Influence of soil warming magnitude and duration on soluble sugar pool in fine roots and rhizomes of subarctic grasslands: Differences at species and plant community level adaptation

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.