The influences of soil sulfate content on the transformations of nitrate and sulfate during the reductive soil disinfestation (RSD) process

The transformations and products of sulfate (SO₄²⁻) and nitrate (NO₃⁻), especially the influences of SO₄²⁻ content on the transformations during RSD process, are unclear. In this study, a series of soil SO₄²⁻ contents (from 333 to 3000 mg S kg⁻¹) were prepared before RSD treatment. The results indicated that nearly all the cumulative NO₃⁻ (>98.6%) was removed and not affected by the soil SO₄²⁻ content. The ¹⁵N recovery results showed that 0.57–1.24% and 2.94–4.59% of NO₃⁻ translated into ammonium (NH₄⁺) and organic N, respectively, and high SO₄²⁻ contents stimulated the processes of NO₃⁻ dissimilatory reduction and NO₃⁻ immobilization. The soluble SO₄²⁻ contents decreased by 397–922 mg S kg⁻¹, but the contents of total sulfur, sulfide, and sulfate precipitation varied slightly after RSD, indicating that the decreased SO₄²⁻ was mainly immobilized into organic sulfur in all soils. In addition, a fraction of decreased SO₄²⁻ was adsorbed to the soil with a relatively high SO₄²⁻ content. The leaching of SO₄²⁻ was high (42.9–602 mg S kg⁻¹) during the RSD process, and the leaching amounts increased with increasing soil SO₄²⁻ content. In terms of the gases emitted from the transformations of NO₃⁻ and SO₄²⁻, the cumulative emissions of nitrous oxide (N₂O) and six sulfurous gases (hydrogen sulfide, carbonyl sulfide, carbon disulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide) were in the ranges of 17.1–21.2 mg N kg⁻¹ and 7.78–23.5 μg S kg⁻¹, respectively, during the whole RSD process. The emissions of sulfurous gases were inhibited by high soil SO₄²⁻ content, but the N₂O emissions were unaffected. In conclusion, the soil SO₄²⁻ content influenced the transformations of NO₃⁻ and SO₄²⁻ during RSD process, and the SO₄²⁻ leaching and N₂O emissions might threaten the environment which should be concerned.