## 5 Conclusions  
We have simulated the response of Arctic vegetation to belowground fertilisation in JULES, which was able to successfully replicate a three-year belowground fertilisation experiment, illustrating the model's ability to simulate belowground nitrogen dynamics in a permafrost-affected ecosystem. JULES simulations show that both shallow- and deep-rooting vegetation responded positively to increased nutrient availability from fertilisation near the soil surface. Increased plant-available inorganic nitrogen pools allowed an increase of nitrogen uptake and subsequent productivity of for both shallow- and deep-rooting species. In particular, the shallow-rooting species was no longer nitrogen-limited due to enhanced NPP. This increase in productivity then resulted in increasing biomass for both species. However, only deep-rooting species responded to fertilisation at the thaw front, in accordance with the observations. Future projections indicate that deep-rooting plant species may have a competitive advantage in response to belowground fertilisation in a warming climate. Results for the model runs where plant roots were allowed to extent their foraging to the thaw front show that deep roots respond more positively to increased available inorganic nitrogen than shallow roots, with simulations showing higher productivity and a further increase of 8% biomass for the deepest rooting species. Furthermore, as inorganic nitrogen becomes available and productivity increases, simulations for shallow-rooting species with extended roots at depth (\(CN_{high}Root_{60}\)) are able to meet their NPP potential, becoming no longer nitrogen-limited, whilst deep-rooting species with extended roots at depth greatly reduced nitrogen limitation. The research presented here is progress towards a more realistic model representation of nitrogen dynamics in permafrost-affected ecosystems. This increases potential to better understand the impacts of climate change on the coupled carbon and nitrogen cycle in the Arctic system. **Author Contributions:** Conceptualization, R.V., E.J.B., S.E.C., F.K. and A.B.H. ; methodology, R.V., E.J.B. and S.E.C. ; software, R.V. and E.J.B. ; formal analysis, R.V. and E.J.B. ; data curation, E.J.B. ; writing--original draft preparation, R.V. ; writing--review and editing, R.V., E.J.B., S.E.C., F.K. and A.B.H. ; visualization, R.V. and E.J.B. All authors have read and agreed to the published version of the manuscript. **Funding:** This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 101003536 (ESM2025--Earth System Models for the Future). E.J.B. was supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). SEC was supported by a Natural Environment Research Council independent research fellowship (grant No. NE/R015791/1). **Institutional Review Board Statement:** Not applicable  
**Informed Consent Statement:** Not applicable  
**Data Availability Statement:** Abisko meteorological data and snow depth data are available on request from the Abisko Scientific Research Station ([[https://polar.se/en/research-in-abisko](https://polar.se/en/research-in-abisko)]([https://polar.se/en/research-in-abisko](https://polar.se/en/research-in-abisko))) (last access: 17 April 2022), and the soil temperature data are archived in the GTN-P database ([[https://gtnp.arcticportal.org/](https://gtnp.arcticportal.org/)]([https://gtnp.arcticportal.org/](https://gtnp.arcticportal.org/))) (last access: 17 April 2022). Soil carbon profiles are available from the following Zenodo repository: [[https://zenodo.org/record/5818180#YmmBftrMKUk](https://zenodo.org/record/5818180#YmmBftrMKUk)]([https://zenodo.org/record/5818180#YmmBftrMKUk](https://zenodo.org/record/5818180#YmmBftrMKUk)) (last access: 17 April 2022). **Acknowledgments:** We thank the Swedish Polar Research Secretariat and SITES for the support of the work done at the Abisko Scientific Research Station.