Background The freeze-thaw cycle is one of the main driving factors of soil erosion. Freeze-thaw cycling changes soil properties and influences the processes of runoff and erosion on slopes, resulting in severe soil and water losses. Studying the effects of freeze-thaw cycles on the characteristics of black soil water-stable aggregates can provide a scientific basis for the prevention and control of soil erosion.

Methods Selecting the cultivated layer of the typical black soil in the Northeast China as our research object, this paper studied the effects of initial water content and the freeze-thaw cycles on the particle size composition of soil water-stable aggregate, the destruction rate of aggregate particle size >0.25 mm (PAD_0.25), the destruction rate of aggregate particle size >1 mm (PAD_1.0), as well as the mean weight diameter and fractal dimension using the simulated freeze-thaw test method.

Results The anhydrous freeze-thaw cycle significantly reduced the water-stable aggregates with particle size >5 mm, but PAD_0.25 and PAD_1.0 of the water-stable aggregates increased by 12.25% and 5.52% respectively than before. Mean weight diameter of the water-stable aggregates reduced by 6.01%, while fractal dimension of the water-stable aggregates increased by 1.61%. The changes of mean weight diameter and fractal dimension promoted the breaking of large aggregates. The water freeze-thaw cycling significantly increased the water-stable aggregates with particle size < 0.5 mm. PAD_0.25 and PAD_1.0 increased by 78.72%-132.31% and 81.44%-184.94% respectively. Mean weight diameter of the water-stable aggregates reduced by 6.03%-8.83% (P < 0.05), but fractal dimension of the water-stable aggregates increased by 69.26%-75.06% (P < 0.05) compared with before. Both of the mean weight diameter and fractal dimension intensified the breaking effect of water-stable aggregates. In addition to the initial moisture content, the freeze-thaw cycling frequency is also a key factor affecting the stability of soil aggregates. With the increase of freeze-thaw cycles, PAD_0.25 and PAD_1.0 increased significantly. On the contrary, the average weight diameter decreased gradually. Especially after 10 freeze-thaw cycles, PAD_1.0 reached 87.04%-96.43%, and the mean weight diameter decreased 62.35%-71.18%. With the further increase of freeze-thaw cycles, no significant changes were observed for the average weight diameter, fractal dimension and PAD_1.0.

Conclusions Generally, the freeze-thaw cycle could change from the large aggregates to small aggregates, and decrease the water stability of the black soil aggregates. The breaking effect of freeze-thaw cycle on water-stable aggregates increases gradually and tends to be stable finally with the increase of initial water content. The water stability of soil aggregates decreases significantly with the increasing freeze-thaw cycles, and gradually stabilizes after 10 freeze-thaw cycles.