Peatlands are a globally significant store of soil carbon and are currently a significant contributor to global methane (CH₄) emissions.  Peatlands primarily occur at high northern latitudes, where climate change models predict the greatest warming.  There is widespread concern that under altered future climate these ecosystems may emit large amounts of carbon dioxide (CO₂) and/or CH₄ and, thus, become a positive feedback to anthropogenic greenhouse gas emissions.  However, the internal ecosystem dynamics that will control carbon accumulation or loss and CH₄ emissions under future climate are poorly known.  

We constructed a large mesocosm facility in northern Minnesota to examine the effects of climate change on peatlands.  Twenty-seven intact peat monoliths (2.1 m², 60-cm depth) were removed each from a bog and intermediate fen and subjected to three infrared-loading treatments and three water-table treatments for eight years.  A wide range of response variables were examined, including soil carbon gain/loss, trace gas emissions, and plant productivity. 

Results/Conclusions

Ecosystem respiration as CO₂ flux increased with warmer soil temperatures, but this effect declined substantially over several years, suggesting the exhaustion of a labile soil pool.  CH₄ fluxes depended on both water-table level and soil temperature, but the effect of water table was much greater in the fen mesocosms, suggesting an enhanced potential for methanotrophy.  The water-table and warming treatments affected CH₄ fluxes as much through changes in porewater chemistry and plant productivity as through the expected direct effects.           

We integrated carbon responses as net carbon gain or loss for the various treatments.  Fen mesocosms either had no change or lost carbon, whereas the bog mesocosms gained carbon.  Wetter conditions enhanced carbon storage in the bog mesocosms and reduced carbon losses in the fen mesocosms.  Rapid initial vertical peat accumulation in the wet bog treatments increased the height of the surface above the water table, and this slowed carbon storage rates over time.  Changes in carbon fluxes in the bog mesocosms were largely due to water-table effects on moss net primary production, whereas the changes in the fen appeared to be due to multiple factors.  Our results suggest large effects of climate change on carbon dynamics and trace gas emissions in peatlands, with important ecosystem feedbacks determining the trajectories of response.