Long-term conservation tillage breaks the plough pan and promotes the development of preferential flow

Macropores and preferential flow critically regulate soil hydrological functions and ecosystem services in agricultural systems. With the intensification of global soil degradation, especially as conventional tillage (CK) exacerbates soil compaction, conservation tillage (CT) has emerged as a crucial strategy for restoring soil quality. However, the ability of CT to alleviate soil compaction and regulate macropore structure and preferential flow remains unclear. This study aimed to clarify how long-term CT alleviates compaction-driven constraints on macropore structure and preferential flow in black soil. We selected an 18-year located experimental field on a typical Phaeozem (clay loam, mixed, mesic) in black soil region of Northeast China. X-ray computed tomography (25 μm resolution) and dye tracer techniques (1.5 g L⁻¹ Brilliant Blue solution) were used to quantify macropores and preferential flow paths in 0–50 cm soil layers. Independent samples t-test (assuming equal variances and normal distributions) was used to compare two treatments. Correlation analysis was also performed to assess variable relationships. Path analysis via structural equation modeling to elucidate direct and indirect effects, and CMIN/DF, GFI, RMSEA were used to evaluate model fit. The results revealed that CT effectively alleviated soil compaction by breaking the plough pan and optimizing pore structure. Compared with CK, CT significantly enhanced macroporosity (500–1000 μm) by 4.8 times and pore connectivity by 21.3 times in the critical 10–20 cm layer, while reducing bulk density by 9.4 % and pore tortuosity by 2.1 % in this layer (p < 0.05). Notably, CT generated continuous pore and staining paths extending below 20 cm, thereby alleviating the plough pan barrier. These structural improvements drove preferential flow development, evidenced by a 21.5 % reduction in dye variation coefficient and enhanced soil dye coverage. Correlation analysis showed dye coverage correlated strongly with soil physicochemical properties and macropore indexes, especially 500–1000 μm macroporosity (p < 0.05). Path analysis further confirmed CT governed preferential flow patterns through macroporosity enhancement and pore tortuosity reduction (p < 0.001). This study demonstrates that CT restructures compacted soil into hydraulically active systems through breaking plough pan and improving pore structure, providing mechanistic insights for combating soil degradation in ecologically vulnerable black soil regions.