From Elemental Sulfur to Hydrogen Sulfide in Agricultural Soils and Plants

Sulfur is an essential element in determining the productivity and quality of agricultural products. It is also an element associated with tolerance to biotic and abiotic stress in plants. In agricultural practice, sulfur has broad use in the form of sulfate fertilizers and, to a lesser extent, as sulfite biostimulants. When used in the form of bulk elemental sulfur, or micro- or nano-sulfur, applied both to the soil and to the canopy, the element undergoes a series of changes in its oxidation state, produced by various intermediaries that apparently act as biostimulants and promoters of stress tolerance. The final result is sulfate S⁺⁶, which is the source of sulfur that all soil organisms assimilate and that plants absorb by their root cells. The changes in the oxidation states of sulfur S⁰ to S⁺⁶ depend on the action of specific groups of edaphic bacteria. In plant cells, S⁺⁶ sulfate is reduced to S^-2 and incorporated into biological molecules. S^-2 is also absorbed by stomata from H₂S, COS, and other atmospheric sources. S^-2 is the precursor of inorganic polysulfides, organic polysulfanes, and H₂S, the action of which has been described in cell signaling and biostimulation in plants. S^-2 is also the basis of essential biological molecules in signaling, metabolism, and stress tolerance, such as reactive sulfur species (RSS), SAM, glutathione, and phytochelatins. The present review describes the dynamics of sulfur in soil and plants, considering elemental sulfur as the starting point, and, as a final point, the sulfur accumulated as S^-2 in biological structures. The factors that modify the behavior of the different components of the sulfur cycle in the soil-plant-atmosphere system, and how these influences the productivity, quality, and stress tolerance of crops, are described. The internal and external factors that influence the cellular production of S^-2 and polysulfides vs. other S species are also described. The impact of elemental sulfur is compared with that of sulfates, in the context of proper soil management. The conclusion is that the use of elemental sulfur is recommended over that of sulfates, since it is beneficial for the soil microbiome, for productivity and nutritional quality of crops, and also allows the increased tolerance of plants to environmental stresses.