Impact of plant community composition and soil characteristics on Mo and V cycling in subarctic habitats at Abisko, Northern Sweden.

Arctic ecosystems are changing rapidly due to climate warming. Increased air temperature increases shrub proportions in plant communities at large scales, and increased soil moisture associated with permafrost thawing favours herbaceous cover at local scales. Changes in vegetation community composition or soil moisture could particularly affect the biogeochemical cycling of elements in arctic environments through the variation in the proportion of slow-cycling woody biomass and in the mineralisation rates of organic matter with soil water saturation. Nitrogen (N), often considered to limit primary production in these ecosystems, could be particularly affected by these changes. Biological fixation of atmospheric dinitrogen represents a major input of N in these systems and is catalysed by nitrogenase enzymes that contain either Molybdenum (Mo) or Vanadium (V). Thus, the concentrations, stocks and bioavailability of these two lithogenic trace elements (TE) may be key factors in alleviating the N constraint on primary productivity and vegetation change. Understanding their biogeochemical cycles is therefore crucial for our comprehension of changes in arctic environments. Our study evaluated the concentrations and stocks of Mo and V   in vegetation and soils of different subarctic habitats with different soil characteristics and vegetation communities in a mire and a tundra ecosystem at Abisko, northern Sweden. Mo was more concentrated in the biomass of herbaceous species than in shrubs, resulting in higher stocks in the biomass of herbaceous-dominated habitats than in shrub-dominated habitats. Conversely, V concentrations and stocks were not different between the two vegetation types. In soils, Mo concentrations were globally the same between deep and surface horizons (0.38 mg kg-1), whereas V concentrations were globally higher in deep horizons than at the surface (61.0 to 22.9 mg kg-1, respectively). Accordingly, enrichment factors of the surface horizons compared to deep horizons showed that Mo was highly enriched at the surface (EF > 1 and up to > 8), highlighting the importance of surface processes on Mo cycling. V was not enriched in the tundra but presented EF values similar to Mo in the mire, indicating a locally higher influence of surface processes in this ecosystem. Exploring the possible reasons behind these behaviours, we found that 1) atmospheric deposition seems to play little role in their concentrations in surface soils, 2) soil pH and redox conditions could partially explain the surface enrichment of these two elements through their sorption on organic matter and metallic oxides. We conclude that global and local changes in plant communities in arctic ecosystems could decrease Mo and V litter fluxes with increased shrub cover, with a greater impact on the Mo cycle than on V due to the stronger influence of surface processes on the Mo cycle. We also highlight the importance of local factors for TE speciation that would control the bioavailability of these elements for organisms. Altogether, these results underline the need to consider the changes in TE cycling in regard to their importance for underlying processes controlling major elements (C, N) dynamics in the changing Arctic.