Variations in soil carbon dioxide efflux across a thaw slump chronosequence in northwestern Alaska

Abstract

Warming of the arctic landscape results in permafrost thaw, which causes ground subsidence or thermokarst. Thermokarst formation on hillslopes leads to the formation of thermal erosion features that dramatically alter soil properties and likely affect soil carbon emissions, but such features have received little study in this regard. In order to assess the magnitude and persistence of altered emissions, we use a space-for-time substitution (thaw slump chronosequence) to quantify and compare peak growing season soil carbon dioxide (CO _2 ) fluxes from undisturbed tundra, active, and stabilized thermal erosion features over two seasons. Measurements of soil temperature and moisture, soil organic matter, and bulk density are used to evaluate the factors controlling soil CO _2 emissions from each of the three chronosequence stages. Soil CO _2 efflux from the active slump is consistently less than half that observed in the undisturbed tundra or stabilized slump (1.8 versus 5.2 g CO _2 −C m ^−2 d ^−1 in 2011; 0.9 versus 3.2 g CO _2 −C m ^−2 d ^−1 in 2012), despite soil temperatures on the floor of the active slump that are 10–15 ^° C warmer than the tundra and stabilized slump. Environmental factors such as soil temperature and moisture do not exert a strong control on CO _2 efflux, rather, local soil physical and chemical properties such as soil organic matter and bulk density, are strongly and inversely related among these chronosequence stages ( r ^2 = 0.97), and explain ∼50% of the variation in soil CO _2 efflux. Thus, despite profound soil warming and rapid exposure of buried carbon in the active slump, the low organic matter content, lack of stable vegetation, and large increases in the bulk densities in the uppermost portion of active slump soils (up to ∼2.2 g ^−1 cm ^−3 ) appear to limit CO _2 efflux from the active slump. Future studies should assess seasonal fluxes across these features and determine whether soil CO _2 fluxes from active features with high organic content are similarly low.

Keywords

• climate change
• CO2 efflux
• arctic
• thaw slump
• thermokarst
• chronosequence