Under flooded conditions, the competition between AsV and P in the rhizosphere deters P uptake and higher concentrations of AsV are absorbed by the plant. Immobilization of As in soils under oxic conditions reduces this competition and P bio availability increases, explaining the higher P concentrations in HS treatments. The nutritional value of rice grain is not typically considered when evaluating the impacts of water management treatments on As uptake, water use, and greenhouse gas emissions. However, potential changes in the grain nutritional content should not be ignored, as rice is a staple food for over half of the world’s population and a critical source of specific nutrients. In this study, in addition to decreasing As grain content, we reveal that II can increase Fe, P, and K levels in rice grain, providing another potential benefit to II management. We confirmed that a single dry down can be a suitable II treatment for minimizing As and Cd concentrations in grain. However, in other rice growing regions different field conditions would exist and II strategies may need to be modified to suit the specific location. To adapt water management regimes to meet specific environmental conditions, a better understanding of the soil and rhizosphere chemistry in rice paddies is crucial. Elucidating the biotic and abiotic changes occurring at the root soil interface and the mechanisms involved in As immobilization and Fe plaque formation, as well as how these are involved in the broader elemental cycling in rice systems,vertical hydroponics can bring us a step closer to defining how we can mitigate the ongoing problem of arsenic in rice while addressing human health concerns for one of the most consumed crops worldwide.

Rice is a staple food for more than half of the world’s population and is particularly susceptible to arsenic accumulation . Elevated As uptake occurs because rice is typically grown under flooded conditions where reduced As predominates and has high mobility and toxicity, leading to accumulation in rice grains and increasing the risk for As consumption by humans . Intermittent irrigation is a water management technique that involves intermittent flooding and draining cycles. Several studies reveal that II is an effective solution for reducing As concentrations in grain, methane gas emissions, and increasing water use efficiency . However, II treatments need additional study to determine their suitability for widespread application in rice paddies. Rice and many other aquatic plants transport oxygen to their roots via aerenchyma, creating an oxidized rhizosphere and resulting in the formation of an iron mineral plaque. This process occurs by oxidation of aqueous FeII present in anoxic soil solution after rice fields are flooded, and the precipitation of iron oxides and hydroxides on the root surface . Iron oxides are extensively studied in numerous disciplines because of their abundance in the environment and high reactivity; they play a vital role in many soil exchange processes. Additionally, Fe plaque formation and composition are affected by environmental factors, such as soil solution composition, pH and redox potential . Fe plaque can account for up to 14% of the dry weight of mature rice roots and can serve as a sink for As . Seyfferth et al., 2017, revealed that Fe and As co occur on the rice root surface. Other studies indicate that rice root iron plaque primarily retains arsenate and someAsIII .

In rice plants grown under continuous flooding, 95% of Fe plaque is composed of ferrihydrite and goethite , and, in lower proportions. lepidocrocite and siderite . The mineral composition of iron plaque is a result of specific biogeochemical factors at the root soil interface, which determine their crystallinity and surface affinity for adsorption of anions and cations . Ferrihydrite is an amorphous mineral that forms via hydrolysis of ferric iron with a large surface area, high adsorptive capacity and a positive charge under most soil conditions, serving as a sink for many oxyanion elements and organic compounds. Ferrihydrite gradually transforms into more thermodynamically stable and more crystalline FeIII oxides, such as goethite, via Ostwald ripening. . Moreover, intermittent irrigation treatments create fluctuating oxic and anoxic conditions in soil, which can alter the speciation of redox active elements at the root soil interface. Iron redox cycling affects the mobility and bio availability of As due to its high affinity for arsenite and arsenate. Oxidizing conditions in the soil can precipitate FeII into FeIII oxides and transform arsenic into AsV , while reducing conditions can lead to the dissolution of iron plaque, resulting in the release of FeII, AsIII and AsV present on root plaque, as well as reduction of AsV . Intermittent irrigation reduces arsenic mobilization by introducing oxic conditions in the soil during the growing season, oxidizing and immobilizing As in soil. Although it is well understood that with redox fluctuations the chemistry of both Fe and As will be affected, we find a gap in understanding how intermittent irrigation treatments of different timing and severity can impact the formation and mineralogy of iron oxides and thus their binding affinity to As. In addition, several authors have studied the sorption of AsIII and AsV on iron oxide surfaces with some evidence for specific binding mechanisms ; however, to our knowledge,hydroponic vertical farming systems a distinction of the binding interactions between AsV and root plaque iron oxides via insitu experiments to evaluate As sorption on the mineral surface have not been conducted.

In Chapter 1, our research demonstrated that II is a suitable strategy to reduce rice As uptake and translocation into the grain, but the chemical changes occurring in the rhizosphere, the mechanisms for As sequestration and the role of Fe plaque remain unclear. The objective of this study is to identify the changes in root plaque chemistry under intermittent irrigation treatments and to understand the binding interactions of synthesized iron oxide minerals present in root plaque with arsenate to facilitate an increased understanding of their impact on rice As uptake.The results from this study demonstrate that the mineralogy of rice root plaque is impacted by water management. Ferrihydrite, goethite, lepidocrocite, and side rite were detected in root plaque samples throughout the growing season, which is consistent with prior studies . In our XRD results, goethite was observed in all plaque samples at the late growth stages . Goethite formation typically follows a mineral ripening process where relatively amorphous and soluble minerals are transformed into a more thermodynamically stable and crystalline phase under oxidizing conditions . Additionally, ferrihydrite was first observed in the XRD spectrum corresponding to the CF treatment, as well as in II treatments at the late growth stages. Several factors can influence the mineralogy of rice root plaque, such as pH and soil solution chemistry. As pH decreases from 7 to 5, ferrihydrite is more likely to form . In the present study, during the 2018 growing season, soil pH increased from 6 to 7.5, making conditions favorable for ferrihydrite formation during the early stages of the growing season. Additionally, this pH range can increase the retention of arsenate by ferrihydrite than goethite . Moreover, Limmer et al., 2018 found that silicon concentrations during the formation of Fe plaque may play a role in its mineralogy. When the ratio of Si/Fe increases, ferrihydrite is more likely to form rather than lepidocrocite . Our SEM results demonstrate that Si and Fe do not co localize in root plaque for all samples. Higher amounts of Si in HS treatment in comparison to CF were found, indicating greater formation of ferrihydrite on roots of II treatments. Under flooded conditions, the transport of oxygen via aerenchyma to the rhizosphere drives the oxidation and precipitation of amorphous iron oxides which are subsequently transformed into goethite. Nevertheless, the identification of ferrihydrite and goethite at different times of the growing season indicates that mineral formation processes are impacted by II, which alters the mineralogy of plaque at different stages of rice growth. Iron plaque formation was studied by Long et al., 2019, revealing that Fe plaque formation occurs progressively; however, it is difficult to identify temporal trends in the mineralogy regarding II treatments when Fe oxide mineral formation begins, during the early stages of rice growth. XRD peak quantification and intensity measurements were limited by the presence of quartz in all samples. Attributed to its highly ordered crystalline structure, absolute intensities of quartz peaks in bulk mineralogy XRD analysis may cause extinction effects and reduce intensities of other mineral peaks, making identification of co existing minerals more challenging .

Additionally, XRD peaks do not often reflect the concentrations of all minerals present in the sample. In some cases, experimental errors or the lattice parameter d= nwill cause the peak positions to vary due to an angle shift. In addition, more crystalline minerals are easier to identify: ferrihydrite being highly amorphous is harder to identify in comparison to a more crystalline mineral such as goethite. For this reason, we focus our discussion on peak identification rather than concentration. Moreover, corresponding XRD peaks from samples collected at the later stages of rice growth, when root plaque mineral composition is less dynamic, are easier to identify in XRD spectra. Furthermore, Liu et al., 2006 found that ferrihydrite and goethite are the iron minerals present in the highest proportion in rice root plaques; therefore, it is critical to understand the binding interactions of these minerals with arsenic. Although the strong adsorption of AsV onto ferrihydrite and goethite has been demonstrated in earlier studies , there is uncertainty about the binding kinetics, and more studies are needed about how the sorption of arsenate compares between these minerals. Thus, insitu ATR FTIR experiments were carried out to elucidate binding mechanisms of AsV to these Fe oxides, which are dominant in the root plaque samples from this study. If specific binding mechanisms can be determined it would improve our understanding of the availability of arsenic for plant uptake under different rice paddy water management. Kinetic analysis results of FTIR spectra for AsV reacted with both goethite and ferrihydrite reveal an increase in peak area as a function of time, indicative of As sorption . These kinetic binding experiments also exhibit a cap on sorption sites of the surface of ferrihydrite based on the peak area change, showing a quick increase of sorption from 15 to 120 minutes. In contrast, for goethite, arsenate sorption increases slower and begins to plateau around 240 minutes. Additionally, we observe a difference in sorption when comparing the delta peak areas of ferrihydrite compared to goethite, being 8 fold higher for ferrihydrite and deducing that it has an increased ability to adsorb arsenate. It is logical to observe that ferrihydrite binds more AsV , as it is a poorly crystalline mineral and its amorphous characteristics are indicative of a greater binding surface and thus can lead to strong affinity for both arsenate and arsenite. These results are in agreement with Huang et al., 2011, which demonstrated higher sorption of iAs for ferrihydrite than goethite, explained by their differences in specific surface area and microporosity. Based on the observed aqueous AsV diagnostic peaks at 907 and 876 cm 1 , we identify distinct peak downshifts in all spectra. When AsV is reacted with goethite IRE coatings, one peak shifts to 871 cm 1 . For AsV reaction with ferrihydrite, a broad peak appears from 883 to 789 cm 1 . A downshift in the location of the FTIR peak for dissolved species upon reaction with a mineral surface is commonly attributed to inner sphere coordination . The reaction of arsenate with iron oxides has been previously studied via H+ /OHrelease stoichiometry titration experiments with strong evidence that arsenate is adsorbed on iron oxide surfaces by forming inner sphere bidentate binuclear complexes . Inner sphere binding, particularly bidentate, is substantially stronger than outer sphere binding and represents a species of Fe that is less bio available. Moreover, prior research has demonstrated that iron and arsenic co occur on the root surface of rice , which is consistent with the positive correlation between Fe and As measured in the DCB extracted plaque for all treatments in the current study .