Heavy metal budgets in two forested catchments with heavy atmospheric deposition pollution in China

To investigate heavy metals (HMs) migration in forests with decreasing atmospheric HM deposition in China, the zinc (Zn), lead (Pb) and cadmium (Cd) fluxes through hydrological pathways were observed in a coniferous forest at Tieshanping (TSP) near Chongqing, a mega city in Southwest China, and a broad-leaved forest at Gaozhuang village (GZV) near Jiyuan, an industrial city with large-scale smelters. The throughfall fluxes of Pb and Cd at GVZ were 109 and 9.57 mg·m^-2·yr^-1, respectively, far exceeding those at TSP, indicating severe pollution by atmospheric deposition. The increasing HM fluxes from rainfall to throughfall highlighted the contribution of dry deposition with coniferous forests exhibiting stronger airbone-HM adsorption. The litter layer on forest floor retarded 40.7 % ~ 70.0 % HMs, with broad-leaved litterfall exhibiting lower retention rates due to faster decomposition. Soil stored 56.0 mg·m^-2·yr^-1 of the three HMs at TSP and 77.2 mg·m^-2·yr^-1 at GZV, resulting in little HM concentration in outflows. GZV's soil with higher pH made it a more effective HM sink than TSP. Despite HM retention rates exceeding 90 % at both forests, HMs in surface soil began to move downward. This study provided the first evidence that following China's substantial reduction of atmospheric HM pollution, polluted forests exhibited vertical HM migration through soil profiles. Fortunately, HMs were further retained by deep soil, preventing watershed-scale export.