| Summary: | Arctic warming and permafrost loss modify northern ecosystems through soil subsidence, changes in soil hydrology, nutrient availability and vegetation succession. In particular, warming and soil moisture conditions influence Arctic tundra vegetation production and distribution: wetter soil conditions favor sedge expansion and drier soil conditions drive woody shrub expansion. While these shifts in vegetation may alter permafrost integrity by modifying the surface energy transfers, they may also be responsible for further changes in the tundra vegetation production and distribution by influencing plant nutrient cycling. Specifically, this PhD thesis investigates the influence of permafrost degradation on mineral nutrient distribution in soils and the influence of tundra vegetation shift on mineral nutrient cycling. Along a permafrost thaw gradient in Interior Alaska, our results demonstrate that permafrost is an important frozen reservoir of nutrients for plants, with stocks of total and exchangeable base cations in the permafrost soil layers more than twice as high as in the seasonally thawed active layer. Moreover, we observe that foliar elemental stocks and annual fluxes from leaf to soil litter change with vegetation shift; sedge expansion promotes Si, P and Fe foliar cycling and shrubification promotes Ca and Mn foliar cycling. Lastly, we highlight that shrub and sedge species both take up nutrients from deeper soil horizons upon permafrost thaw, with the deeply rooted sedge benefitting first from the release of nutrients at depth. These findings provide insights to improve the integration of changes in mineral nutrient cycling into models predicting Arctic ecosystem evolution and feedbacks on climate. (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2022
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