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 (T1), one-third of recommended fertilizer doses (T2), two-thirds of recommended fertilizer doses (T3), full doses of recommended fertilizers (T4), T2 + 5 Mg cow dung (CD) and 2.5 Mg ash ha-1 (T5), and T3 + 5 Mg CD and 2.5 Mg ash ha-1 (T6). The apparent balance of P compared with the initial P status after 10 yr varied from -115 kg ha-1 under T1 to 348 kg ha-1 under T6. 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 NaHCO3 soluble P, NaOH extracted inorganic P (Pi), and acid P were observed under treatments with organic fertilizers (T5 and T6) than with other treatments at 0- to 5-, 5- to 10-, and 10- to 15-cm depths. The concentrations of NaHCO3-P, NaOH-Pi and acid P fractions were lowest under T1 and T2 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 (Po) and residual P (extracted with HNO3 + HClO4) with soil depth was not significant, and the differences in these P fractions under the tested P treatments were not large. The depletion of NaHCO3-P and NaOH-Pi at the 0- to 15-cm depth under control and T2 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 NaHCO3-P and NaOH-Pi (average of 0-15 cm) and it was linearly correlated with acid P (0-15 cm).