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.