Mercury dynamics and mass balance in a subtropical forest in Southwest China
(2.Simian Mountain Forest Resources Administration, Chongqing, China 402296)
(3.Chongqing Key Laboratory of Agricultural Resources and Environment, Southwest University, Chongqing, China 400716)
【Abstract】Forest ecosystem plays an important role in the geochemical cycle of mercury, but it is still not clear which compartment is the major source or sink for mercury cycling in the forest ecosystem. Simian Mountain was selected in our study because it had a subtropical monsoon climate and abundant precipitation ranging from 1 023 mm to 1 586 mm, which is beneficial to the growth of forest vegetation. Moreover, the evergreen broad-leaf forest in Mt. Simian is the most representative vegetation type preserved in the study reserve. Therefore, the input and output of mercury in this forest was monitored for one year. Results showed that THg and TMeHg concentrations in the precipitation were (11.23 ± 2.6) ng/L and (0.35 ± 0.24) ng/L respectively, which were higher than the global background values, probably due to the anthropogenic mercury pollution from ambient cities. The throughfall had higher mercury concentrations than precipitation, which was probably because the precipitation scoured the mercury settled in the canopies by dry deposition. THg concentration [(4.5 ± 2.0) ng/L] in the forest runoff decreased remarkably compared with that in the precipitation, which confirmed that the forest ecosystem had strong interception and fixation effect on mercury from precipitation. The main input pathway of mercury in the forest ecosystem was through litter, because atmospheric mercury can be absorbed by tree leaves. Moreover, when the tree leaves fell on the ground, the mercury in litter was released to the soil during its decomposition. Finally, this part of mercury was accumulated in the top soil due to the high concentrations of dissolved organic matter, leading to higher THg concentration in the top soil. Mercury emitted from the surface forest ground was the main way of output in this forest, which was affected by solar radiation, temperature, mercury concentration in top soil, soil moisture and so on. For the studied forest ecological system, the total amount of mercury input was greater than its total output, so it acted as a “sink” in the biogeochemical mercury cycle.
【Keywords】 mercury; methylmercury; forest ecosystem; Simian Mountain; output/input;
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