Mercury Input, Output And Transport In Forest Ecosystems In Southern China

Mercury(Hg) is a global transport heavy metal of high toxicity and bioaccumulation. Forest is the largest terrestrial ecosystem which plays an important role in Hg biogeochemical cycle. China is currently the world’s top direct emitter of anthropogenic Hg. To know the role of forests in China in Hg transport and cycle is important, which can contribute to estimate the Hg emission from natural sources and evaluate the influence of Chinese anthropogenic Hg emission on global Hg pollution. The objective of this study is to quantify the input, output and transport of Hg in forest ecosystems in southern China and analyze the factors influencing the Hg transport and transformation.The methods of dynamic flux chamber(DFC) were used to observe the Hg exchange of foliage/air and soil/air. Field observations were carried out in four seasons in Qianyanzhou forest station. Some fieldand laboratory control experiments were conducted to study the Hg exchange processes. The results showed needles of masson pine emitted Hg of 10.9 μg·m-2·yr-1 to atmosphere. Hg in soil water could enter into trees and be transported to needles by transpiration, which would be photoreduced to elemental form and reemitted to atmosphere. The Hg emission of needles from transpiration was 19.9 μg·m-2·yr-1 accounted for 44% of the atmospheric Hg deposition. A quarter of the atmospheric Hg deposition would be directly photoreduced and reemitted inforest canopy. Soil emitted Hg of 18.6 μg·m-2·yr-1 to atmosphere. The Hg emission of soil was high in summer but low in winter. Adsorption and desorption are the main processes controlling soil/air Hg exchange, and temperature and Hg gradient is the key factors. Newly deposited Hg(Hg2+and Hg0) can be rapidly and stably accumulated in soil, but the Hg would be reduced and reemited slowly.Micrometeorological method was used to continuously observe the Hg fluxes over forest canopy in Qianyanzhou and Huitong forest stations for a full year. Throughfall, wet deposition, soil waters and litterfall were collected dynamically to estimate the Hg transport. Results showed the Hg fluxes over forest canopy in Qianyanzhou and Huitong were 40.2 μg·m-2·yr-1and 17.9 μg·m-2·yr-1 respectively. High temperature and low air Hg concentration resulted in the high Hg emission in summer. Low temperature in winter and Hg absorption by plant in spring resulted in low Hg emission in the two seasons. The input and output of Hg in Qianyanzhou forest were almost balanced, and theHg flux was-0.92 μg·m-2·yr-1. The forest in Huitong was a Hg sink with Hg flux of-7.88 μg·m-2·yr-1. Although the Hg concentration in soil in Huitong forest was high, the high air Hg concentration caused byWanshan Hg mining area inhibited Hg emission of forest and increased Hg absorption. The output ofHg0 to atmosphere was the main way that forests exported Hg accounted for 89% and 47% of the atmospheric Hg deposition in Qianyanzhou and Huitong respectively. The output of Hg through soil water in the two forests were only accounted for 8% of the atmospheric Hg deposition. The forests in southern Chinacontributed more to atmospherethan to water bodies.