Cultivar-specific methane emissions and yield-scaled greenhouse gas intensity responses to transplanting date: a case study in South Korea

Climate change is shifting rice transplanting calendars, creating a need to identify transplanting schedules that reduce methane (CH 4 ) emissions without a statistically detected reduction in grain yield. We conducted a two-year field experiment in Anseong, South Korea, using two japonica rice cultivars, Koshihikari and Chamdream, transplanted on four dates between 10 May and 25 June under continuous flooding. Seasonal CH 4 emissions, CH 4 -derived global warming potential (GWP_CH 4 ), yield-scaled greenhouse gas intensity (GHGI_CH 4 ), grain yield, and cumulative emission dynamics were evaluated. Grain yield generally did not differ significantly among transplanting dates within each cultivar-year combination, except for Chamdream in 2023, for which the 10 May treatment produced a significantly lower yield than the later treatments. Seasonal CH 4 emissions were consistently higher in 2024 than in 2023, with treatment-specific increases ranging from approximately 1.3- to 2.5-fold, whereas the lowest GWP_CH 4 and GHGI_CH 4 values occurred at the mid-to-late June dates. Among treatments without a statistically detected yield reduction, the lowest observed GHGI_CH 4 was 28–45% lower than that of 10 May for Koshihikari and 54–65% lower for Chamdream. The selected date was 25 June for Koshihikari in both years and shifted from 10 June in 2023 to 25 June in 2024 for Chamdream. These two-year results identify a cultivar-specific mid-to-late June range under the tested conditions rather than establishing a universal optimum.