DECAY POTENTIAL OF HUMMOCK AND HOLLOW SPHAGNUM PEATS AT DIFFERENT DEPTHS IN A SWEDISH RAISED BOG

Samples of organic material derived from two hummock-forming species (Sphagnum fuscum and S. rubellum) and two hollow species (Sphagnum cuspidatum and S. magellanicum) were collected from several depths in a southern Swedish raised bog and rates of CO2 emission were measured in the laboratory under aerated conditions at 18-degrees-C. The rate of CO2 release from the most recently deposited Sphagnum peat litter from the 2.5-5 cm depth layer was nearly twice as high (0.66 mg CO2 g-1 d-1) as that from older organic material from the 10-12.5 cm depth layer (0.35 mg CO2 g-1 d-1) under common conditions of temperature, moisture and aeration. No consistent differences in CO2 emission rates were detected among the four Sphagnum species for the shallow organic material tested. However, when deeper (50-100 cm) Sphagnum-dominated peat samples were exposed to the same aerated conditions, peat formed by the hummock-forming species released CO2 at a slower rate (0.23 mg g-1 d-1) than the two hollow species (0.29 mg g-1 d-1). The results suggest that the amount of previous exposure of Sphagnum peat to decay processes may be more important than the species composition in determining subsequent rates of decay and carbon loss. The observed rates of CO2 emission were used to estimate the annual mass loss from aerobically decaying organic material under the thermal regime at the study site. Based on these estimates, Sphagnum litter loses about 5% of its dry mass annually during the initial stages of decay, but the decay rate decreases to 2-3% annually for deeper Sphagnum peat exposed to aerobic conditions.