Sub-national variability in nitrogen emissions in Canada: Accounting for multiple supply-chain perspectives

Abstract:

Anthropogenic mobilization of reactive nitrogen (Nr) has enhanced global food production yet contributes to a range of issues including climate change, eutrophication, and air pollution. However, understanding drivers of Nr emissions is confounded by factors, such as globalization and urbanization, that spatially decouple production and consumption activities. Varied supply-chain perspectives can therefore inform mitigation measures to reduce excess Nr by accounting for Nr flows across different system boundaries. Using the Canadian context, we compare production (territorial emissions inventories) and consumption (nutrient ‘footprint’) accounting at the sub-national level.

We estimate per capita provincial Nr emissions for food production, wastewater, transport and energy for 2018. For consumption accounting we use a top-down Nr footprint approach, drawing on data from provincial agricultural statistics, fertilizer shipments, and detailed bilateral trade data. We then compare these results to production accounting from provincial greenhouse gas and air pollutant emissions inventories.

Our preliminary estimates show that the relative contribution of transport, energy, and wastewater in each province varies widely. For example, the emissions from the oil and gas sector in Alberta (~170 kg Nr/cap/yr) are substantially higher than the Canadian average (~30 kg Nr/cap/yr). Estimates for these three sectors are similar using both production and consumption perspectives. However, food production-related emissions vary greatly, both between provinces and depending on which accounting methodology is used. This variation results from differences in agricultural activities across the provinces, and whether a province is a net importer or exporter of food. For example, in Saskatchewan, a large grain-producing province, fertilizer application results in elevated territorial emissions (~45 kg N/ cap/yr compared to the Canadian average of ~6 kg N/cap/yr); however, the province’s Nr footprint is driven mainly by consumption of imported meats.

Our study highlights the sub-national variability in drivers of Nr emissions that are not apparent based on national averages, emphasizing the importance of considering multiple accounting perspectives and scales for targeting emissions reductions.