Legacy effect of soil compaction drives post-freeze-thaw rill erosion: Identifying the critical hydro-mechanical threshold in Mollisols

Soil erosion driven by soil compaction and freeze-thaw cycles is a critical global environmental issue in intensively mechanized cold-region agroecosystems. Existing studies predominantly focus on the direct effects of freeze-thaw cycles on soil erosion, yet overlook the legacy effects of pre-freeze-thaw soil compaction, particularly whether a critical threshold exists for such legacy effects. A comparative study was conducted in the Mollisol region of Northeast China by using in situ field erosion experiments and soil property measurements under various compaction levels before and after the freeze-thaw period. Results showed that the pre-freeze-thaw soil compaction exacerbated post-freeze-thaw soil erosion. Before the freeze-thaw period, the influence of soil properties on runoff was greater than their direct effect on sediment mass, and the sediment mass variation was mainly driven by runoff scouring due to soil compaction. After the freeze-thaw period, the decreased soil erosion resistance (aggregate stability and soil strength) and the increased runoff caused by the legacy effects of compaction were the primary reasons for higher sediment mass in compacted soil compared to uncompacted soil. Based on these findings, we propose a conceptual framework to determine a critical bulk density range that maximizes the alleviating effect of freeze-thaw cycles on soil compaction to reduce runoff, thereby avoiding the exacerbation of post-thaw soil erosion. This study underscores how compaction legacy amplifies post-thaw erosion, offering mechanistic insights into optimizing tillage management and soil restoration in seasonally frozen agroecosystems.