Effects of soil physicochemical properties on aggregate stability along a weathering gradient

- Mechanism of aggregate stabilization was investigated for soils along a weathering gradient.
- Aggregate stability and the heterogeneity of pore structure generally decreased with soil depth.
- Slaking was the most disruptive force in aggregate breakdown.
- Amorphous iron oxides, pH and crypto-pores were closely related with aggregate stability.

The mechanism of soil structure stabilization has been rarely investigated from the perspectives of both soil material composition and porosity due to the complexity of soil structure and its formation process. Here, the relative importance of soil physicochemical properties in aggregate stability was evaluated for soils with different weathering degrees. Typical zonal soils derived from quaternary clay were collected from central to south China with increasing precipitation and temperature. Aggregate stability against slaking (MWDFW), mechanical breakdown (MWDWS) and swelling (MWDSW) was determined, as well as basic properties (including clay content, soil organic matter (SOM), different forms of sesquioxides and pore size distribution (PoSD)). The degrees of soil weathering increased from central to south China with increasing free oxides content. PoSD in the eluvium (A) horizon exhibited triple and double peaks, indicating the decreasing heterogeneity of pore structures, while that in parent material (C) horizon was dominantly characterized by textural pores (< 0.05 μm). Slaking was the most disruptive mechanism in aggregate breakdown, followed by the mechanical breakdown. MWDFW decreased with soil depth. Correlation analysis showed that soil binding agents interacted significantly with pore characteristics (P < 0.05). MWDFW and MWDWS increased with an increase in the amorphous iron oxides excluding complex form (Feu) content and a decrease in pH. Moreover, complex iron oxides (Fep) and crypto-pores (0.006-0.1 μm) enhanced the aggregate stability against slaking logarithmically and mechanical strength linearly. MWDSW was positively related with SOM (P < 0.05). The overall results suggest that the combination of amorphous iron oxides (especially Feu), pH and crypto-pores play a synergetic role in the stabilization of soil structure for the highly weathered soils in subtropical and tropical areas.