Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Greenhouse gas emissions from a rice-rice-green manure cropping system in South China Abdulkareem Raheem a , Jun Zhang a , Jing Huang b,c , Yu Jiang a,d , Mohammad Abubakar Siddik a , Aixing Deng a , Jusheng Gao b,c, ⁎ , Weijian Zhang a, ⁎ a Institute of Crop Sciences, Chinese Academy of Agricultural Sciences/Key Laboratory of Crop Physiology & Ecology, Beijing 100081, PR China b Key Laboratory of Plant Nutrition and Fertilizer, Ministry of Agriculture/Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, PR China c Red Soil Experimental Station in Hengyang, Chinese Academy of Agricultural Sciences, Qiyang 426182, PR China d Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4 RJ, UK ARTICLE INFO Handling Editor: David Laird Keywords: Green manure Rice Cropping system Methane Nitrous oxide Global warming potential ABSTRACT Green manure crops play an important role in reducing chemical fertilizer applications while enhancing soil fertility. However, the effects of winter-planted green manure crops on rice paddy greenhouse gas (GHG) emissions are poorly understood. Based on a 35 years experiment, we analyzed the impacts of a rice-rice-green manure cropping system on rice yield and GHG emissions during both rice and winter-green manure seasons. Our experiment consisted of four treatments: ryegrass-rice-rice (Ry-R-R), milkvetch rice-rice (Mv-R-R), rapeseed- rice-rice (Rp-R-R) and fallow-rice-rice (Fa-R-R), arranged in a completely randomized design. Incorporation of winter-planted green manure had more impacts on yield and CH 4 emission in the early rice season than in the late rice season. Compared to the Fa-R-R, the Mv-R-R, Rp-R-R, and Ry-R-R systems increased annual yield by 15.4%, 11.8%, and 6.1%, respectively. However, Ry-R-R and Rp-R-R significantly increased annual CH 4 emis- sions by 116.73% and 52.45%, respectively, while Mv-R-R reduced CH 4 emissions by 16.3%, compared to Fa-R- R. The CH 4 emissions showed significant positive correlations with dissolved organic carbon, as well as the biomass and C/N ratio of the incorporated green manure plant materials. Although green manuring reduced N 2 O emissions in both winter and rice seasons, there was no significant difference in annual cumulative N 2 O emis- sions among the treatments. Ry-R-R and Rp-R-R increased annual yield-scaled GWP by 102.6% and 34.8%, respectively, while Mv-R-R was 27.8% lower, compared to the Fa-R-R. Our results suggest that Mv-R-R is the best choice of cropping system for yield sustainability and eco-friendly rice production over the long-term. 1. Introduction The use of synthetic fertilizer has been instrumental in increasing food production. Without synthetic fertilizers, the food output of agri- cultural systems would be halved (Smil, 2001) and a significant pro- portion of the world's population would go hungry (Springmann et al., 2018). However, its sustainability has been questioned in recent times. Barely half of the quantities of the synthetic fertilizers applied to croplands are utilized by crops (Oenema et al., 2009) while the rest is an environmental liability that disrupt intricate balances in ecosystem functions and biogeochemical cycles. The environmental issues asso- ciated with synthetic fertilizer use, such as: soil acidification, eu- trophication, underground water pollution, degradation of soil quality and yield stagnation (Bashir et al., 2013; Raut et al., 2012; Rossi et al., 2015; Teng et al., 2014), have prompted the call for sustainable and eco-friendly management practices. Thus, agronomic practice innova- tions that promote improved nitrogen use efficiency (Khan et al., 2006; Yousaf et al., 2016), integrated soil fertility management (Gao et al., 2015; Wolie and Admassu, 2016) and lower greenhouse gas emissions (Deng et al., 2016) are key to revolutionizing global food production systems. The use of green manuring to mitigate the negative effects of syn- thetic fertilizers in agricultural systems cannot be overlooked. Green manuring, which is common in organic agriculture, involves the growing of cover crops usually grasses or legumes but may also include other plants such as rapeseed and Azolla (Choi et al., 2014; Lumpkin and Plucknett, 1980). Typically, green manure cover crops are left on the soil, as mulch in the case of conservation agriculture (Jordan, https://doi.org/10.1016/j.geoderma.2019.07.007 Received 23 February 2019; Received in revised form 29 June 2019; Accepted 5 July 2019 ⁎ Corresponding authors at: Institute of Crop Science, Chinese Academy of Agricultural Science, Beijing 100081, PR China. E-mail address: zhangweijian@caas.cn (W. Zhang). Geoderma 353 (2019) 331–339 0016-7061/ © 2019 Elsevier B.V. All rights reserved. T