| The increasing concentration of greenhouse gases in the atmosphere, such as CH4and N2O has been paid much attention by people due to their substantial contribution to global warming, the severe impacts on the global climate. Agroecosytems are important sources of atmosphere greenhouse gases. Many studies about greenhouse gases in cropland based on conventional cropping regimes, while studies about greenhouse gases based on organic cropping regimes have been reported rarely. The study was carried out in rice and vegetable fields with static opaque chamber-gas chromatograph method to simultaneously measure CH4and N2O fluxes. The primary objectives were to examine the differences in characteristics and intensities of greenhouse gas emissions between conventional and organic cropping regimes, seeking for the suitable agriculture management and reducing greenhouse gases emissions in China.In this study, field experiments were accomplished over the rice-growing season in2007, while vegetable field experiment lasted from2009to2011. In2007, a split-plot experiment was conducted to study the effects of cropping regimes and water regimes in the rice-growing season on CH4and N2O emissions. Cropping regimes consisted of conventional and organic cropping regimes, while water regime in paddies consisted of the continous flooding (F) and the flooding-drainage-flooding (F-D-F). In2009-2010, an organic field experiment was conducted to investigate the effect of facilities on soil N2O emissions, and incubation experiments were conducted in laboratory to study nitrogen transformation. In2010-2011, we conducted conventional field experiment to invetagate the impact of environmental factors on CH4and N2O fluxes. Conbined the2009-2010and2020-2011field experiments, we explored the the difference in characteristics and intensities of greenhouse gases emissions between conventional and organic cropping regimes, and to evaluate the mitigation potential of the greenhouse gases emissions in organic cropping regime.The results of this study are presented as follows:1. Soil water status (water regime) was the factor controlling the seasonal pattern of CH4and N2O emission from rice paddies. Meanwhile, temperature (air and soil temperature) was another factor influencing CH4emission, but did’t influence N2O emission. The effects of temperature (air and soil temperature) and soil moisture on CH4emission were not significantly levels despite in greenhouse plots or in open-air plots. The N2O emissions were affected by soil moisture both in greenhouse plots and in open-air plots, the optimum soil moisture for N2O emissions in greenhouse plots was80.1%and74.6%for open-air plots.2. The CH4and N2O fluxes in rice paddies depended on the cropping regimes. Seasonal fluxes of CH4averaged4.44,2.14and1.75mg m-2h-1for the organic rice paddies plots under the water regimes of F, F-D-F and F-D-F-M, respectively. Relative to conventional rice paddies, organic cropping systems increased seasonal CH4emissions by20%,23%, and35%for the plots under the water regimes of F, F-D-F, and F-D-F-M, respectively. Under the water regimes of F-D-F and F-D-F-M, seasonal N2O-N emissions averaged10.85and13.66μg m-2h-1in organic rice paddies, respectively, which were significantly lower than those in conventional rice paddies. The net global warming potentials (GWP) of CH4and N2O emissions from organic rice paddies were significantly higher or comparable under various water regimes relative to conventional rice paddies. The greenhouse gas intensities were greater, while carbon efficiency ratios (CER) were lower in organic relative to conventional rice paddies. The results of this study suggest that organic cropping systems might not be an effective option for mitigating the combined climatic impacts from CH4and N2O in paddy rice production.3. The results of incubation experiments indicate that the mineralization, nitrification and denitrification of greenhouse were more intensive than that of open-air vegetable soil. The results of field show that the facility significantly affected soil N2O emissions (P=0.0015). Greenhouse facitlity increased soil N2O emissions relative to open-air. Fertilizer application increased soil N2O emissions in both greenhouse and open-air plots, fertilizer-induced emission factor for N2O was0.24%for open-air and0.20%for greenhouse.4. In the present study, both CH4and N2O emissions differed between the organic and conventional vegetable cropping systems. Over the whole annual cycle, the soils varied from being minor net sources of CH4in the organic cropping systems to small net sinks for atmospheric CH4in the conventional vegetable cropping systems. Fertilizer application increased N2O emissions both in the organic and conventional vegetable fields, but organic fertilizer-induced N2O emissions in organic vegetable cropping systems were much lower than those induced by synthetic N fertilizer in the conventional vegetable cropping systems. Under the conventional vegetable cropping regime, more soil CH4uptakes and N2O emissions were observed in the greenhouse vegetable regimes compared to the open-air vegetable cropping systems. Under the organic vegetable cropping regime, however, soil CH4and N2O emissions did not significantly differ between the open-air and greenhouse vegetable systems. The combined annual GWP of CH4and N2O emissions were lowest for the organic open-air vegetable control plots while highest for the conventional greenhouse vegetable fertilized plots. The results of this study suggest that the conversion of conventional to organic cropping regimes would benefit for mitigating global warming potentials of CH4and N2O emissions in vegetable fields, while greenhouse instead of open-air vegetable cropping systems would slightly increase their global warming potentials.5. We compiled measurement data of direct N2O emissions from vegetable fields published in peer-reviewed Chinese and English journals, processed data according to certain rules. Model building by available field N2O flux measurements data over or close to1-year indicated a linear relationship between N2O emission and the amount of fertilizer application and that the background N2O emissions from conventional vegetable fields were significantly greater than those estimates from Chinese staple cropping uplands or rice-upland rotation systems. Considering all N2O fluxes data, thereby, we estimated the emission factor of N2O, on average, to be0.59%in conventional vegetable fields. The annual background N2O emission was, on average,2.76kg N2O-N ha-1in vegetable fields. By adopting the emission factor and background emission of N2O in conventional vegetable fields, the present study estimated annual direct N2O emissions to be79.1Gg N2O-N yr-1in Chinese vegetable cropping systems in2008, contributed20-23%of the total N2O emission from croplands in China.Thus, the CH4and N2O fluxes in rice paddies depended on the rice cropping regimes. The net global warming potentials (GWP) of CH4and N2O emissions from organic rice paddies were significantly higher or comparable under various water regimes relative to conventional rice paddies. The greenhouse gas intensities were greater, CER were lower in organic relative to conventional rice paddies. The results of this study suggest that organic cropping systems might not be an effective option for mitigating the combined climatic impacts from CH4and N2O in paddy rice production. Secondly, the results of incubation experiments indicate that the mineralization, nitrification and denitrification of greenhouse were more intensive than that of open-air vegetable soil. Thirdly, both CH4and N2O emissions differed between the organic and conventional vegetable cropping systems. Over the whole annual cycle, the soils varied from being minor net sources of CH4in the organic cropping systems to small net sinks for atmospheric CH4in the conventional vegetable cropping systems. The combined annual GWP of CH4and N2O emissions were lower for organic vegetable plots, meanwhile, greenhouse instead of open-air vegetable cropping systems would slightly increase their global warming potentials. Finally, the present study estimated annual direct N2O emissions to be79.1Gg N2O-N yr-1in Chinese vegetable cropping systems in2008, contributed20-23%of the total N2O emission from croplands in China. |
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