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Background, Aim and Scope:
One of the principal experimental variables which effect the results of phosphorus (P) sorption studies is the ionic composition, both species and concentrations, of the contacting solution. In spite of the realization that ionic species, concentrations and their compositions effect P sorption and/or desorption, most of the salt related studies are confined to Cl- (anion) in association with different cations. While, the knowledge about the comparative response of P to Cl and SO42- ions was lacking. Therefore, current study was conducted to evaluate the comparative effects of anions (in association with cations) on inorganic P release and P fractions in the soil.
Materials and Methods:
The test soil was amended with livestock compost manure (OP); KH2PO4 (IPk) or Ca(H2PO4)2 (IPc) at the rate of 1ppm. Soil was subjected to one salt and nine subsequent water extractions and different P fractions were measured. Four salt types NaCl, Na2SO4, KCl and K2SO4 were used at the rate of 0.5 M.
Results:
Irrespective of P sources, P release was substantially increased in the salt-pretreated soil as compared to the non-saline soil. Sulphate salts released more P in subsequent water extractions than Cl-. Phosphorus release decreased for salt types as Na2SO4> NaCl> K2SO4> KCl and for P sources OP ≈ IPk > Control (without P application) > IPc, respectively.
Discussion:
No previous study was found to compare the results of more P release by SO42- than Cl- salt. Most of the previous studies focused on anion sorption capacities but the mechanism for their adsorption is not fully known. Most of authors suggested that the mechanisms of SO42- and PO43- adsorption are similar, and that both ions compete for the same sorption sites (Kamprath et al. 1956, Couto et al. 1979, Pasricha and Fox 1993). Although adsorbed SO42- does not compete strongly with PO43- but there is likely some competition for sorption between these anions which may cause comparatively more P release by SO42- than Cl- salt. Higher P release by Na-saturation could be due to the release of P associated with oxide surfaces or due to dissolution of Ca-P phases (Curtin et al. 1987).
Conclusions:
Study clearly showed that not only cations species differ for P desorption capacity but associated anions also play a vital role in the fate of P under saline environment. Synergetic effect exists between Na and SO42- ions which enhanced the P release. This study has also confirmed the fact that P from organic source is as available as from inorganic P source. However, P release depends more on the type of P source applied than on total P.
Recommendations and
Perspectives:
It is highly recommended that more than one anion species must be used in the research plans for evaluating P response to saline environment. The results have important implication from research point of view as most of the researchers focuses on different cations only for evaluating P response to salts from environmental point of view. But our study made it clear that anions in association with cations differed for their effects on P release.
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