Geochemical speciation and activation risks of Cd, Ni, and Zn in soils with naturally high background in karst regions of southwestern China

Agricultural soils in karst regions present a remarkable paradox where high geochemical background levels of heavy metals correspond with unexpectedly low crop uptake, challenging traditional risk assessment frameworks and limiting agricultural development. To decode this paradox, we investigated the geochemical speciation of cadmium (Cd), nickel (Ni), and zinc (Zn) in soil-rice systems in southwestern China, which collectively constitute the world's largest continuous karst region and represent diverse soil weathering stages. We employed three chemical extraction methods that revealed reactive pools ranking as Cd (58.74 %) > Zn (7.31 %) > Ni (4.65 %) and risk patterns varying with soil type (Andosols > Cambisols/Gleysols > Lithosols), while multi-surface speciation model (MSM) elucidated the underlying mechanisms. We identified a stage activation-contamination model (SACM) that demonstrates how pH-dependent weathering controlled heavy metal distribution among dissolved, surface-active, semi-stable carbonate, and nonactive species, thereby explaining the observed risk patterns. Specifically, in alkaline soils (pH > 7.50), Cd and Zn were primarily humic acid (HA)- and carbonate-bound, while Ni was goethite-bound. As weathering intensified, the reactive pool shifted to more active HA- and hydrous ferric oxide (HFO)-bound species. In acidic soils (pH < 6.50), dissolved and HA-bound species dominated. Both random forest, offering robust predictions using readily available data, and MSM stepwise regression models, providing high accuracy with mechanistic insights, effectively predicted rice risks. This study from speciation and activation model to prediction model clarifies why standardized risk assessments fail in karst regions and offers practical tools for accurate risk evaluation and management in these agricultural environments.