| 04 Dec 2025
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.
General Comments:
This is a thorough and well-conceived paper describing efforts to use experimental observations to validate a terrestrial biosphere model, with the goal of evaluating how effectively this model describes ecosystem retention of excess nitrogen. The authors use results from a 15N tracer experiment, where plant and soil N retention was measured three times over ten years, to evaluate the QUINCY model’s ability to describe the movement of N through a temperate deciduous forest ecosystem.
The authors are quite thorough in their discussion of the model and how it relates to field observations over time, and do an effective job at explaining where mismatches between simulated and observed N pools occur. The science presented here is sound, and I do not see any critical issues in the methods or interpretation of results in this paper. The work described here is a solid contribution to the literature and should be of interest to BG readers.
My primary critiques are with the contextualization of the study and the results. While the bulk of the paper is spent discussing the model behavior in relation to field observations, there needs to be more attention given to the implications of this work, particularly in the Discussion.
Specific Comments by section:
Introduction
Methods
Results
Discussion
Conclusion
Technical Corrections: