Soil CO2 efflux along a soil warming gradient in subarctic grasslands in Iceland

Cold and high-latitude terrestrial ecosystems, such as the arctic and the subarctic, store large amounts of carbon (C) in the soil. These ecosystems are already experiencing rapid rates of temperature increase compared to other regions of the Earth. It is expected that warmer conditions will increase soil CO₂ efflux by enhancing soil microbial and root respiration. However, our understanding of warming effects on soil C cycling is limited to short-term observations (1-5 y of warming) and under few discrete warming levels.

This study is embedded within the FutureArctic project, and we take advantage of geothermally heated subarctic grasslands in the ForHot research site in Iceland, with 13 years of a stable and constant soil warming gradient. The main objective of this research is to get deeper insights into how rising temperatures will affect soil carbon fluxes in subarctic grasslands ecosystems.

Using automated long-term soil chambers, we are continuously measuring soil CO₂ efflux rates along a soil warming gradient ranging from +0°C to ca. +14°C above ambient soil temperature. Furthermore, complementary manual soil CO₂ efflux measurements allow us to cover higher spatial variability and to assess how soil warming affects the main soil CO₂ source components (i.e., autotrophic, and heterotrophic respiration) via the trenching approach.

Here, we present preliminary results of the soil CO₂ efflux along a soil warming gradient in Iceland, including time series of the first year of the study. Overall, soil CO₂ efflux increased along the soil warming gradient. We found that heterotrophic respiration is the main source component of total soil CO₂ efflux. Both autotrophic and heterotrophic respiration increased with warming, however, the relative contribution of each source component was unresponsive to warming. Ongoing analysis of isotopic soil CO₂ in the automated measurements will allow the partition between biogenic and geogenic sources of soil CO₂ in the studied geothermal system and accurately describe the soil respiration response to warming.