Geographical Patterns of Leaf and Topsoil Mercury in China's Urban Forests

Rapid urbanization, occurring globally, has enhanced emissions and related biogeochemical cycling of multiple elements, including mercury (Hg). However, the geographical distribution of Hg in urban forests remains poorly quantified at multiple spatial scales. Based on field investigations of urban forest parks in China, we explored the urban-rural gradients and latitudinal trends of tree leaf and topsoil (0–10 cm) Hg concentrations. Our results indicate an urban hotspot of leaf and topsoil Hg concentrations across urban-rural transects and show an increase in leaf and topsoil Hg concentrations from higher latitudes toward lower latitudes. The urban-rural gradient and latitudinal trend of leaf Hg concentration are both significantly explained by road density, annual precipitation, and plant life forms. Leaf Hg concentration increases significantly with higher road density and is significantly higher in deciduous trees than in evergreen trees. Leaf Hg concentration decreases with higher annual precipitation across the urban-rural transects but increases with it across latitudes. The urban-rural gradient of topsoil Hg concentration is significantly associated with the spatial variation in traffic sources, while the latitudinal change in topsoil Hg concentration is largely explained by background soil Hg concentration. Our findings demonstrate distinctive patterns of urban forest Hg from local to regional scales and the occurrence of urban Hg hotspot implies a significant imprint on accelerated Hg cycling in urban areas.