Robust silicon extraction method and cycling of arsenic in rice paddy soil

Date
2019
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University of Delaware
Abstract
Arsenic contamination is a great concern for many countries in the world due to its toxic effects and natural occurrence. Arsenic tends to accumulate to higher concentration in rice than in other cereals. Extensive research has been conducted to find ways to lower As in rice plants, and studies have found addition of Si into paddy soil increases plant Si and decreases the uptake of As. However, there are numerous methods to estimate plant-available Si and some have limitations when Si has been applied to soil. In addition, very few studies have examined the impact of Si addition on soil As pools. Because As is associated with poorly-crystalline Fe oxides, which are stabilized with increasing Si, it was hypothesized that Si addition would impact soil As pools by increasing the proportion of As associated with poorly-crystalline Fe oxides. This work describes research on 1) five Si extraction methods (1-h CaCl2, 4-h CaCl2, 16-h CaCl2, 1-h CH3COOH and 24-h PO4) and relation of soil-extractable Si to plant Si in rice straw and husk, and 2) pools of As, Fe and Mn from soil that was amended with different Si treatments. Samples were collected from a 3-year field study in which rice was grown in soil either under a range of soil redox (by manipulating flooding extent) or receiving different Si treatments (nonamended (Control), charred rice husk (Char), rice husk (Husk) or calcium silicate/silicic acid (Silicate)) under reducing conditions (i.e. continuously flooded). Results show that 1) rice straw better correlated with soil Si than rice husk regardless of soil-extraction method; 2) 16-h CaCl2 extraction had the highest correlation (R2=0.67) with straw Si followed by PO4 (R2=0.50), CH3COOH (R2=0.26) and 1-h CaCl2 (R2=0.13); 3) both CH3COOH and to a less extent PO4 extraction overestimated Si from silicate-amended soil while the 1-h CaCl2 extraction correlated poorly with the16-h CaCl2 method (R2=0.37); 4) 4-h CaCl2 extraction method correlated very well with the 16-h CaCl2 (R2=0.82) method and straw Si (R2=0.58); 5) Si addition had no significant effect on plant-available pools or As pool associated with poorly-crystalline Fe oxides (p>0.05); 6) time played an important role in As, Fe and Mn pools; 7) a significant positive correlation between organic matter and poorly-crystalline Fe oxides (R2=0.22, p=0.004); 8) a significant negative relationship between poorly-crystalline Mn oxides and the plant-available pool (R2=0.15, p=0.02). Data suggest that the 4-h CaCl2 extraction method is robust to assess plant-available Si in rice paddy soil regardless of agronomic conditions. Moreover, Si addition does not impact the soil As pool associated with poorly-crystalline Fe oxides in rice paddy soil. Thus, the previously described impact of Si addition increasing ferrihydrite on Fe plaque minerals seems to be localized to the rhizosphere.
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