Integrated and Improved Assessment of Cropland Greenhouse Gas Emissions

Cropland is one of the major greenhouse gas (GHG) emitters contributing to global warming. The integrated effects on net greenhouse gas budget (NGHGB, including non-CO2 GHG emissions and SOC sequestration) and grain yield from different management practices remain poorly defined and varies with environmental and management conditions. In addition, the effect of inclusion of non-growing season emissions on N2O emission factors (EF, percentage of N2O-N emissions arising from applied fertilizer N) has received little attention. This thesis conducted global meta-analyses to a assess the integrated effect of management practices based on 347 observation sets of non-CO2 GHG (CH4 and N2O) emissions and grain yield, and 412 observations of soil organic carbon sequestration rate (SOCSR). In addition, we also conducted a global meta-analysis of the difference in EF between whole-year and growing-season based on 123 observations to assess the magnitude of  EF in response to management-related and environmental factors. Results show that for paddy rice, replacing synthetic nitrogen at the rate of 30 59% with organic fertilizer significantly decreased net GHG emissions while improving rice yield. Intermittent irrigation significantly decreased rice yield and increased net GHG emissions, mostly by the reduction in SOC sequestration especially in alkaline soils. Straw return for paddy rice significantly increased GHG emissions in silt-loam soils, where the increase in CH4 emissions outweighed the increase in SOCSR. For upland cropping systems, mostly by enhancing SOC sequestration, straw return and no-tillage were more effective for GHG mitigation in warm climates, while maintaining yields. For the effect of the inclusion of non-growing season emissions on N2O EF, results indicate that EFs based on whole-year N2O emissions were significantly greater than those based on growing-season emissions for most crop types. Vegetables showed the largest difference in EF among all crops, followed by paddy rice. A higher difference was also identified in areas with rainfall ≥600 mm yr 1, soil with organic carbon ≥1.3% and acidic soils. Neglecting emissions from the non-growing season may underestimate the EF by 30% for paddy fields, almost three times that for non-vegetable upland crops. This thesis highlights the importance of carefully managing croplands to sequester soil organic carbon without sacrificing yield, while limiting CH4 emissions, especially from rice paddies, and shows the importance of the inclusion of the non-growing season in the measurements of N2O fluxes, the compilation of national inventories and the design of mitigation strategies.