Soil pore network response to freeze-thaw cycles in permafrost aggregates
Abstract
Climate change in Arctic landscapes may increase freeze–thaw frequency within the active layer as well as newly thawed permafrost. Freeze-thaw is a highly disruptive process that can deform soil pores and alter the architecture of the soil pore network with varied impacts to water transport and retention, redox conditions, and microbial activity. Our objective was to investigate how freeze–thaw cycles impacted the pore network of newly thawed permafrost aggregates to improve understanding of what type of transformations can be expected from warming Arctic landscapes. We measured the impact of freeze–thaw on pore morphology, pore throat diameter distribution, and pore connectivity with X-ray computed tomography (XCT) using six permafrost aggregates with sizes of 2.5 cm3 from a mineral soil horizon (Bw; 28–50 cm depths) in Toolik, Alaska. Freeze-thaw cycles were performed using a laboratory incubation consisting of five freeze–thaw cycles (−10 ◦C to 20 ◦C) over five weeks. Our findings indicated decreasing spatial connectivity of the pore network across all aggregates with higher frequencies of singly connected pores following freeze–thaw. Water-filled pores that were connected to the pore network decreased in volume while the overall connected pore volumetric fraction was not affected. Shifts in the pore throat diameter distribution were mostly observed in pore throats ranges of 100 µm or less with no cor- responding changes to the pore shape factor of pore throats. Responses of the pore network to freeze–thaw varied by aggregate, suggesting that initial pore morphology may play a role in driving freeze–thaw response. Our research suggests that freeze–thaw alters the microenvironment of permafrost aggregates during the incipient stage of deformation following permafrost thaw, impacting soil properties and function in Arctic landscapes undergoing transition.
TLDR
This paper comes from permafrost (permanently frozen) soil cores out of the (NEON project) and is Erin Rooney’s first chapter of her dissertation. We spent many many hours in the freezer together because these soils needed to be described while still frozen. A hammer and chisel were our best friends to create soil hockey pucks allowing us to see how soil changed in the cores. Erin is particularly interested in the active zone of permafrost, where it’s beginning to thaw due to … drumroll please… climate change! The amount of water and oxygen that can diffuse though soil has a direct impact on how fast, or slow, microbes can chew threw all that locked up carbon. She uses a really fancy technique (X-ray Computed Tomography) to quantify just how much the soil architecture changes due to freeze thaw action which has implications for all the rest of the permafrost that’s thawing.
You can find Erin on Twitter @ECRooney.
Adrian C. Gallo
PhD, He/Him,
Climate Campaign Coordinator
I’m formally trained as a terrestrial biogeochemist (aka I know a lot about how dirt controls ecosystems). My current role involves the intersection of energy and environmental policy, and trying to get the renewable energy transition to hurry up in the most equitable way possible. Outside of the office you can find me running, mountain biking, rock climbing, or playing soccer.