Coupling observational methods and the DayCent model to improve examinations of N₂O production pathways

The traditional calibration approach for process-based models, such as DayCent, consists of the iterative adjustment of model parameters and comparison of the simulated total N₂O flux to measured observations. However, the contributions of individual production pathways, namely nitrification and denitrification, are uncertain. Here, N₂O emissions from the soil of sugar beet plots with control (Null) and mineral (NPK) fertilizer treatments were measured by a static chamber technique. The isotopic composition of emitted N₂O was analyzed to identify the N₂O production pathways. The latter showed that denitrification was the predominant source of N₂O emissions at this site. The model's default settings strongly overestimated the contributions of nitrification. This incorrect allocation of N₂O emissions to nitrification could partly be explained by the model's tendency to underestimate the soil water content during the growing season. DayCent model parameters were also manually adjusted to better represent the observation derived contributions of nitrification and denitrification. Although, this "expert-informed approach", showed a slightly lower performance concerning the cumulative N₂O flux (Null: RMSE = 0.37 kg N ha^-1 yr^-1, NPK: RMSE = 0.50 kg N ha^-1 yr^-1) than the traditional calibration (Null: RMSE = 0.15 kg N ha^-1 yr^-1, NPK: RMSE = 0.10 kg N ha^-1 yr^-1), it may be considered as more representative because it better reflected the higher contribution from denitrification shown by the isotope data. This study demonstrates that the inclusion of observational methods, such as isotope measurements, can provide important insight into model function and improve pathway-specific estimation of N₂O emissions in DayCent.

The online version contains supplementary material available at 10.1007/s10705-026-10504-1.