Effects of seasonal freeze and thaw cycles on the micro-aggregate characteristics of the mechanically compacted black soil

Based on the simulation of mechanical compaction in the field, the impacts of seasonal freeze and thaw cycles on soil micro-aggregates composition and stability were revealed in the vertical range by measuring and analyzing the soil micro-aggregates distribution(≥0.25 mm, 0.25-0.05 mm, 0.05-0.02 mm, 0.02-0.002 mm, < 0.002 mm), fractal dimension (D) and dispersion coefficient in two consecutive years at different soil depths (0-20 cm, 20-40 cm, 40-80 cm) of different compaction passes (no compaction, 3 passes, 12 passes), and the interaction effects between seasonal freezing-thawing cycles and mechanical compaction were discussed emphatically. The results showed that the effects of seasonal freeze and thaw cycles on the composition and stability of micro-aggregates in black soil were different at varied soil depths, the stability of soil micro-aggregates mainly increased in the surface soil layer (0-20 cm) after seasonal freeze and thaw cycles, while decreased in other soil layers. Simultaneously, the effects of seasonal freeze and thaw cycles were also different from year to year. The first seasonal freezing-thawing process mainly increased the micro-aggregates stability of the uncompacted soil, but the second seasonal freezing-thawing processes reduced the stability of the soil micro-aggregates significantly. However, it is worth noting that the interaction effects between seasonal freezing-thawing cycles and mechanical compaction on the soil micro-aggregates characteristics were more complicated. After two seasonal freezing-thawing processes, the D value of soil micro-aggregates and dispersion coefficient were significantly higher than control condition (P < 0.05) when the mechanical compaction treatment was only 3 compaction passes, although the soil dispersion coefficient was also higher than control (P < 0.05) when the mechanical compaction passes increased to 12 times, the D value of soil micro-aggregates was no significant difference compared with the control. In general, the seasonal freeze and thaw cycles generally exacerbated the damage of micro-aggregates and reduced the stability when the soil was compacted only a few times, but displayed a certain recovery characteristic when the mechanical compaction passes were extremely increased.