An innovative utilization approach for by-products of biogas desulfurization：Co-hydrothermal treatment of sulfur with biogas slurry to prepare sulfur-enriched liquid fertilizer

The sulfur bioavailability enhancement introduced by the co-hydrothermal treatment of sulfur with corn straw biogas slurry was evaluated, and the effects of different sulfur speciation on plant and soil were explored via kale pot experiment. Based on this, an innovative methodology was proposed to prepare sulfur-enriched liquid fertilizer via co-hydrothermal treatment of BS and sulfur. The addition of sulfur element promoted the yield and quality of kale, and significantly impacted soil microorganisms. Notably, after co-hydrothermal treatment of biogas slurry and elemental sulfur, such effect on kale and microorganisms was more pronounced among different treatments due to the diverse sulfur speciation, where the maximum fresh weight, dry weight, and anthocyanin content of seeding kale increased by 115.52 %, 79.87 %, and 357 % compared with the control group. Moreover, co-hydrothermal treatment improved the bioavailability of sulfur. After applying hydrothermal sulfur, the sulfur content of kale increased by 18.65 % and 21.95 % compared to the experimental group treated with elemental sulfur and sulfate, respectively. The enhancement of hydrothermal sulfur on the kale growth was attributed to the improved sulfur utilization efficiency, the alleviated salt stress, the reduced plant pathogens, and the enriched plant growth-promoting microbes.