Alternating wet-dry cycles rather than sulfate fertilization control pathways of methanogenesis and methane turnover in rice straw-amended paddy soil
<p>Alternate wet-drying (AWD) and sulfate fertilization have been considered as effective management practices for lowering CH<sub>4</sub> emissions from paddy soils. However, the effects of management practices on in situ belowground CH<sub>4</sub> turnover (production and oxidation) are not yet fully understood. Here, soil CO<sub>2</sub> and CH<sub>4</sub> 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. CH<sub>4</sub> concentration reached 51.0 mg C L<sup>-1</sup> at flowering stage under flooded conditions, while it decreased to 0.04 mg C L<sup>-1</sup> under AWD. Relative enrichment of &#948;<sup>13</sup>C in CH<sub>4</sub> and depletion of &#948;<sup>13</sup>C in CO<sub>2</sub> under AWD indicated CH<sub>4</sub> oxidation. Sulfate addition had no significant effect on CH<sub>4</sub> 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 CH<sub>4</sub> production. The &#948;<sup>13</sup>C-CO<sub>2</sub> enrichment over time (7 &#8240; and 5&#8240; with and without sulfate fertilizer, respectively) under flooded conditions likely indicates an increasing contribution of hydrogenotrophic methanogenesis to CH<sub>4</sub> production with ongoing rice growth. Overall, the results showed that AWD could more efficiently reduce CH<sub>4</sub> production than sulfate fertilization in rice-straw-amended paddy soils.</p><p>&#160;</p>