Stable isotope method reveals the role of abiotic source of carbon dioxide efflux from geothermally warmed soil in southern Iceland

Natural temperature gradients which can appear within geothermal areas have been used to study effects of warming on carbon dioxide (CO 2 ) fluxes from soils and thus to study climate feedbacks on natural unwarmed ecosystems.However, at least among ecologists, it is less known that geothermal areas also release abiotic CO 2 and thus confound the interpretations of temperature dependencies of respiratory fluxes.Carbon dioxide efflux and its δ 13 C values (which differ between biotic and abiotic CO 2 ) were thus measured using the static chamber method in a Sitka spruce forest floor in Iceland in 2014 and 2016, along a natural geothermal soil temperature (T s ) gradient, which at 10 cm depth ranged from the ambient temperature up to 65°C warming.In 2014, soil CO 2 efflux increased steadily (260 -3900 mg CO 2 m -2 h -1 ) with increasing T s (10-52°C).The ratio of 13 C/ 12 C in CO 2 flux suggested that an increasing proportion of the CO 2 emitted at the higher temperatures was geothermally derived.However, in 2016 the highest geothermal source of CO 2 had moved and the location was not connected to the highest soil temperature.At that time the maximum CO 2 efflux was measured at 44 o C T s warming (2100 mg CO 2 m -2 h -1 ), instead of a maximum 65°C.Our study showed that a significant amount of CO 2 emitted from the geothermal temperature gradients can have a non-biotic origin.These abiotic CO 2 fluxes have to be considered when interpreting temperature effects on soil respiration rates from geothermal areas or close to active volcanos.The only way to separate the biotic and abiotic CO 2 fluxes is by implementing isotope techniques, as done here.