Extractable Potentially Toxic Elements Across Soil Profiles in East Kazakhstan: Depth Patterns and Soil-Property Associations

The vertical distribution of operationally extractable potentially toxic elements may vary substantially among soil profiles, limiting the value of surface-only assessment. This study examined ammonium acetate buffer (AAB)-extractable Zn, Cu, Cd, and Pb in 486 soil samples from 133 profiles in the East Kazakhstan Region. Samples were obtained from four depth intervals: 0–5, 5–20, 20–50, and 50–100 cm. Soil-property analyses as core properties were assessed across 0–100 cm, nutrients across 0–50 cm, and humus and exchangeable cations within 0–20 cm. Depth-related patterns and associations with soil properties were evaluated using non-parametric tests, Spearman correlations, linear mixed-effects models, redundancy analysis and pooled percentile screening. Metal concentrations were strongly right-skewed and varied markedly among profiles. From 0–5 to 50–100 cm, median concentrations declined from 55.2 to 4.5 mg kg−1 for Zn, 5.7 to 1.7 mg kg−1 for Cu, 2.5 to 1.4 mg kg−1 for Cd, and 3.9 to 1.2 mg kg−1 for Pb. Depth-only mixed-effects models confirmed negative average depth trends for all four metals, alongside substantial between-profile heterogeneity. Sand content showed positive associations with all four metals across the analysed depth scopes; available P2O5 was positively associated with all four in the 0–50 cm models, while humus was positively associated with Zn, Cd, and Pb in the upper 20 cm. Observations at or above the pooled P90 and P95 thresholds were most frequent near the surface, although P95-level observations also occurred at 50–100 cm. These findings show that depth-related median declines coexist with substantial profile-specific heterogeneity and support depth-resolved monitoring of AAB-extractable metals in polymetallic mining and metallurgical landscapes.