Alternating wet-dry cycles rather than sulfate fertilization control pathways of methanogenesis and methane turnover in rice straw-amended paddy soil

Alternate wet-drying (AWD) and sulfate fertilization have been considered as effective management practices for lowering CH₄ emissions from paddy soils. However, the effects of management practices on in situ belowground CH₄ turnover (production and oxidation) are not yet fully understood. Here, soil CO₂ and CH₄ concentrations and their C isotope compositions were measured at three rice growing stages in straw-amended paddy soils with and without sulfate fertilization under continuously flooded conditions and two wet-dry-cycles. CH₄ concentration reached 51.0 mg C L^-1 at flowering stage under flooded conditions, while it decreased to 0.04 mg C L^-1 under AWD. Relative enrichment of δ¹³C in CH₄ and depletion of δ¹³C in CO₂ under AWD indicated CH₄ oxidation. Sulfate addition had no significant effect on CH₄ concentration. The ample substrate supply might have prevented sulfate-reducing bacteria from out-competing methanogenic archaea and could therefore explain the absence of a fall in CH₄ production. The δ¹³C-CO₂ enrichment over time (7 ‰ and 5‰ with and without sulfate fertilizer, respectively) under flooded conditions likely indicates an increasing contribution of hydrogenotrophic methanogenesis to CH₄ production with ongoing rice growth. Overall, the results showed that AWD could more efficiently reduce CH₄ production than sulfate fertilization in rice-straw-amended paddy soils.