Influence of slope position on soil organic carbon, soil structure and hydraulic properties in cultivated humid tropical coastal plain sands

Abstract

Slope position plays a key role in regulating soil redistribution, soil structural and hydraulic functioning in cultivated landscapes, but the effect of these interrelated aspects has not been properly examined together. Therefore, soil samples were collected from upper, middle, and lower slope positions and analysed for soil organic carbon (SOC), soil structural and hydraulic parameters. Although all slope positions exhibited a loamy sand texture, particle-size fractions varied along the slope. Sand content was highest at the middle slope, whereas silt and clay were enriched at the lower slope, indicating downslope deposition of fine materials. The SOC concentration increased from 12.5 g kg⁻¹ at the upper slope to 19.3 g kg⁻¹ at the lower slope, with SOC stocks increasing from 13.3 to 18.6 Mg ha⁻¹. Bulk density declined downslope (1.48 to 1.34 Mg m⁻³), while total porosity increased, reflecting improved soil structure in depositional areas. Aggregate stability was highest at the lower slope, where larger macroaggregates and greater mean and geometric aggregate diameters were observed, with saturation and field capacity water contents showing similar trends. Regression analysis showed that bulk density and macroporosity decreased with SOC stocks, whereas total porosity, aggregate stability, saturated water content, and field capacity increased with SOC, with field capacity showing the strongest response (R² = 0.46). Saturated hydraulic conductivity showed little variation among slope positions. In general, the findings show that slope position controls soil structural and hydraulic behaviour via erosion, organic carbon distribution, and aggregate-driven pore development, underscoring the importance of slope-specific soil management.