## 4 Discussion
Therefore, under future warming, nitrogen mineralisation at the thaw front could be impacted through the response of such factors. Nitrogen mineralisation is known to be highly sensitive to temperature change [66, 67, 68, 35]. In shallow soil layers, plant-available nitrogen is enhanced due to increased mineralisation due to air temperatures [67, 68]. Whilst warmer temperatures in deeper layers of permafrost soils can lead to warming-induced increased microbial activity and increased mineralisation rates of organic matter [66]. A study conducted by [35] investigating mineralisation sensitivity to increased temperatures in permafrost soils found a five-fold increase in mineralisation rates over 0.5-11 \({}^{\circ}\)C, highlighting the importance that nitrogen mineralisation might play in providing sources of inorganic nitrogen for plant uptake at the thaw front in the future. Thawing permafrost may also provide soil drainage and new oxygen availability which could further enhance mineralisation rates [66]. However, mineralisation rates may respond differently by different thawing scenarios [35]. For example, abrupt permafrost thaw can lead to the formation of permafrost ponds and wetland which, in turn, might limit mineralisation through soil moisture [69]. The effects of increased nitrogen availability and uptake are enhanced when the simulated root depth is allowed to increase with increased summer thaw depth. Previous work suggests that a mismatch in time between nutrient demand and nitrogen availability will result in a negligible impact on plant root dynamics with future permafrost thaw [64, 63, 70]. However, recent research suggests that plant rooting depth can increase in thawing permafrost ecosystems [40, 42, 43, 44], suggesting that having deep roots could be more advantageous under future warming and could result in vegetation shifts [71, 72]. In our simulations, the largest pools of plant-available inorganic nitrogen were seen when species roots were extended to account for future root plasticity (Table 4), resulting in higher uptake and productivity compared to species with no root growth into greater depths. Inorganic nitrogen uptake by the species with no root adaptation increased by the end of the century (54% and 75% for \(CN_{high}Root_{25}\) and \(CN_{low}Root_{60}\), respectively). However, once the roots were allowed to adapt and grow deeper in the soil, the inorganic nitrogen uptake increased substantially--an increase of up to 300% when compared to present (Table A3). Increasing inorganic nitrogen uptake reduces nitrogen-limitation, and consequently, simulations with increased root depths had the largest increases in plant productivity and vegetation biomass (Figure 9). In fact, the \(CN_{high}Root_{25}\) species with adapted roots (\(CN_{high}Root_{60}\)) became no longer nitrogen-limited in the future (Figure 8) and had an extra 16% increase in aboveground biomass compared with the \(CN_{high}Root_{25}\) species. This suggests that species that are able to extend roots deep into the soil profile will have a competitive advantage under future warming. This has been observed in Arctic permafrost peatlands. For example, [73] found a change from shrub-dominated hummock sites to graminoid-dominated in Sweden over a 30-year period in response changes in frozen ground. There are around 1.7 million km\({}^{2}\) of permafrost peatlands [74]. The shallow-rooting species (\(CN_{high}Root_{25}\)) has about 90 gm\({}^{2}\) aboveground biomass in the present day, which converts to 0.15 Gt C over the permafrost peatlands. An increase of 22% gives an extra 0.033 Gt C of aboveground biomass for this species over the permafrost peatlands. If this species grows deeper roots, there is an increase of 33% in the aboveground biomass which gives an extra 0.05 Gt C. The extent of the permafrost region is estimated to be 21 million km\({}^{2}\) with approximately 14 million km\({}^{2}\) underlain by permafrost [75].