Comment on bg-2021-338

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 <span class="cit" id="xref_paren.1">(<a href="#bib1.bibx17">Ilyina and Friedlingstein</a>, <a href="#bib1.bibx17">2016</a>; <a href="#bib1.bibx35">Shi et al.</a>, <a href="#bib1.bibx35">2018</a>)</span>. Using natural geothermal soil warming gradients of up to <span class="inline-formula">+6.4</span> <span class="inline-formula">^∘</span>C in subarctic grasslands <span class="cit" id="xref_paren.2">(<a href="#bib1.bibx37">Sigurdsson et al.</a>, <a href="#bib1.bibx37">2016</a>)</span>, we show that soil organic carbon (SOC) stocks decline strongly and linearly with warming (<span class="inline-formula">−2.8</span> t ha<span class="inline-formula">^−1</span> <span class="inline-formula">^∘</span>C<span class="inline-formula">^−1</span>). Comparison of SOC stock changes following medium-term (5 and 10 years) and long-term (<span class="inline-formula">&gt;50</span> 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 <span class="inline-formula">&gt;50</span> 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.