Soil pore network response to freeze-thaw cycles in permafrost aggregates

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 corresponding 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.