Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 23, 2004
Publication Date: July 1, 2004
Citation: Dao, T.H. 2004. Ligands and phytase hydrolysis of organic phosphorus in soils amended with dairy manure. Agronomy Journal. 96(4):1188-1195.
Interpretive Summary: Animal manures are major sources of organic phosphorus. In soils amended with animal manure, phytate and other organic P are added to the soil P pool, in addition to inorganic phosphorus. Desorption of soil soluble P plays an important role in plant nutrition, transfer to runoff, and potential environmental transport, particularly in manure nutrient-laden soils. This study was conducted with the objectives of developing a novel in situ soil phosphorus desorption and PHP procedures, and determining various fractions of potentially bioactive P in soil that are present as water-soluble and insoluble complexed P. Various sources and types of phosphatases exist and provide an opportunity to identify organic P forms. The LIGND-PHP method is simple to use; it measures desorbed uncomplexed and complexed soil P, yielding valuable insights into the distribution of bioactive P forms that are water-extractable. The simplicity of the LIGND-PHP procedure may increase the practicality of widespread measurements of composition and P bioavailability in soil to rapidly develop mitigation practices to detect soil conditions favorable to and prevent potential offsite discharges of bioactive P from high-P soils or soils amended with animal manure in watersheds with intensive animal agriculture near major surface water bodies and fragile estuary ecosystems. In long-term manure-amended soils under a permanent orchardgrass-red clover stand, the LIGND-PHP method showed that soils treated with large amounts of animal manure accumulated inorganic and organic P. While soil PHP concentrations continue to increase with manure loading, added organic P was increasingly unextractable by water. The use of an enzymatic method showed that the organic P substrates were biologically active. Thus, the practice of repeated applications of animal manure to permanent pastures and grasslands caused an accumulation of soil P in the near-surface zone, adding to the risks of potential mobilization of bioactive P and transfer to runoff water during rainfall.
Information about orthophosphate and organic phosphorus desorption in soil amended with animal manure is needed because the build up of P following repeated applications of manure on agricultural lands poses risks of potential offsite discharges of dissolved P. An enzymatic hydrolysis and soil P desorption study was conducted to quantify water-extractable uncomplexed and complexed P forms (i.e. bioactive P) in two Typic Hapludults as affected by organic ligands (LIGND) and PO4-P anion sinks. The effects of four LIGNDs on P desorption were determined to develop a novel LIGND-based phytase-hydrolyzable P (PHP) method to assess the impact of long-term dairy manure management on soil P storage in Christiana silt loam collected from an orchardgrass-red clover pasture. Extractability of soil inorganic P (LEP) increased with increasing LIGND concentrations in the order of CDTA = EDTA > DTPA >> oxalate. A large soil organic P fraction was hydrolyzed in situ by Aspergillus ficuum (Reichardt) Henn. phytase EC 126.96.36.199, corroborating the presence of myo-inositol phosphomonoesters that includes phytic acid in both soils. Adding CDTA and EDTA significantly increased the efficacy of EC 188.8.131.52 and that of the phytase-hydrolyzable P assay for measuring previously inaccessible complexed organic P. Soil LIGND-PHP recovered up 40.8 ± 0.2 % of total P as organic P. In long-term manure-amended soils, the LIGND-PHP method showed that repeated land applications caused a soil LEP buildup and an increasingly difficult to extract organic PHP fraction. The accumulation of LEP increases risks of potential mobilization of bioactive P in the top 10 cm of these soils.