Radiocarbon constraints on lateral export of soil organic carbon in rivers of Switzerland

Lateral carbon transport through inland waters plays a critical yet often overlooked role in the delicate balance of carbon exchanges controlling the net uptake and storage of anthropogenic carbon in terrestrial ecosystems. Though new terrestrial carbon cycle models are increasingly making an effort to explicitly represent these traditionally neglected lateral fluxes, the parameters governing lateral carbon transport, particularly the composition and ages of exported soil carbon, remain ill-constrained. In this study, we explore the power of combined river and soil ¹⁴C datasets as a parameter constraint when calibrating a novel carbon cycle model connecting the terrestrial and riverine systems in catchments of Switzerland. For the riverine data, we use ¹⁴C measurements of particulate organic carbon (POC), dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) for water samples collected during high-discharge events in 2021 across Switzerland. Within those river catchments, we have forest soil ¹⁴C data at various depths down to 60cm for up to three samples in time (mid 1990s, 2014, and 2022). The soil samples were split into the following representative fractions: soil dissolved organic carbon (soil-DOC), particulate organic carbon (soil-POC), and mineral-associated organic carbon (soil-MOC), which represents the older and more recalcitrant component of soil carbon. This study investigates to what extent using these fraction-specific ¹⁴C measurements together with ¹³C and nitrogen data for both rivers and soils allows us to not only distinguish the different sources of riverine carbon but further analyze the composition and age of the soil organic carbon ending up in Swiss rivers.