## 1 Introduction  
Over recent decades, both the atmosphere and oceans have warmed, sea level has arisen, snow cover and ice sheets have diminished, all whilst the atmospheric concentration of greenhouse gases have continued to increase [1]. Although changes have been observed globally, it is mostly agreed that northern high latitude and alpine regions are amongst the most vulnerable to climate change--currently warming at 0.6 \({}^{\circ}\)C per decade; over twice as fast as the global average [1, 2, 3, 4]. Such regions hold a large portion of global organic carbon with approximately twice as much carbon being stored in frozen Arctic soil or permafrost as is currently in the atmosphere [5, 6, 7, 8]. There is the potential for this carbon to be released into the atmosphere as greenhouse gases under permafrost thaw, creating a positive feedback loop onto climate [9, 10, 11, 12]. There are an increasing number of both modelling and experimental studies which are beginning to explore the impact of an interactive nitrogen (N) cycle on the permafrost carbon feedback (for example, [13, 14, 15, 16]). Generally, Arctic vegetation is strongly nitrogen-limited due to the supply of available soil mineral N being lower than the demands of the plant, reducing net primary productivity (NPP) [17, 18, 19, 20]. However, with rising temperatures,there is increased 'greenness' around the circumpolar Arctic [21, 22], which is likely a result of increased biomass [21], changes in plant community composition [21, 23, 24, 25, 26], and changing plant phenology [21, 27, 28]. Although there are reports of background increases in tundra vegetation [21, 25, 29, 30, 31], Arctic shrub expansion is widely reported throughout the literature [21, 22, 32], with shrub species, such as birch, willow, and alder increasing in abundance, cover and height across the region [22]. Increases in shrub abundance can be attributed to various processes, such as rising temperatures, permafrost thaw, tundra fires, anthropogenic activity, etc. [22]. Rising temperatures can promote shrub growth either directly through physiological processes or indirectly by enhancing soil microbial activity such as increased nitrogen mineralisation which supplies nutrients for shrub uptake [22]. Arctic amplification of shrubs and other Arctic tundra also varies between species. For example, taller shrub species have been observed to have greater nitrogen availability and faster nitrogen cycling compared to shorter, low shrubs [22, 32, 33]. Whilst shrubs that are able to grow in cover or height can restrict the growth of other species by limiting light [22, 34], suggesting some species may have a competitive advantage under further future warming. The interactions between this enhanced vegetation growth and the availability of mineral nitrogen is uncertain and will impact the magnitude and timing of the permafrost carbon feedback [6, 13]. Recent experimental studies at selected sites suggest that nitrogen at depth in the soil (organic and inorganic), previously unavailable to plants due to its frozen state, could be made accessible by warming-induced permafrost thaw and reduce plant nitrogen limitation in the future [13, 14, 15, 35, 36, 37, 38, 39, 40]. Furthermore, there is evidence that future warming may result in an increased abundance of Arctic plant roots with depth, further increasing plant uptake of previously unavailable nutrients [41, 40, 42, 43, 44]. For example, certain shrub species such as the dwarf birth _Betula nana_ is able to take advantage of increased temperatures and subsequent thaw depth, by rapidly elongating its short shoots [22]. This suggests that increased nitrogen availability could reduce or even outpace nitrogen demand of Arctic vegetation and hence nitrogen limitation [36].