Using radiogenic Sr isotopes to trace nutrient uplift from permafrost thaw: a field-based soil warming experiment

The amplified warming at the poles have largely impacted arctic and subarctic ecosystem by accelerating permafrost degradation. The resulting topographical and hydrological consequences have induced significant shifts in vegetation development and composition with a clear trend of increased productivity since the early 1980’s. This trend, referred to as Arctic greening, causes significant feedback to climate dynamics by altering ground albedo, solar radiation, and shading, as well as the ecosystem net C balance through respiration, photosynthesis and litter degradation.

Firstly, Arctic greening is characterized by increased productivity, resulting from warmer temperatures, longer growing seasons, increased precipitation, atmospheric CO₂ concentrations, and access to newly thawed nutrients from deeper soil horizons. Secondly, over the past decades, a notable shift in vegetation has been occuring with an overall increase in shrub dominance accompanied by local increase in graminoid expansion in subsided and poorly drained areas. Given that changing nutrient sources for tundra vegetation has major implications for vegetation changes in the Arctic, and thereby on vegetation-climate feedback, there is a need to identify the processes controlling changes in nutrient sources and mobility for Arctic tundra vegetation upon permafrost thaw. We hypothesize the release and uplift of essential nutrients at depth to result from vegetation cycling and/or water table rise.

To test this hypothesis, we compared radiogenic Sr isotopes composition of three typical tundra plants with different rooting depth subjected to an eight-year soil warming experiment at the Eight-Mile Lake study site in Alaska. We show that plants subjected to soil warming exhibit access to a different nutrient source than that of the control plants, representative of a deeper, recently thawed soil layer. This shift is observed regardless of rooting depth, indicating uplift of thawed nutrients. Therefore, to identify the dominant process governing nutrient mobility upon permafrost thaw, we used vegetation composition survey and mass balance equation to model the magnitude of nutrient transfer from vegetation cycling and water table rise.