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 methods for lowering CH emissions from paddy soils. However, there is a clear knowledge gap between field studies that focus on the quantification of emissions and laboratory studies that investigate mechanisms. To elucidate mechanisms of CH production and oxidation under field conditions, rice was planted in straw-amended mesocosms with or without sulfate fertilization under continuously flooded conditions (FL) or two wet-dry cycles. CO and CH concentrations in soil air and their natural C isotope compositions were measured at stem elongation, booting, and flowering stages. CH concentration reached 51 mg C L at the flowering stage under FL, while it decreased to 0.04 mg C L under AWD. Relative C enrichment in CH and depletion in CO under AWD indicated CH oxidation. Ample organic substrate supply may have reduced competition between sulfate-reducing bacteria and methanogenic archaea, and therefore, it explains the absence of a decrease in CH concentrations in sulfate treatments. C enrichment in CO over time (6 and 7‰ with and without sulfate fertilizers, respectively) under FL indicates continuous contribution of hydrogenotrophic methanogenesis to CH production with ongoing rice growth. Overall, AWD could more efficiently reduce CH production than sulfate fertilization in rice straw-amended paddy soils.