Many plants acquire sulfur through gaseous atmospheric compounds. Leaves of trees have δS values lying between those of air and soil, suggesting that there is uptake occurring from atmospheric and soil sources. The δS values of trees has also been demonstrated to be height dependent, with the foliage at the tops of conifers, bull rushes and deciduous trees having δS values more reflective of the atmosphere and lower foliage having δS values closer to that of soil. It has been proposed that this is due to upper foliage exerting a canopy action on the lower branches, taking up atmospheric sulfur before it can reach lower levels. This is further supported with the epiphytic lichens and mosses having δS values close to atmospheric S compounds. This occurs due to lichens and mosses having no access to soil and relying on the direct uptake of gaseous sulfur, dissolved sulfur through rainfall and dry fall accumulation, providing a cumulative record of atmospheric sulfur isotope composition.
Main forms of atmospheric sulfur come from the natural sulfur emissions formed biologically and emitted as HS or organic sulfur gases such as DMS (dimethyl sulfide), COS (carbonyl sulfide), and CS (carbon disulfide). These gases are predominantly formed over oceans, wetlands, salt marshes, and estuaries by algae and bacteria. Anthropogenic emissions have increased the concentration of sulfur in the atmosphere mainly through emissions of SO, from coal, oil, industrial processes, and biomass burning. In 2000, global anthropogenic emission of sulfur was estimated of 55.2–68 Tg S per year, which is much higher than the natural sulfur emissions estimated to be 34 Tg S per year. In the event of excess sulfur in plant tissue, it has been demonstrated that when exposed to high doses of sulfur dioxide, plants emit hydrogen sulfide (HS) and possibly other reduced sulfur compounds in response to high sulfur loading