How Rice (Oryza sativa L.) Responds to Elevated As under Different Si-Rich Soil Amendments

Abstract

Several strategies exist to mitigate As impacts on rice and each has its set of trade-offs with respect to yield, inorganic As content in grain, and CH emissions. The addition of Si to paddy soil can decrease As uptake by rice but how rice will respond to elevated As when soil is amended with Si-rich materials is unresolved. Here, we evaluated yield impacts and grain As content and speciation in rice exposed to elevated As in response to different Si-rich soil amendments including rice husk, rice husk ash, and CaSiO in a pot study. We found that As-induced yield losses were alleviated by Husk amendment, partially alleviated by Ash amendment, and not affected by CaSiO amendment. Furthermore, Husk was the only tested Si-amendment to significantly decrease grain As concentrations. Husk amendment was likely effective at decreasing grain As and improving yield because it provided more plant-available Si, particularly during the reproductive and ripening phases. Both Husk and Ash provided K, which also played a role in yield improvement. This study demonstrates that while Si-rich amendments can affect rice uptake of As, the kinetics of Si dissolution and nutrient availability can also affect As uptake and toxicity in rice.

Si and Water Management Drives Changes in Fe and Mn Pools that Affect As Cycling and Uptake in Rice

“…As we hypothesized, soil Si amendments leading to the highest porewater Si (Figure 1) favored root plaque comprised of proportionally more ferrihydrite and consequently higher concentrations of As in root plaques (Figures 2-4). While the impact of increasing porewater Si on ferrihydrite stabilization in plaque has been previously reported with smaller data sets [34][35][36], here we report this phenomenon across studies, which suggests the finding is not an artifact of experimental design or dependent upon rice cultivar. Carefully controlled laboratory studies have illustrated that Si retards the transformation of ferrihydrite to more ordered phases [40,41,53], which supports the increase in ferrihydrite and decrease in lepidocrocite and goethite observed due to Si-rich amendment (Figure 2).…”

“…Straw As levels reflect conditions of these processes over a majority of the~4-month growth cycle of rice, whereas grain levels are more reflective of rhizosphere conditions during reproduction. For example, Teasley et al [35] showed that while both Silicate and Husk amendments increased porewater Si, the differences in Si solubility between the amendments resulted in Husk being better able to provide porewater Si at grain filling to compete with As for uptake and grain storage and therefore resulted in higher grain yield. Thus, while estimates of plant-available Si and As provide snapshots of conditions at harvest, it is important to consider the dynamic nature of the rhizosphere conditions over the growth cycle [31,62].…”

“…Data were synthesized from three previously published pot studies [34,35,38,46] in which rice was grown in soil amended with different forms of Si under differing soil flooding conditions. Soil contained either background levels of~5 mg/kg As (Low As, n = 18), this Low As soil spiked with an 80/20 mixture of As(V)/As(III) to achieve~25 mg/kg As (Spiked As, n = 16), or elevated levels of~16 mg/kg As (High As, n = 27).…”

“…iAs(III) adsorbs as an outer-sphere complex, which is more easily prone to desorption than iAs(V), which adsorbs as an inner sphere complex and thus more tightly bound. Moreover, the Fe plaque coatings on rice roots are discontinuous [32] and are comprised of a mixture of Fe phases [32][33][34][35][36], which change in response to solution chemistry [34][35][36][37]. Thus, the ability of Fe oxides in Fe plaque to retain As and lower its uptake into rice depends strongly on the biogeochemical conditions of the rice paddy, which affect the composition and quantity of Fe plaque.…”

Opportunities and Challenges for Dietary Arsenic Intervention

“…2016; Seyfferth et al. 2016; Teasley et al. 2017)] has great potential because silicon fertilizers are too expensive to be implemented at many smallholding farms, and, if used, would potentially drive up the price of rice.…”