SOIL CARBON DYNAMICS AND GREENHOUSE GAS EMISSIONS IN CONSERVATION TILLAGE SYSTEMS AT MULTIPLE SCALES

Conservation tillage practices like no-tillage and reduced tillage have been widely implemented worldwide, with expectations they would provide multiple benefits (e.g., yield enhancement and soil carbon sequestration) for food security and climate adaptation and mitigation. However, the adoption of conservation tillage faces both opportunities and challenges. A knowledge gap still exists regarding the effects of conservation tillage on the carbon cycle in agroecosystems. This dissertation reflects a comprehensive evaluation of conservation tillage at multiple scales using an integrated systems approach, a combination of data synthesis, the agriculture ecosystem model, and field observations and measurements. I first conducted a meta-analysis to assess the effects of no-tillage (one widespread conservation tillage) on crop yield, greenhouse gas (i.e., CO2, CH4, and N2O) emissions, and the global warming potential of major cereal cropping systems in the world. Compared to conventional tillage, no-tillage reduced greenhouse gas emissions and increased crop yield in dry climate conditions. It reduced the global warming potential at sites with acidic soils. Considering the crucial role of soil organic carbon in providing ecosystem services, I further analyzed conservation tillage effects on soil carbon sequestration and the environmental controlling factors. Based on the meta-analysis review, I developed a conceptual tillage module accordingly and integrated it into a process-based agroecosystem model, the DLEM-Ag. At a long-term tillage experiment site in Lexington, KY, the improved model captured the changes and trends in soil organic carbon under different tillage treatments during 1970-2018, with no-tillage retaining more soil carbon than moldboard plow. Model factorial analyses revealed that this was mainly due to the lower CO2 emissions in no-tillage than in the moldboard plow treatments. Then, I expanded the simulation to the maize and soybean croplands in Kentucky to explore the conservation tillage effects on greenhouse gas emissions at the regional scale. Sensitivity analyses showed that, compared to conventional tillage, no-tillage significantly reduced CO2 and N2O emissions in both croplands. Lastly, the effects of conservation tillage on the coupled carbon and water cycles at the Ohio River Basin were examined using the improved DLEM-Ag model. Simulation results suggested higher crop water productivity in maize and soybean croplands under conservation tillage than under conventional tillage at the basin level. This dissertation is based on and adapted from three articles recently published in peer-review journals and two manuscripts prepared for publication