Effect of freeze-thaw on the stability of aggregates in mechanically compacted soil

Background As an important commodity grain base, the agricultural mechanization rate in black soil area of Northeast China is 97%. At the same time, the soil is significantly affected by freezing and thawing in winter and spring. Although freeze-thaw can alleviate soil compaction, the effect varies with the degree of compaction, and the mechanism remains unclear. Therefore, this study took the black soil aggregate of Heshan Farm as the object to explore the influence mechanism of compaction, freeze-thaw and their interaction, and put forward soil management measures to provide basis for black soil protection and sustainable agricultural development.

Methods This study employed controlled laboratory tests with soil samples at an initial soil density of 0.9 g/cm³ and a mass water content of 20%. A factorial design was implemented, incorporating three key variables: mechanical compaction levels (0Y, 1Y, 3Y, 5Y, 7Y, and 9Y), freeze-thaw cycle levels (0D, 1D, 5D, 10D, 15D, and 20D), and their interactive treatments, with a total of 36 treatments. The study examined changes in the dry-sieved and wet-sieved aggregate size distribution and mean weight diameter (MWD), and evaluated the individual and main effects of compaction and freeze-thaw cycles on aggregate stability using an ecological data-oriented classification method.

Results 1) At 0D, as the compaction increased, macroaggregate in dry sieving showed an increasing trend. However, water-stable macroaggregate and MWD in wet sieving initially increased then decreased. 2) Under different compaction passes, macroaggregate in dry sieving generally decreased with increasing freeze-thaw cycles. For 0Y, 1Y, 3Y, and 5Y, water-stable macroaggregate and MWD initially increased and then decreased with freeze-thaw cycles. In contrast, for 7Y and 9Y, both measures fluctuated but showed an overall increasing trend. Specifically, for 7Y and 9Y, MWD significantly increased by 35.99%, 16.46%, 93.26%, and 91.84% at 5D, 10D, 15D, and 20D, respectively, and by 87.00%, 79.54%, 169.77%, and 151.90% at the same freeze-thaw cycles. 3) Fewer compaction frequencies (1Y, 3Y, and 5Y) enhanced aggregate stability, while multiple compaction (7Y and 9Y) reduced it. Similarly, 5D and 10D freeze-thaw cycles improved aggregate stability, whereas 15D and 20D cycles decreased it. Under fewer compaction and freeze-thaw interaction, compaction was the dominant factor. However, under multiple compaction and freeze-thaw interaction, freeze-thaw cycles became dominant.

Conclusions The results showed that soil subjected to fewer compaction (1Y, 3Y, and 5Y) exhibited increased aggregate stability under fewer freeze-thaw cycles (1D, 5D, and 10D), whereas multiple compacted soil (7Y and 9Y) showed improved stability under more freeze-thaw cycles (15D and 20D). For future land management, soil aggregate stability can be improved by adjusting mechanical compaction frequency in areas with severe seasonal freeze-thaw.