Age and source of different forms of carbon released from boreal peatland streams during spring snowmelt in E. Finland

Isotopic data are increasingly being used to quantify and understand the processes that control the release of carbon (C) from northern peatlands. We used delta C-13 and C-14 measurements to investigate the source and age of different forms of aquatic C (DOC, POC and evasion CO2) released from 2 contrasting (undrained v drained) forested peatland catchments at the end of the winter snowmelt period in boreal E Finland. The delta C-13(VPDB) values of DOC (range -28.3 to -28.8 aEuro degrees) were generally more C-13 depleted than evasion CO2 (range -22.7 to -31.5 aEuro degrees) and showed no clear differences between the pre-flood, flood and post-flood periods. Both forms of C had evidence of bomb-C-14 (i.e. > 100%modern), indicating that they contained substantial quantities of C fixed since the mid AD 1950s. However, DOC was C-14 enriched compared to evasion CO2, with C-14 concentrations suggesting that, on average, DOC-C was similar to 5-6 years younger than evasion CO2-C, with the most recently fixed C being released when temperatures were highest. POC was significantly depleted in C-14 with conventional (uncalibrated) radiocarbon ages of 805-1135 BP. In contrast to other studies, the isotopic compositions of DOC and evasion CO2 were very similar, suggesting a predominantly single and consistent C source (decomposition of soil organic matter; SOM) during the snowmelt period. Whilst we found no evidence to suggest that old (pre-bomb) C was being released at the end of the winter period, the drained site was associated with more C-14 depleted and C-13 enriched evasion CO2, suggesting a closer link to the atmospheric CO2 pool. Our isotopic data suggest that the various forms of C released to the aquatic system from these forested Finnish peatlands are closely related, largely unaffected by drainage and (at least in the case of evasion CO2 and DOC) indicate strong connectivity between C cycling in the soil-plant-water system.