Soil nutrient cycling response to thermokarst in northern Alaska

Background/Question/Methods

Arctic soils store large pools of carbon (C) that are sensitive to a warming climate. When upland permafrost thaws, soil organic matter, C and nutrients are mobilized by the resulting landscape erosion. Ecosystem recovery in the short-term (~ 50 y) is characterized by strongly enhanced above-ground biomass (shrubbiness) relative to undisturbed, early or late successional tundra. Yet, upland arctic terrestrial ecosystems are very strongly nitrogen (N) limited to plant growth and microbial activity, and may also be co-limited with phosphorus (P). Our goal was to determine the source of the soil nutrients for these above-ground successional patterns. We hypothesized that ecosystem succession is driven primarily by enhanced microbial processing of soil organic matter within the thermokarst scar. To test this, we measured C, N and P availability, net cycling, and gross N cycling across three chronosequences on the Alaskan North Slope, spanning more than 100 years of ecosystem recovery after thermokarst.

Results/Conclusions

Nutrient pools accumulated in the mineral soils over time since thermokarst. Across all chronosequences, over the fifty plus years, extractable organic carbon and nitrogen declined in the organic soil and accumulated in the mineral soil. Extractable phosphate and microbial biomass phosphate also accumulated in the mineral soil. We found enhanced ammonium availability in the organic soils, which may support enhanced biomass in the fifty-year old thermokarsts. Generally, nitrogen cycling rates were high, pools were low, and these rates and pools did not vary along the gradient of recovery. Further patterns in cycling and pools were site-specific and most likely a response to variation in soil parent material, pH, aspect and slope in combination with thermokarst recovery. Nutrient turnover as a result of enhanced microbial processing of soil organic matter within a thermokarst scar may be inadequate to explain the enhanced above-ground biomass that is evident mid-succession. We are continuing investigation of other possible nutrient inputs to thermokarst scars, such as N-fixation and downslope nutrient inputs at thaw.