Behaviour of available phosphorus during submerged condtion in rice paddy soils by adding phosphorus fertilizer

Background and objectives: Phosphorus (P), next to nitrogen is the most limiting factor of rice growth and development. Phosphorus deficiency have severe negative impact on rice yield. Despite of vital importance of P on rice production in paddy fields, its recovery is too low -less than 25% of applied P fertilizers- and the rest unrecovered P will fix in paddy soils through various aging process and fractions, or/ and lost along inefficient off stream and runoff movement. The one year project was conducted to identify and improve these trends. The main objectives of current study are: To characterize P desorption behaviors on six different types of paddy soils, to explore the P kinetics equations and their coefficients and to study the possibility of P fertilizers splitting through results of P kinetics findings.
Materials and Methods: A two factors factorial experiment was done based on complete random design with six levels of soils and two levels of P fertilizer (0 and 45 kg ha-1 P2O5 in source of pure K2HPO4). 2.5 g of sieved and air dried soil samples were submerged by 5 ml for 30 days in 25 oC. The submerged soils treated by P levels and available P sequentially extracted in the following time sequences: 2, 4, 6, 8, 10 and 12 hours, and 1, 2, 4, 6, 8, 10, 14, 17, 20, 24, 28, 32, 36, 40, 45, 50, 55, 60, 65, 70 and 75 days after P treatment. The extracted available P was run with the zero, first, second and third order equations, Elovich kinetic equation, and power function equation, and parabolic diffusion equation to calculate the coefficients of equations. The best fitted equation was selected according to the determination coefficient (R2) and the standard error of estimate (SEE).
Results: The findings of this research showed that in spite of different physical and chemical soil characters, the available P increased significantly through submerging (72%) compared to no flooded condition, averagely about 6.7 to 8.2 mg kg-1 in all soils. Moreover, the desorption curves of available P indicated a rapid decrease of P concentration (until 48 hours after adding P), and followed by gradual reduction until 600 hours after adding P and continued to end of experiment by very slow and constant slope. The first and second order equation (highest R2), zero and power function equations (highest SEE) could describe the P desorption process. Finally, the second order model were selected to express the P behavior in all soils because of the high R2 values, lower SEE and the significant correlation with soil properties. The slope of second order equation indicated significant correlation with pH (-0.51**), organic carbon (-.51**), calcium carbonate content (0.68**), available P of un- flooded condition (-0.51**), available P of submerged soils (-0.51**), Sand (0.68**) and clay content (-0.60).
Conclusion: Also P desorption curves showed three different reactions by time, 50 reduction of P before 12 days and 70 reduction before 60 days. Therefore, it might be concluded that P splitting at the begging of vegetative and reproductive stages is achievable.