Rates of emission of H₂S from plants and patterns of stable sulphur isotope fractionation

In all green plants both HSO^-₃ and SO ₄^2- are reduced during photosynthesis, and many species can emit the product as H₂S gas. Plants will emit H₂S after fumigation with acute doses of SO₂ gas^1,2, irrigation with 5% K₂SO₄ solutions³, or immersion of roots in HSO^-₃ and SO₄ ^2- solutions⁴. If such emissions of H₂S by plants occur in nature, then this process adds sulphur to the atmosphere and contributes to the global sulphur budget. Oxidation of biogenic H₂S in the atmosphere may also contribute to the formation of acid rain. We report here that irradiated plants with roots immersed in HSO^-₃ and SO₄^2- solutions at concentrations found in nature emit H₂S from their leaves, and that the fractionation of stable isotopes of sulphur during H₂S emission may be useful for identifying atmospheric sulphur that has been generated by photosynthetic reduction of sulphur.