The oxygen and sulfur isotopic compositions of soluble sulfate in the needles of Pinus massoniana Lamb.: Source discrimination and contribution estimation

Abstract

In order to identify the sources of sulfate in plant leaves, needles of Pinus massoniana Lamb. were collected from Guiyang (an area seriously affected by acid rain) and Yunnan areas (where acid rain barely occurred) in southwestern China to measure the isotopic compositions of the water-soluble sulfate (δ34SSO4 and δ18OSO4) and leaf water (δ18OH2O). Compared to atmospheric sulfate, all needle samples were quite depleted in 18O, indicating that sulfate in needles was rarely derived from atmospheric sulfate. The δ18OSO4 values in the needles (−5.3‰) were similar to those in the soil (−5.2‰) in the uncontaminated mountain area of Jinning, suggesting that sulfate in needles originated mainly from soil sulfate in barely SO2 pollution areas. However, the δ18OSO4 values in the needles were significantly more negative than that of soil in most areas with relatively severe SO2 pollution, and the δ18OSO4 values in the needles were positively correlated with the δ18OH2O values of leaf water, indicating that a substantial part of the sulfate in the needles might be derived from atmospheric SO2 oxidation in polluted areas. The estimated contributions of atmospheric SO2 oxidation to the sulfate in the needles increased significantly with increasing atmospheric SO2 and needle sulfate concentration. When the atmospheric SO2 concentrations were >30 μg m−3, the contributions of SO2 to needle sulfate were >50%. The δ18OSO4 values in needles decreased obviously with the increase of the distance from both the Steel Plant and the Power Plant, implying that part of needle sulfate likely to be derived from primary sulfate which generated in the industrial plant flues and secondary sulfate which generated in atmosphere.