Effect of Organic Matter Removal on Kinetics and Thermodynamic of Phosphorus Desorption in some calcareous soils of East Azerbaijan Province

Abstract
Background and objectives:
The phosphorus is second limiting nutrient in soil for crop production after nitrogen. Mobility and availability of phosphorus (P) in soils is affected by phosphorus desorption characteristics. Information about the effect of organic matter on kinetics and thermodynamics of P desorption in calcareous soils of East Azerbaijan province is limited. The objective of this study was to evaluate the effect of organic matter on phosphorus desorption kinetics and thermodynamics parameters. P desorption kinetics and thermodynamic was studied in 4 soil samples (0-30 cm) collected from East Azerbaijan province before and after organic matter removal by NaOCl.
Materials and methods:
According general characteristics, 4 soil composite samples (0-30) were selected so that as much as possible had a wide range of SOM, available P and sand and clay content. Soil samples were air dried, to pass a 2 mm sive and some physical and chemical properties analysed.Desorption kinetics of P was studied at 100 mg P L-1 solution as initial concentration with CaCl2 0.01 M solution before and after SOM removal. Statistical analysis using SPSS software and fitting nonlinear kinetics equations were performed by using Data fit software.
Results:
The P desorption kinetics were best described by a simple Elovich equation (r2≥0.94).The mean values of constant “ “was 9.69 before of organic matter removal and 8.05 was after of organic matter. According to the results, P desorption pattern based on the simple Elovich equation revealed that the rate of P desorption was higher in the first 2 hours followed by a slower release rate, which may suggests two different mechanisms are involved. To evaluate temperature effects, desorption thermodynamics of P was studied at various temperatures of 10, 20, 30 and 400C. Results suggested that the P desorption, before and after SOM removal, was a spontaneous and endothermic process.
The Positive values of enthalpy for the retention of P, indicated that the retention process was endothermic. The mean values of ∆G0 ranged from -32.12 to -39/.3 KJmol-1 in NaOCl untreated soils, while it ranged from -33.05 to -40.21 KJmol-1 in NaOCl treated soils when temperature increased from 100C to 400C.

Conclusion: Regarding the rate of P retention by soil and decreasing P desorption after SOM removal, application of organic matter, or at least prevent its loss can increase efficiency and rate of P release to soil solution.

Key words: Enthalpy, Entropy, Hydrogen peroxide (H2O2), phosphorus retention, Sodium Hypochlorite (NaOCl).