Method for the partitioning of organic sulphur in forest soil O layers

A new method for the partitioning of total sulphur and organic S species (C-bonded S, ester sulphate S) in different humus fractions of forest soil O layers is presented and discussed. Our method combines an adopted version of the sequential humus fractionation procedure of Schnitzer and Schuppli (1989; Soil Science Society of America Journal 53, 1418–1424) with wet-chemical procedures for analysis of Corg, Stot, and hydriodic acid (HI)-reducible S in soils. The procedures were modified for the determination of micro-amounts of these compounds in aqueous and non-aqueous solvents. The applicability of our method was tested with samples taken from Of and Oh layers of two acidic forest soils and compared with results of a traditional humic acid:fulvic acid:humin fractionation. The average recoveries of Corg and Stot for the proposed fractionation method were 91% (Corg) and 100% (Stot), compared to 96% (Corg) and 87% (Stot) for the traditional procedure. Average HI-reducible S recoveries of >110% in both methods suggest a conversion of C-bonded S during the sequential extraction into ester sulphate or SO42−-S. On average, polycondensed humins accounted for 70% of the organic matter of the O layers from acid forest soils. The percentage of hydrophilic compounds soluble in alkaline extractants was 18%, that of hydrophobic substances extractable with solvents of low polarity 12%. About 37% of the forest floor S pool were bound in ecologically labile hydrophilic humus compounds, indicating a considerable S enrichment of the latter fraction. Contrarily, hydrophobic organic matter extractable with solvents of low polarity was depleted in total S and particularly in HI-reducible S.