Suppressive Effect of Sulfate on Establishment of Rice Seedlings in Submerged Soil May be Due to Sulfide Generation around the Seeds

Direct seeding is a labor-saving rice cultivation method, but poor establishment is a major obstacle to the widespread use of this practice. This study examined the suppressive effect of ammonium sulfate, a nitrogen fertilizer, on seedling establishment in submerged soil. Establishment was impaired even at 20ºC by the application of sulfate salts, including ammonium sulfate and potassium sulfate, but not by ammonium chloride. Impairment by ammonium sulfate did not occur in autoclaved soil. In addition, the redox potential in the soil near the seeds decreased rapidly to levels that allowed sulfide ion generation, even at 20ºC. The sulfur concentration in the soil near the seeds decreased with the decline in redox potential, suggesting that sulfide ions were transformed from soluble sulfate ions into insoluble iron sulfide through binding with ferrous ions near the seeds. These results imply that sulfate ions may be transformed into sulfide ions by bacteria only around the seeds if sufficient sulfate ions are present in the soil, thereby impairing the establishment of seeds sown directly in submerged soil, even under cool springtime conditions and in paddy fields that do not dry out prior to submergence