The long-term experiment (10 yr) was conducted with the following six treatments: absolute control, (T1), one-third of recommended fertilizer doses, (40-8-12-5 kg ha-1 N, P, K and S in dry season rice and 30-6-9-4 kg ha-1 N, P, K, and S in wet season rice, respectively) (T2), two-thirds of recommended fertilizer doses, (80-16-24-10 kg ha-1 N, P, K and S in dry season rice and 60-12-18-8 kg ha-1 N, P, K, and S in wet season rice, respectively) (T3), full doses of recommended fertilizers (120-24-36-15 kg ha-1 N, P, K and S in dry season rice and 90-18-27-12 kg ha-1 N,P, K, and S in wet season rice, respectively) (T4), T2 + 5 Mg CD and 2.5 Mg ash ha-1 (T5), and T3 + 5 Mg CD and 2.5 Mg ash ha-1 (T6) for 10 yr. The N, P, K and S content in the CD was 12.9, 05.8, 17.6 and 2.6 g kg-1, respectively. The ash contained 0.20 g kg-1 N, 1.0 g kg-1 P, and 10.0 g kg-1 K. The total P inputs in treatments T1 through T6 were 0, 14, 28, 42, 45.5, and 59.5 kg ha-1 yr-1. The experiment was conducted as a randomized complete block design with four replications. The long-term rice cropping with various nutrient management practices created apparent soil P balance varied from -115 kg ha-1 to 348 kg ha-1. The soils for the study were collected from five depths: 0 to 5, 5 to 10, 10 to 15, 15 to 30, and 30 to 50 cm from each 5 by 4 m plot. The soil samples were air-dried, crushed, and passed through 2-mm sieve and stored in polyethylene bags at room temperature, before P fractionation. Fraction of inorganic and organic P was performed on each soil by a modified P fractionation scheme of Sui and Thompson (1999) stated as following sequence:
(1) Solution P, by shaking 1 g soil in 30 mL of 0.05MCaCl2 for 16 h, centrifuging, filtering, and measuring P in the filtrate.
(2) NaHCO3–P, by shaking the residue from (1) in 30 mL of 0.5 M NaHCO3 for 16 h, centrifuging, filtering, and measuring P in the filtrate.
(3) NaOH-Pi–P, by shaking the residue from (2) in 30 mL of 0.1 M NaOH, centrifuging, filtering, and measuring P in the filtrate after acidifying 5mL (with concentrated HCl) and centrifuging.
(4) NaOH-P0–P, by digesting 5 mL of the filtrate from (2) in 6 mL of concentrated H2SO4 for 1 h, cooling, adding 5 mL of H2O2, and reheating until the residue became white, determining P in the digest, and subtracting NaOH-Pi–P from it (Hedley et al., 1982).
(5) Acid P, by shaking the residue from (3) in 30 mL of 1:1 mixture of 1 M HCl/1 M H2SO4, centrifuging, filtering, and measuring P in the filtrate.
(6) Residual P, by refluxing the soil residue from (5) in 6 mL of a 5:2 mixture of concentrated HNO3 and HClO4, and determining P from the digest (Hedley et al., 1982).
All P was determined colorimetrically (Murphy and Riley, 1962) after neutralization when necessary with dilute HCl and NaOH and the neutral pH indicated by the light yellow color of the solution in the presence of P-nitrophenol indicator. Absorbance for P was determined at a wavelength of 712 nm by spectrophotometer. Data were analyzed by ANOVA using IRRISTAT 3.0 (Bartolome et al., 1998).