Effect of mechanical compaction on soil micro-aggregate composition and stability of black soil.

Aggregates are secondary particles formed through the combination of mineral particles glued by organic and inorganic substances. Soil compaction caused by mechanical tillage practices mainly affects its structure and function by the dispersing, braking, re-proportioning, re-arranging and re-combining aggregates. Although mechanical compaction was reported to reduce the proportion of soil water stable aggregate with size smaller than 0.25 mm, how further impact of mechanical compaction on composition and stability of soil micro-aggregate is still unknown. Comparing to the study of soil macro-aggregates, research on soil micro-aggregates can contribute to a better understanding of disperse strength and structural performance of primary aggregates under particular conditions. Based on the cropland soil in the typical black soil region of Northeast China, the influence of mechanical compaction on soil micro-aggregate composition and stability was studied by measuring and analyzing the soil micro-aggregate distribution, mean weight soil specific area (MWSSA), fractal dimension (D) and dispersion coefficient at different soil depths (0-20 cm, > 20-40 cm, and >40-80 cm). The results showed that mechanical compaction mainly reduced the proportion of macro-aggregate (≥0.25 mm) and increased the proportion of small-sized (<0.002 mm) micro-aggregate in the 0-20 cm soil depth. The MWSSA, D value and dispersion coefficient were significantly higher than control condition (P<0.05) when the number of mechanical compaction passes increased 12 times, indicating that repeated compaction could cause a significant reduction on soil micro-aggregate stability. In the soil depth > 20-40 cm, mechanical compaction primarily affected the proportion of micro-aggregates with diameter of 0.05-0.25 mm and <0.002 mm. The MWSSA, and D value of soil micro-aggregates decreased significantly (P<0.05) when the mechanical compaction treatment was only 3 passes but the proportion of each size aggregate distributed uniformly. However, these measurements increased significantly when the compaction passes increased to 12 times (P<0.05), which also indicated that repeated compaction caused a significant reduction on soil micro-aggregate stability. The only difference was that a few times of compaction (3 passes) could promote the reunion of the small-sized micro-aggregates and extenuate the destruction of relative bigger sized micro-aggregates. Within the >40-80 cm soil layer, the proportion of middle-class micro-aggregates (> 0.05-0.25 mm and > 0.02-0.05 mm) reduced, but macro-aggregate (≥0.25 mm) and smaller sized micro-aggregate (<0.002 mm) increased, which resulted in a non -uniformed distribution pattern of different sized micro-aggregates. Soil MWSSA, D value and dispersion coefficient increased gradually with increasing the passes of traffic, indicating even few passes of compaction could significantly reduce the stability of the soil micro-aggregates, and the mechanical compaction had a significant cumulative effect on deeper soil layer (>40-80 cm). This result confirmed the former research conducted in this area with the same traffic machine. The proportion of <0.002 mm micro-aggregate (clay content) had a highly significant correlation with the other micro-aggregates characteristic indices (P<0.05), which suggested to be an effective parameter to reflect soil micro-aggregates dispersion, reunion and stability. In summary, mechanical tillage compaction tended to increase the smaller sized micro-aggregate composition and reduce its stability, especially for the deeper soil (>40-80 cm). The findings showed that cumulative compaction effects on subsurface area and deeper soil layer can not be neglected, and may serve as theoretical basis for scientifically evaluating soil compaction and quality evolution in typical black soil region. [ABSTRACT FROM AUTHOR]