title: Assessment of bioavailable organic phosphorus in tropical forest soils by organic acid extraction and phosphatase hydrolysis.
abstract: Soil organic phosphorus contributes to the nutrition of tropical trees, but is not accounted for in standard soil phosphorus tests. Plants and microbes can release organic anions to solubilize organic phosphorus from soil surfaces, and synthesize phosphatases to release inorganic phosphate from the solubilized compounds. We developed a procedure to estimate bioavailable organic phosphorus in tropical forest soils by simulating the secretion processes of organic acids and phosphatases. Five lowland tropical forest soils with contrasting properties (pH 4.4-6.1, total P 86-429 mg P kg- 1) were extracted with 2 mM citric acid (i.e., 10 μmol g- 1, approximating rhizosphere concentrations) adjusted to soil pH in a 4:1 solution to soil ratio for 1 h. Three phosphatase enzymes were then added to the soil extract to determine the forms of hydrolysable organic phosphorus. Total phosphorus extracted by the procedure ranged between 3.22 and 8.06 mg P kg- 1(mean 5.55 ± 0.42 mg P kg- 1), of which on average three quarters was unreactive phosphorus (i.e., organic phosphorus plus inorganic polyphosphate). Of the enzyme-hydrolysable unreactive phosphorus, 28% was simple phosphomonoesters hydrolyzed by phosphomonoesterase from bovine intestinal mucosa, a further 18% was phosphodiesters hydrolyzed by a combination of nuclease fromPenicillium citrinumand phosphomonoesterase, and the remaining 51% was hydrolyzed by a broad-spectrum phytase from wheat. We conclude that soil organic phosphorus can be solubilized and hydrolyzed by a combination of organic acids and phosphatase enzymes in lowland tropical forest soils, indicating that this pathway could make a significant contribution to biological phosphorus acquisition in tropical forests. Furthermore, we have developed a method that can be used to assess the bioavailability of this soil organic phosphorus.
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: Tegan
authors.initials: T
authors.lastname: Darch
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: Martin S A
authors.initials: MS
authors.lastname: Blackwell
authors.affiliation: School of Environment, Natural Resources and Geography, Environment Centre Wales, Deiniol Road, Bangor University, Bangor LL57 2UW, UK.
authors.forename: David
authors.initials: D
authors.lastname: Chadwick
authors.affiliation: Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Haygarth
authors.affiliation: Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK.
authors.forename: Jane M B
authors.initials: JM
authors.lastname: Hawkins
authors.affiliation: Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panama.
authors.forename: Benjamin L
authors.initials: BL
authors.lastname: Turner
keywords: (IP6), myo-inositol hexakisphosphate; (P), phosphorus; (RP), reactive phosphorus; (TP), total phosphorus; (UP), unreactive phosphorus; Bioavailable; Citric acid; Organic acid; Organic phosphorus; Phosphatase hydrolysis; Tropical