Effects of topographic factors on soil bioavailable cadmium and arsenic in typical microtopographic rice-growing areas of hilly southern China.

Topographic factors critically regulate the bioavailability of heavy metals in paddy soils; however, their direct and indirect effects, particularly at the microtopographic scale, remain poorly quantified. In this study, focusing on a typical microtopographic rice-growing area (2.40 ha) in the hilly region of southern China, we collected 51 surface soil samples and analyzed key soil properties including pH, soil organic matter (SOM), active iron (A-Fe), active manganese (A-Mn), active sulfur (A-S), and active zinc (A-Zn), to explore the effects of topographic factors on bioavailable cadmium (Bio-Cd) and arsenic (Bio-As), and a combination of partial least squares structural equation modeling (PLS-SEM) and spatial clustering analysis (LISA, Kappa, and Dice coefficients) was employed to disentangle the direct and indirect effects of topographic factors and to validate their spatial consistency. The results revealed that elevation was the most influential topographic factor, with total effects of 0.220 on Bio-Cd and 0.339 on Bio-As. Elevation directly positive association with Bio-Cd accumulation and indirectly regulated both Bio-Cd and Bio-As by modulating soil moisture, thereby affecting the speciation of active iron (A-Fe) and manganese (A-Mn). Spatial consistency analysis demonstrated strong concordance between elevation and Bio-Cd (Kappa = 0.7), suggesting that the spatial distribution trend of the dominant factor is consistent with that of Bio-Cd. Collectively, these findings suggest that microtopography is associated with the spatial heterogeneity of Bio-Cd and Bio-As, with mediated by soil properties. Such insights offer a foundational reference for the acquisition of accurate parameters in analogous regions, as well as for deciphering the intrinsic mechanisms governing heavy metal spatial variability in more topographically complex settings.