Speciation And Isotopic Composition Of Sulfur In Limestone Soil And Yellow Soil In Karst Areas Of Southwest China: Implications Of Different Responses To Acid Deposition

The contents and stable S isotope ratio (delta S-34) values of total S, organic S, SO42--S, and total reduced inorganic S (TRS) in typical limestone soil and yellow soil were analyzed in this study to examine the general distributions of S forms and their delta S-34 values in soils in karst areas of southwest China. Under a similar level of acid deposition, the vertical profiles of the S forms and their delta S-34 values differed in limesto\ne soil and yellow soil, indicating the different geochemical responses of these soils to acid deposition. The deposited SO42- was retained as organic S in both soils. The depletion in S-34 of TRS relative to SO42- and the parallel increasing delta S-34 values of TRS and SO42- indicate a bacterial reduction process of sulfate in both soils. The different extents of C-bonded S mineralization and organic sulfate transport explain the different vertical profiles of total S and organic S contents in both soils. Sulfate adsorption in limestone soil was negligible because of high pH values. Sulfate adsorption in yellow soil was another important S retention process in addition to biological S retention to form organic S and TRS because of low pH values. The effect of acid deposition on yellow soil appeared more serious because of the accumulation and leaching of deposited SO42-, which can result in soil acidification and accelerate the loss of basic cations from yellow soil. However, compared with yellow soil, limestone soil released more S into rivers by organic S mineralization after a large decrease in annual S deposition rate.