عنوان مقاله [English]
Zinc (Zn), an essential micronutrient for both plants and humans, is involved in a number of physiological and biochemical processes. Calcareous soils cover more than 30% of the earth’s land and are characterized by the high pH and low availability of plant nutrients. Zinc (Zn) that is freely available in acid soils is only sparingly available in calcareous soils, due to their poor solubility at high pH. Zinc deficiency in most of the world’s soils has resulted in significant loss of agricultural yields. Information about Zn availability in soils is very important in the view point of Zn nutritional status of plant and human. Several soil physicochemical properties including organic matter, CaCO3, pH, moisture and total Zn concentration affect soil Zn availability to plants. Under Zn deficiency, plants tend to release organic acid in the rhizosphere which in turn increases soil Zn availability. Oxalic acid is the simplest dicarboxylic acid with two pKa values, 1.23 and 4.19 and it occurs in sediments, forest soils, and agricultural soils, especially in the rhizosphere. Oxalic acid is able to chelate with the poorly soluble nutrients in the soil and consequently influence their bioavailability. It is known that Zn availability is controlled by adsorption, release, precipitation and dissolution reactions. Study of kinetic models is a useful method to describe the changes in the nutrient availability with time. A knowledge of desorption kinetics may provide important information concerning the nature of reaction and the rate of Zn supply to plants via soil solution.
Materials and methods Composite samples of the two soils were collected from 0-30 cm depth of agricultural areas in Kerman province, Eastern Iran. The samples were air dried, crushed and passed through a 2mm sieve. Some soil chemical and physical properties of soil sample including Particle size distribution, Electrical conductivity and pH, Organic carbon, carbonate calcium equivalent, cation exchange capacity, available Zn and Total content of Zn were done according to standard procedures. For the kinetic study, soil samples were weighed (1.5g), placed in a 20 mL centrifuge tube and then 15 mL of oxalic acid with two concentrations of 1.1 and 2.2 mµ L-1 was added. The tubes were shaken from 1 to 72h at 25±2°C. Two drops of toluene were added to each tube to inhibit microbial activity. After shaking, the solutions were centrifuged and filtered through Whatman filter paper No. 42. Zinc concentration was determined in the filtrate using a Vario atomic absorption spectrometer. Several kinetic equations including zero-, first-, second- and third order, parabolic diffusion, Power function and simple elovich were also fitted to experimental data.
Results and discussion Zn release by oxalic acid increased with time and the amount of Zn release differed between soils. The difference in the amount of Zn release may be attributed to differences in (i) the total amount of labile Zn which sorbed in the soil; (ii) types, quantities and relative proportions of the soil components by which the Zn is retained and (iii) other soil properties such pH and CEC. The release pattern of Zn included an initial fast reaction followed by a slow reaction that continued up to 72 h. The two phases of Zn release can be due to the heterogeneity of adsorption site with different adsorption affinities. The release kinetic of Zn in soils was poorly described by first- and second-order equations while Time dependent Zn release was best modeled by the simple Elovich, power function and parabolic diffusion equations. Based on the relatively higher values of r2 and the lower values of S.E., the simple Elovich showed the best fitness on the cumulative release of Zn. At each specified time, the lower dose of oxalic acid released Zn from soil more than the higher dose. Organic acids may increase the sorption of metal ions on soil particles through electrostatic interactions, ternary metal–ligand–surface complex formation or surface precipitation. It seems that Zn may interact with oxalic acid where adsorbed to solid phases and resulted in decreased Zn release. The rate parameters derived from the best-fitted model were used to compare Zn release by different concentrations of oxalic acid. The results showed that the rate parameters “ab”, Kp and β decreased with the oxalic acid concentration.
Conclusion From the present study, oxalic acid, especially at the lower rate, can increase Zn release and its bioavailability in calcareous soils.