Source and age of dissolved and gaseous carbon in a peatland–riparian–stream continuum: a dual isotope (14C and δ13C) analysis

Abstract

Radiocarbon isotopes are increasingly being used to investigate the age and source of carbon released from peatlands. Here we use combined 14C and δ13C measurements to determine the isotopic composition of soil and soil decomposition products [dissolved organic carbon (DOC), CO2 and CH4] in a peatland–riparian–stream transect, to establish the isotopic signature and potential connectivity between carbon pools. Sampling was conducted during two time periods in 2012 to investigate processes under different temperature, hydrological and flux conditions. Isotopic differences existed in the peatland and riparian zone soil organic matter as a result of the riparian depositional formation. The peatland had a mean radiocarbon age of 551 ± 133 years BP, with age increasing with depth, and δ13C values consistent with C3 plant material as the primary source. In contrast the riparian zone had a much older radiocarbon age of 1,055 ± 107 years BP and showed no age/depth relationship; δ13C in the riparian zone was also consistent with C3 plant material. With the exception of DOC in September, soil decomposition products were predominately >100 %modern with 14C values consistent with derivation from organic matter fixed in the previous 5 years. Emissions of CO2 and CH4 from the soil surface were also modern. In contrast, CO2 and CH4 evaded from the stream surface was older (CH4: 310–537 years BP, CO2: 36 years BP to modern) and contained a more complex mix of sources combining soil organic matter and geogenic carbon. The results suggest considerable vertical transport of modern carbon to depth within the soil profile. The importance of modern recently fixed carbon and the differences between riparian and stream isotopic signatures suggests that the peatland (not the riparian zone) is the most important source of carbon to stream water.