Ph.D. Thesis/ Oral Defense_ Aria Duncan

Stanford University

*** Ph.D. Thesis/ Oral Defense ***

Unveiling Alternate Wetting and Drying Effects on Rice Production Under Future Climate Conditions

Aria Duncan

Friday, July 5, 9:00am

Green 365

Zoom Link is available upon request!

 Department of Earth System Science

Advisors: Scott Fendorf

 Rice is an important staple crop—essential for global food security. However, climate change threatens future rice production. Rising temperatures cause heat stress and alter soil biogeochemical processes exacerbating the toxic effects of widespread endemic soil arsenic and challenging future rice production. Alternate wetting and drying irrigation (AWD) has the potential to decrease grain arsenic, water use, and methane emissions, thus helping to mitigate the impacts of a changing climate. This dissertation experimentally explores the combined effects of AWD and climate change (rising temperatures and atmospheric CO2) on the soil-rice system. I assess the temporal dynamics of arsenic bioavailability in the rice root zone and find decreases in dissolved arsenic following drainage events. Differences in rice root arsenic exposure due to irrigation management led to a 32% decrease (p = 9.1 x 10-5) in grain total arsenic with AWD under current climate conditions (daily high of 33 ºC and 420 ppmv CO2). Under a severe warming scenario (+5 ºC and 850 ppmv CO2), grain arsenic decreased by 22% (p = 0.025) with AWD. I illustrate that AWD can partially compensate for climate-induced yield losses, despite evidence of abiotic stress. With conventional, continuously flooded irrigation, severe warming led to a 55% yield loss (p = 0.04); whereas, with AWD, warming led to a 9% yield loss (p = 0.70), which was not statistically significant. The combination of AWD and elevated CO2 also enhanced root system growth. Finally, I explore the spatial distribution of arsenic in and around rice roots using X-ray fluorescence mapping. Our findings reveal that both climate and irrigation strongly influence arsenic distribution in the root zone. Continuously flooded irrigation encourages arsenic enrichment on root surfaces, and AWD results in more uniformly distributed arsenic. This dissertation indicates that from rice yield to grain arsenic, AWD appears advantageous for rice production and consumption even under severe warming.