Soil carbon loss in warmed subarctic grasslands is rapid and restricted to topsoil

Abstract

Abstract. Global warming may lead to carbon transfers from soils to the atmosphere, yet this positive feedback to the cli- mate system remains highly uncertain, especially in subsoils (Ilyina and Friedlingstein, 2016; Shi et al., 2018). Using natural geothermal soil warming gradients of up to +6.4 °C in subarctic grasslands (Sigurdsson et al., 2016), we show that soil organic carbon (SOC) stocks decline strongly and linearly with warming (−2.8 ton ha−1 °C−1). Comparison of SOC stock changes following medium-term (5 and 10 years) and long-term (> 50 years) warming revealed that all SOC loss occurred within the first five years of warming, after which continued warming no longer reduced SOC stocks. This rapid equilibration of SOC observed in Andosol suggests a critical role for ecosystem adaptations to warming and could imply short-lived soil carbon-climate feedbacks. Our data further revealed that the soil C loss occurred in all aggregate size fractions, and that SOC losses only occurred in topsoil (0–10 cm). SOC stocks in subsoil (10–30 cm), where plant roots were absent, remained unaltered, even after > 50 years of warming. The observed depth-dependent warming responses indicate that explicit vertical resolution is a prerequisite for global models to accurately project future SOC stocks for this soil type and should be investigated for soils with other mineralogies.

Community adaptation to temperature explains abrupt soil bacterial community shift along a geothermal gradient on Iceland

“…This system has been used to study the effects of warming on a wide range of processes in soil ecology and biogeochemistry (Walker et al, 2018, 2020; Marañón-Jiménez et al, 2019; Poeplau et al, 2020; Zhang et al, 2020) Focussing on the microbial community composition, Radujković et al (2018) found that bacterial and fungal communities changed only at warming levels exceeding +6-8 °C above ambient. However, the authors could not decisively conclude whether this change was a direct effect of temperature or indirect effects due to, for example, effects on vegetation growth and phenology (Leblans et al, 2017), or an observed reduction in soil organic matter concentration, and soil texture (Poeplau et al, 2017; Verbrigghe et al, 2022). It is difficult to distinguish between potential drivers using community data alone, given that the relationship between specific taxonomic groups and ecological functions is largely unknown (Prosser, 2012).…”