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 (δS) values of total S, organic S, SO-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 δS 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 δS values differed in limesto\ne soil and yellow soil, indicating the different geochemical responses of these soils to acid deposition. The deposited SO was retained as organic S in both soils. The depletion in S of TRS relative to SO and the parallel increasing δS values of TRS and SO 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 SO, 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.