Soil microbial ecology and biogeochemical cycling of arsenic and iron in flooded rice paddies
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Rice is a critical food source for over half the world’s population, but rice quality and yield are threatened by arsenic (As) uptake into grains. Arsenic and iron (Fe) biogeochemical cycling control the speciation and amount of As taken up into rice grains, and are controlled in part by soil microbes. Therefore, it is critical to understand As and Fe biogeochemical cycling and the microbial ecology of involved organisms in flooded rice paddy pore waters. This dissertation focuses on understanding organisms involved with As methylation, Fe oxidation, and Fe reduction in flooded rice paddy soils. ☐ The first study presented in this dissertation investigates the influence of silicon (Si) addition to soils on As availability and resulting impacts on the microbial community of arsenic-methylating microbes. Addition of Si to soils has previously been reported to increase grain organic As while decreasing grain inorganic As, but the mechanism by which this occurs is not understood. We hypothesized that Si addition to soils would release inorganic As, spurring the microbial community of arsenic-methylating microbes. We found Si did not induce release of methylated As species, as has previously been debated in the literature. Instead, Si likely released inorganic As, but inorganic As increases did not influence the total abundance of microorganisms, or the abundance of organisms harboring arsM. When compared across a variety of soils, arsM abundance did not correspond with total soil As or porewater As, calling to question the function of arsM in microbial genomes. ☐ The second study presented in this dissertation investigates the influence of plant-based Si-amendments on the microbial community composition and the community composition of arsM-bearing organisms in a field study. We hypothesized that rice husk amendment would drive lower redox and higher organic carbon in soils, driving higher grain dimethylarsinic acid (DMA) and selecting for a distinct microbial community and community of arsM-bearing organisms compared to other treatments. We found that different Si amendments drove change in both the total microbial community and that of arsM-bearing organisms, but this did not correspond with differences in grain DMA. Instead, low redox, high porewater As, and high methane flux were correlated with high grain DMA. Together these results indicate that the composition of arsM-bearing organisms and the whole soil microbial community are less important factors than redox and arsenic availability in determining grain As speciation. In addition, methanogen activity may play an important role in As methylation in flooded rice paddies. ☐ The third study presented in this dissertation investigates the microbial community of the Fe plaque. The Fe plaque is an amalgamation of Fe-(oxyhydr)oxides at the root surface that may serve as a source or sink of As, but better understanding of the development of the plaque is needed. We hypothesized that Fe-oxidizing bacteria would be critical members of the plaque community. We found Fe-oxidizing bacteria and Fe-reducing bacteria were enriched and found in high abundance in the plaque over the entire course of the rice growing season, and that throughout the season the abundance of an Fe-oxidizer (Ferrigenium) and Fe-reducer (Anaeromyxobacter) correlated with the amount of plaque. This implies that Fe-cycling organisms are involved in plaque formation and suggests that cryptic Fe cycling occurs at the rice root surface. ☐ Rice As mitigation strategies are critical for improving the quality of rice and for ensuring high yields for this critical crop. This dissertation provides better understanding of As mitigation strategies by elucidating their underlying mechanistic processes.
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Keywords
Arsenic, Iron, Rice paddies, Soil, Microbial ecology, Biogeochemical cycling