Climate warming causes increased ecosystem carbon fluxes without decreasing soil carbon stocks in boreal forests

Background/Question/Methods

Soil carbon (C) stores in boreal forests are globally significant and recent investigations suggest they may be vulnerable to climate warming representing a potentially significant climate feedback.  It remains unclear, however, how the balance between soil respiratory loss and productivity will be impacted by future warming in these cooler, high latitude forests. Laboratory and in situ field experiments are not able to integrate the impacts of climate on both respiratory and productivity responses and their interactions on time scales relevant to climate change. Natural climate gradient studies, able to capture integrated responses on appropriate time scales, have yielded varied results regarding soil C stocks likely due to the variation in other significant factors, such as soil moisture and forest composition, that regulate soil inputs and losses. Here we report the soil C reservoirs, fluxes into and out of those reservoirs, and the chemical composition of inputs and soil organic matter pools along a mesic boreal forest climate transect.  The sites studied consist of similar forest composition, successional stage, and soil moisture but differ by 5.2˚C mean annual temperature. 

Results/Conclusions

Although a major influence on total soil respiration, temperature was not a driver of net soil C loss but rather one of multiple factors, including precipitation, that increases ecosystem metabolism and turnover of C pools in these forests.  Respiration rates increased by over 40% and the flux of dissolved organic carbon from the organic to mineral soil horizons tripled across this climate gradient.  The 2-fold increase in litterfall inputs to these soils across this climate gradient coincided with a significant increase in the organic horizon C stock with warming, however, no significant difference in the surface mineral soil C stocks was observed. The ratio of alkyl to o-alkyl-C increased in the warmer relative to the colder region organic and mineral soils, and the amino acid compositions were similar despite the younger mean age of the mineral soil C (~70 versus ~330 YBP) signifying the greater turnover of soil organic matter pools in the warmer climate soils. Our results suggest potential for greater terrestrial-to-aquatic fluxes and the need to better constrain those fluxes relevant to carbon loss and sequestration in boreal forested landscapes in a warmer and wetter world.