Exploring the consequences of winter versus summer permafrost soil warming using a microbial physiology-explicit decomposition model

Background/Question/Methods

Arctic soils are among the largest stores of terrestrial organic carbon (C) globally. Climate models unambiguously predict that the Arctic will continue to warm over the next century; therefore, there is substantial interest in developing mechanistic descriptions of Arctic systems’ responses to warming, in particular C storage potential. Previous studies have suggested that nitrogen (N)-limitation regulates both plant and microbial growth in tundra soils in what may be a seasonally-dependent pattern; however, there is a little information on the impact of the seasonality of warming on long-term soil biogeochemical dynamics or plant-soil feedbacks. We are using a combined measurement-mechanistic modeling approach to address this knowledge gap, by exploring the consequences of long-term warming on tundra soil microbial dynamics, nutrient cycling and net C storage.

The model includes three classes of soil organic matter (SOM), microbially-synthesized extracellular enzymes specific to these SOM pools,and a microbial biomass with a variable C:N ratio. The plant biomass, which contributes to the SOM pools, dynamically allocates growth effort towards wood, root and leaf biomass, based on N-uptake. The microbial community acclimates between a more bacterial-like (lower C:N, faster turnover) and fungal-like (higher C:N, slower turnover) community, depending upon the SOM environment and inorganic nutrient availability the microbes see at a given timestep. This stoichiometeric flexibility allows for the microbial C and N use efficiency to vary, feeding back into system decomposition and productivity dynamics. These feedbacks scale from the microbial to ecosystem level, including changes in the relative allocation to oxidative and hydrolytic extracellular enzyme synthesis, nutrient turnover rates, plant growth, and net C storage. In order to test the effects of predicted Arctic warming on decomposition dynamics and net ecosystem C storage, we simulated winter, summer and year round soil warming over a 50 year period, and observed the effects of this on the model system's biogeochemical dynamics.

Results/Conclusions

This research highlights the potential for the seasonal nature of warming to be a highly significant factor in regulating microbial activity and thus the potential magnitude of tundra soils’ decomposition with warming. In particular, our model system reveals that winter warming potentially has a greater net effect on ecosystem C loss than summer warming, due to changes in microbial nutrient use efficiency. This net C loss with winter warming occurs despite increased plant growth in both the summer and winter warming scenarios, echoing recent experimentally-derived data from field studies.