An updated nitrogen budget for Canadian agroecosystems

Abstract

Nitrogen (N) is an essential ingredient for the production of food in agricultural systems. As the use of industrial fertilizers has risen over time, so too have concerns relating to N losses from these systems and the contamination of air and water, climate change impacts and biodiversity losses these cause. In order to balance continuing and improving agricultural productivity with the potential for N losses, improved estimates of N additions, N use efficiency (NUE, the ratio between N inputs and N exported in products) and N losses to the environment are crucial. We estimated the N balance for Canadian agroecosystems based on 2016 data and used similar but updated assumptions and coefficients to recalculate estimates from a previous study based on 1996 data. This approach calculated the overall recovery of N in Canadian agriculture by imposing a system boundary around all farmlands, and then estimating the N balance within that area. In 2016, a total of 3.80 Tg of N were input to the agroecosystems from biological (0.94 Tg), industrial (2.54 Tg) and atmospheric (0.32 Tg) fixation, compared with 2.42 Tg in 1996 (a 57 % increase). Between 1996 and 2016, crop NUE increased from 46.7 to 50.8 %, livestock NUE from 21.2 to 22.8 %, and agroecosystem NUE from 58.3 to 59.5 %. Due to higher total N inputs and a decrease in the area of agricultural land, the total mass of N added to the system and not exported in crop and livestock products rose by over 50 % (from 1.01 to 1.54 Tg). This indicates that although N use efficiencies improved over the 20-year period, the total mass of N potentially lost to the environment increased significantly. To improve N use efficiencies and reduce N losses, management strategies must be tailored to different conditions across Canada’s agricultural landscape, adopted at all stages of agricultural production and consider the multiple flows and feedbacks of N within the agroecosystem. Future research should aim not just at improving N use efficiency, but also at tracing more precisely the fate of N unaccounted for, so that losses can at least be steered away from more pernicious routes.