## 2 Methods
### Model Description  
JULES forms the land-surface component of the UK Earth System Model (UKESM, [50]) and can be run either in a coupled mode, or offline using observed meteorology, at point, regional or global scales [51, 52, 53]. The model simulates fluxes of energy, water, carbon, and nitrogen. It has a dynamic vegetation model, namely, the Top-down Representation of Interactive Foliage and Flora Including Dynamics (TRIFFID) where different plant functional types (PFTs) compete for space based on their height [54]. Recent developments within JULES include the addition of a terrestrial nitrogen cycle which has a representation of all of the key terrestrial nitrogen processes. This closely follows the structure of the carbon cycle [53]. Nitrogen inputs to the land surface are via biological fixation, fertilisation, and nitrogen deposition, with losses from the land surface via leaching and gas loss. JULES simulates a nitrogen-limited ecosystem by reducing the NPP if there is insufficient available nitrogen. Any excess carbon is assumed to be lost from the system. The soil biogeochemistry is represented using the standard four-pool RothC soil carbon model [52] in which the total litter carbon flux (taken as a sum from leaves, roots, and stem) is split into either a decomposable plant material (DPM) pool or resistant plant \begin{table}
\begin{tabular}{l c c} \hline \hline
**Type** & _Rubus Chamaemorus_ & _Empertrum Hernaphroditium_ \\ \hline Common name & Cloudberry & Mountain Crowberry \\ Description & perennial deciduous herb & low-growing evergreen shrub \\ Biomass (\%) & 12 & 78 \\ Aboveground biomass (gDW m\({}^{-2}\)) \({}^{*}\) & 8 & 55 \\ Root depth (m) & 0.45 & 0.25 \\ Root density in top layer (gm\({}^{-2}\)) & 350 & 30 \\ Leaf C:N ratio & 20 & 70 \\ Aboveground C:N ratio \({}^{*}\) & 10 & 40 \\ \hline Equivalent JULES simulations & \(CN_{low}Root_{60}\) & \(CN_{high}Root_{25}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the observed plant characteristics of the two dominant vegetation types observed in the plots. Root content was evaluated at a top soil layer depth of 15 cm and included all living roots larger than 0.1 mm. \({}^{*}\) Estimated from Figure 3 in [40]. material (RPM) pool according to an approximated fraction defined for each vegetation type within the model. These pools then feed into microbial biomass (BIO) and humus (HUM) pools via decomposition. For each soil carbon pool, there is an equivalent soil nitrogen pool [53]. Additionally, [55] added a representation of the vertical distribution of soil carbon with each of the pools represented in each of the soil layers. This model was extended by [53] who additionally included a vertical representation of organic and inorganic nitrogen. Nitrogen is transferred between the organic and inorganic pools depending on the respiration and the carbon-nitrogen ratio (C:N) of the organic pool. For each soil layer, the inorganic nitrogen pool (\(N_{in}(z)\) in kg[N] m\({}^{-2}\)) is the balance of biological nitrogen fixation (first term in Equation (1)), plant nitrogen uptake (second term in Equation (1)), net mineralisation (\(M_{net}(z)\) in kg[N] m\({}^{-2}\)s\({}^{-1}\)), external inputs via deposition and fertilisation (\(N_{add}(z)\) in kg[N] m\({}^{-2}\)s\({}^{-1}\)) transfer of dissolved inorganic N between layers (\(N_{flux}(z)\) in kg[N] m\({}^{-2}\)s\({}^{-1}\)), and gaseous emission (\(N_{gas}(z)\) in kg[N] m\({}^{-2}\)s\({}^{-1}\)). This is represented by the following equation:  
\[\frac{dN_{in}(z)}{dt}= \sum_{i}v_{i}BNF_{i}f_{R,i}(z)-\sum_{i}v_{i}\Phi_{i}f_{I,i}(z)+M_{ net}(z)+N_{add}(z)-N_{flux}(z)-N_{gas}(z) \tag{1}\]  
where \(v_{i}\) is the PFT fraction for PFT \(i\) and \(BNF_{i}\) is the biological nitrogen fixation for PFT \(i\) in kg[N] m\({}^{-2}\) s\({}^{-1}\). The inputs from biological nitrogen fixation for PFT \(i\) are distributed vertically according to the normalised fraction of roots in each soil layer (\(f_{R,i}(z)\)) described below and shown in Equation (2).