Polyacrylamide-Induced Trade-Offs in Soil Stability and Ecological Function: A Multifunctional Assessment in Granite-Derived Sandy Material

Soil erosion in granite-derived weathering mantles poses serious threats to slope stability and ecological sustainability in subtropical regions. While polyacrylamide (PAM) is widely used to improve soil structure, its concentration-dependent effects on multiple soil functions remain unclear. This study developed a multifunctional Soil Function Index (SFI) framework integrating erosion resistance (SFI1), water regulation (SFI2), and ecological function (SFI3) to evaluate the effects of PAM application (0‰, 1‰, 3‰, 5‰, 7‰) on gully-prone sandy material. Herein, SFI1 was quantified through shear strength (τ) and soil erodibility (Kr); SFI2 was assessed using soil hydraulic parameters (saturated hydraulic conductivity and water retention curves) and SFI3 was derived from the grass root system analysis. The results showed that SFI1 and SFI2 increased nonlinearly with PAM concentration, reaching maximum values of 0.983 and 0.980 at 7‰, with Kr reduced by 77.3% and non-capillary porosity (NAP) increased by 8.1%. In contrast, SFI3 peaked at 0.858 under 3‰ and declined sharply to 0.000 at 7‰, due to micropore over-compaction, reduced aeration, and limited plant-available water. The total SFI exhibited a unimodal trend, with a maximum of 0.755 at 3‰, beyond which ecological suppression offset physical improvements. These findings demonstrate that PAM modifies soil multifunctionality through pore-scale restructuring, inducing function-specific thresholds and trade-offs. A PAM concentration of 3‰ is identified as optimal, achieving a balance between erosion control, hydrological performance, and ecological viability in the management of subtropical granite-derived sandy slopes.