TRANSFORMATION OF SULFUR IN SOIL BY MICROBES 169 



TABLE 42 



Disappearance of Sulfate During Growth of Aspergillus Niger on 

 A Carbohydrate (from Rippel) 



Sulfate-sulfur, 

 Time mgm. 



At start 3. 19 



After 5 days 1 • 58 



After 7 days 0.64 



After 12 days 



is the sulfur carrier. When proteins are hydrolyzed by micro- 

 organisms, the cystine is first hberated. When the cystine mole- 

 cule is decomposed by microorganisms in the soil, the sulfur is 

 usually liberated in the form of hydrogen sulfide: 



CHo — S — S — CH2 



I I 



CH • NHo CH • NHo + 4H2O 



I I 



COOH COOH 



Cystine 



= 2CH3COOH + HCOOH + CO2 + 2NH3 + 2H2S 



Acetic acid Formic acid 



Other organic compounds containing sulfur are frequently intro- 

 duced into the soil, but generally in very small amounts. These 

 include taurine, among the animal products, and certain gluco- 

 sides, among the plant residues. 



The nature of the compound as well as the environmental con- 

 ditions in which it is decomposed, whether aerobic or anaerobic, 

 will determine to a large extent the resultant form of the sulfur 

 when it appears in the inorganic state. Under anaerobic condi- 

 tions sulfides are frequently produced in considerable quantities. 

 This is the case in water-logged soils, ditches, stagnant pools, and 

 seas that receive appreciable contributions of organic materials 

 and sulfates. Coloration of soils may be determined by sulfides 

 of iron under anaerobic conditions where organic matter is not so 

 abundant as to conceal this coloration. These sulfides are formed 

 from the decomposition of organic compounds containing sulfur 

 and from reduction of sulfates, sulfur, or other inorganic, incom- 

 pletely oxidized sulfur compounds. 



