『Abstract
　Long-term effects of rice (Oryza sativa L.) cultivating 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 fertilizer (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 epth 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 (abverage of 0-15 cm) and it was linearly correlated with acid P (0-15 cm).』

Abbreviations
(Introduction)
Materials and methods
　Soil and location
　Sequential phosphorus fractionation
Results and discussion
　Solution soil phosphorus
　NaHCO3-P
　NaOH-Pi
　NaOH-Po fraction
　Acid phosphorus
　Residual phosphorus
Conclusions
References

連続的なリンの留分化（抽出法）
　無機と有機のリンの留分化は、Sui and Thompson（1999）によるものを修正した方法を各土壌試料に適用した。具体的には以下の手順である：
（１）土壌試料１gを0.05M塩化カルシウム（CaCl2）溶液30mLに加えて16時間攪拌し、遠心分離の後にろ過して、溶液中のリンを測定する。⇒溶液リン
（２）（１）の残渣を0.5M炭酸水素ナトリウム（重炭酸ナトリウム、重炭酸ソーダ）（NaHCO3）溶液30mLに加えて16時間攪拌し、遠心分離の後にろ過して、溶液中のリンを測定する。⇒NaHCO3−リン
（３）（２）の残渣を0.1M水酸化ナトリウム（苛性ソーダ）（NaOH）溶液30mLに加えて攪拌し、遠心分離の後にろ過して、溶液に濃塩酸（HCｌ）5mLで酸性にした後に遠心分離を行い、測定する。⇒NaOH-Pi（無機物）−リン
（４）（２）の残渣を濃硫酸（H2SO4）6mlに加え、1時間温浸して5mLにする。その後、冷却する。過酸化水素（H2O2）5mLを加え、残渣が白色になるまで再加熱する。その後、溶液を測定し、その値から（３）の値を差し引く。（Hedley et al., 1982）⇒NaOH-Po（有機物）−リン
（５）（３）の残渣を1M塩酸（HCｌ）と１M硫酸（H2SO4）の１：１溶液30mLに加えて攪拌し、遠心分離の後にろ過して、測定する。⇒酸性リン
（６）（５）の残渣を濃硝酸（HNO3）と過塩素酸（HClO4）の５：２混合液よう6mLに加え、温浸の後に測定する。（Hedley et al., 1982）⇒残留リン
リンの測定は、試料液を中性に調節した後に、比色分析法（Murphy and Riley, 1962）で行える（リンの吸光度は712 nmの波長）。