Response of CO₂ and CH₄ emissions from peatlands to warming and water table manipulation

Projected changes in climate could shift northern peatlands from their current status as net C sinks toward that of being net C sources by changing soil temperatures and hydrology. We assessed the importance of water table and soil temperature as controls over ecosystem respiration in a bog and sedge fen in northern Minnesota, USA, by means of a manipulative mesocosm experiment. Fifty-four intact monoliths were removed from a bog and a fen and installed in insulated tanks that permitted control of the water table and were heated by overhead infrared heaters. The experimental design was a fully crossed factorial combination of two communities, three water tables, and three heat levels. Ecosystem respiration as indicated by emission of CO₂ and CH₄, dissolved nutrient fluxes, and productivity were measured and summarized for each growing season from 1995 to 1997. Seasonal ecosystem respiration (ER) as indicated by CO₂ emissions responded almost exclusively to soil temperature and did not differ between community types (∼630 g C/ m²) or with water table level. These results suggest that community type, within certain limits, will not be an important factor in predicting temperature-driven increases in ER. The response of CH₄ flux to soil temperature and water table setting became progressively stronger in each succeeding growing season. Seasonal CH₄ emissions were on average three times higher in the bog than in the fen mesocosms (21 vs. 7 g C/m²). Aboveground net primary productivity and dissolved N retention were also higher in the bog mesocosms. There were strong correlations between CH₄ flux and N retention, but generally weak correlations between CH₄ and plant primary production. The relatively lower CH₄ emissions from the fen mesocosms appear to result mainly from higher rates of methanotrophy in the aerated zone, possibly reinforced by the effects of higher pore water N concentrations and lower primary productivity compared to the bogs. The results confirm the existence of strong environmental controls over ER and methanogenesis, which are modulated by complex interactions between plant community and soil nutrient dynamics. The differential responses of these ecosystem functions to climate change may complicate efforts to predict future changes in C dynamics in these important repositories of soil C.