Formation of organic sulfur in forest soils: a biologically mediated process

Abstract

The ability of soils from hardwood, clear-cut, and pine forests to incorporate sulfur from added inorganic sulfate into salt-extractable (adsorbed) and nonsalt-extractable forms was investigated. At least 65% of the added sulfate was adsorbed while 8–27% of the sulfate added was recovered only after treatment of salt-extracted samples with acid and base (nonsalt-extractable sulfur). The incorporation of sulfur into this latter fraction was dependent upon incubation time, temperature, and depth and exhibited both spatial as well as seasonal variation in samples taken along a transect of one of the watersheds. Sulfur incorporation into the nonsalt-extractable fraction was inhibited 75–87% by sodium azide, 62–84% by erythromycin, and 41–68% by candicidin suggesting that the process is mediated by bacteria and fungi. Data on factors influencing sulfur incorporation suggest that sulfate was incorporated into organic matter as a covalent linkage and released after rupture of this linkage during acid and base treatment. The observations that 35S incorporation was inhibited 93–99% by unlabelled sulfate and stimulated 21–65% by increased carbon availability are consistent with this suggestion.