  Global rice production systems face two opposing challenges: the need to
increase production to accommodate the world's growing population while
simultaneously reducing greenhouse gas (GHG) emissions. Adaptations to drainage
regimes are one of the most promising options for methane mitigation in rice
production. Whereas several studies have focused on mid-season drainage (MD) to
mitigate GHG emissions, early-season drainage (ED) varying in timing and
duration has not been extensively studied. However, such ED periods could
potentially be very effective since initial available Carbon levels (and
thereby the potential for methanogenesis) can be very high in paddy systems
with rice straw incorporation. This study tested the effectiveness of seven
drainage regimes varying in their timing and duration (combinations of ED and
MD) to mitigate methane (CH4) and nitrous oxide (N2O) emissions in a 101-day
growth chamber experiment. Emissions were considerably reduced by early-season
drainage compared to both conventional continuous flooding (CF) and the MD
drainage regime. The results suggest that ED + MD drainage may have the
potential to reduce methane emissions and yield-scaled global warming potential
by 85 to 90% compared to CF and by 75 to 77% compared to MD only. A combination
of (short or long) ED drainage and one MD drainage episode was found to be the
most effective in mitigating methane emissions without negatively affecting
yield. In particular, compared with CF, the long early-season drainage
treatments LE + SM and LE + LM significantly decreased yield scaled global
warming potential by 85% and 87% respectively. This was associated with carbon
being stabilised early in the season, thereby reducing available carbon for
methanogenesis. Overall nitrous oxide emissions were small and not
significantly affected by ED.
