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

Abstract. Nitrogen availability constrains the 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 ¹⁵N tracer applications allow 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 ¹⁵N fluxes and pools. We used observations from a long-term (10-year) ¹⁵N tracer experiment in a temperate deciduous forest to evaluate the nitrogen dynamics simulated by QUINCY. Recovery in soil N dominated overall ecosystem ¹⁵N recovery in both observations and simulations over the long-term. The observed gradual movement of the ¹⁵N tracer to lower soil layers was also captured by the model. However, in the short-term modeled uptake and losses of ¹⁵N into leaves and fine roots were too fast, and recovery in litter and surface soil was too slow, indicating that the model likely overestimates plant competitiveness for newly added N relative to soil microbes. Downward vertical transport of ¹⁵N tracer in the soil was slow compared to measurements, which may be indicative either of too low bioturbation or vertical transport via leaching. Overall, the QUINCY model results showed good agreement with the observations making it a valuable tool to study long term nitrogen dynamics. Running the model for an extended period for example indicated that the ecosystem retained a very large share of the added ¹⁵N tracer (>90 %), and that this retention persisted at multi-decadal timescales. This study shows that explicit inclusion of isotopic tracers allows for a more profound evaluation of carbon-nitrogen turnover and dynamics and thereby can contribute to reduce uncertainties in modelling nitrogen flow and constrains in terrestrial ecosystems.