Stimulating sulfate reducing bacteria by sulfate-based amendments to reduce methane emissions and immobilize arsenic in paddy soils

Anaerobic paddy soils are a major source of methane (CH₄) emissions. When the soil is contaminated with arsenic (As), the reducing environments favor the predominance of the highly mobile and toxic As(III). This indicates that flooded paddy soils can simultaneously pose risks associated with greenhouse gas emissions and As toxicity. Sulfate-reducing bacteria (SRB) compete with methanogens for electron donors, while the sulfide produced during sulfate reduction can react with As to form stable As-sulfide phases. Therefore, this study aimed to evaluate the simultaneous mitigation of CH₄ emissions and immobilization of As in paddy soils by applying sulfate-based amendments, including ammonium sulfate (NS), iron sulfate (FS), or potassium sulfate (KS). Ten grams of soil were treated with 0.3% NS, FS or KS, with untreated soil used as a control. The soil was amended with either 30 mL of 1000 mg/L deionized water or As(V) solution, and incubated under N₂-purged anaerobic conditions at 25°C in the dark. Greenhouse gas emissions were monitored and As concentration and speciation in soil solution were analyzed. Ammonium sulfate, FS, and KS reduced CH₄ emissions by 95.6%, 81.4%, and 94.2%, respectively, compared with the control under deionized water conditions, while under As solution conditions, CH₄ reduction reached 100% for NS and KS, and 21.3% for FS. At the same time, total As immobilization increased by 26.9%, 31.2%, and 7.23% compared with the control, and the reduction of As(V) to As(III) was suppressed by 55.0%, 19.3%, and 100% in the NS, FS, and KS treatments, respectively. Iron sulfate was less effective at CH₄ mitigation than NS and KS because Fe acted as an additional electron donor, enhancing methanogenesis. In addition, FS induced the strongest reduction of As(V) to As(III) through active Fe redox reactions, but showed the highest total As immobilization. Dehydrogenase activity followed the order NS > FS > KS under both deionized water and As solution conditions, likely because NS supplies both sulfate and readily available nitrogen, strongly stimulating microbial metabolic activity. Overall, sulfate-based amendments effectively suppressed CH₄ emissions and enhanced As immobilization in paddy soils by stimulating SRB-driven processes. These results indicate that sulfate-based amendments represent a promising strategy for simultaneously mitigating greenhouse gas emissions and As risks in rice paddy systems.

 

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2024-00414790).