Peatland carbon afflux partitioning reveals that Sphagnum photosynthate contributes to the DOC pool

Over half of the world's peat originated from Sphagnum, representing 10–15% of the terrestrial carbon stock. However, information regarding the release and exudation of organic carbon by living Sphagnum plants into the surface peat is scarce. In this study, we examined the contribution of recent Sphagnum subnitens (Russ. and Warnst.) photosynthate carbon to the peatland dissolved organic carbon (DOC) pool. This was done using a 13CO2 pulse-chase experimental approach during the growing season. Despite the importance of Sphagnum in long-term carbon accumulation, results showed that the Sphagnum community rapidly contributes recently synthesized carbon to the peatland DOC pool. We estimate that by 4 h up to 4% of the total DOC in peat leachate was derived from 13CO2 pulse labelling at ambient CO2 concentrations. Nonetheless, a huge 64% of the 13C initially assimilated by photosynthesis was retained in Sphagnum subnitens for 23 days after labelling, consistent with the role of Sphagnum in peatland carbon accumulation. The majority of 13C loss as respired CO2 came within the few days post 13CO2 labelling, suggesting that it was derived from plant respiration of photosynthates.