NATURE OF THE HCL-SOLUBLE SULFATE IN THE SEQUENTIAL EXTRACTION FOR SULFUR SPECIATION IN SOILS

Thirty soils collected from different regions of China were used to in vestigate the nature of HCl-soluble sulfate (HCl-S) and to evaluate th e importance of HCl-S for sulfur speciation in soils. The soils were f irst extracted with NaH2PO4 solution to remove water-soluble and adsor bed sulfate, followed by extraction in 1 mol/l HCl at room temperature , minimizing the hydrolysis of organic sulfur into sulfate. Excellent linear correlations (r = 0.986) were found between the HCl-S and the c arbonate contents in these soils, indicating HCl-S was mainly the carb onate-occluded inorganic sulfate in calcareous soils, averaging for 39 % of total sulfur. However, the traditionally recog nized major form o f soil sulfate (water-soluble and adsorbed sulfate) only account for 7 .5% of total sulfur. The non-calcareous soils were also found to conta in some of HCl-S (6.9% of total sulfur). Large errors will occur in th e determination of ester sulfur if HCl-S is not subtracted from the HI -reducible sulfur, especially in calcareous soils and soils containing little organic matter, which was the common practice in sulfur specia tion studies. The reasonableness of including HCl-S in the subtraction method to determine ester sulfur was further verified by the good cor relations between ester sulfur and organic carbon content in the soils .