Shifting carbon dynamics in a warmer world: Increasing respiration from frozen soils

Background/Question/Methods

Northern tundra ecosystems have been accumulating carbon for thousands of years as a result of cold and moist soil conditions that have limited microbial decomposition.  However, there is a strong potential for changes in carbon storage as the climate warms.  Climate models project that the rate of warming in northern latitudes will be about twice the average global rate and that the greatest warming will occur during the winter months.  An increase in wintertime soil temperature may profoundly affect carbon storage due to enhanced microbial respiration during the snow-covered period.  While wintertime respiration rates in tundra are relatively low, because of the extended duration of the snow-covered period, CO₂ losses under the snowpack are an important component of annual respiratory loss.  This study, which focuses on the effects of warming on ‘wintertime’ (i.e., October through April) respiration in Alaskan tundra, was conducted at the Carbon in Permafrost Experimental Heating Research (CiPEHR) project, an ecosystem warming experiment located in Interior Alaska.  Wintertime respiration was estimated using an exponential temperature model based on CO₂ flux measurements collected in the vicinity of the warming experiment, as well as from direct measurements collected under the snowpack from CiPEHR plots.  Cumulative wintertime respiration estimates were placed within the context of annual carbon balance under current and warmed scenarios. 

Results/Conclusions

Experimental warming increased wintertime soil temperatures (5-40 cm) by 2-3^o C during the snow covered period.  Despite sub-zero soil temperatures, which averaged -4^o C in ambient plots, wintertime respiration accounted for 20-30% of annual respiratory CO₂ loss from the ecosystem.  Modeled estimates of cumulative wintertime respiration were 64 ± 6 g CO₂-C m^-2 in ambient plots and 96 ± 15 g CO₂-C m^-2 in warmed plots.  However, the estimated effect of warming on wintertime respiration varied based on measurement method, ranging from a 50% (modeled) to 200% increase (measured) in respiration as a result of soil warming.  Reducing this uncertainty in CO₂ exchange is essential because wintertime carbon losses can shift a system from a net carbon sink during the growing season to a net source on an annual basis.  We found that warming also significantly increased net growing season carbon uptake, but wintertime respiration completely offset these gains.  These results highlight the importance of wintertime respiration to annual carbon budgets, as well as the potential for a warming-mediated increase in winter respiration to shift tundra ecosystems from a net carbon sink to a carbon source.