Spatial variations of aggregate stability in relation to sesquioxides for zonal soils, South-central China

Soil aggregate stability is an important soil property affecting soil functions and erosion. However, there exists some divergence on the relationship between different forms of sesquioxides and aggregate stability, and limited knowledge is available about spatial evolution of aggregate stability under climate conditions (temperature and precipitation) and anthropogenic activities. In this study, the spatial variation and profile distribution of soil aggregate stability and its relation to sesquioxides under cultivation in different zonal soils were investigated. Typical zonal soils in different weathering degrees were selected that were exposed to an increasing trend of annual average temperature and precipitation (from central to south China). These soils contained low organic matter (<3%) and high variation of sesquioxides in different forms (coefficient of variations CVs in 34–104%). Soil free oxides (Fed and Ald) contents showed an increasing trend across the zonal soils with the content of Fed higher than that of Ald. Both amorphous oxides (Feo and Alo) and complex oxides (Fep and Alp) did not present the obvious increasing variation unlike free oxides across the zonal soils. Soil Alp content was higher than Fep content and their complex degrees decreased with soil depth due to complexation with soil organic matter. Soil aggregate stability showed a distinct unimodal trend across the zonal soils and water aggregate stability decreased with soil depth. In general, slaking was the main disruptive force in disaggregation, followed by mechanical breakdown. Multiple stepwise regression analysis showed that complex oxides especially the Alp were strongly related with aggregate stability, synthetically contributing to dry aggregate stability logarithmically and to water stability in a power function. Soil organic matter had a negative effect on dry aggregate stability. Interestingly, it was found that non-complex Fe oxides in amorphous oxides (Feo–Fep) improved water aggregate stability against slaking. More studies are needed to gain more insights into the interaction between soil aggregate stabilization and sesquioxides in future.