The Impact of Ryegrass on a Fragipan Soil

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Theses and Dissertations- Plant and Soil Sciences


Fragipans are soil horizons which negatively impact row crop yields due to their cemented nature. Annual ryegrass has been proposed as a cover crop to help remediate these dense horizons. The objectives of this study were to (1) evaluate potential ryegrass root exudates and their ability to solubilize proposed fragipan cementing agents, aluminum (Al), iron (Fe), and silica (Si) in stirred-batch reactor studies; and (2) evaluate annual ryegrass’ ability to remediate the fragipan through an 8-week greenhouse growth study using non-destructive, mini-rhizon samplers installed on soil cores for in situ pore water sampling, and non-invasive soil porosity calculations using X-ray computed tomography (CT). Mineralogical characterization of the fragipan horizon revealed the presence of opal-CT as a possible source of Si with some evidence of goethite with possible Al-substitution as a source of Fe. Results from the stirred-batch studies indicated that the potential root exudate 3-(3,4-Dihydroxyphenyl)propionic acid (DHPPA) was more reactive than azelaic acid because of its ability to form chelated soluble complexes with Al or Fe, whereas Si remained largely unaffected. Analysis of soil solution from the soil columns showed that the columns planted with ryegrass had lower dissolved Al, increased dissolved Fe(II), and modified sulfate levels when compared with unplanted controls. CT analysis showed that these trends were correlated with greater porosity (in Marshall ryegrass cores) compared to the unplanted control column. These results help improve our understanding of the possible mechanisms responsible for fragipan decomposition when planted in ryegrass and will prove useful when assessing the possibility of making ryegrass a recommendation for producers to help remediate fields impacted by fragipans.


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