Arsenic (As) contamination from abandoned gold mines threatens adjacent ecosystems through leaching and erosion. This study investigated how soil physicochemical properties regulate As binding forms upon initial contamination and associated ecotoxicological effects on soil invertebrates. Forest soils (0-10 cm depth) were collected from four mountainous sites across Korea with varying physicochemical properties. Four forest soils were spiked with 20, 50, or 100 mg kg⁻¹ of As. Sequential extraction revealed that newly introduced As primarily bound to nonspecifically and specifically sorbed fractions (F1 and F2) and amorphous iron and aluminum oxide fractions (F3). F1 and F2 showed strong correlations with cation exchange capacity, available phosphorus, and dithionite-citrate-bicarbonate-extractable aluminum. Toxicity tests using Allonychiurus kimi (Collembola) demonstrated distinct life stage-specific differences in As uptake pathways. Correlation analysis and partial least squares path modeling (PLS-PM) revealed that juvenile reduction showed high correlations with highly mobile fractions (F1, F2) and As effects varied according to soil physicochemical properties. In contrast, adult survival and As accumulation showed the strongest responses to the F3 fraction in correlation analysis and were influenced by total As concentration rather than soil physicochemical properties in PLS-PM. These findings enhance understanding of As-organism toxicity relationships in forest soils and contribute to improved ecotoxicological assessment. By linking soil properties, As fractionation, and life stage-specific sensitivity, this study provides foundation for effective remediation strategies.