Methanogenic Pathway and Fraction of CH(4) Oxidized in Paddy Fields: Seasonal Variation and Effect of Water Management in Winter Fallow Season

A 2-year field and incubation experiment was conducted to investigate δ(13)C during the processes of CH(4) emission from the fields subjected to two water managements (flooding and drainage) in the winter fallow season, and further to estimate relative contribution of acetate to total methanogenesis (F(ac)) and fraction of CH(4) oxidized (F(ox)) based on the isotopic data. Compared with flooding, drainage generally caused CH(4), either anaerobically or aerobically produced, depleted in (13)C. There was no obvious difference between the two in transport fractionation factor (ε(transport)) and δ(13)C-value of emitted CH(4). CH(4) emission was negatively related to its δ(13)C-value in seasonal variation (P<0.01). Acetate-dependent methanogenesis in soil was dominant (60–70%) in the late season, while drainage decreased F(ac)-value by 5–10%. On roots however, CH(4) was mostly produced through H(2)/CO(2) reduction (60–100%) over the season. CH(4) oxidation mainly occurred in the first half of the season and roughly 10–90% of the CH(4) was oxidized in the rhizosphere. Drainage increased F(ox)-value by 5–15%, which is possibly attributed to a significant decrease in production while no simultaneous decrease in oxidation. Around 30–70% of the CH(4) was oxidized at the soil-water interface when CH(4) in pore water was released into floodwater, although the amount of CH(4) oxidized therein might be negligible relative to that in the rhizosphere. CH(4) oxidation was also more important in the first half of the season in lab conditions and about 5–50% of the CH(4) was oxidized in soil while almost 100% on roots. Drainage decreased F(ox)-value on roots by 15% as their CH(4) oxidation potential was highly reduced. The findings suggest that water management in the winter fallow season substantially affects F(ac) in the soil and F(ox) in the rhizosphere and roots rather than F(ac) on roots and F(ox) at the soil-water interface.