Evaluating the GHG mitigation-potential of alternate wetting and drying in rice through life cycle assessment.

Alternate wetting and drying (AWD), has gained increasing attention as a promising strategy for mitigating greenhouse gas emissions (GHG) in flooded rice systems. AWD involves periodic drainage of rice paddies in order to inhibit methane (CH4) emissions. To date, studies evaluating this practice have been limited in their scope and resolution. Our study evaluates the mitigation potential of AWD from a life cycle perspective using high-resolution CH4 modeling to more accurately estimate the mitigation potential of this practice. We simulated California rice production under continuous flooding and under five AWD schedules ranging in the severity and frequency of dry-downs. Production models were coupled with the Peatland Ecosystem Photosynthesis Respiration and Methane Transport (PEPRMT) model to simulate CH4 fluxes at daily intervals. We then evaluated the GHG mitigation potential of AWD using life cycle assessment models. Frequent or severe dry-downs reduced simulated grain yields, which negated some of the benefits of AWD when assessed on a yield-scaled basis. We also found AWD-induced mitigation of CH4 emissions modeled with PEPRMT to be roughly half the magnitude reported from up-scaling of chamber measurements, highlighting the importance of high resolution field data to better characterize GHGs in rice systems. Reduced yields and conservative CH4 mitigation in our model lessened the overall mitigation potential of AWD. When the entire rice life cycle was considered, mitigation of overall global warming potential (GWP) was further reduced by the presence of additional GHG sources, which comprised roughly half of life cycle GWP. Our simulations resulted in ≤12% reductions in GWP kg-1 across all AWD scenarios and saw an increase in GWP when yields were severely reduced. Our results highlight the importance of constraining uncertainties in CH4 emissions and considering a life cycle perspective expressed on a yield-scaled basis in characterizing the mitigation potential of AWD.