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

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.