Diurnal and seasonal variation of carbon dioxide exchange from a former true raised bog

Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half year period in 1994 and 1995. The measurements took place over a former true raised bog, characterized by a shallow peat layer and a vegetation dominated by Molinia caerulea. The growing season extende d from May until late October, with a maximum LAI in August of 1.7. The carbon balance shows a net release of 97 g C m(-2) y(-1) (265 kg C ha(- 1) y(-1)) from the peat bog ecosystem to the atmosphere. During June, July and August there is net consumption of CO2, while during the rest of the ye ar there is net production of CO2. The average daytime assimilation rates r anged between - 0.2 and - 0.5 mg CO2 m(-2) s(-1) (- 45 and -11.3 mu mol CO2 m(-2) s(-1)), in a period where the LAI ranged between 1 and 1.7. A high v apour pressure deficit (> 15 hPa) corresponding with high temperatures was found to reduce the assimilation rate by on average 50%. Apart from these f actors, LAI and the soil temperature codetermine the net exchange of CO2. The total nocturnal respiration during the growing season lies within the s ame order as the average daytime net assimilation rate. Temperature was fou nd to be the main fatter controlling soil respiration, with a Q(10) of 4.8.