Relative movement and soil fixation of soluble organic and inorganic phosphorus

There is considerable concern about pollution of surface waters with P. Although most of the research has focused on inorganic P in surface runoff, it has recently become possible to easily follow the fate of soluble organic P forms in soils and waters. Two experiments were performed to compare the relative mobility and soil fixation affinity of orthophosphate monoesters, orthophosphate diesters, and soluble inorganic P. We used three P substrates, 4-methylumbelliferyl phosphate (MUP), deoxyribonucleic acid (DNA), and KH2PO4 in (i) a soil column experiment and (ii) a soil P adsorption test tube experiment. Shortly after columns were prepared, approximately two pore volumes of 0.005 M CaCl2 were passed through 25 cm length columns containing 10 cm of loamy sand amended with approximately 10 mg P as MUP, DNA, or KH2PO4 above 15 cm of nonamended loamy sand. The total net quantity of 757.8 microgram P 2L(-1) of orthophosphate diesters in the leachate from the DNA columns exceeded the net quantity of orthophosphate monoesters in leachate from the MUP columns (4.6 microgram P 2L(-1)) and soluble inorganic P from the KH2PO4 columns (34.0 microgram P 2L(-1)). Adsorption of soluble organic and inorganic P in the test tube experiment yielded similar results: DNA, containing orthophosphate diesters, had a relatively low affinity for soils. In both experiments, high concentrations of other P compounds were identified in samples treated with organic P substrates, suggesting enzymatic hydrolysis by native soil phosphatase enzymes. These findings indicate that repeated application of organic forms of P could lead to significant leaching of P to ground water.