Print PageArctic warming promotes permafrost degradation and thaw. Formerly frozen soils destabilize, leading to hillslope thermokarst failures, which erode organic matter from terrestrial ecosystems. Thermokarst failures are abundant and appear to have become more numerous in northern Alaska. This study is part of an ongoing larger NSF-funded study that is investigating the impact of thermokarst failures on aquatic and terrestrial systems.
After the initial episodic loss of nutrients and organic matter, ecological theory predicts that secondary succession will proceed with an initial increase in aboveground biomass and heightened retention of limiting nutrients, whereas ecosystem development after primary succession is much more dependent on external inputs of nutrients. Along several separate chronosequences of thermokarst hillslope failures in Northern Alaska, the early landscape resembles primary succession, with a few isolated rafts of floating tundra on thawed sediment. However, short-term (0-50 y) ecosystem recovery is clearly associated with increased above-ground biomass relative to control tundra, as predicted by the secondary succession model. Our goal was to determine the soil processes driving the observed successional patterns. We hypothesized that ecosystem succession is driven by a combination of enhanced internal nutrient cycling and enhanced nutrient inputs from upslope. To test this, we measured seasonal net cycling of carbon (C), nitrogen (N) and phosphorus (P), seasonal inputs and losses of CNP, and gross N cycling across three chronosequences of ecosystem recovery after thermokarst.
Results/Conclusions
Initial growing season results indicate that both enhanced nutrient cycling and enhanced upslope nutrient inputs are important mechanisms in vegetated ecosystems relative to early-succession thermokarsts or control tundra. Nutrient fluxes to and from isolated soil cores were high relative to pool sizes across all ecosystem ages in both the organic and mineral soils. However, only the shallow organic horizon displayed enhanced nutrient cycling as compared to the mineral soil, the floating tundra rafts, or the control tundra. Together these results indicate a strong and relatively rapid positive feedback from early vegetation establishment, both for soil organic matter production and for nutrient retention. Sampling and analysis is continuing to establish the importance of biological N fixation, exposed mineral soil P weathering, winter snow depth, soil microbial community change, and spring thaw nutrient flushing on ecosystem development. Our preliminary results indicate that nutrient-limited Arctic ecosystems can rapidly mobilize soil nutrients after disturbance, and that patterns of ecosystem recovery from thermokarst may differ from a classic primary or secondary succession model.
