The Potential Hidden Age of Dissolved Organic Carbon Exported by Peatland Streams

Radiocarbon (C-14) is a key tracer for detecting the mobilization of previously stored terrestrial organic carbon (C) into aquatic systems. Old C (>1,000years BP) may be masked by postbomb C (fixed from the atmosphere post-1950 CE), potentially rendering bulk aquatic dissolved organic C (DOC) C-14 measurements insensitive to old C. We collected DOC with a modern C-14 signature from a temperate Scottish peatland stream and decomposed it to produce CO2 under simulated natural conditions over 140days. We measured the C-14 of both DOC and CO2 at seven time points and found that while DOC remained close to modern in age, the resultant CO2 progressively increased in age up to 2,356767years BP. The results of this experiment demonstrate that the bulk (DOC)-C-14 pool can hide the presence of old C within peatland stream DOC export, demonstrating that bulk (DOC)-C-14 measurements can be an insensitive indicator of peatland disturbance. Our experiment also demonstrates that this old C component is biologically and photochemically available for conversion to the greenhouse gas CO2, and as such, bulk (DOC)-C-14 measurements do not reflect the C-14 signature of the labile organic C pool exported by inland water systems more broadly. Moreover, our experiment suggests that old C may be an important component of CO2 emissions to the atmosphere from peatland aquatic systems, with implications for tracing and modeling interactions between the hydrological and terrestrial C cycles. Plain Language Summary The introduction of old carbon previously stored in soils for thousands of years into rivers can increase the net flux of greenhouse gases to the atmosphere, impacting global climate. This is because rivers transport the equivalent of one third of human carbon emissions annually from land to the oceans. Much river-borne carbon is plant and soil (organic) matter that can decompose during transport, releasing greenhouse gases to the atmosphere. Radiocarbon dating can reveal the age of river-borne carbon, but previous measurements may have underestimated the age of carbon released into rivers by not considering the potential for old carbon hidden within individual bulk water samples. Using an incubation experiment, we demonstrate that dissolved organic carbon from a Scottish peatland stream that would be considered modern in age using traditional bulk radiocarbon dating can readily decompose to produce carbon dioxide with an old radiocarbon signature up to similar to 2,500years old. This demonstrates that radiocarbon dating of bulk riverine dissolved organic carbon can hide the presence of old carbon. Furthermore, these results indicate that old carbon may be more common in the global carbon cycle than previously thought, with important implications for our understanding and modeling of peatland ecosystems.