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Rice is a staple food for nearly half of the world's population, but rice paddies constitute a major source of anthropogenic CH 4 emissions. Root exudates from growing rice plants are an important substrate for methane-producing microorganisms. Therefore, breeding efforts optimizing rice plant photosynthate allocation to grains, i.e., increasing harvest index (HI), are widely expected to reduce CH 4 emissions with higher yield. Here we show, by combining a series of experiments, meta-analyses and an expert survey, that the potential of CH 4 mitigation from rice paddies through HI improvement is in fact small. Whereas HI improvement reduced CH 4 emissions under continuously flooded (CF) irrigation, it did not affect CH 4 emissions in systems with intermittent irrigation (II). We estimate that future plant breeding efforts aimed at HI improvement to the theoretical maximum value will reduce CH 4 emissions in CF systems by 4.4%. However, CF systems currently make up only a small fraction of the total rice growing area (i.e., 27% of the Chinese rice paddy area). Thus, to achieve substantial CH 4 mitigation from rice agriculture, alternative plant breeding strategies may be needed, along with alternative management., (© 2018 John Wiley & Sons Ltd.)

Breeding high-yielding rice cultivars through increasing biomass is a key strategy to meet rising global food demands. Yet, increasing rice growth can stimulate methane (CH 4 ) emissions, exacerbating global climate change, as rice cultivation is a major source of this powerful greenhouse gas. Here, we show in a series of experiments that high-yielding rice cultivars actually reduce CH 4 emissions from typical paddy soils. Averaged across 33 rice cultivars, a biomass increase of 10% resulted in a 10.3% decrease in CH 4 emissions in a soil with a high carbon (C) content. Compared to a low-yielding cultivar, a high-yielding cultivar significantly increased root porosity and the abundance of methane-consuming microorganisms, suggesting that the larger and more porous root systems of high-yielding cultivars facilitated CH 4 oxidation by promoting O 2 transport to soils. Our results were further supported by a meta-analysis, showing that high-yielding rice cultivars strongly decrease CH 4 emissions from paddy soils with high organic C contents. Based on our results, increasing rice biomass by 10% could reduce annual CH 4 emissions from Chinese rice agriculture by 7.1%. Our findings suggest that modern rice breeding strategies for high-yielding cultivars can substantially mitigate paddy CH 4 emission in China and other rice growing regions., (© 2017 John Wiley & Sons Ltd.)

Crop straw and N fertilizer applications impact paddy rice yield and greenhouse gas (GHG) emissions. However, their interactive effects have not been well documented. This study investigated the effects of straw (S), no straw incorporation (NS), and three levels of N fertilization rates (N0, N1, and N2) on single rice (SR), double rice (DR), and rice-wheat (RW) cropping systems. Straw incorporation significantly increased total CH4 emissions by 118.6%, 8.0%, and 79.0% in the SR, DR, and RW, respectively, compared to the NS. The total GHG emissions in DR are significantly 72.6% and 83.5% higher than those in RW and SR, respectively. Compared to NS, straw incorporation significantly increased yield-scaled emissions by 27.8%, 15.0%, and 89.0% in SR, DR, and RW, respectively. Straw with N application significantly increased average rice yield over N1 and N2 by 39.4%, 50.0%, and 6.7% in SR, DR, and RW, respectively. There was a significant correlation between methyl coenzyme M reductase (mcrA) and CH4 emissions in rSR = 0.87 (p < 0.05) and rRW = 0.85 (p < 0.05), except in rDR = 0.06 (p > 0.05). This study scientifically supports straw incorporation combined with a moderate N application rate in rice-based cropping systems to maintain high rice yields and mitigate GHG emissions. [ABSTRACT FROM AUTHOR]

Straw incorporation promotes rice productivity and soil fertility. However, the effects of tillage practice with straw on GHG emissions in paddy fields are not well documented. Under three rice-based cropping systems of China (single rice, double rice and rice-wheat cropping systems), we investigated rice yield, CH4, N2O, area and yield-scaled emissions arising from different straw-incorporated tillage patterns. Tillage with straw affected rice yield by −6.8~3.2%, −9.1~9.0% and −9.8~2.1% in single rice, rice-wheat and double rice cropping systems respectively. Straw impacted CH4 emission but tillage influenced its impact irrespective of the rice cropping system. The highest CH4 emissions occurred in RedT + S, RoT + S and RoT + S under single rice, rice-wheat and double rice cropping systems respectively. Cumulative CH4 emission of PT + S decreased by 46.8% (p < 0.05) compared to RedT + S in the single cropping system, while under rice-wheat and double rice cropping systems, cumulative CH4 emission of PT + S decreased by 19.0% (p < 0.05) and 13.2% (p > 0.05) respectively compared with RoT + S. Lower methanogenic abundance of PT + S translated into the lowest cumulative CH4, area and yield scaled emissions in single rice and double rice cropping systems. To maintain high rice yield and reduce GHG emissions from straw incorporation, PT + S is recommended for a rice-based cropping system. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 (eCO2) generally increases carbon input in rice paddy soils and stimulates the growth of methane‐producing microorganisms. Therefore, eCO2 is widely expected to increase methane (CH4) emissions from rice agriculture, a major source of anthropogenic CH4. Agricultural practices strongly affect CH4 emissions from rice paddies as well, but whether these practices modulate effects of eCO2 is unclear. Here we show, by combining a series of experiments and meta‐analyses, that whereas eCO2 strongly increased CH4 emissions from paddies without straw incorporation, it tended to reduce CH4 emissions from paddy soils with straw incorporation. Our experiments also identified the microbial processes underlying these results: eCO2 increased methane‐consuming microorganisms more strongly in soils with straw incorporation than in soils without straw, with the opposite pattern for methane‐producing microorganisms. Accounting for the interaction between CO2 and straw management, we estimate that eCO2 increases global CH4 emissions from rice paddies by 3.7%, an order of magnitude lower than previous estimates. Our results suggest that the effect of eCO2 on CH4 emissions from rice paddies is smaller than previously thought and underline the need for judicious agricultural management to curb future CH4 emissions. [ABSTRACT FROM AUTHOR]

Both spikelet removal and low temperature stress largely reduced photosynthate allocation to wheat grain; the former stimulated the average N2O emissions by 31.3% and 33.2% under the field and pot conditions; the latter increased average N2O emission by 19.9% under pot condition. Spikelet removal and low temperature stress significantly reduced total plant N uptake, thereby increasing soil NO3–-N content, and increased root biomass. Our findings indicated that photosynthate allocation affects the soil N2O emissions from wheat fields through altering plant N uptake and root growth, and suggest that promoting photosynthate allocation to grain may not only benefit higher wheat yield but also mitigate greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

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]

Dense planting and less basal nitrogen (N) fertilization have been recommended to further increase rice ( Oryza sativa L.) grain yield and N use efficiency (NUE), respectively. The objective of this study was to evaluate the integrative impacts of dense planting with reduced basal N application (DR) on rice yield, NUE and greenhouse gas (GHG) emissions. Field experiments with one conventional sparse planting (CK) and four treatments of dense planting (increased seedlings per hill) with less basal N application were conducted in northeast China from 2012 to 2013. In addition, a two-factor experiment was conducted to isolate the effect of planting density and basal N rate on CH 4 emission in 2013. Our results show that an increase in planting density by about 50% with a correspondingly reduction in basal N rate by about 30% (DR1 and DR2) enhanced NUE by 14.3–50.6% and rice grain yield by 0.5–7.4% over CK. Meanwhile, DR1 and DR2 reduced GWP by 6.4–12.6% and yield-scaled GWP by 7.0–17.0% over CK. According to the two-factor experiment, soil CH 4 production and oxidation and CH 4 emission were not affected by planting density. However, reduced basal N rate decreased CH 4 emission due to it significantly reduced soil CH 4 production with a smaller reduction in soil CH 4 oxidation. The above results indicate that moderate dense planting with less basal N application might be an environment friendly mode for rice cropping for high yield and NUE with less GHG emissions. [ABSTRACT FROM AUTHOR]

Atmospheric CO 2 concentrations and water management practices both affect greenhouse gas (GHG) emissions from rice paddies, but interactive effects between these two factors are still unknown. Here, we show the first study to compare the impact of elevated atmospheric CO 2 (eCO 2) on GHG emissions under continuously flooded irrigation (CF) and under intermittently flooded (IF) conditions. Elevated CO 2 stimulated CH 4 emissions under CF by 50% in a field experiment and by 46% in a pot experiment, but it had no effect under IF in both experiments. Elevated CO 2 had no effect on N 2 O emissions in either the field or pot experiment. Rice root biomass, aboveground biomass and grain yield increased with eCO 2 , but were not affected by water management. Elevated CO 2 only stimulated the abundance of methanogens under CF, suggesting that increased soil O 2 availability with IF limited methanogenic activity under eCO 2. Our findings suggest that estimates of CH 4 emissions from global rice agriculture with eCO 2 need to account for recent changes in water management. • The first study to compare the effect of elevated CO 2 on GHG emissions under CF and IF. • Elevated CO 2 stimulated CH 4 emissions under CF, but had no effect under IF. • Elevated CO 2 did not affect N 2 O emissions under CF and IF. • Elevated CO 2 increased the abundance of methanogens under CF only. • Current estimates of CO 2 effects on CH 4 emissions from rice paddies may be too high. [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]

Organic amendment often affects both crop yield and greenhouse gas (GHG) emission. Yet the impact of organic amendment on yield gains and GHG emissions is still unclear, particularly in long-term soybean-wheat cropping system. Based on a 30-year soybean-wheat cropping system experiment, the impact of organic amendments (i.e. straw and manure) on soil properties, crop yield, N 2 O and CH 4 emissions, and net ecosystem economic budget (NEEB) were investigated. The treatments were: chemical NPK fertilizer (NPK), chemical NPK plus low amount of wheat straw (NPKLS), chemical NPK plus high amount of wheat straw (NPKHS), chemical NPK plus pig manure (NPKPM), chemical NPK plus cattle manure (NPKCM), and a control with no fertilizer applied (CK). The long-term straw and manure amendments enhanced soybean yield by 13.1% and 44.0%, and improved wheat yield by 6.4% and 9.9%, respectively. Meanwhile, they stimulated N 2 O emissions by 25.1% and 49.0% respectively, without significant effect on CH 4 emissions, compared to the chemical NPK fertilizer treatment (NPK). The results of structural equation model showed that soil properties tested explained 76% of the variations observed in N 2 O emissions, which were directly affected by soil nitrification capacity, pH and denitrification capacity. Although straw and manure amendments increased the total GHG emissions investigated by 25.7% and 48.6% respectively, they showed insignificant effects on the greenhouse gas intensity (GHGI), and increased NEEB by 8.6% and 21.1%, respectively, compared to the NPK treatment. This indicates that the benefits arising from crop yield improvement through the use of organic amendments can cover the carbon fees caused by increased GHG emissions. Our findings suggest that organic amendment can be an effective strategy to simultaneously increase crop yield while mitigating greenhouse gas emissions from the perspective of carbon fees. [Display omitted] • Organic amendments stimulated total GHG emissions of N 2 O and CH 4. • Soil properties tested explained 76% of the variation in N 2 O emissions. • Organic amendments improved yield without increasing the greenhouse gas intensity. • Straw and manure amendments increased the net ecosystem economic budget. [ABSTRACT FROM AUTHOR]

Abstract: Recently increasing studies suggest that a comprehensive assessment of cropping practices impacts on greenhouse gas (GHG) emissions at yield-scale will benefit cropping technique innovation and policy selection for higher-yield with less-emissions. In this paper, we conducted a meta-analysis to quantify the impacts of rice cropping practices on the global warming potential (GWP) of GHG emissions at yield-scale rather than area-scale in China. The results showed that the yield-scaled GWP of Chinese major rice cropping systems during rice growing season was in the order: double rice cropping system (1188.9kg CO2 equiv.Mg−1)>rice-upland crop rotation system (777.0kg CO2 equiv.Mg−1)>single rice cropping system (346.7kg CO2 equiv.Mg−1). Nitrogen fertilization (50–300kgNha−1) significantly increased rice yield with slight increments in the GWP of CH4 and N2O emissions, resulting in significant reductions in the yield-scaled GWP. The greatest reduction occurred at the application rate of 150–200kgNha−1 by 37% as compared to the non-fertilization control. For organic amendments, biogas residue application had no significant effect on the yield-scaled GWP, while manure application and straw recycling significantly increased the yield-scaled GWP by 54% and 154%, respectively. Intermittent irrigation significantly reduced yield-scaled GWP by 59% largely because of a significant reduction in CH4 emission and a significant increment in rice yield. No-tillage showed lower CH4 emission with similar rice yield in comparison with the conventional tillage, resulting in a reduction in yield-scaled GWP by 20%. These results indicate that there is a great potential to meet the new objective of higher-yield with less-GHG emissions through innovating rice cropping technique. More efforts should be paid on the field observations of GHG emissions during non-rice growing season, so as to perform a life-cycling assessment of GHG emissions from rice fields at yield scale under different cropping systems. [Copyright &y& Elsevier]