Sulfur fractions and retention mechanisms in forest soils

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Abstract
The relevance of organosulfur formation and sulfate adsorption as S retention mechanisms in forest soils based upon the sulfur status of samples collected by horizon was investigated. Several forests of varying elevation, vegetation, location, and soil type were considered. Organic S was found to constitute over 78% of total S in the uppermost mineral (0–20 cm; A,E) horizons. This trend was also observed for both intermediate (20–40 cm; primarily A/B) and deeper (40+ cm; B,C) horizons, where organic S exceeded 65% of total S in all but one site examined. Adsorbed sulfate generally constituted only a minor component of the S pool in the uppermost mineral horizons (<13% of total S in all sites examined). This trend was also observed with increasing depth, although in the lowermost horizons adsorbed sulfate increased on a percentage basis. In almost all cases, the adsorbed anion constituted substantially less of the total S than did organic S, irrespective of depth. Carbon-bonded S was the most prevalent form of organic S for most sites at all depths examined. Adsorbed ester sulfate, recovered by extraction with basic phosphate, generally constituted a substantial portion of the adsorbed S pool for both O1 and O2 components of the forest floor and for the uppermost mineral horizon of most sites examined. This trend did not hold true with increasing depth. Because organic S was the dominant form of S, irrespective of horizon, the data suggest that organosulfur formation, not sulfate adsorption, may represent the primary mechanism for S retention in forest soil.