Denitrification in biogeochemical models—challenges and advances in simulating N$_2$ and N$_2$O emission in agricultural systems

Denitrification represents a major nitrogen (N) loss pathway in agriculture, reducing plant N uptake, lowering crop N use efficiency, while emitting the potent greenhouse gas nitrous oxide (N$_2$O). However, due to methodological challenges, field measurements of N$_2$ emissions remain rare, leaving denitrification losses poorly characterised across most cropping systems and comprehensive N budgets lacking. This constrains parameterization and validation of biogeochemical models, impeding efforts to forecast denitrification losses under changing conditions and design effective mitigation strategies. This study (i) reviewed the current literature on field-based N$_2$ measurements, (ii) tested the ability of five different models to accurately simulate denitrification losses, and (iii) simulated denitrification losses from agricultural soils in Germany using gridded and point-based modelling frameworks. Results show that field studies suitable for model calibration remain scarce and mostly limited to temperate systems. Simulated cumulative N$_2$ emissions varied widely between models, ranging from 0.89 to 6.07 kg N ha$^{−1}$, far below the observed N$_2$ emissions of 21 ± 3 kg N ha$^{−1}$. ... mehrThe ratio of N$_2$O to (N$_2$O + N$_2$) emitted (R$_{N_2O}$), a key determinant of the climate impact of denitrification, was not only strongly overestimated by all models but also exhibited considerable variations, ranging from 0.056 to 0.707, compared to the measured ratio of 0.006. Models also failed to capture expected responses of to environmental drivers. For Germany, simulated average N$_2$ emissions from agricultural soils ranged from 1.7 to 14.5 kg N ha$^{−1}$yr$^{−1}$, with corresponding R$_{N_2O}$ ratios ranging from 0.18 to 0.59. Spatial patterns of emissions also differed significantly across modelling frameworks, reflecting large structural uncertainties. These findings highlight that both N$_2$ emissions and RN$_2$O ratios remain poorly represented in current models, severely limiting our ability to establish reliable N budgets and develop targeted N management strategies. Coordinated advancements in field-based measurements and modelling are urgently needed to improve the representation of denitrification in biogeochemical models.