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

Projected changes in climate could shift northern peatlands from their curr ent status as net C sinks toward that of being net C sources by changing so il temperatures and hydrology. We assessed the importance of water table an d soil temperature as controls over ecosystem respiration in a bog and sedg e fen in northern Minnesota, USA, by means of a manipulative mesocosm exper iment. Fifty-four intact monoliths were removed from a bog and a fen and in stalled in insulated tanks that permitted control of the water table and we re heated by overhead infrared heaters. The experimental design was a fully crossed factorial combination of two communities, three water tables, and three heat levels. Fcosystem respiration as indicated by emission of CO2 an d CH4, dissolved nutrient fluxes, and productivity were measured and summar ized 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 communit y types (similar to 630 g C/ m(2)) or with water table level. These results suggest that community type, within certain limits, will not be an importa nt 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 emi ssions were on average three times higher in the bog than in the fen mesoco sms (21 vs. 7 g C/m(2)). Aboveground net primary productivity and dissolved N retention were also higher in the bog mesocosms. There were strong corre lations between CH4 flux and N retention, but generally weak correlations b etween CH4 and plant primary production. The relatively lower CH4 emissions from the fen mesocosms appear to result mainly from higher rates of methan otrophy in the aerated zone, possibly reinforced by the effects of higher p orewater N concentrations and lower primary productivity compared to the bo gs. The results confirm the existence of strong environmental controls over ER and methanogenesis, which are modulated by complex interactions between pla nt community and soil nutrient dynamics. The differential responses of thes e ecosystem functions to climate change may complicate efforts to predict f uture changes in C dynamics in these important repositories of soil C.