Saturday, 15 July 2006

Mixing of Anthropogenic and Geogenic Lead Through Biogeochemical Cycling at a Temperate Moist Forest Ecosystem in Japan.

Yuko Itoh, Kyotaro Noguchi, Masamichi Takahashi, and Shuichiro Yoshinaga. Forestry and Forest Products Research Institute, Japan, Matsunosato 1, Tsukuba, Japan

Industrial pollution and the combustion of leaded gasoline result in markedly increased atmospheric concentrations of lead (Pb). Once emitted, Pb can be transported for long distances in air masses and can contaminate ecosystems (Pacyna & Pacyna 2001). In Europe, North America, and Japan, emissions of Pb have been decreasing over the last two decades. In contrast, emissions in continental Asia have increased and are now the highest in the world. In forest ecosystems, atmospheric deposition is the main source of anthropogenic Pb input. To assess the environmental impacts of this pollutant, we determined the distribution and cycling of atmospherically derived Pb in a forest ecosystem in Japan. We measured Pb concentrations and Pb isotope ratios in bulk precipitation, throughfall, vegetation, litterfall, the forest floor, mineral soil, and bedrock in a rural forest in Japan. The annual input–output budget indicated that atmospherically deposited Pb was retained within the forest catchment. The Pb concentrations in the forest floor ranged from 1.8 to 17.6 mg kg–1 and those in the mineral soil ranged from 0.4 to 44.3 mg kg–1. The Pb concentration was highest in the surface soil and it decreased with increasing depth. The 206Pb/207Pb isotope ratios of the surface soil, forest floor, and vegetation were almost same and were intermediate between those of rainfall and the subsoil. The Pb isotope ratios in the rainfall were generally influenced by the anthropogenic origin of the atmospheric Pb. The ratios in the subsoil were similar to those of the parent material of the mineral soil. In the surface soil, mixing of anthropogenic and natural geogenic Pb was observed. This indicates that anthropogenic Pb is accumulated in the forest floor and the surface soil, and migration of anthropogenic Pb to the deep soil layer is very limited. These results suggest that biogeochemical cycling actively functions to retain Pb derived from anthropogenic sources within this forest ecosystem.

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