Effects of different soil moisture regimes and P application on available and inorganic P forms were studied in four paddy soils, north of Iran. A factorial experiment in a completely randomized design with two replications was performed in a laboratory study. Factors of soil at four levels (two acid and two alkaline-calcareous soils), soil moisture contents at three different levels (air-dry, 40% Saturation Percentage (SP), and submerged) as well as P fertilizer at two levels (with and without P fertilizer application). Phosphorus fertilizer was added at a rate of 40 mg P per kg of soil as monocalcium phosphate. The treatments were incubated for one month at 25?C and then analyzed for P fractions employing the method of Jiang and Gu (1989) for calcareous soils and of Kuo (1996) for non-calcareous soils. Available P, was determined through Olsen method. The results indicated that the level of available P, in all soils, increased on the average more than two times following submergence. In acidic soils, the level of available P increased more than that in alkaline-calcareous soils following submergence. After one month of incubation at 40% SP and with no P application, in three out of four soils, the available-P increased, while in one soil it did not change significantly. The average recovery of added P under submerged conditions was lower than that under unsubmerged conditions. The average recovery of added P in alkaline-calcareous soils was higher than that in acidic soils. Effects of soil moisture regimes and P fertilizer on all inorganic P fractions were significant in all soils studied. Higher levels of added P were converted to Fe-P under unsubmerged conditions. Higher amounts of added P were transformed to Al-P in alkaline-calcareous soils relative to acidic soils under both nonsubmerged and submerged conditions. Higher levels of added P were converted to Fe-P in acidic soils relative to alkaline-calcareous ones under at both nonsubmerged vs. submerged conditions. In air dried alkaline-calcareous soils, the following order was observed for inorganic phosphorus fractions: Fe-P< Ca2-P < Ca8-P < Al-P < Ca10-P; and in acidic soils, under the same conditions, the order observed was as follows: NH4Cl-P < Al-P < Fe-P < Ca10-P.