variability in drivers of reactive nitrogen emissions in Canada

Abstract

Nitrogen (N) footprints are one method to quantify consumer driven reactive nitrogen (Nr) emissions. Canada is a highly urbanized yet economically natural resource-dependent country, providing an illustrative case study to examine attribution of Nr emissions to per capita consumption, either domestically or abroad. Yet, considered only at the national scale, N footprints may obscure absolute and relative contributions of local drivers to Nr emissions. We apply a top-down N footprint approach drawing from national N budgets, emissions inventories, and agricultural statistics to estimate sub-national (provincial) drivers of Nr emissions across Canada. We calculate per capita provincial Nr footprints from four primary sectors in 2018: (a) crop production, (b) animal production, (c) wastewater treatment, and (d) fossil fuel burning. We estimate that Canada’s total N footprint is 995.7 Gg Nr yr ^−1 , which equates to an average per capita footprint nationally of 27.1 kg Nr capita ^−1 yr ^−1 . The largest national contributions come from a few key (sub)sectors, including transport, beef consumption, and wastewater treatment. Provincial per capita N footprints vary widely, with the largest (Saskatchewan 50.3 kg Nr cap ^−1 yr ^−1 ) more than double the smallest (Ontario 22.0 kg Nr cap ^−1 yr ^−1 ). Most variation across provinces is due to the fossil fuels sector, including emissions from energy generation and the oil and gas industry. We therefore compare our top-down approach for the fossil fuels sector with bottom-up N footprints and territorial emissions methodologies. Per capita N emissions vary considerably across these approaches. For example, Alberta’s per-capita fossil fuel Nr emissions are 45.9, 23.0, and 6.3 kg Nr cap ^−1 yr ^−1 using territorial, top-down and bottom-up footprint approaches, respectively. This analysis demonstrates the challenges of attributing Nr emission for export-oriented economies. Our study provides novel insights on sub-national drivers of Nr emissions, emphasizing the need to consider how heterogeneous geographic contexts contribute to national N footprints.

Keywords

• nitrogen
• footprint
• virtual nitrogen
• agriculture
• fossil fuels
• greenhouse gas emissions