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• Arctic tundra experiences strong climatic seasonality, with cold and long winters, but effective insulation by snow might enable plants and microbes to remain active in winter.
• We investigated year-round seasonality of plant and microbial inorganic nitrogen (N) uptake, within-plant allocation, and their dependence on snow depth in low-Arctic tundra, using monthly in situ 15N-pulse-labelling.
• Plants and microbes took up the 15N-label throughout the year, likely due to sufficiently deep and early snow that stabilised soil temperatures c. 0°C in both study sites. Surprisingly, we found only a few indications for higher plant and microbial 15N-uptake and (estimated) plant total inorganic N-uptake during the growing season compared to the cold season (i.e. the non-growing season). Instead, N-acquisition strongly varied within the cold season, with clearly higher uptake during the cold deep-winter (November–March) than the warmer spring-winter (April–May/June).
• Taken together, our results suggest that 78–82% of the annual plant inorganic N-uptake may take place during the long cold season and as much as 50–56% during deep-winter. This demonstrates the importance of nutrient dynamics during the cold season in the Arctic and challenges the general assumption that arctic plants remain dormant during the winter.

Subarctic regions are warming faster than other parts of the globe, and warming is expected to impact carbon (C) assimilation and its allocation into plant biomass and soluble sugars in plant tissues. We analyzed the concentration of soluble sugars (fructose, glucose, and sucrose) in fine roots and rhizomes for three dominant species: Anthoxanthum odoratum, Equisetum spp., and Ranunculus acris. We also examined the concentration and pool of soluble sugars at the plant community level with the aim to investigate the impact of soil warming duration [medium-term (11 years, MTW) vs. long-term (> 60 years, LTW)] and magnitude on soluble sugars in geothermally warmed subarctic grasslands. Among three species, R. acris exhibited the highest concentration of soluble sugars in both fine roots and rhizomes. Comparing total soluble sugar (TSS) between fine roots and rhizomes, rhizomes exhibited a higher concentration in A. odoratum and Equisetum. spp., whereas fine roots had a higher concentration in R. acris. Soil warming did not affect TSS in E. spp. and R. acris, while in A. odoratum, it increased TSS in fine roots and rhizomes in MTW and only in fine roots in LTW. At the plant community level in MTW, soil warming did not affect the soluble sugar concentration in fine roots. However, it increased the TSS and sucrose concentration in rhizomes, which positively correlated with the abundance of grasses. The TSS pool in fine roots decreased with soil warming in MTW, mainly due to a decline in fine root biomass that described 70 % of the decline in the TSS pool. Also, in LTW, soil warming decreased the TSS pool in fine roots, but 74 % of the decline was mainly driven by decreased soluble sugar concentration, specifically that of sucrose, and not by the change in fine root biomass. The decrease in sucrose concentration in fine roots in LTW was related to a decrease in the abundance of A. odoratum. We highlight the species-specific and organ-specific differences in soluble sugar concentration in subarctic grasslands. We observed elevated soluble sugars in A. odoratum's fine roots and rhizomes due to soil warming, while the overall community-level soluble sugar pool in fine roots decreased. We conclude that in warmed subarctic grasslands, the community-level soluble sugar pool in fine roots and rhizomes depends upon changes in biomass, soluble sugar concentration, and plant community structure.

• Below and aboveground vegetation dynamics are crucial in understanding how climate warming may affect terrestrial ecosystem carbon cycling. In contrast to aboveground biomass, the response of belowground biomass to long-term warming has been poorly studied.
• Here, we characterized the impacts of decadal geothermal warming at two levels (on average +3.3°C and +7.9°C) on below and aboveground plant biomass stocks and production in a subarctic grassland.
• Soil warming did not change standing root biomass and even decreased fine root production and reduced aboveground biomass and production. Decadal soil warming also did not significantly alter the root–shoot ratio. The linear stepwise regression model suggested that following 10 yr of soil warming, temperature was no longer the direct driver of these responses, but losses of soil N were. Soil N losses, due to warming-induced decreases in organic matter and water retention capacity, were identified as key driver of the decreased above and belowground production. The reduction in fine root production was accompanied by thinner roots with increased specific root area.
• These results indicate that after a decade of soil warming, plant productivity in the studied subarctic grassland was affected by soil warming mainly by the reduction in soil N.

BACKGROUND AND AIMS: The response of subarctic grassland's below-ground to soil warming is key to understanding this ecosystem's adaptation to future climate. Functionally different below-ground plant organs can respond differently to changes in soil temperature (Ts). We aimed to understand the below-ground adaptation mechanisms by analysing the dynamics and chemistry of fine roots and rhizomes in relation to plant community composition and soil chemistry, along with the duration and magnitude of soil warming.

METHODS: We investigated the effects of the duration [medium-term warming (MTW; 11 years) and long-term warming (LTW; > 60 years)] and magnitude (0-8.4 °C) of soil warming on below-ground plant biomass (BPB), fine root biomass (FRB) and rhizome biomass (RHB) in geothermally warmed subarctic grasslands. We evaluated the changes in BPB, FRB and RHB and the corresponding carbon (C) and nitrogen (N) pools in the context of ambient, Ts < +2 °C and Ts > +2 °C scenarios.

KEY RESULTS: BPB decreased exponentially in response to an increase in Ts under MTW, whereas FRB declined under both MTW and LTW. The proportion of rhizomes increased and the C-N ratio in rhizomes decreased under LTW. The C and N pools in BPB in highly warmed plots under MTW were 50 % less than in the ambient plots, whereas under LTW, C and N pools in warmed plots were similar to those in non-warmed plots. Approximately 78 % of the variation in FRB, RHB, and C and N concentration and pools in fine roots and rhizomes was explained by the duration and magnitude of soil warming, soil chemistry, plant community functional composition, and above-ground biomass. Plant's below-ground biomass, chemistry and pools were related to a shift in the grassland's plant community composition - the abundance of ferns increased and BPB decreased towards higher Ts under MTW, while the recovery of below-ground C and N pools under LTW was related to a higher plant diversity.

CONCLUSION: Our results indicate that plant community-level adaptation of below ground to soil warming occurs over long periods. We provide insight into the potential adaptation phases of subarctic grasslands.

Global warming may lead to carbon transfers from soils to the atmosphere, yet this positive feedback to the climate system remains highly uncertain, especially in subsoils . Using natural geothermal soil warming gradients of up to +6.4 °C in subarctic grasslands , we show that soil organic carbon (SOC) stocks decline strongly and linearly with warming (-2.8tha^-1 °C^-1). Comparison of SOC stock changes following medium-term (5 and 10 years) and long-term (>50 years) warming revealed that all SOC stock reduction occurred within the first 5 years of warming, after which continued warming no longer reduced SOC stocks. This rapid equilibration of SOC observed in Andosol suggests a critical role for ecosystem adaptations to warming and could imply short-lived soil carbon-climate feedbacks. Our data further revealed that the soil C loss occurred in all aggregate size fractions and that SOC stock reduction was only visible in topsoil (0-10cm). SOC stocks in subsoil (10-30cm), where plant roots were absent, showed apparent conservation after >50 years of warming. The observed depth-dependent warming responses indicate that explicit vertical resolution is a prerequisite for global models to accurately project future SOC stocks for this soil type and should be investigated for soils with other mineralogies.