Quantifying soil aggregate hierarchy through energy-based dispersion modelling in contrasting soil types and land uses

Aggregate hierarchy, the organization by which microaggregates form progressively larger, structurally distinct macroaggregates, is central to soil stability, governing resistance to erosion and response to disturbance. However, the mechanisms and extent of hierarchical breakdown remain poorly quantified across different soil types and land management. In this study, we addressed this gap by evaluating the stepwise breakdown of soil structure into aggregates, driven by incremental sonication energy, across a range of soils differing in mineral composition and management practices. By applying a quantitative modelling framework, we derived three key parameters: the disruption constant (k₁), reflecting the rate of aggregate breakdown; the dispersion constant (k₂), which describes particle release; and the critical energy threshold (E crit ), which denotes the transition point between aggregate disruption and full particle dispersion. These parameters were used to evaluate the degree of hierarchy in aggregate breakdown, whereby higher k₁/k₂ ratios signal a pronounced stepwise (hierarchical) disintegration, and ratios near unity indicate direct dispersion into clay-sized particles. Our results indicated that soils enriched in 2:1 phyllosilicate clay minerals, such as Luvisols, exhibited markedly higher k₁/k₂ ratios in larger aggregates, demonstrating a structured, multi-step breakdown process. In contrast, oxide-rich soils like Ferralsols and Andosols typically lacked such hierarchy, dispersing rapidly into smaller fractions, which is consistent with a stronger role of mineral-mineral binding relative to organic-mediated aggregation. In the studied Luvisols, direct seeding was associated with higher stability (higher E crit ) and a greater degree of hierarchy than conventional tillage, emphasizing the synergistic effects of organic matter input and reduced disturbance on soil structural integrity. These findings highlight the mechanistic roles of distinct pedogenic groups and management practices in controlling aggregate hierarchy and stability. Our study shows that sonication-derived indicators can differentiate not only distinct pedogenic groups but also soil management. These indices can be further developed to provide a quantitative insight into the structural organization that underpins water retention, erosion resistance, and other soil functions critical for conservation.