Thiosulfate Inhibition of Urea Hydrolysis in Soils: Tetrathionate as a Urease Inhibitor
- D. M. Sullivan * and
- J. L. Havlin
Ammonium thiosulfate [ATS, (NH4)2S2O3, 12-0-0-26S] added to urea-ammonium nitrate (UAN) inhibits urea hydrolysis after application to soil. Thiosulfate itself does not inhibit purified jackbean urease, so products of S2O2-3 oxidation in soil (tetrathionate [S4O2-6], Fe2+, and Mn2+) are the probable urease inhibitor(s). The objectives of this study were to (i) identify the primary urease inhibitor produced by ATS in soil and (ii) determine why the efficacy of the inhibitor is improved by increasing UAN droplet size. Jackbean urease (urea amidohydrolase, Type III from jackbean, EC 3.5 1.5) was inhibited by Fe2+ (100% inhibition at 0.3 mM), Mn2+ (100% at 6.3 mM), Fe3+ (97% at 6.3 mM), and S4O2-6 (39% at 7.8 mM). Thiosulfate did not inhibit jackbean urease at concentrations up to 18.8 mM. When applied in a simulated UAN fertilizer band, S4O2-6 was as effective as S2O2-3; it inhibited soil urease (24%) to the same extent as S2O2-3 (26%) but more than FeSO4 (0%) in two soils. Urease inhibition by all species was low in unbuffered 5-h soil incubations (Fe2+ [14%], Mn2+ [3%], S4O2-6 [10%], and S2O3 [7%]) with 25 mmol S kg−1. Urease inhibition by S4O2-6 increased from 9% at pH 7 to 30% at pH 8.5 in buffered incubations. Since pH during urea hydrolysis increases with UAN droplet size, the increased efficacy of ATS with application of large UAN droplets is probably due to greater activity of S4O2-6 as a urease inhibitor at pH > 7. Tetrathionate is the primary urease inhibitor produced by ATS in soil. Reaction of urease sulfhydryl groups with S4O2-6 is proposed as an inhibitory mode of action.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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