Multiple environmental factors but not land use affect topsoil mercury levels across the urbanized mosaic of the Hengduan Mountain region

Various studies have looked at how patterns of topsoil mercury concentrations related to land-use and influenced by climate, vegetation, elevation, and soil conditions; however, a pivotal question remains for urban land management and toxicology: to what extent do natural factors continue to drive mercury accumulation in the type of moderately populated, urbanized mosaic landscapes that represent the optimal compromise between nature and development. We investigated total mercury (THg) concentrations in urban topsoils across the principal cities of fourteen counties in the Hengduan Mountains at the eastern margin of the Qinghai-Tibetan Plateau, spanning an elevation range from c. 360 m to 3380 m. We found that land use type (i.e., parkland, woodland, cropland, or vacant land) had no significant effect on topsoil THg concentrations; however, THg levels were sensitive to climate (temperature, but no precipitation or UV-B), vegetation, and soil organic matter. These results reveals that natural factors appear to dominate the distribution pattern of urban topsoil mercury in ecologically fragile areas with mild human interference. Therefore, there is likely a threshold of urbanization intensity at which topsoil mercury dynamics shift to become more highly land-use dependent.