Soil health index–based assessment of cadmium ecological risk–fertility coupling in seasonally frozen agricultural soils

Heavy metal contamination in seasonally frozen agricultural regions poses potential risks to food security in cold-climate croplands. However, the structural relationship between ecological risk and soil fertility under seasonal freeze-thaw climatic background remains insufficiently evaluated within an integrated soil health framework. In this study, five representative cropland soil types surrounding the Greater Khingan Range were investigated based on 370 soil samples collected from 0 to 40 cm profiles during the non-freezing season. The geo-accumulation index (Igeo), potential ecological risk index (RI), and a principal component-based soil fertility index (SFI) were integrated to construct a weight-adjustable Soil Health Index (SHI). Self-organizing feature mapping (SOFM) and hierarchical cluster analysis (HCA) were applied to identify coordinated variation patterns among heavy metals.Cadmium (Cd) exhibited pronounced enrichment relative to regional background values (40-146 times) and showed significant surface accumulation in the plow layer (0-10 cm), with concentrations 2.98-5.97 times higher than those at 30-40 cm depth. Although sampling was conducted outside the active freeze-thaw period and no direct freeze-thaw monitoring was performed, the observed vertical differentiation patterns are statistically consistent with redistribution processes reported in freeze-thaw-affected soils. Alternative explanations such as long-term surface inputs, tillage mixing, and soil texture effects cannot be excluded.SHI evaluation revealed structural mismatches between fertility and ecological risk, particularly in soils characterized by relatively high fertility but elevated Cd risk. SOFM and HCA consistently grouped Cd and Ni as co-varying elements, whereas Cr and Cu displayed relatively stable patterns indicative of stronger lithogenic control. The findings provide a process-constrained statistical assessment framework for evaluating pollution-fertility interactions in seasonally frozen croplands. The results should be interpreted as pattern-based inference under freeze-thaw background conditions rather than direct mechanistic verification of freeze-thaw effects.