Impact of drying-wetting cycles on the soil aggregate stability of Alfisols in southwestern China

Drying-wetting cycles are important environmental factors controlling potential changes in aggregate stability. However, the relationships between soil aggregate stability and drying-wetting cycles under different breakdown mechanisms are not clear. We conducted a simulation study to investigate the effects of drying-wetting cycles on aggregate stability under the three main breakdown mechanisms (slaking, microcracking, and mechanical breakdown) without soil organic matter inputs. Four initial aggregate size classes (i.e., 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm) within a clay-loam soil, which undergoes intense drying-wetting cycles, were selected and subjected to eight levels of drying-wetting cycles (i.e., 0, 1, 2, 3, 5, 7, 10, and 15 cycles). Aggregate stability was measured with three treatment methods involving fast wetting (FW), slow wetting (SW), and shaking after prewetting treatments (ST). Our study showed higher resistance to mechanical breakdown and lower resistance to slaking. Compared with the mean weight diameter (MWD), the normalized mean weight diameter (NMWD) was a better indicator of comparing the susceptibility of different initial size aggregates to breaking up into smaller aggregates. The values of NMWD for 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm aggregates were 0.1, 0.1, 0.09, and 0.09, respectively, for FW; 0.35, 0.34, 0.33, and 0.32, respectively, for SW; and 0.46, 0.45, 0.44, and 0.42, respectively, for ST. This indicated that as initial aggregate size increased, aggregates were more prone to breaking down. The effects of drying-wetting cycles on aggregate stability varied greatly among different aggregate breakdown mechanisms. For 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm size aggregates, drying-wetting cycles decreased MWD by 61%, 67%, 70%, and 53%, respectively, for SW, and 69%, 72%, 80%, and 78%, respectively, for ST. For FW, MWD increased by 5%, 7%, 8%, and 36% after the first two cycles, respectively, while it decreased by 36%, 36%, 44%, and 15% after 15 cycles. The results showed that different aggregate breakdown mechanisms influence the effects of drying-wetting cycles on aggregate stability. This study provides an important basis for better soil management practices and yields insights for an improved understanding of the role of drying-wetting cycles on aggregate stability.