## 4 Discussion  
It has been shown that subarctic permafrost peatlands contain large amounts of plant-available nitrogen which may be released upon future thawing [13, 35]. Such change in nutrient availability will impact (usually nitrogen-limited) vegetation by altering composition, productivity, and biomass [40] and could potentially counteract some of the carbon loss from thawing permafrost. Here, we show that it is possible to simulate belowground fertilisation in JULES by successfully replicating observations seen from previous fertilisation experiments. Our results show that near-surface fertilisation increases nitrogen available to both shallow- and deep-rooting species (Figure 4). However, only the deep-rooting species benefit from nitrogen applied at the permafrost have front. This is consistent with the experimental study conducted by [40]. In their study, [40] also found that deep-rooting plants were able to take up nutrients at the end of the growing season at the thaw front. This can be seen in our results by the substantial amount of nitrogen available to the deep-rooting plant species (\(CN_{low}Root_{60}\)) near the thaw front where subsurface fertiliser has been applied (Figure 4(iii,iv)) and by the increase in NPP in Figure 5. Since the largest thaw depth is reached at the end of the growing season, this ability to take up nitrogen late in the season is key to whether or not plant species can benefit from the nitrogen released from thawing permafrost in the future. These results contradict the assumption from both [63, 64] that seasonal asynchrony between nitrogen-availability at the thaw front and plant nitrogen-demand means that nitrogen released from thawing permafrost will not affect biomass, and our study shows that the impact on biomass can be significant indeed. Although broadly our results are similar to [40], the simulated increase in aboveground biomass in response to fertilisation differs from the values found in the [40] study. In particular, [40] showed the fertilisation at the thaw front had a larger affect on the aboveground biomass of the deep-rooting species (_R. chamaerorus_) than near-surface fertilisation, whereas our \(CN_{low}Root_{60}\) saw the opposite response. This difference is likely explained by  
Figure 9: Vegetation aboveground biomass for \(CN_{low}Root_{60}\) and \(CN_{high}Root_{25}\) and \(CN_{low}Root_{60}\) for the present (2001–2010) and future (2009–2099). Also shown are the two future scenarios where the roots have responded to the increase in thaw depth \(CN_{low}Root_{90}\) (labelled Future-Root\({}_{90}\)) and \(CN_{high}Root_{60}\) (labelled Future-Root\({}_{90}\)). the relative amounts of roots in the soil profile, where larger a larger abundance of roots is observed at the thaw front for deep-rooting species compared to what is assumed in JULES. In addition, the deep-rooting PFT (\(CN_{low}Root_{60}\)) has approximately 10 gm\({}^{-2}\) more aboveground biomass than our shallow-rooting PFT (\(CN_{high}Root_{25}\)), which is contrary to that found by [40]. There is also an approximately double relative increase in aboveground biomass for \(CN_{high}Root_{25}\) compared to \(CN_{low}Root_{60}\) when near surface fertiliser was added. This is likely caused by differences in nitrogen-limitation, with shallow-rooting species being more nitrogen-limited near the surface (Figure 5). Future projections show that increases in plant-available inorganic nitrogen from present to the end of the century under climate warming are broadly similar to the changes obtained from the fertilisation at the thaw front in the belowground fertilisation simulations (Table 4). This results in increases of productivity and subsequent changes in vegetation. Nitrogen mineralisation is a key process that provides plant-available inorganic nitrogen at depth and is affected by multiple factors, such as hydrology, biochemistry, oxygen availability, and soil temperatures [2, 11, 65].