Soil Moisture Controls the Thermal Habitat of Active Layer Soils in the McMurdo Dry Valleys, Antarctica

Abstract

Antarctic soil ecosystems are strongly controlled by abiotic habitat variables. Regional climate change in the McMurdo Dry Valleys is expected to cause warming over the next century, leading to an increase in frequency of freeze‐thaw cycling in the soil habitat. Previous studies show that physiological stress associated with freeze‐thaw cycling adversely affects invertebrate populations by decreasing abundance and positively selecting for larger body sizes. However, it remains unclear whether or not climate warming will indeed enhance the frequency of annual freeze‐thaw cycling and associated physiological stresses. This research quantifies the frequency, rate, and spatial heterogeneity of active layer freezing to better understand how regional climate change may affect active layer soil thermodynamics, and, in turn, affect soil macroinvertebrate communities. Shallow active layer temperature, specific conductance, and soil moisture were observed along natural wetness gradients. Field observations show that the frequency and rate of freeze events are nonlinearly related to freezable soil moisture (θf). Over a 2 year period, soils at θf < 0.080 m3/m3 experienced between 15 and 35 freeze events and froze rapidly compared to soils with θf > 0.080 m3/m3, which experienced between 2 and 6 freeze events and froze more gradually. A numerical soil thermodynamic model is able to simulate observed freezing rates across a range of θf, reinforcing a well‐known causal relationship between soil moisture and active layer freezing dynamics. Findings show that slight increases in soil moisture can potentially offset the effect of climate warming on exacerbating soil freeze‐thaw cycling.

Transit Times and Rapid Chemical Equilibrium Explain Chemostasis in Glacial Meltwater Streams in the McMurdo Dry Valleys, Antarctica

“…Similarly, expansion and contraction of the wetted perimeter between high and low flows may cause the dissolution and precipitation of channel‐margin salt crusts. Additionally, periodic freezing and thawing of stream corridor waters can alter solute concentrations (Anderson, ; Wlostowski et al, ). Incrementally more complex, yet parametrically constrained, models will be needed to assess the influence of additional physiochemical processes on C‐q relationships.…”

“…We assume w c and w HZ to be 4 and 5 m, respectively, based on observations of channel and wetted margin widths (Northcott et al, 2009). Thaw depth (z thaw ) is set to 1 m, which is approximately the maximum annual thaw depth observed beneath stream channels (Wlostowski et al, 2018).…”

Connectivity: insights from the U.S. Long Term Ecological Research Network

“…The ecosystem is poised at the phase change of water with multiple freeze–thaw cycles occurring over daily timescales during the brief austral summer (Fountain et al 1999). Therefore, this ecosystem and its resident biota are sensitive to small changes in climate (Fountain et al 2016, Knox et al 2017, Wlostowski et al 2018).…”

The Hydroecology of an Ephemeral Wetland in the McMurdo Dry Valleys, Antarctica

“…As the summer progresses, an active layer of less than 1 m thaws above permanently frozen ground, limiting potential groundwater flow to the upper shallow zone. The soils are typically extremely dry, and often highly saline (Wlostowski et al, ).…”