Transformation of phosphate in rice (Oryza sativa L.) rhizosphere and its influence on phosphorus nutrition of rice

Rice (Oryza sativa L.) nutrition is greatly dependent on the phosphorus (P) status in the rhizosphere. A greenhouse experiment was carried out with rice grown in four low-pH soils (granitic lateritic red earth, a paddy soil from the red earth, a basaltic latosol, and a paddy soil from the latosol) to investigate the transformation of phosphate in the rice rhizosphere. Rice was grown in pots with the roots confined in rhizobags, and the rhizosphere soil and nonrhizosphere soil were analyzed for iron (Fe)-bound P, aluminum (Al)-bound P, and clacium (Ca)-bound P by Chang's method. The concentration of three different forms of P decreased in the order of Fe-P > Al-P > Ca-P for the granitic soils, whereas the order was Fe-P > Ca-P > Al-P for the basaltic soils. The difference between the rhizosphere soil and the nonrhizosphere was statistically significant for Fe-P and Al-P but not for Ca-P. The delta Pi was defined as the difference in P of form "i" between the rhizosphere soil and nonrhizosphere soil, which could be used as the index of efficiency of rice use for the "i" form of P. For all the soils, delta P(Fe-P) was much higher than delta P(Al-P) and delta P(Ca-P) indicating that Fe-P is probably the main source of P supply to rice. The delta P(Al-P) was much higher in granitic soils than that in basaltic soils, while the delta P(Ca-P) was higher in paddy soils than that in the corresponding upland soils, indicating that Al-P was also an important source of P supply to rice in granitic soils and Ca-P could be used by rice to some degree in paddy soils. The sigma delta P was distinctly different, and it was coincident with the rice growth in the four soils. It could be concluded that the inorganic P fractions in the rhizosphere soil determined the rice use efficiency for different P fractions, thus affecting the rice growth.