Spatial Distribution Trends and Influencing Factors of Typical Heavy Metals in Subtropical Alpine Forest Soils: A Case Study from Ailao Mountain in Yunnan Province

Abstract

In this study, we hypothesized that the distribution patterns of trace metal elements in montane regions would be controlled by the local climate, human activities, vegetation distribution and soil development. Hence, we selected nine forest sites among the elevation of 850 m to 2650 m on the windward (western slope) and leeward (eastern slope) slopes of Ailao Mountain in Yunnan Province of China to determine the typical heavy metal concentrations in forest surface soils, and their differences for distribution trends along with the increasing elevations, and the corresponding environmental factors. The results showed that surface soil Cd, Sb, As, Cr, and Pb had relatively higher concentrations and distinct enrichment factors both on eastern and western slopes. Specifically, the concentrations of Cd and Sb in surface soils of the eastern and western slopes and Pb on the western slope all had obvious altitude distribution trends, which showed their concentrations increased with the increasing elevation. In addition, we found the concentrations of Cd, Sb, As, and Cr in surface soils of the western slope were significantly higher than those of the eastern slope at the same altitude. The correlation analysis and principal component analysis further depicted that the elevation trends of Cd and Sb were mainly controlled by the strong combining effects from long-range atmospheric input and soil organic matter strong absorption, and there was an obvious "cold trapping" effect in high altitude areas. The distribution variations of As and Cr were mainly derived from the weathering difference of soil parent material, while the variations for Pb were controlled by the weathering of bedrock minerals. This study highlights the important role of combining effects from long-range atmospheric input, and from precipitation and vegetation (such as "cold trapping" effect, promoting canopy filtration, litterfall deposition and bedrock weathering) on the distribution trends of trace metals in remote alpine soils.