Seasonal dynamics and temperature sensitivity of soil CO2 efflux in a medium-term warmed subarctic grassland

Climate warming is expected to occur stronger and faster in high-latitude terrestrial ecosystems compared to other regions of the world. It has been suggested that high-latitude systems are characterized by large soil C stocks which are highly vulnerable to warming. Warmer conditions can stimulate soil microbes to decompose more soil organic matter and increase the activity of plant roots, therefore, increasing soil CO₂ emissions. However, our current understanding of soil warming effects on soil CO₂ efflux is largely restricted to short-term warming observations which could over- or under-estimate warming effects. Additionally, the warming effects could vary seasonally, and it is important to consider this variation to better predict how natural ecosystems will respond to prolonged warming.

In this study, we aim to assess the seasonal dynamics of soil CO₂ efflux in a subarctic grassland under medium-term warming (>10 years) and examine how soil warming modifies the temperature sensitivity of soil CO₂ efflux. We take advantage of a geothermally heated grassland in Iceland for 13 years that is part of the ForHot and FutureArctic Research Network. We measured soil CO₂ efflux in ambient and warmed plots (from +1°C to +10°C above ambient soil temperature) between mid-2020 and early 2022 and during various field campaigns, we analysed the isotopic composition (δ¹³C) of soil CO₂ efflux for partitioning between biogenic and geogenic soil CO₂ and disentangle the soil respiration (a biological process) response to medium-term warming. We found strong seasonality in soil CO₂ efflux, with particularly higher fluxes during the growing season. After accounting for the geogenic soil CO₂ efflux, we found that long-term warming increases soil respiration during winter, spring, and fall. That means that warmer conditions keep microbes more active during the colder months. However, during summer the response was the opposite and soil respiration was unexpectedly lower in warmed plots. This can be related to decreasing soil C stocks in the topsoil during the first years of warming and lower root biomass and soil microbial biomass, limiting soil CO₂ efflux under warming during the growing season. Additionally, we found that the apparent temperature sensitivity of soil CO₂ efflux (Q₁₀) decreases with warming in a non-linear way, inducing thresholds at different soil warming levels. From these results, we conclude that medium-term warming effects on soil CO₂ efflux vary seasonally and warming decreases the temperature sensitivity of soil respiration.