Mechanisms of aggregate breakdown in (sub) tropical soils: Effects of the hierarchical resistance

The soil system is a dynamic component of the earth. Aggregate stability is critical for soil quality and ecosystem services. In tropical and subtropical soils, the role of clay, soil organic carbon, aluminum, and iron oxides on aggregate stability is somewhat controversial. In addition, aggregates develop a hierarchical organization; however, this process is poorly evaluated in the context of aggregate stability on different breakdown mechanisms such as raindrop impact, slaking (entrapped air), breakdown by differential swelling, and abrasion. Therefore, the objective of this study was: 1) to evaluate the aggregate stability of (sub) tropical soils in terms of four different breakdown mechanisms; 2) to ascertain the existence of hierarchical resistance to break down mechanism of pre-selected aggregates classes; 3) to determine whether soil chemical-mineralogical attributes influence aggregate stability. Five Oxisols originated from igneous rocks along a transect were evaluated. The soil aggregate stability responded according to hierarchical resistance, but differently to each breakdown mechanism: abrasion, slaking, differential swelling, and raindrop impact. Overall, the larger aggregates (4-8 mm) corresponded to a lower resistance to abrasion and raindrop impact. In contrast, the smaller aggregates (1-2 mm) were more affected by slaking than raindrop impact. Soil organic carbon, except for large aggregates, and soil texture did not show a significant role on aggregate stability. Silicon oxides increased the resistance to abrasion, while iron oxides increased the resistance to slaking. In contrast, aluminum oxides decreased the aggregate stability (slaking). The aggregate stability was influenced by method deployed and pre-selected classes of aggregate size. In this study, rainfall simulation (raindrop impact) and wet sieving (fast wetting) were the recommended methods to evaluate (sub) tropical soil aggregate stability. An aggregate size of 2-4 mm is preferable if a single class is selected.

Automated summary

The soil system is a dynamic component of the earth, and aggregate stability is crucial for soil quality and ecosystem services. This study found that the impact of clay, soil organic carbon, aluminum, and iron oxides on aggregate stability in tropical and subtropical soils is controversial, and the stability of aggregates varies depending on different breakdown mechanisms. Soil organic carbon and soil texture do not play a significant role in aggregate stability, while silicon oxides and iron oxides increase resistance to breakdown, and aluminum oxides decrease stability. The recommended methods for evaluating (sub)tropical soil aggregate stability are rainfall simulation and wet sieving.