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 climate 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 t 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 stock reduction occurred within the first 5 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 stock reduction was only visible in topsoil (0–10 cm). SOC stocks in subsoil (10–30 cm), where plant roots were absent, showed apparent conservation 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.

The legacy of one hundred years of climate change for organic carbon stocks in global agricultural topsoils

“…In a century-long geothermal soil warming gradient in subarctic Canada, whole-soil SOC stocks have been found to decline by 3% per 1°C 24 in a deciduous forest. This was also observed for a grassland topsoil in a geothermal gradient in Iceland (2.8 Mg C ha − 1 °C) 25 . Finally, the Harvard Forest soil, the longestrunning soil warming experiment, lost about 3 Mg C ha − 1 °C− 1 in 26 years of warming, which is also similar to the estimated absolute values in the present study 26 .…”

The legacy of one hundred years of climate change for organic carbon stocks in global agricultural topsoils

“…In a century-long geothermal soil warming gradient in subarctic Canada, whole-soil SOC stocks have been found to decline by 3% per 1°C 23 in a deciduous forest. This was also observed for a grassland topsoil in a geothermal gradient in Iceland (2.8 Mg C ha − 1 °C) 24 . Finally, the Harvard Forest soil, the longestrunning soil warming experiment, lost about 3 Mg C ha − 1 °C− 1 in 26 years of warming, which is also similar to the estimated absolute values in the present study 25 .…”

Decadal soil warming decreased vascular plant above and belowground production in a subarctic grassland by inducing nitrogen limitation

“…At the same sites, soil warming accelerated SOM decomposition (Marañón‐Jiménez et al ., 2018; Walker et al ., 2018), resulting in decreased SOM stocks (Verbrigghe et al ., 2022), while the soil C–N ratio remained unaltered, suggesting that plants and microbes were not able to retain the mineralized N within the ecosystem. Warming thus induced substantial losses of soil N (Fig.…”

“…In contrast to these hypotheses, we found that the increase in soil temperature reduced both below and aboveground production similarly and did not change root–shoot ratio (Figs 1 and 2). Our results revealed that, in contrast to what was expected (warming accelerating nutrient cycling and thereby stimulating productivity), warming induced a loss of soil nitrogen and an increase in soil bulk density (both driven by a substantial loss of SOM; Marañón‐Jiménez et al ., 2019; Verbrigghe et al ., 2022), which elicited a reduction in both below and aboveground productivity.…”