Drying-wetting cycles promote greater soil structure recovery after compaction more than freezing-thawing, but laboratory tests overestimate field responses

Traffic-induced soil compaction is a major threat to soil functioning, and recovery of compacted subsoils is particularly challenging. Drying–wetting (DW) and freezing–thawing (FT) cycles are recognized as natural mechanisms of soil structural recovery, but their relative importance under field conditions remains poorly understood. Therefore, this study quantified the effects of FT and DW processes on the recovery of compacted clay loam soil under field and laboratory conditions. A field experiment was set up in which one set of compacted subsoil samples was exposed to natural weather conditions for 13.5 months, followed by one FT and one DW cycle in the laboratory. Meanwhile, a second set of samples underwent multiple FT cycles followed by DW cycles under controlled laboratory conditions. Soil structure was evaluated using air-filled porosity (ɛ), air permeability (k a ) and X-ray computed tomography (CT)-derived pore characteristics measured at a soil matric potential of pF 2. No soil freezing was observed in the field below the soil surface; however, several DW cycles of varying intensities occurred. No freezing occurred below the soil surface during field exposure, whereas multiple DW cycles of varying intensity were observed. Field exposure significantly increased ɛ by 175% and altered pore architecture, as indicated by increases in CT-derived macropores and pore tortuosity. In the laboratory, repeated FT and DW cycles increased both ɛ and k a , but the response was substantially greater under DW, with k a increasing by up to 2420% after 15 cycles. CT analyses indicated that both processes modified pore-size distribution and connectivity, likely through aggregate fragmentation and pore-network reorganization. Repeated DW cycles produced larger and more persistent changes in pore architecture than FT cycles, resulting in a more connected pore network. These findings demonstrate that DW is likely the dominant mechanism of structural recovery in compacted clay loam soil under temperate conditions. However, laboratory FT and DW experiments substantially overestimated recovery, particularly for pore connectivity and gas transport, compared with field exposure. Controlled experiments therefore provide valuable mechanistic insights but may not accurately predict the magnitude of natural soil structure recovery under field conditions.