Aged carbon mineralisation from headwater peatland floodplains in the Peak District, UK

Floodplains are dynamic ecosystems that cycle carbon, which is both delivered from upstream catchment sources and produced in-situ by pedogenesis. These landforms are being progressively acknowledged as important environments of carbon processing, with the capacity for both substantial carbon sequestration in addition to acting as hotspots of carbon mineralisation. The balance between storage and release is dependent on a number of controls including landscape position, environmental conditions and soil characteristics. This study focuses on three headwater floodplains in a single catchment (approximately 10km in length), downstream of a highly eroded blanket bog peatland in the Peak District, UK. Aged organic carbon of peatland origin has been found in floodplains in this area based on prior research, and therefore we aimed to understand whether the allochthonous carbon was being mineralised in this context. We examined sediment cores and analysed the radiocarbon (¹⁴C) content of CO₂ respired from the floodplain soils using a partitioning approach to scrutinise the depth and age relations of respiration in the individual floodplains and patterns of age distributions downstream. As such, we examined whether soil heterogeneity as a function of distance downstream and within individual floodplain profiles had an impact on age of respired CO₂.

Aged organic carbon was released from the upper and mid floodplain sites (¹⁴C ages of 682 and 232 years BP, respectively), whereas only modern dates were recorded at the lower site. The sedimentology was in accordance with the radiocarbon dates, suggesting primarily allochthonous deposition at the upper sites, but a dominance of in-situ soil development at the lower site. There was no age-depth relationship within individual floodplains, suggesting that the floodplain sediments were well-mixed and that aged organic matter was being processed both at the surface and at depth in the uppermost sites. An isotope mass balance mixing model indicated the control of two sources of CO₂; recently fixed C3 organic matter and CO₂ produced by methanogenesis. The results indicate that floodplains in a relatively narrow halo around eroding headwater peatlands could be important sites of aged carbon turnover originally derived from upstream sources, with those further downstream playing a different role. Reworked carbon does not transfer passively through the system and experiences periods of deposition where it can be subject to microbial action. In areas where organic carbon has previously been ‘locked up’ (e.g., permafrost regions) but is now under the threat of release due to climate change, this is an important consideration.