Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. David H. McNear Jr.

Abstract

Serpentine soils are formed from weathering of serpentinite, the ultramafic parent material that provides serpentine soils with their unique identity. Weathering of serpentinite results in a plethora of edaphic factors that impose strong selection pressures on plant life, with high magnesium (Mg) to calcium (Ca) ratio, low fertility, and high levels of geologically derived metals such as cobalt (Co), chromium (Cr), and nickel (Ni). Appropriately, plant life on serpentine soils is often specialized, sparse, and endemic which in turn affects the ecosystems that develop on serpentine soils from the landscape scale down to the physiology of their inhabitants. At the landscape scale, selection by serpentine edaphic factors may result in sharply delineated changes in plant community structure, resulting in abrupt changes in ground cover and habitat. At the physiological scale, plant adaptation to ultramafic soils is thought to have resulted in the emergence of heavy metal hyperaccumulation.

Interestingly, plant species closely related to hyperaccumulators may be found living adjacent to them on serpentine soils, but the opposite strategy for coping with elevated levels of heavy metals whereby heavy metals are excluded from the vascular system. Studying the development and differentiation of the rhizosphere of serpentine adapted plants may lead to a better understanding of how plants have evolved to cope with the varied abiotic stresses posed by, but not exclusive to, serpentine soils.

This research seeks to contribute to the understanding of hyperaccumulation by asking, what are the fundamental differences between the rhizospheres of hyperaccumulating and non-accumulating serpentine adapted plants, and could those differences be related to these contrasting strategies for inhabiting serpentine soils? In the research reported here, rhizospheres of Ni hyperaccumulating and non-hyperaccumulating plants growing in two serpentine soils, Neshaminy silt loam and Chrome silt loam, were interrogated using a multidisciplinary approach including x-ray absorption spectroscopy for in-situ Ni speciation, ratiometric fluorescent imaging for in-situ spatially resolved O2 concentrations and pH, and bacterial community structure via 16S rRNA gene amplicon sequencing and phospholipid fatty acid analysis. Overall, the results of this research suggest that Ni speciation in the rhizosphere of some Ni hyperaccumulators may contain a greater abundance of mineral sored species compared to a non-accumulator, hypertolerant plant. Oxygen content and pH did not differ among plant types and does not appear to explain differences in Ni speciation among rhizospheres. In Neshaminy silt loam soils, a clear differentiation in rhizosphere microbial community structure was observed, while in Chrome silt loams the differences were more subtle. Due to the lack of evidence for differing pH and redox environments across the rhizosphere of Ni hyperaccumulating and non-accumulating plants presented in this study and others, along with support for the differentiation of these environments along biological lines, it appears more likely that differences in Ni speciation and availability are driven by differences in root and microbial biochemistry.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.154

Funding Information

This research was supported by the National Science Foundation Graduate Research Fellowship Program from August 2015 - August 2018.

Available for download on Saturday, November 14, 2020

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