Variability in above- and belowground carbon stocks in a Siberian larch watershed

Abstract

Permafrost soils store between 1330 and 1580g Pg carbon (C), which is 3 times the amount of C in global vegetation, almost twice the amount of C in the atmosphere, and half of the global soil organic C pool. Despite the massive amount of C in permafrost, estimates of soil C storage in the high-latitude permafrost region are highly uncertain, primarily due to undersampling at all spatial scales; circumpolar soil C estimates lack sufficient continental spatial diversity, regional intensity, and replication at the field-site level. Siberian forests are particularly undersampled, yet the larch forests that dominate this region may store more than twice as much soil C as all other boreal forest types in the continuous permafrost zone combined. Here we present above- and belowground C stocks from 20 sites representing a gradient of stand age and structure in a larch watershed of the Kolyma River, near Chersky, Sakha Republic, Russia. We found that the majority of C stored in the top 1g m of the watershed was stored belowground (92g %), with 19g % in the top 10g cm of soil and 40g % in the top 30g cm. Carbon was more variable in surface soils (10g cm; coefficient of variation (CV)g Combining double low line g 0.35 between stands) than in the top 30g cm (CVg Combining double low line g 0.14) or soil profile to 1g m (CVg Combining double low line g 0.20). Combined active-layer and deep frozen deposits (surface - 15g m) contained 205g kgg Cg mg '2 (yedoma, non-ice wedge) and 331g kgg Cg mg '2 (alas), which, even when accounting for landscape-level ice content, is an order of magnitude more C than that stored in the top meter of soil and 2 orders of magnitude more C than in aboveground biomass. Aboveground biomass was composed of primarily larch (53g %) but also included understory vegetation (30g %), woody debris (11g %) and snag (6g %) biomass. While aboveground biomass contained relatively little (8g %) of the C stocks in the watershed, aboveground processes were linked to thaw depth and belowground C storage. Thaw depth was negatively related to stand age, and soil C density (top 10g cm) was positively related to soil moisture and negatively related to moss and lichen cover. These results suggest that, as the climate warms, changes in stand age and structure may be as important as direct climate effects on belowground environmental conditions and permafrost C vulnerability.