Sulfur Isotopic Ratios Indicating Sulfur Cycling in Slope Soils of Karst Areas

Sequential extraction methods for soil sulfur were used to determine δ~34S ratios and sulfur contents of total sulfur,organic sulfur,SO~2-_4 and FeS_2 in slope soils of karst areas.In general,FeS_2 has the lowest δ~34S ratios,ranging from-6.86‰ to-4.22‰,followed in ascending order by SO~2-_4(-2.64‰——1.34‰),total sulfur(-3.25‰——1.03‰) and organic sulfur(-1.63‰-0.50‰) in surface soils of profiles,and δ~34S ratios in different sulfur forms all show increasing trend with profiles deepening.Covariations of δ~34S ratios of SO~2-_4 and FeS_2 with increasing depth are related to SO~2-_4dissimilatory reduction,while the increase in parallel of δ~34S ratios of total sulfur and organic sulfur could be resulted from organic sulfur cycling.δ~34S ratios have been extensively used to indicate sulfur sources,moreover,SO~2-_4 dissimilatory reduction and organic sulfur mineralization result in significant sulfur isotopic fractionation,and sulfides oxidation and SO~2-_4 assimilation have no isotopic fractionation occurred,the vertical variations of δ~34S ratios in different sulfur forms can therefore be good records for depth-dependant sulfur cycling processes.Furthermore,by comparing depth distributions of sulfur contents and δ~34S ratios in different sulfur forms,it is easily to discuss the migration of SO~2-_4 and organic sulfur fractions in soil profiles.