Evaluation of simulated N cycling using observations from a 15 N tracer experiment in a mixed deciduous forest

Nitrogen availability constrains terrestrial carbon uptake and storage, yet large uncertainties remain in the magnitude of the effect, because the interactions of the carbon and nitrogen (N) dynamics are challenging to observe in undisturbed ecosystems at relevant timescales. Long-term experiments with 15 N tracer applications allow the study of the nitrogen cycle in a fairly undisturbed manner, and they are therefore a valuable data source to test the biogeochemical dynamics simulated by terrestrial biosphere models. In this study we applied the model QUINCY (QUantifying Interactions between Terrestrial Nutrient CYcles and the climate system), which includes an explicit representation of terrestrial 15 N fluxes and pools. We used observations from a long-term (10-year) 15 N tracer experiment in a temperate deciduous forest to evaluate the nitrogen dynamics simulated by QUINCY. Recovery in soil N dominated overall ecosystem 15 N recovery in both observations and simulations over the long-term. The observed gradual movement of the 15 N tracer to lower soil layers was also captured by the model. However, in the short-term the modeled uptake and losses of 15 N for leaves and fine roots were too fast, and recovery in litter and surface soil was too slow, indicating that the model likely overestimated plant competitiveness for newly added N relative to soil microbes. Downward vertical transport of 15 N tracer in the soil was slower in the model compared to measurements, which may be indicative either of too little bioturbation or vertical transport via leaching. Overall, the QUINCY model results showed good agreement with the observations, making it a valuable tool for studying long-term nitrogen dynamics. Running the model over an extended period indicated that the ecosystem retained a very large share of the added 15 N tracer (>90 %), and that this retention persisted over multi-decadal timescales. This study shows that explicit inclusion of isotopic tracers allows for a more thorough evaluation of carbon-nitrogen turnover and dynamics and thereby can contribute to reduce uncertainties in modelling nitrogen cycling and constraints in terrestrial ecosystems.