Impact of freeze-thaw cycles on hydro-physical properties of peat

The ongoing climate warming is likely to increase the frequency of freeze-thaw cycles (FTC) in cool-temperate regions, which could modify the properties as well as carbon sequestration and release of peat soils. Despite the importance of soil hydro-physical properties for the water and carbon cycles in peatlands, the impact of FTC on selected soil properties has only poorly been investigated. In this study, we collected undisturbed topsoil samples from two drained peatlands. The soil samples were subject to five different freeze-thaw treatments, including a no freeze-thaw cycle (FTC0), one (FTC1), three (FTC3), five (FTC5), and ten (FTC10) freeze-thaw cycles. Each freeze-thaw cycle was composed of 24 hours of freezing (5 C) and 24 hours of thawing (5 C) and the soil moisture during the freeze-thaw experiment was adjusted according to field capacity. The saturated hydraulic conductivity (Ks) and macroporosity of peat were determined before and after each freeze-thaw treatment. The results showed that the Ks generally decreased with increasing FTC and the maximum changes in Ks (50% reduction) occurred after five FTC. For peat soils with low Ks values prior treatment the opposite was observed and the Ks increased after freeze-thaw cycles. The changes in Ks differed between the two study sites and highly depend on the initial Ks of peat. The changes in macroporosity showed a similar pattern with Ks after freeze-thaw treatments, however, the magnitude of the changes was minor (<3 vol%). After FTC treatment, the coefficient of variation for Ks was generally lower by appr. 50%. In conclusion, the FTC alters peat soil properties, thereby affecting hotspots of water flow and as a consequence also solute transport, and biogeochemical reactions in peatlands.