Methanogenic Pathway and Fraction of CH4 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 CH4 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 (Fac ) and fraction of CH4 oxidized (Fox ) based on the isotopic data. Compared with flooding, drainage generally caused CH4, either anaerobically or aerobically produced, depleted in (13)C. There was no obvious difference between the two in transport fractionation factor (εtransport ) and δ(13)C-value of emitted CH4. CH4 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 Fac -value by 5-10%. On roots however, CH4 was mostly produced through H2/CO2 reduction (60-100%) over the season. CH4 oxidation mainly occurred in the first half of the season and roughly 10-90% of the CH4 was oxidized in the rhizosphere. Drainage increased Fox -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 CH4 was oxidized at the soil-water interface when CH4 in pore water was released into floodwater, although the amount of CH4 oxidized therein might be negligible relative to that in the rhizosphere. CH4 oxidation was also more important in the first half of the season in lab conditions and about 5-50% of the CH4 was oxidized in soil while almost 100% on roots. Drainage decreased Fox -value on roots by 15% as their CH4 oxidation potential was highly reduced. The findings suggest that water management in the winter fallow season substantially affects Fac in the soil and Fox in the rhizosphere and roots rather than Fac on roots and Fox at the soil-water interface.