Influence of manganese abundances on iron and arsenic solubility in rice paddy soil

Authors: MAGUFFIN, SCOTT - Southern Oregon University; ABU-ALI, LENA - Cornell University; TAPPERO, RYAN - Brookhaven National Laboratory; WOLL, ARTHUR - Cornell University; PENA, JASQUELIN - University Of Lausanne; Rohila, Jai; McClung, Anna; REID, MATTHEW - Cornell University

Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2020
Publication Date: 3/20/2020
Citation: Maguffin, S.C., Abu-Ali, L., Tappero, R., Woll, A., Pena, J., Rohila, J.S., McClung, A.M., Reid, M.C. 2020. Influence of manganese abundances on iron and arsenic solubility in rice paddy soil. Geochimica et Cosmochimica Acta. https://doi.org/10.1016/j.gca.2020.02.012.
DOI: https://doi.org/10.1016/j.gca.2020.02.012

Interpretive Summary: Alternate wetting-drying (AWD) is an attractive water saving irrigation management practice that involves wet- and dry- phases in the rice field. AWD is known to reduce accumulation of Arsenic (As), a toxic element, in rice grains. The accumulation of As in rice grain is known to be affected plant genotype and cultural management practices, however the contribution of biogeochemical factors in the rhizosphere (i.e. the soil around the roots) on As availability is critically lacking. When flooded paddy soils are subjected to a “drying phase” , the redox status [i. e., combination of reduction and oxidation processes] of the soil changes affecting the coupled biogeochemical cycling of manganese (Mn), iron (Fe) and As. Rice paddy soils are known to vary widely in Mn/Fe ratios which may have consequences on As mobility, but their role in controlling As availability to the plant for uptake is poorly known. Replicated field studies, lab experiments (synchrotron-based imaging and spectroscopy of soil thin sections, microcosm experiments, microbial biomass quantification via QPCR), and geochemical modeling approaches were used to understand the roles of Mn oxide minerals in controlling As mobilization in rice paddy soils with varying temporal redox potentials as a result of irrigation management. Further, the results were validated in lab experiments (microcosms) using same field soil, but varying Mn/Fe ratios created by amending the soil with synthetic birnessite (d-MnO2). Results from field experiments demonstrate that a single soil dry-down that reached up to -30 centibars was effective at reducing dissolved As in the soil solution (porewater) for about a month due to the near-complete re-oxidation of dissolved Fe and Mn to Fe- and Mn-oxides during the dry-down phase. Results from µXRF mapping and field porewater monitoring experiments suggested that adsorption of As to Mn oxides and subsequent release during microbial Mn reduction may have a very limited role in releasing As to porewater. Laboratory microcosm experiments indicated that higher initial Mn/Fe ratios in soil may inhibit Fe and As mobilization into porewater. Thus, lower Mn/Fe ratios in paddy soils may be of lesser importance for minimizing As solubility in porewater during dry down phases of AWD. Probably Mn is playing a role as a redox buffer delaying the onset of reducing conditions. Geochemical modeling suggested that pH increase, driven by microbial MnO2 reduction, may lead to precipitation of a Mn(II) arsenate phase along with siderite and mackinawite. This pH driven process may have a greater importance for minimizing As solubility in porewater. A key outcome of this study was improving our understanding of the importance of direct (i. e., Mn as an As oxidant and adsorbent) vs. indirect (i.e., Mn as a driver of redox or pH) processes through which Mn regulates As mobilization, and the implications for plant-availability of As in the context of rice paddy soils with variable abundances of Fe and Mn.

Technical Abstract: Arsenic (As) mobilization in rice paddy soils under fluctuating redox conditions is influenced by the biogeochemical cycling of redox sensitive elements such as iron (Fe) and manganese (Mn). While rice paddy soils are characterized by a wide range of Mn abundances and Mn/Fe ratios, the consequences of this variability on As mobility in paddy soils irrigated through alternate wetting and drying cycles have received little attention. In this contribution, we developed a complementary set of field and laboratory experiments designed to evaluate the impact of Mn on interconnected Fe and As solubilization in rice paddy soils experiencing wetting-drying cycles through controlled irrigation. Porewater monitoring and synchrotron-based imaging and spectroscopy of thin sections prepared from an Arkansas paddy soil confirmed that As release was primarily governed by reductive dissolution of Fe (oxy)hydroxide phases. Experiments with laboratory soil microcosms amended with the synthetic nanocrystalline Mn oxide, d-MnO2, showed that higher initial Mn/Fe inhibited Fe and As mobilization into porewater relative to unamended soil by up to 95% and 45%, respectively. Geochemical modeling suggests that pH increases driven by microbial MnO2 reduction, in conjunction with microbial Fe- and sulfate-reduction in carbonate-rich porewater, enhanced the precipitation of siderite (FeCO3(s)), mackinawite (FeS(s)), and potentially a Mn(II) arsenate phase. These secondary mineral phases played a greater role in controlling As solubilization than the role of Mn as a redox buffer delaying the onset of reducing conditions. Soil dry-downs in both field and laboratory experiments showed that alternate wetting and drying approaches with a single dry-down can be effective at reducing dissolved As concentrations in porewater through the oxidation of Fe. Differences in soil Mn/Fe ratios had no clear impact on the effectiveness of dry-downs as a strategy to reduce As mobilization.