Sulfur speciation and bioaccumulation in camphor tree leaves as atmospheric sulfur indicator analyzed by synchrotron radiation XRF and XANES

Analyzing and understanding the effects of ambient pollution on plants is getting more and more attention as a topic of environmental biology. A method based on synchrotron radiation X-ray fluorescence and X-ray absorption near edge structure spectroscopy was established to analyze the sulfur concentration and speciation in mature camphor tree leaves (CTLs), which were sampled from 5 local fields in Shanghai, China. Annual SO₂ concentration, SO₄^2- concentration in atmospheric particulate, SO₄^2- and sulfur concentration in soil were also analyzed to explore the relationship between ambient sulfur sources and the sulfur nutrient cycling in CTLs. Total sulfur concentration in mature camphor tree leaves was 766-1704 mg/kg. The mainly detected sulfur states and their corresponding compounds were +6 (sulfate, include inorganic sulfate and organic sulfate), +5.2 (sulfonate), +2.2 (suloxides), +0.6 (thiols and thiothers), +0.2 (organic sulfides). Total sulfur concentration was strongly correlated with sulfate proportion with a linear correlation coefficient up to 0.977, which suggested that sulfur accumulated in CTLs as sulfate form. Reduced sulfur compounds (organic sulfides, thiols, thioethers, sulfoxide and sulfonate) assimilation was sufficed to meet the nutrient requirement for growth at a balanced level around 526 mg/kg. The sulfate accumulation mainly caused by atmospheric sulfur pollution such as SO₂ and airborne sulfate particulate instead of soil contamination. From urban to suburb place, sulfate in mature CTLs decreased as the atmospheric sulfur pollution reduced, but a dramatic increase presented near the seashore, where the marine sulfate emission and maritime activity pollution were significant. The sulfur concentration and speciation in mature CTLs effectively represented the long-term biological accumulation of atmospheric sulfur pollution in local environment.