Dairy manure-component effects on phosphorus release from sandy soils

Phosphorus (P) in heavily manure-amended soils can be labile even years or decades after manure input cease. Knowledge of manure-derived components and their associations with P is pertinent to nutrient management for sandy soils with minimum P sorbing capacity. The overall objective of this research was to understand the effects of manure-derived components such as Mg, Si, and dissolved organic carbon (DOC) on P solubility in manure-amended sandy soils. Soil samples (0--25 cm) from manure-impacted areas were collected from the Suwannee and Okeechobee Basins of Florida. The soil release of P, Ca and Mg was studied using repeated water extractions and 1.0 M NH4Cl extractions. Columns of the soils were leached with deionized water and leachate speciation was modeled using MINTEQ. Solid state assessments of dairy manure and manure-amended soils were done using x-ray diffraction, scanning electron microscopy, and elemental microanalysis. The inhibitory effects of Mg, Si, and manure-derived DOC on Ca-P crystallization were studied by incubating solutions with and without clay-sized solids for 20 weeks. Repeated water and ammonium chloride extractions and speciation of column leachates confirm that sparingly-soluble phases of P associated with Mg and Ca control P release from the manure-amended soils and maintain elevated P concentrations in soil solutions even years after abandonment of the dairies. Solid state assessments suggested Mg-P and Ca-P associations in dairy manure and manure-amended soils. Formation of the most stable Ca-P mineral, hydroxyapatite, was inhibited by Mg and/or DOC, but not Si, in dairy manure amended soils. Mg-P associations in manure and manure-amended soils could maintain elevated P solubility, and Mg in soil solution could inhibit formation of stable forms of Ca-P. Therefore, consideration of Mg and Ca is necessary to explain the nature of P in manure-amended soils. Preemptive dietary controls to maximize Ca-P and minimize Mg-P in manure would be a strategy to reduce P loss from these soils in the future. Application of Al-based water treatment residuals could minimize the release of P from the manure-amended soils. DOC inhibition of Ca-P precipitation and competitive effects on P sorption reduce prospects for stabilizing P reactions in heavily manure-amended soils.