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PHOSPHORUS MANAGEMENT

Ligands and Phytase Hydrolysis of Organic Phosphorus in Soils Amended with Dairy Manure

USDA-ARS, AMBL, BARC-East, Beltsville, MD 20705-2350

^* Corresponding author (thdao{at}anri.barc.usda.gov).

Received for publication December 15, 2003. Information about orthophosphate and organic phosphorus (P) desorption in soil amended with animal manure is needed because a build-up of P following repeated manure applications poses risks of offsite dissolved P discharges. An enzymatic hydrolysis and soil P desorption study was conducted to develop a novel ligand-based phytase-hydrolyzable P (PHP) method and quantify water-extractable uncomplexed and complexed P forms (i.e., bioactive P) in Unicorn and Christiana soils (Typic Hapludults). Extractability of inorganic P with ligands alone (LEP) increased from 2 and 22 mg kg^–1 (0.3 and 3.1 ± 0.16% total P) up to sevenfold with increasing ligand (LIGND) concentrations in the order of CDTA = EDTA > DTPA >> oxalate. The LIGNDs nullified the benefits of using an anion sink in soil P desorption measurements. Soil organic P was hydrolyzed in situ by Aspergillus ficuum (Reichardt) Henn. phytase EC 3.1.3.8, corroborating the presence of myo-inositol phosphomonoesters including IP6. Adding CDTA and EDTA increased phytases' efficacy and that of the PHP assay for measuring previously inaccessible complexed organic P; soil PHP increased from 2.3 and 6.4% to peak at 32.0 ± 0.7 and up to 40.8 ± 0.2% of total P in Unicorn and Christiana soils, respectively. In long-term manure-amended Christiana silt loam under orchardgrass–red clover stands, the LIGND-PHP method showed that repeated manure applications resulted in soil storage of unextractable complexed PHP and a buildup of inorganic LEP. Accumulation of LEP increases risks of mobilization of bioactive P in the top 10 cm of these soils.

Abbreviations: CDTA, 1,2-cyclohexane diamino-tetraacetate • DTPA, diethylenetriamine-N, N, N', N'', N''-pentaacetate • EDTA, ethylenediamine-N, N, N', N'-tetraacetate • LIGND, other, non-IP6 organic ligand • IP, myo-inositol phosphate monoesters • IP6, myo-inositol 1, 2, 3, 5/4, 6-hexakis dihydrogenphosphate • LEP, inorganic ligand-extractable P • PHP, phytase-hydrolyzable P • WEP, water-extractable P • ZVMA, zerovalent metal alloy

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