Effects of Rhizosphere of Rice Plant on the Inorganic Phosphorus Fractions in the Paddy Soils (in North Iran) Following P Fertilizer Application

In a greenhouse experiment, changes in inorganic phosphorus fractions as well as their availability in the rhizosphere of rice (Oryza sativa c.v. Khazar) crop, following an application of 40 mg P (as monocalcium phosphate per kg of soil) were investigated in paddy soils north of Iran. The study was performed as a 14×2 factorial in a completely randomized design of two factors namely: soil at 14 levels (10 calcareous vs. 4 non-calcareous) and cultivation at two levels (cultivated vs. uncultivated). Each level of soil and cultivation were performed in examined two replicates. Following a period of three months, specially designed soil tubes, buried in soil at the beginning of the experiment, were simultaneously drawn out of the cultivated and of the uncultivated pots and then immediately analyzed for P fractionation. The method of Jiang and Gu (1989) was employed for determinations in calcareous soils, of Kuo (1996) for non-calcareous soils) while the available-P was determined through Olsen method. The results indicated that in all the 14 soils, the available soil P (Olsen-P) in the rhizosphere of rice was significantly lower than that in the non-rhizosphere (uncultivated soil). But, following the plant P uptake and its addition to the extractable-P (Olsen-P) in the rhizosphere soil, the total available-P in the rhizosphere soil was higher than that in the non-rhizosphere one. In calcareous soils, dicalcium-phosphates, octacalcium-phosphates, and Al-phosphates (Al-P) in the rhizosphere of the rice crop were significantly lower than those in the non-rhizosphere soil; whereas iron oxides occluded-phosphates and Fe-phosphates (Fe-P) in the rhizosphere of rice did not significantly differ from those in the non-rhizosphere (uncultivated) soil. Only in two calcareous soils, the apatite-P in the rhizosphere of rice was significantly lower than that in the non-rhizosphere soil. In non-calcareous and acidic soils, readily soluble-P, Fe-P, and Al-P in the rhizosphere of the rice crop were significantly lower than those in the non-rhizosphere soil. Apatite-P and Fe oxide occluded-P did not significantly differ from those in the non-rhizosphere soil. The results of the research finally revealed that the Olsen extractant may underestimate the availability of P under laboratory (air dry) conditions.