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Giles, Courtney D.

Civil and Environmental Engineering

2012

PhD

Modern agricultural practices depend heavily on mineral phosphate fertilization to maintain food crop productivity world-wide. This has placed pressure on mineral phosphate reserves, with resource depletion expected within 30 years. Additionally, agricultural non-point source nutrient pollution has been exacerbated by high rates of agricultural phosphorus (P) application and loss through runoff and percolation processes. By improving crop access to native soil P, fertilizer application rates could be reduced and non-point source losses minimized. The dissertation work that will be discussed addresses biological organic P cycling by focusing on the most abundant (less than or equal to 50% total P) organic P compound in soils (i.e. myo-inositol hexakisphosphate; a.k.a., phytate, IHP). In order for plants to access the six phosphate groups in phytate, the compound must first be hydrolyzed to orthophosphate by phytases. This process is limited by low phytate and enzyme solubility resulting from sorption and precipitation reactions in the soil environment. Processes which increase the solubility of phytate in soils should therefore enhance biodegradation and plant utilization of this organic P form.
The dissertationis composed of six sections: (I) A review of inositol phosphate cycling in agricultural soils; (2) A study of organic P transport and cycling in leached, poultry manure amended soil columns; (3) An assessment of plant organic anion exudation and the influence of this process on the bioavailability of sorbed phytate to tobacco (Nicotiana tabacum); (4) The specific role of Pseudomonas spp. gluconic acid production on the bioavailability of precipitated phytate to tobacco; (5) Implications for agricultural nutrient management and (6) a summary of findings.
The results of the dissertation studies show that (1) preferential leaching of organic P from manure-amended soils and biodegradation of phytate occurs when soils are saturated in P, as often occurs in soils with a history of manure-amendment, (2) the bioavailability of sorbed phytate will depend on the unique organic anion and phytase exudation patterns of the model plant, (3) Pseudomonas organic acid production promotes calcium-phytate solubility and plant availability, but gluconic acid alone does not account for the observed plant growth promotion. In summary, the solubility of phytate plays a central role in its transport and cycling in soils. Organic anion-mediated solubilization of sorbed and precipitated phytate may therefore improve plant access to native soil P, or conversely, contribute to nutrient pollution at the field and watershed scale.