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Thiosulfate Production in Cysteine-Amended Soil

Division of Soil Science, Univ. of Idaho, Moscow, ID 83844

*Corresponding author (mmorra{at}marvin.ag.uidaho.edu).

ABSTRACT

Thiol-labile compounds are produced in cysteine-amended soil, as evidenced by the production of H₂S following addition of reagents containing free sulfhydryl groups (–SH). Thiosulfate (S₂O^2–₃) is a likely reactant candidate because it is an intermediate in biogeochemical S cycling and is reduced to H₂S by -SH, but its presence in cysteine-amended soils has not been documented previously. Four soils were amended with L-cysteine (15 mmol kg^–1 soil) incubated at 0.033 MPa soil water suction or under saturated conditions, and then extracted with 0.1 M LiCl. Colorimetric analysis of the extracts using cyanolysis with and without addition of a Cu²⁺ catalyst indicated the presence of S₂^2–₃ and a polythionate such as S₄O^2–₆; positive interferences from the cyanolysis of S^2– were minimal. Ion chromatographic analysis of the extracts showed a well-resolved peak with the same retention time as S₂O^2–₃ and yielded concentration estimates similar to those obtained by colorimetry, providing positive identification of S₂O^2–₃ in cysteine-amended soils. Thiosulfate production occurred under both saturated and unsaturated conditions, varied among soils from 0 to 35.2 mg S₂O^2–₃-S kg^–1, and was nearly eliminated by autoclaving the soil before incubation or by addition of NaN₃. Although the exact pathway for S₂O^2–₃ production is unknown, it is likely that H₂S formation and subsequent oxidation are responsible.

Funding provided by the Idaho Agric. Exp. Stn.

Received for publication July 26, 1993.