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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.

<p><strong>Abstract.</strong> Carbon dioxide (CO₂) efflux and δ¹³C in CO₂ were measured along a natural geothermal soil temperature (T_s) gradient in upland Sitka spruce forest soil in a volcanic area in Iceland in July 2014 and 2016. The gradient that reaches from ambient soil temperature up to 40&amp;thinsp;&amp;deg;C warming at 10&amp;thinsp;cm depth was originally formed in May 2008, following a major earthquake. The CO₂ efflux from the forest floor was measured using the static chamber method. In addition, subsurface soil CO₂ concentrations and δ¹³C values of CO₂ were studied. In summer 2014, soil surface CO₂ efflux increased steadily with increasing soil temperature across a temperature gradient of 40&amp;thinsp;&amp;deg;C (from 260 to 3900&amp;thinsp;mg&amp;thinsp;m^&amp;minus;2&amp;thinsp;h^&amp;minus;1). In 2016 the trend had changed; the maximum CO₂ efflux (2100&amp;thinsp;mg&amp;thinsp;m^&amp;minus;2&amp;thinsp;h^&amp;minus;1) was measured at 20&amp;thinsp;&amp;deg;C T_s warming and a similar nonlinear trend was observed in soil CO₂ concentrations in 2016. The ¹³C isotope analysis of CO₂ suggested that a proportion of the CO₂ emitted from the warmer plots was geothermally derived. The plot with the highest geothermal source was different in 2014 and 2016, which explained the shift in the temperature dependence of the total CO₂ efflux. Our study showed that a significant amount of CO₂ emitted from the higher warming levels of geothermal temperature gradients can have non-biotic origin and this has to be taken into account when measuring respiration fluxes on such volcanic sites.</p>

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