Combining measurements and modelling to reveal long-term effects of nitrogen fertilizer application timing on N2O emissions in corn

The impact of nitrogen fertilizer (N) application timing on nitrous oxide (N 2 O) emissions is inconsistent in the literature. This inconsistency is attributed to year-to-year weather variations, which affect soil conditions around N application time. Planting dates (PD) also vary year-to-year based on weather, and PD can influence N timing decisions. The study aims to evaluate: i) the long-term effects of different N application timings on N 2 O emissions and, ii) how variations in PD influence the relative performance of different N timing strategies. We used the DeNitirifcation-DeComposition (DNDC) model, calibrated with field measurements from Elora, Ontario, Canada, to simulate 39 growing seasons using historical weather data. Three N timing strategies were tested: spring application one day before planting, in-season application at the V6 growth stage, and a split-N strategy with N applied at both times. PDs were either dynamically adjusted each year based on rainfall or fixed to one of three typical corn ( Zea mays L.) planting dates in Ontario: April 25, May 5, and May 15. For the first objective, the long-term simulation found that average N 2 O emissions were greatest when N was applied at V6 (3.2 kg N ha −1 ) compared to when N was applied pre-plant (2.3 kg N ha −1 ) or split-applied (2.0 kg N ha −1 ). This was caused by slightly greater rainfall around V6 than planting. For the second objective, the relative performance of different N-timing strategies was affected by PD. Earlier PDs resulted in lower N₂O emissions compared to later PDs, primarily due to lower soil temperatures around the time of N fertilizer application. Earlier PDs also led to the largest differences in N 2 O emissions among the N timing strategies, with PD delays leading to smaller differences among N timing strategies. Large single N applications, particularly those applied in-season, resulted in greater N 2 O emissions than split and at-planting N applications in a long-term simulation. Early PDs consistently reduced N 2 O emissions by creating less favourable conditions for N 2 O production. Moreover, the relative performance of N timing strategies was mediated by PD. This study highlights the interconnected nature of cropping systems, where one management practice, PD, can influence a seemingly unrelated outcome, N 2 O emissions. Long-term climatic, social, economic, and technological changes that influence PD will also influence N 2 O emissions from spring and summer-applied N fertilizer. • Early planting dates reduce N 2 O emissions from nitrogen applications. • Split nitrogen applications result in the lowest average N 2 O emissions. • N 2 O emissions peak with large nitrogen applications at the V6 growth stage. • Effect of nitrogen timing on N2O emissions depends on planting date. • Long-term simulations can reveal the true effect of different fertilizer strategies for emission reduction.

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