Reactive nitrogen losses from Canadian agricultural soils over 36 years

Agriculture is a major source of reactive nitrogen (Nr) losses through ammonia (NH3) volatilization, nitrous oxide (N2O) emissions and nitrate (NO3−) leaching. A Canadian Agricultural Nitrogen Budget for Reactive N (CANBNr) model was developed to estimate the nitrogen (N) balance in soils, including N removals by harvested crops and Nr losses for the years 1981–2016 across Canada. Annual N inputs to farmland include commercial fertilizer N, livestock manure N, symbiotic and asymbiotic biological N fixation and atmospheric N deposition of NOx and NH3. The total annual N input for Canadian farmland was 5528 Gg N (103.2 kg N ha−1) in 2016 where N removal by crops and Nr accounted for 72.5 % (74.9 kg N ha−1) and 13.4 % (13.9 kg N ha−1), respectively. The Nr losses from N2O emissions, NH3 volatilization and NO3− leaching accounted for 64 Gg N (1.2 kg N ha−1), 330 Gg N (6.2 kg N ha−1) and 348 Gg N (6.5 kg N ha−1), which represents between1.2–6.3 % of total N input. A total of 777 Gg N (14.5 kg N ha−1) remained in the soil as surplus N (14.1 % of total N input), which could be available to subsequent crops in dryer regions but might be subject to N2O loss through nitrification or denitrification processes or NO3− leaching following heavy rains in humid regions. Nitrous oxide and ammonia emissions increased over a 36-year period due to increased fertilizer N inputs. The percentage of N inputs that was estimated to be lost as Nr increased from 17.9 % in 1981 to the peak level of 19.8 % in 2001 and then declined to 13.4 % in 2016. Total N removal by crops increased at a greater rate than N input during the 2001–2016 period resulting in an increased N uptake by crops over the last 15 years. The improved management of fertilizer N for agricultural systems represents a key opportunity for both farmers and policy makers to further reduce Nr losses from Canadian farmland without negatively impacting productivity.