The effects of freeze-thaw processes on crusting, aggregation and the interaction with erosive level winds in the Mollisol region of Northeast China

Wind erosion is widely recognised as one of the causes of soil degradation, which is exacerbated by the effects of freezing and thawing, and poses a serious threat to the sustainability of agricultural production. The mechanisms of freeze-thaw effects on wind erosion in the Mollisols region have been the subject of considerable investigation from the perspective of what the impact of freeze-thaw processes have on soil aggregates. In contrast, the role of the physical crust in the mechanism of freeze-thaw effects on wind erosion remains uncertain. In this study, for disentangling the changes in environmental conditions (freeze-thaw cycles (FTCs), initial soil moisture (M)) on aggregate size distribution, mean weight diameter (MWD), crust formation and their properties, and the roles played by these changes in influencing the magnitude of wind erosion (W), a wind tunnel simulation experiment was used to measure the wind erosion rate of erodible soil aggregates with four diameter ranges (D). The relationship between the variations in the distribution of aggregate sizes and the properties of the crust, as well as the impact of freezing and thawing on the distribution of aggregate sizes, were examined. The findings indicated that both aggregates and crust were susceptible to damage during the freeze-thaw cycle. The MWD of the aggregates exhibited a notable alteration following the 1st freeze-thaw cycle (p < 0.05). There exists a good exponential correlation between the strength of the crust and the number of freeze-thaw cycles (R² > 0.70). The crust strength demonstrated a decline significantly with an increase in the number of freeze-thaw cycles. The variation tendency of crust strength tended to be flat and towards a minimum crust strength of 4.27 kPa (D_0.5–1), 2.87 kPa (D_0.25–0.5), and 2.82 kPa (D _{< 0.25}) beyond 6th freeze-thaw cycles. The initial moisture content had a significant impact on the variation in aggregate sizes, with higher moisture leading to greater fluctuations in the variation percentage of aggregates breaking or aggregating. The percentage of de-aggregation (disintegration of soil aggregates) varied from 12.68% to 20.64%, while the percentage of re-aggregation (recombination of soil aggregates) varied from 0.84% to 10.78%. When the moisture content of the soil was greater than or equal to 12%, a physical crust formed on the surface of the constructed soil samples, with an approximate thickness of 1 mm. When D ≥ 0.25 mm, the freezing-thawing effect was the primary cause of aggregate breakage, resulting in a reduction in MWD. When D < 0.25 mm, the primary phenomenon was aggregation, which resulted in an increase in MWD. When D < 1 mm, the formation of a physical crust on the constructed soil sample surface was facilitated. De-aggregation of aggregates increased the wind erosion rate by an average of 12.31% (M_4%), 12.21% (M_8%), 37.15% (M_12%) and 43.47% (M_16%) respectively. Conversely, re-aggregation led to a reduction in wind erosion rate by an average of 20.60% (M_4%), 24.22% (M_8%), 44.21% (M_12%) and 34.46% (M_16%), respectively. The process of de-aggregation makes the aggregate size smaller, leading to an increase in wind soil erosion losses at the same wind speed. Re-aggregation process showed the opposite tendency. The formation of a crust greatly enhanced the soil surface strength and significantly reduced the degree of wind erosion, which decreased by 0.96% (M_4%), 14.98% (M_8%), 79.15% (M_12%) and 107.23% (M_16%), respectively, after crust formation. Although the constructed soil samples formed crusts under different initial soil moisture levels, all samples had naturally air-dried to approximately 4% prior to the wind erosion experimental study was conducted. Therefore, there was no significant change in threshold wind velocity due to different sample moisture levels. In conclusion, we constructed a path model based on the effects of environmental conditions on the wind erosion rate. The results indicated that the initial soil moisture and freeze-thaw effect exerted an indirect influence on wind erosion, by mediating their impact on aggregate variation, MWD, crust strength and crust thickness. The relationship between wind erosion rate and topsoil (2 cm) properties was significant. The initial moisture content and the freeze-thaw effect contributed 30.4% and 49.2%, respectively, to the rate of wind erosion. This study clarifies the role played by the crust and aggregation in the influence of wind erosion under freeze-thaw conditions in the Mollisols region, and provides a scientific theoretical basis for the mechanism of spring wind erosion in cold regions.