## 3 Results
### Impact of Nitrogen Addition on Vegetation
JULES simulated much higher biomass for \(CN_{low}Root_{60}\) (the equivalent of \(R\). _chamaemorus_) at \(\sim\)95 gm\({}^{-2}\) and fairly similar values (\(\sim\)77 gm\({}^{-2}\)) for \(CN_{high}Root_{25}\) (the equivalent of \(E\). _hermaphrotitum_). One reason for this difference may be that JULES assumes that the root carbon is the same as the leaf carbon. However, [40] suggested that the root biomass is much higher than the aboveground biomass--between 254 and 403 gm\({}^{-2}\) for \(R\). _chamaemorus_ and 53 and 200 gm\({}^{-2}\) for \(E\). _hermaphrotitum_. Figure 6 shows that impact of fertilisation on the aboveground biomass after the nitrogen addition. In all cases, as might be expected, there is an increase in aboveground biomass. Near surface fertilisation (shown in red) causes an increase in biomass for \(CN_{high}Root_{25}\) which is approximately twice that compared with \(CN_{low}Root_{60}\). This is fairly well aligned with the observations, although the observed response of _E. hermaphrottum_ to near-surface fertilisation is smaller than the response of \(CN_{high}Root_{25}\) in JULES. \(CN_{high}Root_{25}\) has a very small response (\(<\)1 gm\({}^{-2}\)) to thaw-front fertilisation both in JULES and in its equivalent observations for _E. hermaphrottum_. \(CN_{low}Root_{60}\) responds more to thaw-front fertilisation, increasing by over 9 gm\({}^{-2}\) although this response in the model is less than observed for _R. chamaemorus_ and less than that for the equivalent near-surface fertilisation. This is likely because the linearly decreasing profile of the root distribution prescribed in the model is not an accurate representation of the root profile of the deep-rooting species _R. chamaemorus_ which was found to be relatively evenly distributed over the soil profile in [40]. Another possible cause of the differences is that addition of inorganic nitrogen to the thaw front only increased NPP beyond the control for one month (August) in the simulation, whilst, in reality, we would expect to observe longer impacts. Responses of both \(CN_{low}Root_{60}\) and \(CN_{high}Root_{25}\) show approximately additive behaviour with both near-surface and thaw-front fertilisation scenario (purple bars Figure 6) very similar to the near-surface and thaw-front fertilisation combined. This is comparable to the response of NPP in Figure 5. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \(CN_{high}Root_{25}\) & \(CN_{low}Root_{60}\) \\ \hline \(Control\) & 1.9 & 3.3 \\ \(N_{NearSurface}^{+}\) & 1.0 & 2.8 \\ \(N_{ThawFront}^{+}\) & 1.9 & 2.4 \\ \(N_{Both}^{+}\) & 1.0 & 2.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The response ratio which is defined as the ratio of the potential amount of carbon that can be allocated to growth and spreading of the vegetation (\(NPP_{pot}\)) compared with the actual amount achieved in the natural state (dimensionless). A value of 1.0 indicates the vegetation is not nitrogen-limited. The larger the value, the more nitrogen-limited the vegetation. Figure 5: NPP for \(CN_{high}Root_{25}\) (**left**) and \(CN_{low}Root_{60}\) (**right**). The impact on the NPP of the different N addition experiments are shown in both cases throughout the year 2010. For \(CN_{high}Root_{25}\) (left) \(Control\) and \(N_{ThawFront}^{+}\) are the same, so only \(N_{ThawFront}^{+}\) is visible. Similarly, \(N_{NearSurface}^{+}\) and \(N_{Both}^{+}\) are the same.