Abiotic CO₂ sources confound interpretation of temperature responses of in situ respiration in geothermally warmed forest soils of Iceland

Abstract. 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 °C warming at 10 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 °C (from 260 to 3900 mg m⁻² h⁻¹). In 2016 the trend had changed; the maximum CO₂ efflux (2100 mg m⁻² h⁻¹) was measured at 20 °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.