Objective The aims were to study the distribution characteristics of soil organic carbon (SOC) and iron and aluminum oxides in soil aggregates of pomelo orchards with different parent materials and their influences on the stability characteristics of aggregates, in order to provide a theoretical basis for the regulation of the stability characteristics of soil aggregates in pomelo orchards in the south hilly areas.

Methods The development soil of weathered products of SLATE shale and granite weathering products were taken at different slope positions in the pomelo orchard as the research object. Soil aggregates were fractionated into four size classes (> 2 mm, 2 ~ 0.25 mm, 0.25 ~ 0.053 mm, and < 0.053 mm) using the wet sieving method. The content changes of components such as SOC, total nitrogen (TN), amorphous iron-aluminum oxides (Fe_o-Al_o), complex iron-aluminum oxides (Fe_p-Al_p), and free iron-aluminum oxides (Fe_d-Al_d) of aggregates of each particle size were analyzed. Pearson correlation analysis was employed to explore the relationships between organic carbon and iron-aluminum oxides and aggregate stability in soils derived from the two parent materials.

Results The mean weight diameter (MWD), geometric mean diameter (GMD), and specific gravity of aggregates > 0.25 mm (R_0.25) of the soil aggregates developed from granite weathering products on the slope were 7.48%, 15.01%, and 7.89% higher than those of the soil developed from weathered products of SLATE shale, respectively. At the lower slope position, these values for granite weathering products derived soils increased by 48.20%, 73.34% and 50.93%, respectively, compared to weathered products of SLATE shale derived soils. At the same slope position, weathered products of SLATE shale derived aggregates generally exhibited higher contents of SOC, C/N ratio, Fe_o-Al_o, Fe_p-Al_p, and Fe_d-Al_d than granite weathering products derived aggregates. In granite weathering products derived soils, SOC and TN contents decreased from upper to lower slope positions, whereas Fe_o-Al_o, Fe_p-Al_p, and Al_d contents in both parent materials increased from upper to lower slopes. For weathered products of SLATE shale derived aggregates, SOC, C/N ratio, Fe_o-Al_o, Fe_p-Al_p, and Al_d showed negative correlations with MWD but positive correlations with < 0.25 mm aggregates. In contrast, Fe_o exhibited a significant positive correlation with MWD in granite weathering products derived aggregates. At upper slopes, Fe_o-Al_o, SOC, and C/N ratio positively correlated with MWD, while at lower slopes, these components negatively correlated with MWD.

Conclusion In the tested soil, the stability of soil aggregates developed from granite weathering at the same slope was higher than that of soil developed from SLATE weathering. The organic carbon and iron and aluminum oxides in the soil developed from the two parent materials interacted and promoted the stability of aggregates < 0.25 mm. However, the contents of soil organic carbon and iron and aluminum oxides vary in different terrain areas, resulting in significant differences in the stability and particle size distribution of aggregates.