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

Abstract

Natural temperature gradients which can appear within geothermal areas have been used to study effects of warming on carbon dioxide (CO2) 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 CO2and thus confound the interpretations of temperature dependencies of respiratory fluxes. Carbon dioxide efflux and its δ13C values (which differ between biotic and abiotic CO2) 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 (Ts) gradient, which at 10 cm depth ranged from the ambient temperature up to 65°C warming. In 2014, soil CO2efflux increased steadily (260 - 3900 mg CO2m-2h-1) with increasing Ts(10-52°C). The ratio of 13C/12C in CO2flux suggested that an increasing proportion of the CO2emitted at the higher temperatures was geothermally derived. However, in 2016 the highest geothermal source of CO2had moved and the location was not connected to the highest soil temperature. At that time the maximum CO2efflux was measured at 44°C Tswarming (2100 mg CO2m-2h-1), instead of a maximum 65°C. Our study showed that a significant amount of CO2emitted from the geothermal temperature gradients can have a non-biotic origin. These abiotic CO2fluxes 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 CO2fluxes is by implementing isotope techniques, as done here.