Effect of Landscape Position and Dairy Manure Addition on Bioavailable Forms of Soil Phosphorus Using Enzyme Hydrolysis

Bioavailable phosphorus (P) has traditionally been measured as inorganic orthophosphate (PO₄) while organic P (P_o) has been considered of limited relevance to short-term biological consumption. However, enzymes secreted by bacteria and fungi which serve to mineralize P_o 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 P_o 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 P_o 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.