Disentangling long-term and short-term temperature response of carbon fluxes in a subarctic grassland ecosystem exposed to long-term, geothermal warming

The impact of rising global temperatures on carbon cycling in some of our most sensitive ecosystems, such as the Arctic, is critical yet challenging to quantify because short- and long-term effects of warming may be different. The observed short-term temperature response of soil organic carbon (SOC) decomposition may be altered in the long-term due to changes in substrate availability and possible acclimation of soil communities. 

Testing how the temperature response of soil decomposition changes over time with warming under open-air, field-scale conditions presents another obstacle for researchers, as warming experiments at different temperatures and over long measurement periods are expensive and rare. Fortunately, the ForHot grassland site in Iceland has provided us with a unique opportunity to explore soil warming effects by measuring along a naturally occurring geothermal gradient that resulted from an earthquake back in 2008.

As part of the FutureArctic project at the ForHot site, we collected five replicate soil cores to a soil depth of 5 cm (100cm3 per sample), from fourteen plots along the geothermal gradient spanning a soil warming gradient from 0°C to +80°C. The samples were incubated in the lab at 5, 15, 25, and 35°C, and flux rates of CO2 and CH4 were measured using a LGR-ICOS M-GGA-918 to produce observational Q10 temperature response relationships. 

Temperature response rates and curvature appeared to be driven primarily by substrate availability. Samples containing the greatest totals of carbon and nutrients produced the highest rates of CO2 emission and CH4 consumption at all temperatures. This likely being an effect of the 13 years of warming where organic content and spatial proximity to the hotspot are inversely correlated. From this, we can then present an analysis of the potential linearity or nonlinearity that temperature responses can have over a rather extensive temperature gradient and how these responses can change over time.

Samples collected from the hotspot, where previous in-situ chamber measurements have shown the highest emissions of CO2 and CH4, had significantly lower CO2 emissions and essentially no flux of CH4 during the laboratory incubations. This suggests a significant contribution of geogenic sourcing to in situ measurements. We present an analysis of the potential use for laboratory incubations at different temperatures to infer geogenic/biogenic flux contributions for in situ measurements where geogenic CO2 and CH4 emissions are present. This will allow us to construct a corrected biogenic carbon budget of the ForHot ecosystem and improve our fundamental understanding of the long-term effects that rising temperatures have on the carbon cycle in subarctic ecosystems.