Date of Award
Master of Science
Environmental and Soil Sciences
Amy M. Johnson
Mark Radosevich, Jennifer M. DeBruyn; Forbes Walker
Bioavailable phosphorus (P) has traditionally been measured as inorganic orthophosphate (PO4) while organic P (Po) has been considered of limited relevance to short-term biological consumption. However, enzymes secreted by bacteria and fungi which serve to mineralize Po are ubiquitous in the environment and may contribute significant bioavailable P over time. In order to assess environmentally-relevant, potentially bioavailable P in soils with added dairy wastes, microcosms of identical soil series but differing management histories and landscape positions were incubated at 24 C° for three weeks. Subsamples were taken weekly from microcosms and analyzed by: 1) Mehlich-3 extractant, 2) NaOH-EDTA subjected to sulfuric acid total digest, and 3) NaOH-EDTA subjected to enzymatic hydrolysis by one, two or three commercially-available phosphatase enzymes. A high throughput, microplate reader method (Johnson and Hill, 2010) was adapted to quantify the enzyme-hydrolysable P. There were significant microcosm changes (P <0.05), with three week 24 C° incubation, in Mehlich-3 extractable P, with soils having low total P (TP) showing decreases and soils in both upland and depositional field positions increasing in Mehlich-3 extractable P. Also, over three weeks of study, low TP control soils subjected to three enzymes had a 58% increase in MRP over initial MRP in NaOH-EDTA soil extracts, whereas dairy farm upland soils had a 9% increase and depositional soils had an increase of 3%. Soils with higher levels of TP had smaller amounts of enzymatically hydrolysable P than soils containing low levels of TP. This points to limitations of this enzyme hydrolysis method for assessing potentially available Po in soils with very high Mehlich-3 and TP (Mehlich-3 P: >120 mg/kg; total P: >1,200 mg/kg). In conclusion, NaOH-EDTA extracts a range of Po species that can be transformed in the environment into P available to aquatic algae, but results from the Johnson-Hill method differ widely, even in identical series soils, possibly depending on field position and animal waste additions leading to high copper, zinc and TP.
Stone, Themis Mahalia, "Effect of Landscape Position and Dairy Manure Addition on Bioavailable Forms of Soil Phosphorus Using Enzyme Hydrolysis. " Master's Thesis, University of Tennessee, 2011.