Transfer of belowground carbon into forest vegetation in Icelandic volcanic geothermal fields: Implications for ¹⁴C...

Belowground carbon transfer from non-equilibrium sources can alter isotopic signatures in terrestrial ecosystems, yet the transfer of such carbon into vegetation under natural field conditions remains poorly constrained. Understanding these processes is essential for improving radiocarbon (¹⁴C) transfer models used in environmental risk assessments. Geothermal fields offer natural analogue systems where isotopically enriched ¹³CO₂ emitted from the subsurface can be used as a proxy for potential belowground ¹⁴C releases. This study determined the contribution of geogenic versus biogenic carbon to soil gas, near-surface air, and plant functional groups along a geothermal warming gradient in a forested volcanic field in Iceland. Stable isotope analyses (δ¹³C) with two-endmember mixing models revealed that geogenic CO₂ dominated soil gas in all warmed plots (≥98%). Near-surface air (5 cm aboveground) showed geothermal influence, with geogenic CO₂ contributing up to 45% in the warmest plots. Despite this pronounced subsurface signal, only lichens assimilated geogenic carbon (10-25%), while bryophytes and all vascular plants assimilated their carbon from biogenic sources. These results highlight physiological and microclimatic differences in carbon acquisition pathways among plant groups, while identifying lichens as sensitive integrators of subsurface CO₂ due to their thallus structure, boundary layer positioning, and hydration dynamics. Overall, these findings provide field-based evidence of non-equilibrium carbon pools in geothermal soils and highlight the limitations of assuming homogeneous isotopic signatures across environmental compartments in most current ¹⁴C biosphere assessment models. Incorporating such species-specific and spatially explicit carbon transfer dynamics will improve the accuracy of radiological impact assessments from subsurface ¹⁴C releases.