Formation and mineralization of organic sulfur in forest soils

The incorporation of sulfur from inorganic sulfate into organic matter was examined using³⁵S for O1, O2 and A-horizon samples from two hardwood forests located at the Coweeta hydrologic laboratory, near Franklin, NC. This temperature-dependent transformation was stimulated by increased availability of sulfate or energy and the process was inhibited by sodium azide, erythromycin and candicidin. These data suggest that bacteria and fungi mediate sulfur incorporation via the formation of the covalent linkages. The latter possibility was confirmed by characterization of the organic sulfur fraction after isolation and partial purification from O2 layer material. Evidence is presented which indicates that the sulfur of this fraction is subject to mineralization after depolymerization of the carbon matrix and methods are given for the direct and indirect assessment of potential turnover rates. The availability of sulfate from mineralization appears to depend upon the rate of incorporation of sulfur into organic matter. Thus, substantially higher levels of extractable sulfate were detected when turnover of the isolated organosulfur fraction was assayed for in the presence of azide, an inhibitor of sulfate incoporation. However, the reverse was true when turnover was monitored in the presence of glucose and succinate which stimulate sulfate incorporation.Contribution from a symposium on the role of sulfur in ecosystem processes held August 10, 1983, at the annual meeting of the A.I.B.S., Grand Forks, ND; Myron Mitchell, convenor.