RAPID CARBON RESPONSE OF PEATLANDS TO CLIMATE CHANGE

Peatlands comprise a globally important carbon pool whose input-output budgets may be significantly altered by climate change. To experimentally determine the sensitivity of the carbon stored in peatlands to climate change, we constructed a mesocosm facility with 54 peat monoliths from a bog and fen in northern Minnesota, USA. These mesocosms were subjected to nine combinations of heat and water-table levels over eight years. Bog mesocosms initially accumulated soil carbon, with greater gains in wetter mesocosms, but after three years no further water-table effects occurred. In contrast, fen mesocosms lost or had no change in soil carbon, with the greatest losses in drier and warmer mesocosms. Changes in soil-carbon storage resulted in concomitant changes in water-table depth, so that water-table depths were similar to those in the natural source sites by the end of the experiment regardless of the initial treatment. These results were primarily due to water-table effects on Sphagnum moss production in the bog mesocosms and to a more complicated suite of warming and water-table effects on production and decomposition in the fen mesocosms. We show that different kinds of peatlands will rapidly gain or lose carbon following hydrological disturbance until they return to their characteristic ("equilibrium'') water-table levels. Our results illustrate the potential for a rapid homeostatic response of these ecosystems to future climate change at small spatial scales. Climate change will likely also interact with other carbon cycle-hydrological feedbacks at the scale of the entire peatland over longer time frames and larger spatial scales.