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The experiment was conducted to study the transformation of phosphorus (P) in slurry from pigs fed two different diets, a regular corn soybean meal diet with or without supplementation of 40% soybean hulls (SBH). The slurry was collected and treated either without aeration or continuous aeration at an airflow rate of 0.5 l/m3/s for 21 days. Two slurries and two treatments constituted a 2 2 factorial design. The slurry was sampled and measured on days 1, 2, 3, 4, 7, 14, and 21. Results indicated that the pH of aerated slurries was increased (P < 0.01) by 0.9 unit within 24 hours, but the pH of non-aerated slurries was unchanged. Aeration decreased total inorganic P, insoluble inorganic P, and soluble P, but increased organic P by approximately 163 mg/l. The average organic P in the slurry from both diets with aeration was about 34% higher than that in the same slurry without aeration. Aeration decreased insoluble inorganic P by about 10.6% and soluble P by about 4.2%. The mass balance of P fractions showed transformation of insoluble inorganic P into organic forms during the aeration stage. The insoluble inorganic P takes about 68% of the total P in the slurry, so it is essential to perform solid-liquid separation prior to aeration to enhance the efficiency of soluble P removal because insoluble inorganic P is mainly contained in the slurry solids.