Restoration of compacted soils using artificial pores under freeze–thaw conditions

Abstract

Soil compaction caused by the use of farm machinery is a widespread issue. Freeze–thaw cycles can improve the soil structure after compaction; however, the effect decreases as soil depth increases. Herein, we applied freeze–thaw cycle treatments to re-moulded compacted sandy loam soil (bulk density of 1.6 g/cm³) in two water content states (80 % and 30 % field capacities). Artificial perforation was performed to create long, straight pores in soil, which ensured that the soil mass was largely intact and unbroken, leaving the freeze–thaw cycles to complete the structural remediation and monitoring soil structure recovery. We measured the soil temperature, heat flux and thermal properties to explore the mechanisms of soil temperature regulation using artificial pores during freeze–thaw cycles. The pore and aggregate structure parameters before and after the freeze–thaw cycle treatment were measured. Under the freeze–thaw cycle treatment, the temperature in the bottom layer of compacted soil with artificial pores rapidly dropped below 0°C during the third and second cycles under high- and low-water-content conditions, respectively, whereas the temperature of soil without artificial pores decreased during the seventh and fourth cycles at the same water content states. Results indicated that the heat flux during the freezing phase was larger in the soil with artificial pores. However, no significant differences were observed in the thermal parameters, including thermal conductivity, volumetric heat capacity and thermal diffusivity, of soils with and without artificial pores at each water content state. The air-filled porosity, aggregate mean weight diameter and structure coefficient of the surface and bottom layers of the compacted soil columns were generally better in soil with artificial pores than in soil without artificial pores after repeated freeze–thaw cycles. This indicates that the artificial pores facilitated the restoration of compacted soil in the bottom layer during freeze–thaw cycles owing to the rapid drop in soil temperature. We deduced that the artificial long, straight pores in compacted soil created additional soil heat exchange areas in the heat transfer process to increase the rate of heat transfer, thus increasing soil heat exchange and causing the soil temperature of the bottom layer to drop rapidly during repeat freezing and thawing. However, further studies are required to investigate the remediation of artificial pores on compacted soils in fields and the optimal process for creating artificial pores in agricultural settings.