Controlling variables and emission factors of methane from global rice fields

Abstract. Rice cultivation has long been known as one of the dominant anthropogenic contributors to methane (CH₄) emissions, yet there is still uncertainty when estimating its emissions at the global or regional scale. An increasing number of rice field measurements have been conducted globally, which allow us to reassess the major variables controlling CH₄ emissions and develop region- and country-specific emission factors (EFs). The results of our statistical analysis show that the CH₄ flux from rice fields was closely related to organic amendments, the water regime during and before the rice-growing season, soil properties and agroecological conditions. The average CH₄ fluxes from fields with single and multiple drainage were 71 % and 55 % that of continuously flooded rice fields. The CH₄ flux from fields that were flooded in the previous season were 2.4 and 2.7 times that of fields previously drained for a short and long season, respectively. Rice straw applied at 6 t ha⁻¹ in the preseason can decrease CH₄ emissions by half when compared to that applied shortly before rice transplanting. The global default EF was estimated to be 1.19 kg CH₄ ha⁻¹ day⁻¹ with a 95 % confidence interval of 0.80 to 1.76 kg CH₄ ha⁻¹ day⁻¹ for continuously flooded rice fields without organic amendment and with a preseason water status of short drainage. The lower EFs were found in countries from South Asia (0.85 kg CH₄ ha⁻¹ day⁻¹) and North America (0.65 kg CH₄ ha⁻¹ day⁻¹) relative to other regions, indicative of geographical variations at sub-regional and country levels. In conclusion, these findings can provide a sound basis for developing national inventories and mitigation strategies of CH₄ emission from rice fields.