Chemical Characteristics and Sulfur Isotopic Compositions of Forest Precipitations and Lake Water in the Yuanyang Lake Forest Ecosystem

Since the beginning of industrialization, the environmental acidification becomes a serious issue to influence the ecosystem stability. Of potential compounds related to acidification, the oxidized sulfur compounds are one of the important contributors of acid deposition. While sulfur is one of the important nutrients in forest ecosystems and mainly derives from atmospheric deposition, acid deposition may have great influence on the forest ecosystem. Previous studies indicated that the Yuanyang Lake Nature Preserve is an acidic cloud forest ecosystem and, therefore, may serve as an ideal studying site. In this study, different types of water were collected from the Yuanyang Lake and adjacent forest ecosystem from April 2005 to December 2006, and analyzed with the major anion, cation and sulfur isotopic composition. The goal of this study is to understand the variation and dynamics of chemical and sulfur isotopic compositions of forest precipitations and lake water and to evaluate the source and process with respect to sulfur in the Yuanyang Lake forest ecosystem.ulfate concentrations of bulk precipitation, throughfall and stemflow were higher in spring and winter, which may result from the winter monsoon. However, sulfate concentrations of fog water were higher in summer, which may relate to the typhoon events. Both the sulfate sources of the forest precipitation and the lake water were dominated by non-sea salt sulfate, an estimate based on the sodium contribution. The sulfate concentrations of lake water were higher in winter and lower in summer, suggesting that an important polluted source of sulfate transported by the winter monsoon was required. Under this circumstance, pollution would be also the main source of potassium in the forest precipitations and lake water. The source of calcium in the forest precipitations and lake water include ocean, dry deposition and rock weathering. Good correlations between calcium and potassium concentrations in stemflow, fog and soil water were evident, suggesting that they may have the same source. Magnesium concentrations were generally low in all types of water and higher concentrations were only measured in several throughfall samples. Magnesium may derive from the ocean and extra magnesium in stemflow was provided by dry deposition. A good correlation between calcium and potassium concentrations in lake water may infer the same source for these two ions.ulfur isotopic compositions of dissolved sulfate in the Yuanyang Lake ranged from 1.9 to 3.0‰. The inverse correlation between δ34S values and sulfate concentrations of lake water with depth indicate the microbial sulfate reduction in the lake sediments. Sulfur isotopic compositions of dissolved sulfate in the fog deposition ranged from 3.4 to 7.4‰. The value is significantly higher than the value of lake water, inferring different sources or various processes involved in the change of sulfur isotopic composition. The interaction between fog water and leaves may enrich sulfate with heavier sulfur isotopic compositions. The δ34S values of fog water increase through time, which may result from the polluted sources with smaller proportion or heavier sulfur isotopic compositions; however, it cannot be clarified with limited background observation of polluted sources.