Carbon Thaw Rate Doubles When Accounting for Subsidence in a Permafrost Warming Experiment

Abstract

Permafrost thaw is typically measured with active layer thickness, or the maximum seasonal thaw measured from the ground surface. However, previous work has shown that this measurement alone fails to account for ground subsidence and therefore underestimates permafrost thaw. To determine the impact of subsidence on observed permafrost thaw and thawed soil carbon stocks, we quantified subsidence using high‐accuracy GPS and identified its environmental drivers in a permafrost warming experiment near the southern limit of permafrost in Alaska. With permafrost temperatures near 0°C, 10.8 cm of subsidence was observed in control plots over 9 years. Experimental air and soil warming increased subsidence by five times and created inundated microsites. Across treatments, ice and soil loss drove 85–91% and 9–15% of subsidence, respectively. Accounting for subsidence, permafrost thawed between 19% (control) and 49% (warming) deeper than active layer thickness indicated, and the amount of newly thawed carbon within the active layer was between 37% (control) and 113% (warming) greater. As additional carbon thaws as the active layer deepens, carbon fluxes to the atmosphere and lateral transport of carbon in groundwater could increase. The magnitude of this impact is uncertain at the landscape scale, though, due to limited subsidence measurements. Therefore, to determine the full extent of permafrost thaw across the circumpolar region and its feedback on the carbon cycle, it is necessary to quantify subsidence more broadly across the circumpolar region.

Lower soil moisture and deep soil temperatures in thermokarst features increase old soil carbon loss after 10 years of experimental permafrost warming

“…After a decade of experimental soil warming, permafrost degradation induced significant soil subsidence ranging from 1.20 to 6.12 cm yr −1 , causing the soil to collapse and the average water table depth to be 74% closer to the soil surface in 2018 relative to 2009 (Rodenhizer et al, 2020). By 2016, plot‐level soil VWC reached a maximum of 90%, and ~25% of the plots measured from 2016 to 2018 had soil VWC greater than 60%.…”

“…At CiPEHR, 10 years of experimental soil warming using snow fences and open‐top chambers led to a two‐fold increase in thaw depth (TD) relative to control plots (Mauritz et al, 2017). The degradation of ice‐rich permafrost has resulted in thermokarst formation across the landscape, caused by soil subsidence from the loss of ice structures (Plaza et al, 2019; Rodenhizer et al, 2020). As the surface of the soil moves closer to the water table, soil becomes waterlogged (Lawrence et al, 2015; Nauta et al, 2015).…”

Long‐term field measurements of climate‐induced thaw subsidence above ice wedges on hillslopes, western Arctic Canada

“…8 This paper focusses on observed rates of thaw subsidence, which is a principal concern for the integrity of infrastructure foundations built in permafrost during a time of climate warming. 19,20 Thaw depth is a well-monitored variable 21 but there are relatively few measurements of thaw subsidence accompanying active-layer thickening in flat ground under natural conditions, [22][23][24][25] two reports from experimental manipulations, 26,27 and none from hillslopes.…”

Disproportionate microbial responses to decadal drainage on a Siberian floodplain

“…Higher temperature increases decomposition rates of carbon stored in soils (Bond‐Lamberty & Thomson, 2010; Davidson & Janssens, 2006; Walker et al, 2018). Additionally, melting of bulk ice is followed by geomorphological changes and surface water redistribution (Aas et al, 2019; Avis et al, 2011; Liljedahl et al, 2016), and this changing soil hydrology between dry and wet conditions adds complexity in estimating the amount and forms of released C (Schädel et al, 2016; Schuur et al, 2015): wetter soil condition after permafrost thaw, such as development of thermokarst features and bogs (Osterkamp et al, 2009; Rodenhizer et al, 2020), decreases CO 2 but increases CH 4 emissions (Johnston et al, 2014; McCalley et al, 2014), whereas drier soil conditions increase CO 2 but decrease CH 4 emissions (Kittler et al, 2017; Lawrence et al, 2015), which is potentially counterbalancing the effect of preserving old C by impeding permafrost thaw (Kwon et al, 2019).…”