Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland

Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO2-C with the atmosphere of 0.1-0.5 Pg yr(-1) and of CH4-C of 10-25 Tg yr(-1). Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6-year balance computed from continuous net ecosystem CO2 exchange (NEE), regular instantaneous measurements of methane (CH4) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6-year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and age-depth profiles from two peat cores. The 6-year mean NEE-C and CH4-C exchange, and net DOC loss are -40.2 +/- 40.5 (+/- 1 SD), 3.7 +/- 0.5, and 14.9 +/- 3.1 g m(-2) yr(-1), giving a 6-year mean balance of -21.5 +/- 39.0 g m(-2) yr(-1) (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH4 had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi-maximum limits of the annual C balance between 50 and -105 g m(-2) yr(-1). The net C accumulation rate obtained from the two peatland cores for the interval 400-3000 bp (samples from the anoxic layer only) were 21.9 +/- 2.8 and 14.0 +/- 37.6 g m(-2) yr(-1), which are not significantly different from the 6-year mean contemporary exchange.