Li, Changsheng (Univ. of New Hampshire, Durham, NH, Complex Systems Research Center, Durham, NH, 03824; Phone: 603-862-1771; Fax: 603-862-0188; Email: changsheng.li@unh.edu)

Assessing Alternatives for Mitigating Net Greenhouse Gas Emissions and Increasing Yields from Rice Production in China Over the Next 20 Years

C. Li *, W. Salas, B. DeAngelo, S. Rose

Mitigation assessment of greenhouse gas (GHG) emissions from rice paddy production has typically been based on a limited series of field studies. However, extrapolating the GHG mitigation potential to watershed, province and national scales, while capturing heterogeneous conditions, requires the use of spatially explicit process models, like the DeNitrification and DeComposition (DNDC) model. DNDC is a unique soil biogeochemical model that simulates aerobic and anaerobic soil conditions, estimates crop yields based on a crop physiology-phenology model, and assesses the net effect of alternative management (mitigation strategies) on short- and long-term soil organic carbon (SOC) dynamics and emissions of N₂O, NO, CH₄, and NH₃ from both upland and wetland agricultural ecosystems. This paper quantifies the effect on net GHG emissions and rice yields for several mitigation alternatives, including changing water management, fertilizer practices, and utilization of rice straw. For each mitigation scenario, we ran DNDC for rice-involved cropping systems in China’s ~2500 counties over a 21-year period. Besides mid-season drainage, which has been widely adopted in China and was incorporated to various degrees in the baseline, shallow flooding (also known as marginal flooding) and upland rice scenarios significantly reduced national CH₄ emissions since the two practices universally depressed CH₄ fluxes across climate zones, soil types and crop rotation regimes. Applications of ammonium sulfate and off-season straw only slightly decreased national CH₄ emissions, although there were significant impacts under certain climate/soil/management conditions. All of the alternative practices except off-season straw amendment reduced national N₂O emissions. The N₂O reductions were caused by elevation of soil redox potential due to the practices of shallow flooding and upland rice as well as by alteration of soil N dynamics by applying sulfate fertilizer. The rice paddies in China will be a carbon sink in the coming 20 years, although the sequestration rates vary inter-annually due to the variation of predicted crop residue incorporation rate and soil organic matter storage level. Based on the net effects on CH₄, N₂O and CO₂, the order of GHG mitigation effectiveness is upland rice greater than shallow flooding greater than ammonium sulfate greater than off-season straw amendment. The order is not the same if only individual GHGs are considered. However, use of upland rice decreases yields relative to the baseline; the other mitigation options increase yields. DNDC’s GHG and yield results can be used as inputs for other analyses and models to assess the costs, potential market effects and adoption feasibility of the mitigation options.