Mitigation of methane emissions from paddy fields by prolonging midseason drainage

In order to analyze the mitigation of methane (CH4) emissions and the global warming potentials (GWPs) of CH4 and nitrous oxide (N2O) emissions from paddy fields by modifying the adopted water-management technique, we conducted field experiments to measure the CH4 and N2O fluxes at nine sites across Japan. Over 2 years, we tested different water-management strategies such as prolonged midseason drainage (MD) in each site. The CH4 emission rates at each site varied considerably; the rates were dependent on the ratio of reductive and oxidative capacities of the fields. Seasonal CH4 emission was effectively reduced at most sites by prolonging MD beyond its conventional duration, especially at sites where organic matter was added to the soil before the cultivation. We attribute this result to the effective suppression of the CH4 emission peak that occurs early in the cultivation period. Despite the large variation in seasonal CH4 emissions among the sites, the rate of CH4 emission resulting from alternative water-management strategies relative to that resulting from conventional water-management strategies is highly dependent on the degree of drainage during the MD period. N2O emission at most sites, in terms of GWP-based CO2-equivalent, was much smaller than that of CH4 emission. Compared to conventional water-management strategies, the seasonal CH4 emissions and the net 100-year GWPs (CH4 + N2O) can be suppressed to 69.5 ± 3.4 (SE)% and 72.0 ± 3.1% while maintaining grain yields as high as 96.2 ± 2.0% by prolonging MD on average by employing the selected alternative water-management strategies that satisfied the following conditions: the percent of CH4 emission of alternative water-management strategies was less than 90% and the grain yield was greater than 85% relative to conventional water-management strategies.