Controlling variables and emission factors of methane from global rice fields

Jinyang Wang,Kazuyuki Yagi,Xiaoyuan Yan,Hiroko Akiyama

doi:10.5194/acp-18-10419-2018

Abstract

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.

Highlights

Atmospheric methane (CH4) is an important greenhouse gas (GHG), and its global mean concentration has increased by a factor of 2.5 since the pre-industrial era (Dlugokencky et al, 2011)
Empirical or mechanistic models are encouraged to be used for estimating CH4 emissions during rice cultivation, only a few countries such as China (CH4MOD; Huang et al, 2004), the US (DAYCENT; Cheng et al, 2014) and Japan (DNDC-Rice; Katayanagi et al, 2016) used this approach in their submitted national reports to the Conference of the Parties (UNFCCC, 2017)
We find that our estimate agrees with the overall average of 0.59 ± 0.35 kg CH4 ha−1 day−1 (± standard deviation, the length of the rice season was assumed to be 125 days), which was used for the CH4 emission inventory from Indian rice cultivation (MoEFCC of India, 2015)

Summary

Introduction

Atmospheric methane (CH4) is an important greenhouse gas (GHG), and its global mean concentration has increased by a factor of 2.5 since the pre-industrial era (Dlugokencky et al, 2011). The observed expansion of rice fields was the dominant factor for the increase of global CH4 emissions from rice cultivation (Fuller et al, 2011; Zhang et al, 2016). Owing to the increasing area of rice grown globally, the increase in CH4 emission is expected to continue in the near future (EPA, 2012; FAO, 2016). Over the last 3 decades, substantial progress has been made in estimating CH4 emissions from global rice fields, but large discrepancies in magnitude exist among various studies (range: 20.8 to 170 Tg CH4 yr−1; Cicerone and Oremland, 1988; EPA, 2012; Frankenberg, 2005; Neue et al, 1990; Yan et al, 2009). Previous studies have shown that the magnitude of estimated CH4 emissions from rice cultivation has followed a downward trend, suggesting that

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