Inorganic and Organic Phosphorus Fertilizer Effects on the Phosphorus Fractionation in Wetland Rice Soils

Long‐term effects of rice ( Oryza sativa L.) cultivation with varying nutrient management on soil P fraction are important to understand from soil nutritional and environmental point of view. Soil P fractionation gives an idea about the soil P supplying capacity to plants. The present experiment was conducted to evaluate the effect of different nutrient management in wetland rice on the changes of soil P fraction at different depths. Soil samples from five depths (0–5, 5–10, 10–15, 15–30, and 30–50 cm) were collected from a long‐term experimental field classified as a Chhiata clay loam, hyperthermic Vertic Endoaquept. The field received six treatments for 10 yr: absolute control with no fertilizer applied (T 1 ), one‐third of recommended fertilizer doses (T 2 ), two‐thirds of recommended fertilizer doses (T 3 ), full doses of recommended fertilizers (T 4 ), T 2 + 5 Mg cow dung (CD) and 2.5 Mg ash ha −1 (T 5 ), and T 3 + 5 Mg CD and 2.5 Mg ash ha −1 (T 6 ). The apparent balance of P compared with the initial P status after 10 yr varied from −115 kg ha −1 under T 1 to 348 kg ha −1 under T 6 The P fractionation study was conducted over the treatments and soil depth. Treatment and depth had no significant effect on solution P. Larger concentrations of NaHCO 3 soluble P, NaOH extracted inorganic P (P i ), and acid P were observed under treatments with organic fertilizers (T 5 and T 6 ) than with other treatments at 0‐ to 5‐, 5‐ to 10‐, and 10‐ to 15‐cm depths. The concentrations of NaHCO 3 –P, NaOH‐P i and acid P fractions were lowest under T 1 and T 2 treatments. At 15 to 30 cm or lower soil depths, none of the P fractions were affected by treatments. The change in NaOH organic P (P o ) and residual P (extracted with HNO 3 + HClO 4 ) with soil depth was not significant, and the differences in these P fractions under the tested P treatments were not large. The depletion of NaHCO 3 –P and NaOH‐P i at the 0‐ to 15‐cm depth under control and T 2 suggests that the rice plant depends upon these fractions of P. The P depletion profile in wetland rice appears to be confined within the first 15‐cm depth. The mean P uptake by rice showed a polynomial relationship with NaHCO 3 –P and NaOH‐P i (average of 0–15 cm) and it was linearly correlated with acid P (0–15 cm).