Abstract
Abstract. Permafrost soils store between 1330 and 1580 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 1 m of the watershed was stored belowground (92 %), with 19 % in the top 10 cm of soil and 40 % in the top 30 cm. Carbon was more variable in surface soils (10 cm; coefficient of variation (CV) = 0.35 between stands) than in the top 30 cm (CV = 0.14) or soil profile to 1 m (CV = 0.20). Combined active-layer and deep frozen deposits (surface – 15 m) contained 205 kg C m−2 (yedoma, non-ice wedge) and 331 kg C m−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 (53 %) but also included understory vegetation (30 %), woody debris (11 %) and snag (6 %) biomass. While aboveground biomass contained relatively little (8 %) 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 10 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.
Highlights
Boreal forests cover roughly 22 % of the earth’s terrestrial landscape (Chapin et al, 2000) and account for approximately 9% of the global vegetation carbon (C) stock (Carvalhais et al, 2014)
The Y4 watershed is located near Chersky, Sakha Republic, Russia, approximately 130 km south of the Arctic Ocean and is underlain by yedoma, which is widespread across the region (Grosse et al, 2013)
After collinear explanatory variables were removed from analysis using a variance inflation factor of 3, we considered densitometry, organic layer depth, stand age, live shrub biomass, woody debris, tree density, snag density, summer insolation, percent herbaceous cover, percent moss cover, percent lichen cover, percent other cover, soil C, basal diameter (BD), and root C, as explanatory variables for the thaw depth model
Summary
Boreal forests cover roughly 22 % of the earth’s terrestrial landscape (Chapin et al, 2000) and account for approximately 9% of the global vegetation carbon (C) stock (Carvalhais et al, 2014). Despite the massive amount of C present in the boreal region, the quantity of C stored here and the magnitude of the change in C stocks that will result from climate change is one of the least understood carbon–climate feedbacks (Schuur et al, 2015). Over the past 50 years, air temperatures in the Arctic have risen nearly twice the global average as a result of climate change (Christensen et al, 2013), and this accelerated rate of warming means that the vast amount of C stored in high-latitude systems is vulnerable to loss to the atmosphere (Koven et al, 2015; Schuur et al, 2015). Despite the vulnerability of permafrost soils to increased thaw and C release due to climate change, there is a lack of data quantifying the C stocks at northern latitudes compared to other regions
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