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

Context: The impact of nitrogen fertilizer (N) application timing on nitrous oxide (N2O) 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. Objective: The study aims to evaluate: i) the long-term effects of different N application timings on N2O emissions and, ii) how variations in PD influence the relative performance of different N timing strategies. Methods: 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 splitN 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. Results: For the first objective, the long-term simulation found that average N2O 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 N2O emissions among the N timing strategies, with PD delays leading to smaller differences among N timing strategies. Conclusions: Large single N applications, particularly those applied in-season, resulted in greater N2O emissions than split and at-planting N applications in a long-term simulation. Early PDs consistently reduced N2O emissions by creating less favourable conditions for N2O production. Moreover, the relative performance of N timing strategies was mediated by PD. Implications: This study highlights the interconnected nature of cropping systems, where one management practice, PD, can influence a seemingly unrelated outcome, N2O emissions. Long-term climatic, social, economic, and technological changes that influence PD will also influence N2O emissions from spring and summer-applied N fertilizer.