Source-driven vertical partitioning and stratified risk assessment of trace metals in a subtropical forest soil...

Understanding the vertical partitioning of trace metals is essential for assessing their biogeochemical cycles and ecological risks in forest ecosystems. This study systematically investigated the distribution, storage, sources, and risks of eight trace metals (arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), zinc (Zn)) across organic (Oi, Oe + Oa) and mineral soil layers (0-100 cm) in a subtropical evergreen broad-leaved forest in the Ailao Mountains, Southwest China. Results revealed distinct source-driven distribution patterns: Cd and Pb were enriched in surface organic horizons and topsoil, indicating dominant atmospheric anthropogenic inputs; As, Cr, Cu, Ni, and Zn generally increased with depth, reflecting a pedogenic origin from parent material weathering; and Hg exhibited a dual-source signature, with surface enrichment indicating atmospheric deposition and a deep-concentration profile influenced by geogenic background and unique geochemical behavior. The forest floor, particularly the Oi horizon, showing extreme enrichment (enrichment factor >6) for Cd, Cu, Hg, Ni, Pb, and Zn, highlighting its role as an efficient filter for atmospheric deposition. In contrast, the mineral soil constituted the dominant long-term reservoir. Ecological risk assessment identified the topsoil (0-5 cm) as a high-risk zone (potential ecological risk index (RI) > 300). The root enrichment index revealed that Cd's phytoavailability remained elevated even in deeper soil layers where its total content was lower, indicating that root-mediated processes could enhance metal bioavailability independently of total storage. Soil properties (pH and bulk density) exerted heterogeneous impacts on metal distribution, which depended on the sources of the metals. These findings emphasized the need for a stratified risk management approach that considered both the interception capacity of organic layers and the potential for subsoil contaminant mobilization in forest ecosystems under persistent anthropogenic pressure.