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

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.

Automated summary

Nitrogen footprints are used to quantify consumer-driven reactive nitrogen emissions, with Canada providing a case study to examine attribution of these emissions to per capita consumption. The study found that different provinces in Canada have varying levels of nitrogen footprints, with key sectors such as transportation, beef consumption, and wastewater treatment making significant contributions. The analysis highlights the challenges of attributing nitrogen emissions in export-oriented economies and emphasizes the importance of considering heterogeneous geographic contexts in national nitrogen footprints.