Estimation of total CH₄ emission from Japanese rice paddies using a new estimation method based on the DNDC-Rice simulation model

Methane (CH₄) is a greenhouse gas, and paddy fields are one of its main anthropogenic sources. In Japan, country-specific emission factors (EFs) have been applied since 2003 to estimate national-scale CH₄ emission from paddy field. However, these EFs did not consider the effects of factors that influence CH₄ emission (e.g., amount of organic C inputs, field drainage rate, climate) and can therefore produce estimates with high uncertainty. To improve the reliability of national-scale estimates, we revised the EFs based on simulations by the DeNitrification-DeComposition-Rice (DNDC-Rice) model in a previous study. Here, we estimated total CH₄ emission from paddy fields in Japan from 1990 to 2010 using these revised EFs and databases on independent variables that influence emission (organic C application rate, paddy area, proportions of paddy area for each drainage rate class and water management regime). CH₄ emission ranged from 323 to 455ktCyr^-1 (1.1 to 2.2 times the range of 206 to 285ktCyr^-1 calculated using previous EFs). Although our method may have overestimated CH₄ emissions, most of the abovementioned differences were presumably caused by underestimation by the previous method due to a lack of emission data from slow-drainage fields, lower organic C inputs than recent levels, neglect of regional climatic differences, and underestimation of the area of continuously flooded paddies. Our estimate (406ktC in 2000) was higher than that by the IPCC Tier 1 method (305ktC in 2000), presumably because regional variations in CH₄ emission rates are not accounted for by the Tier 1 method.