|Maguffin, Scott - Cornell University - New York|
|Abu-ali, Lena - Cornell University - New York|
|Woll, Arthur - Cornell University - New York|
|Smieska, Louisa - Cornell University - New York|
|Reid, Matthew - Cornell University - New York|
Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 12/14/2018
Publication Date: 12/14/2018
Citation: Maguffin, S.C., Abu-Ali, L., Rohila, J.S., McClung, A.M., Woll, A., Smieska, L., Reid, M.C. 2018. Periodic dry-downs affect interactions between manganese redox dynamics and arsenic mobility in rice paddy soils. American Geophysical Union Abstracts. Available: https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/421853.
Technical Abstract: Flooded rice paddy conditions promote arsenic (As) mobilization and subsequent uptake by plants via reductive dissolution of iron (Fe) and manganese (Mn) oxide minerals. Alternative irrigation methods like alternate wetting and drying (AWD) have been shown to be effective at reducing As concentrations in the soil solution and its accumulation in rice grains. The effectiveness of AWD practices in reducing As mobility is typically explained in the context of re-oxidation of Fe and the formation of As-sorbing Fe-oxide minerals during dry-down cycles of AWD, yet the importance of Mn-oxide redox processes in AWD is unexplored. Furthermore, the geochemical effects of intensity and frequency during AWD dry-down on As, Fe, and Mn redox transformations remains uncharacterized. Here, we present results from three field-scale studies conducted in experimental paddy fields at Dale Bumpers National Rice Research Center in Stuttgart, Arkansas, USA. Fields were maintained under: (i) continuously flooded conditions (control) and with (ii) one and (iii) two dry-down events. We combined monitoring of depth-resolved soil physical properties (redox potential, temperature, volumetric water content) through a soil sensor network, analysis of soil solution chemistry, and synchrotron-based micro X-ray fluorescence (µXRF) analysis of soil cores collected before, during, after dry-downs to probe the effects of dry-downs on Fe and Mn redox dynamics and As mobility. Dry-downs were effective at reducing soil solution As concentrations, and concomitant increases in redox potential and decreases in dissolved Fe and Mn support the hypothesis that the formation of Fe and/or Mn oxide phases was responsible for the decrease in As concentration. However, µXRF analysis revealed that the highest As concentrations are co-located with Mn and not Fe, in contrast to the accepted conceptual model that Fe oxides drive As immobilization. The role of Mn in As mobilization was further explored in birnessite-amended, simulated dry-down laboratory mesocosms and higher frequency pore water sampling. The integration of spatially- and temporally-resolved monitoring of soil in situ physical-chemical conditions with synchrotron-based X-ray fluorescence imaging analysis provides new insights into the effects of AWD on Mn-Fe-As soil redox processes.