EFFECT OF SULFUR SOURCE ON SULFUR OXIDATION

The rate of elemental sulfur (S-degrees) oxidation in soils may be affected by the source of S-degrees. This study was conducted to determine the effect of S-degrees source on S-degrees oxidation. Liquid culture studies with Thiobacillus thiooxidans and T. thioparus, laboratory incubations with a Holloman soil (loamy, gypsic, thermic, shallow Typic Torriorthent) and a Belen soil (clayey over loamy, montmorillonitic [calcareous], thermic Typic Torrifluvent), and a field study on the Belen soil were conducted. Periodic measurements included pH as a measure of S-degrees oxidation to H2SO4, electrical conductivity (EC) as a measure of dissolution of CaCO3 from H2SO4 production, and available Fe, Zn, P, and SO4 at the end of soil experiments as a measure of S-degrees-oxidation effects on nutrient availability. The S-degrees sources included pure S-degrees (Sp), Wettable S (Sw), flowable S-degrees (Sf), and Disper-Sul S-degrees (Sd). In liquid culture, Sw, Sf, and Sd inhibited or slowed S-degrees oxidation, compared with Sp, suggesting an ingredient of the commercial S-degrees sources was responsible for oxidation inhibition. Oxidation of Sw, Sf, and Sd were similar to Sp oxidation in laboratory soil incubations and in the field. Results indicate that the S-degrees source is not the cause of slow S-degrees oxidation in some New Mexico soils. Chemoautotrophic S-degrees oxidizers were present in unamended soils and their numbers increased in S-degrees-amended soils. Although S-degrees amendment (25 g S-degrees kg-1) in the field decreased pH approximately 0.5 unit and increased SO4 tenfold, S-degrees amendment did not increase available Fe, Zn, or P. The slow rate of S-degrees oxidation in some New Mexico soils and failure of S-degrees amendment to increase nutrient levels are attributed to the buffering capacity of the calcareous soils.