Industrial development in China has accelerated rapidly in the last few decades. This has led to a range of environmental problems. Deposition of trace metals to forest ecosystems via the atmosphere is a potential regional concern.
In this study, levels of trace metals (As, Cd, Co, Cr, Cu, Fe, Hg, Ni, Mn, Pb, Sn, V and Zn) have been measured in the soil samples from A and B horizons in acid-sensitive forest soils at 3 different sites in China. Two of these sites, Tie Shan Ping (TSP) and Liu Chong Guan (LCG), are located close to the large cosmopolitan city of Chongqing and the less developed city of Guiyang, respectively. The third site, Lei Gong Shan (LGS), is situated in a remote region of Guizhou province with no large local emission source.
Total Hg concentrations have been determined using a Direct Mercury Analyzer. For the other trace metals, total content of each element has been determined by Inductively Coupled Plasma Mass Spectrometry after microwave digestion. With a few exceptions, the metal concentrations can be characterized as moderate. Compared with the soil environmental background values and Chinese Class I standards, Cd, Pb, Hg, and to some extent As, show higher concentrations in the upper horizon; Cd, Pb and Hg with median values 0.25, 47 and 0.28 mg kg-1; 0.17, 30 and 0.39 mg kg-1; and 0.42, 28 and 0.37 mg kg-1 at TSP, LCG and LGS respectively, and As with median concentrations 14 and 16 mg kg-1 at TSP and LGS respectively. Average metal concentrations in this study are generally similar to results available from studies in China and Europe. Significantly higher concentration of Cd (0.59 mg kg-1) in a previous study in the same area may be due to poor accuracy of the semi-quantitative analytical method used in the previous study.
Metal concentrations are rather similar among the studied sites. High contents of Cd and Pb at TSP and Pb and V (in a couple of samples) at LCG may be related to proximity to the big cities Chongqing and Guiyang and large emission sources. At the remote site (LGS), metal levels are fairly similar or even higher than at the other sites; this can be mostly associated with higher metal content in the minerals (shale). In addition, there may be some contributions from long-range transport of trace metals and high deposition flux due to high precipitation at this site.
Evaluation of variations among and within the macroplots (10 m x 10 m area) generally shows the lowest variations for As and Cd with RSD% < 25% and the largest variations for Fe, Mn and V with RSD% > 25%.
Investigation of the behavior of metals in the soil profile and association with soil characteristics using statistical approaches (Principal Component Analysis and t-test) indicates accumulation of Hg, Pb and Cd at TSP and Hg at LCG in the A horizons. Accumulation of metals in the upper horizon can be related to high atmospheric deposition due to proximity to urban areas. Since Pb and Hg are strongly complexed to organic matter, their accumulation might be associated with higher organic content of A horizons. However, the PCA shows only a weak correlation between Hg and Pb and soil organic content. This could be affected by the significantly different depositions of these metals among the studied sites which are not considered in the PCA. Accumulation of Co, Mn and Ni in the B horizon is found at TSP and may be the result of mobilization at low pH-values in the A horizon. At the remote site (LGS), most of the investigated metals (As, Cd, Co, Cu, Ni, Pb, Sn and Zn) show higher concentrations in the A horizon than in the B horizon. Except for Cd, the differences are rather small.
Increased mobilization of Cd due to acidification in combination with high Cd levels in the studied area can be a potential concern as mobilization of Cd may lead to ecotoxicological impacts.