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Reactive nitrogen (Nr) losses to the air (ammonia NH 3 , nitrous oxide N 2 O) and to water (nitrate NO 3 - leaching) have been reported for Canadian agricultural systems. However, there is very little information on N use efficiency (NUE) and reactive N losses for major crop types. The objectives of this study were to evaluate the NUE and reactive N losses for spring wheat, grain corn, and canola from 1981 to 2021 using the Canadian Agricultural Nitrogen Budget for Reactive N (CANBNr) model. The provincial averages of residual soil N (RSN) at harvest increased by over 3.8-fold for spring wheat, 2.7-fold for canola (grown in Alberta, Saskatchewan and Manitoba), but decreased by 10.8% for corn (mainly grown in Ontario and Quebec) from 1981–1985 to 2017–2021. The RSN was higher in Ontario and Quebec than in the Prairie provinces (Alberta, Saskatchewan and Manitoba). The NUE decreased from 1981–1985 to 2017–2021 for spring wheat and canola whereas it increased from 40.2% to 58.4% for corn. Over the same period, reactive N losses increased by 86.9% for spring wheat and 65.8% for canola, whereas it decreased by 6.8% for corn. Over the 41-year period, reactive N losses for corn were higher than for spring wheat and canola due in part to higher N inputs and greater precipitation in corn-production regions in Ontario and Quebec compared to spring wheat production in the Prairie provinces. Management practices should first target corn to reduce N losses to the environment while improving NUE and maintaining yields for all three crops. • Residual soil N increased 2.7-3.8 fold for spring wheat and canola over 41 years. • Residual soil N decreased by 10.8% for corn over 41 years. • N use efficiency (NUE) increased for corn but decreased in spring wheat and canola. • Reactive N losses increased by 86.9% for spring wheat and 65.8% for canola whereas they decreased by 6.8% for corn over the 41-year period. • Strong correlations were identified between NUE and crop yields for corn (R 2 = 0.59-0.93).
Jiang et al. (Fri,) studied this question.