How the insulating properties of snow affect soil carbon 2 distribution in the continental pan - Arctic area

6 [1] We demonstrate the effect of an ecosystem differentiated insulation by snow on the soil 7 thermal regime and on the terrestrial soil carbon distribution in the pan-Arctic area. This is 8 done by means of a sensitivity study performed with the land surface model ORCHIDEE, 9 which furthermore provides a first quantification of this effect. Based on field campaigns 10 reporting higher thermal conductivities and densities for the tundra snowpack than for taiga 11 snow, two distributions of near-equilibrium soil carbon stocks are computed, one relying on 12 uniform snow thermal properties and the other using ecosystem-differentiated snow thermal 13 properties. Those modeled distributions strongly depend on soil temperature through 14 decomposition processes. Considering higher insulation by snow in taiga areas induces 15 warmer soil temperatures by up to 12 K in winter at 50 cm depth. This warmer soil signal 16 persists over summer with a temperature difference of up to 4 K at 50 cm depth, especially in 17 areas exhibiting a thick, enduring snow cover. These thermal changes have implications 18 on the modeled soil carbon stocks, which are reduced by 8% in the pan-Arctic continental 19 area when the vegetation-induced variations of snow thermal properties are accounted 20 for. This is the result of diverse and spatially heterogeneous ecosystem processes: where 21 higher soil temperatures lift nitrogen limitation on plant productivity, tree plant functional 22 types thrive whereas light limitation and enhanced water stress are the new constrains 23 on lower vegetation, resulting in a reduced net productivity at the pan-Arctic scale. 24 Concomitantly, higher soil temperatures yield increased respiration rates (+22% over the 25 study area) and result in reduced permafrost extent and deeper active layers which expose 26 greater volumes of soil to microbial decomposition. The three effects combine to produce 27 lower soil carbon stocks in the pan-Arctic terrestrial area. Our study highlights the role 28 of snow in combination with vegetation in shaping the distribution of soil carbon and 29 permafrost at high latitudes.