| Summary: | Permafrost thaw modifies resource acquisition for tundra vegetation in two major directions of vegetation shift across the Arctic: the expansion of deeply rooted sedges and the widespread increase in shallowly rooted woody shrubs. Assessing to what extent nutrient access is changing for the Arctic tundra vegetation development is crucial given the feedbacks of vegetation shifts on Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. In this study, we evaluate the influence of permafrost degradation on the nutrient sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source for plant nutrient, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determine the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compare the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (B. nana, V. vitis-idaea, and E. vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) reflect a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflect a source of Sr from deeper soil horizons. Importantly, our data highlight a shift in the range of foliar 87Sr/86Sr ratios towards lower values in plants grown on more deeply thawed permafrost soils, thereby supporting that the three Arctic tundra plant species access nutrients from deeper soil horizons upon permafrost thaw. The differences in plant strategy for nutrient acquisition is therefore expected to influence largely interactions between deeply and shallowly rooted plant species, and thereby the future shift in Arctic tundra vegetation.
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