Effect of Organic Matter Removal on Phosphorus Hysteresis Indices (HI) in Calcareous Soils

Sorption and desorption are important processes that influence phosphorus (P) chemistry in soil. Desorption is a process more complex than sorption and usually not all that is adsorbed is desorbed. This indicates that adsorption and desorption mechanisms are not similar and it seems that such reactions are irreversible. Such irreversibility is usually called hysteresis. Major factors such as chemical changes in the structure of minerals, non-equivalent processes, inflation of adsorbent material, changes in the strength of crystals, irreversible fixation of adsorbed molecules in fine pores and equilibrium time less than its true value lead to hysteresis phenomenon. The concentration of phosphate in soil solution and thus its availability for plant are closely related to sorption processes by soil components. This relationship can be explicated by sorption isotherms. Soil organic matter (SOM) especially in arid and semiarid regions is one of the important indices of soil quality and plays important role in phosphate chemistry and fertility. Organic matter could decrease P sorption, maximum buffering capacity, and bonding energy and could increase P concentration in calcareous soils solution. Organic matter and organic acids resulted from its decomposition may coat calcium carbonate surfaces and prevent the formation of apatite precipitation. There are several methods to remove soil organic matter including using hydrogen peroxide and sodium hypochlorite solutions. It has been reported that H2O2 is penetrated into the interlayer spaces of phlogopite and vermiculite through exchange with water and cations and decomposes into H2O and O2. Therefore, this study was conducted to quantify the hysteresis indices, to investigate the effect of organic matter removal on phosphorus (P) hysteresis indices and to evaluate the relationship between hysteresis indices and soil characteristics and selection of index with the close correlation.

This study was carried out to obtain soil organic matter (SOM) removal with sodium hypochlorite solution (NaOCl, pH=8) effects on P hysteresis indices in 12 calcareous soils of Iran with different characteristics. For experiment of P sorption, 2 gr of soil subsamples was placed in separate 50 mL centrifuge tubes, to which were added 20 ml of monocalcium phosphate containing 5, 10, 15, 20, 30, 40, 60, 80 and 100 mg P L-1, which had been prepared in 0.01 M CaCl2 solution as background. Centrifuge tubes were shaken in a shaker incubator for 48-hour period to reach an equilibrium. Then, they were centrifuged at 4000 rpm for 5 minutes. The supernatant was filtered through a filter paper and the P concentration of filtrates determined using a spectrophotometer. The difference between initial and final P concentrations was assumed to be the amount of P adsorbed by the soil. Desorption experiments were assumed at the end of sorption experiments at the highest initial concentration of P with 0.01 M CaCl2 solution. The tubes were shaken to reach phosphate desorption equilibrium time (24 hours) at 25 °C in incubator shaker. Then, it was centrifuged for 5 minutes at 4000 rpm and 15 ml of the supernatant solution was pipetted and then 15 ml of solution of 0.01 M CaCl2 was added to tubes and the above steps continued to 9 steps. Freundlich model was used to describe the sorption – desorption isotherms data. DataFit 9.0.59 software (1995-2008) was used for nonlinear fitting of Freundlich to sorption data.

According to the results, P sorption and desorption data showed hysteresis which indicates adsorption and desorption mechanisms are not the same. As expected, nonlinear Freundlich equation showed a best fit (R2=0.96) to the data. The mean value of desorbed P in studied soils after SOM removal was decreased by 40%, so it was concluded that P sorption was more irreversible. In NaOCl treated soils, the mean values of seven studied hysteresis indices (HI) increased. Regression analysis indicated that the fourth hysteresis index, obtained from the distribution coefficient (Kd), had close relation with clay (r = 0.69, p < 0.05) and active calcium carbonate (r = 0.7, p < 0.05) concentration. Moreover, this hysteresis index showed significant (p<0.01) positive correlation with Kfsorb and Kfdesorb, which suggests that increasing bonding energy in sorption and desorption isotherms decreased desorption amount due to the strong interaction between adsorbed P and absorbent surface, increasing this hysteresis index.

It was concluded that among seven used hysteresis indices, HI4 can be introduced as the best index for the studied calcareous soils. It is predicted that using organic matter or preventing its reduction in arid and semi-arid calcareous soils may increase the efficiency of P fertilizer, given an increase in hysteresis index after the removal of the organic matter.