The effects of permafrost thaw and climate on decomposition in subarctic tundra

Background/Question/Methods

Whether arctic and boreal ecosystems become carbon sources causing a positive feedback to climate change is of global importance. These ecosystems have been carbon sinks for thousands of years because plant productivity has outpaced decomposition, which was slowed by the cold temperatures and frozen soils (permafrost). However, these ecosystems are warming with 7- 8°C temperature increases predicted over the next century. This warming will increase decomposition rates, altering the carbon balance. We investigated the effects of permafrost thaw on annual decomposition of a common substrate in Healy, AK from 2004 through 2011. Furthermore, we set up a common garden to investigate the decomposition of Betula nana, Rubus chamaemorus, Vaccinium uliginosum, Vaccinium vitis-idaea, Rhododendrum tomentosum, Eriophorum vaginatum, Dicranum and Sphagnum litter from 2008 through 2011. We compared decomposition rates of Healy, AK litter to decomposition rates of the same species in more northern tundra around Toolik, AK. We hypothesized that decomposition rates in Healy would be greater than in Toolik because Healy has a warmer climate with a mean annual temperature (MAT) of -1°C compared to Toolik’s MAT of -10°C. Lastly, we measured initial litter quality and ran a leaching experiment to measure potential abiotic mass loss. 

Results/Conclusions

In the permafrost thaw gradient, areas with more extensive thaw had significantly greater annual decomposition of a common substrate. Year was also a significant effect. Model comparisons will be done to investigate the effect of summer temperature, winter temperature, and summer precipitation on decomposition within the permafrost thaw gradient. Despite the warmer climate, the decomposition of Healy litter occurred at the same rate or even slower than the decomposition of Toolik litter from the same species. Percent mass loss due to leaching was significantly greater in Toolik litter (7 to 34%) than in Healy litter (6 to 13%). Toolik litter’s greater abiotic losses may therefore have caused similar decomposition rates among Healy and Toolik litter despite faster microbial decomposition in Healy. Mass loss due to leaching was significantly related to initial litter nitrogen (R²=0.75), and Toolik litter had greater initial nitrogen than Healy litter. Toolik plants may therefore have lower retranslocation efficiencies than Healy plants leading to more cell solubles and nitrogen being left in the litter and thus greater leaching losses.  Initial litter carbon:nitrogen ratios (R²=0.53) and percent cellulose (R²=0.48) were the best predictors of decomposition constant (k) for a combined dataset of Healy and Toolik litter.