ALBERT

Description: Abstract The isotopic composition (ic) of soil Nitrogen (N) and, more recently, the intramolecular distribution of 15N in the N2O molecule (site preference, SP) are powerful instruments to identify dominant N turnover processes, and to attribute N2O emissions to their source processes. Despite the process information contained in the ic of N species and the associated potential for model validation, the implementation of isotopes in ecosystem models has lagged behind. To foster the validation of ecosystem models based on the ic of N species, we developed the Stable Isotope MOdel for Nutrient cyclEs (SIMONE). SIMONE uses fluxes between ecosystem N pools (soil organic N, mineral N, plants, microbes) calculated by biogeochemical models, and literature isotope effects for these processes to calculate the ic of N species. Here we present the concept of SIMONE, apply it to simulations of the biogeochemical model LandscapeDNDC, and assess the capability of 15N‐N2O and, to our knowledge for the first time, SP to constrain simulated N fluxes by LandscapeDNDC. LandscapeDNDC successfully simulated N2O emission, soil nitrate and ammonium as well as soil environmental conditions of an intensively managed grassland site in Switzerland. Accordingly the dynamics of 15N‐N2O and SP of soil N2O fluxes as simulated by SIMONE agreed well with measurements, though 15N‐N2O was on average under‐ and SP overestimated (RMSE of 8.4 ‰ and 7.3 ‰, respectively). While 15N‐N2O could not constrain the N cycling process descriptions of LandscapeDNDC, the overestimation of SP indicated an overestimation of simulated nitrification rates by 10 to 59% at low water content, suggesting the revision of the corresponding model parameterization. Our findings show that N isotope modeling in combination with only recently available high frequency measurements of the N2O ic are promising tools to identify and address weaknesses in N cycling of ecosystem models. This will finally contribute to augmenting the development of model‐based strategies for mitigating N pollution. This article is protected by copyright. All rights reserved.