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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 emissions 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 emissions 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. • The impact of green manure crops on greenhouse gas emissions was studied. • Ry-R-R and Rp-R-R significantly increased CH 4 emissions by 116.73% and 52.45%, respectively. • Mv-R-R significantly increased rice yield by 15.4% and reduced CH 4 emission. • Mv-R-R is the most suitable cropping system for higher rice yield and lower CH 4 emission. [ABSTRACT FROM AUTHOR]

Highlights • A meta-analysis on GHG emissions and rice yield as affected by water management. • Non-continuous flooding reduced CH 4 emissions by 53%, but it increased N 2 O emissions by 105%. • Severe soil drying reduced rice yield, but mild soil drying did not. • Non-continuous flooding reduced GWP by 44% and yield-scaled GWP by 42%. • IPCC underestimates the impact of multiple (i.e. ≥ 2) drying events on CH 4 emissions. Abstract Rice is a main staple food for roughly half of the world's population, but rice agriculture is also a main source of anthropogenic greenhouse gas (GHG) emissions. Many studies have reported that water management (e.g. alternate wetting and drying, intermittent irrigation, mid-season drain, aerobic rice) affects rice yields and methane (CH 4) and nitrous oxide (N 2 O) emissions from rice paddies. However, these studies span a variety of practices and vary in experimental design and results, making it difficult to determine their global response from individual experiments. Here we conducted a meta-analysis using 201 paired observations from 52 studies to assess the effects of water management practices on GHG emissions and rice yield. Overall, compared to continuous flooding, non-continuous flooding practices reduced CH 4 emissions by 53%, increased N 2 O emissions by 105%, and decreased yield by 3.6%. Importantly, N 2 O emissions were low, contributing, on average, 12% to the combined global warming potential (GWP; CH 4 + N 2 O). As a result, non-continuous flooding reduced GWP (-44%) and yield-scaled GWP (-42%). However, non-continuous flooding practices stimulated N 2 O emissions to a greater degree in soils with high organic carbon or with manure additions. The reduction in CH 4 emissions increased with the number of drying events, soil drying severity, and the number of unflooded days. Currently, Intergovernmental Panel on Climate Change (IPCC) scaling factors for single and multiple (≥ 2) drying events are 0.6 and 0.52. Based on this analysis using actual side-by- side field studies, we suggest changing these to 0.67 for a single event and 0.36 for multiple events. [ABSTRACT FROM AUTHOR]

Mechanical planting is replacing manual transplanting in the major Asian rice cropping areas, however, few are known about the impacts of mechanical planting on rice yield and greenhouse gas (GHG) emission. Therefore, a two-year experiment was conducted to examine the impacts of dry direct-seeding (MDS), wet direct-seeding (MWS) and transplanting (MTP) in Jianghuai area, China. The results showed that CH 4 emission was significantly lower in the MDS than the MWS and MTP both with and without straw incorporation. No significant difference in N 2 O emission was found among the planting methods. Straw incorporation significantly stimulated CH 4 emission in the MWS and MTP, but not in the MDS. The lowest rice yield was found in the MWS, while there was no significant difference in grain yield between the MDS and MTP methods. The area-scaled emission of CH 4 and N 2 O in CO 2 -equivalent unit was respectively 78.5 and 89.6% lower in the MDS than those in the MWS and MTP without straw incorporation, and 87.7 and 94.1% lower with straw incorporation. The yield-scaled emission was correspondingly 80.4 and 88.1% lower without straw incorporation, and 89.0 and 93.4% lower with straw incorporation, respectively. Our results indicate that MDS can get an acceptable yield with a large reduction in GHGs emissions in the test location. [ABSTRACT FROM AUTHOR]