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Previously, we found that organic amendments decreased P sorption that was related to changes in some soil chemical properties (e.g., pH and exchangeable Al). However, addition of organic residue to soils may also affect P sorption by adding PO4 (Pi) or releasing organic P (Po) during mineralization of residues and affect redistribution of P among P pools in soils. The objective of this study was to determine the effects of organic amendments (manure, alfalfa (Medicago sativa) and wheat straw (Triticum estivum) compared with inorganic amendments (CaCO3 and CaSO4) on P fractions after 28-day incubations. The five acid soils with high P fixation characteristics used in the study were Jory (clayey, mixed, mesic xeric Haplohumult) and Tolo (medial over loamy, mixed, frigid typic Vitrandept) from Oregon, and Mata (Sombrihumult), Kinigi (Placandept), and Kibeho (Paleudult) from Rwanda. Manure and alfalfa residues increased resin Pi and NaHCO3-Pi (biologically available) and NaHCO3-Po (readily mineralizable) and NaOH-Pi (chemisorbed) in all soils. The Tolo soil showed the least increase of chemisorbed Pi and the greatest increase of resin and NaHCO3-Pi, which corresponded to previous findings of lower P sorption by Tolo than by the other soils. Wheat straw residues had relatively little effect on P fractions as a consequence of its low total P content. In all soils, both NaOH-Pi (and to lesser extent NaOH-Po) increased with manure or alfalfa amendments. These fractions are sites for P sorption; thus, increases in these fractions provide evidence that P-rich organic amendments can decrease future P sorption in soils by reacting with these sorption sites. Calcium carbonate or CaSO4·2H2O had little or no effect on P distribution, indicating that redistribution of P fraction does not occur with reduction in exchangeable Al and Fe (CaSO4 and CaCO3) or change in pH (CaCO3).

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