Canopy Influences of Three Palm Cultivars on Release Rates of Native Zinc from Different Depths of a Calcareous Soil

Zinc (Zn) is an important nutrient element for plants growth, which plays an important role in the metabolism of plant. Zn uptake by plants requires release of Zn adsorbed onto the soil constituents and even Zn containing minerals. The release of Zn from soil surface to the soil solution is an important factor that controls availability of Zn for growing plants. Kinetics of Zn release from soil could indicate ability of soil to Zn supply to plants. The purpose of present study was to investigate the effects of soil depth and canopy of three cultivars of palm including Shahani, Ghasab and Halavani on release kinetics of native Zn by AB-DTPA extractant.

In order to investigate the effects of depth and vegetation on the pattern of Zn release,  54 soil samples were collected from inside and outside canopy of different palm cultivars (Shahani, Halavani and Ghasab), three depths (0-20, 20-40 and 40-60 cm) with three replications. These samples were collected from village of Simakan, located in the city of Jahrom. Extraction was performed after eight shaking times (5, 15, 30, 60, 120, 240, 480, and 1440 min) with ammonium bicarbonate-diethylene triamine penta acetic acid (AB-DTPA). After each shaking time, samples were immediately centrifuged for 15 min, and then filtered through filter paper. The concentration of Zn in solutions were determined using an atomic absorption spectrophotometer (AA-67OG). Seven commonly used kinetics models in nutrient release studies were used to describe Zn release including zero-order, first-order, pseudo-first-order, pseudo-second-order, power function, parabolic diffusion, and simple elovich. Data analysis and drawing of charts were done by SAS software and Excel program, respectively. Relatively high value of coefficient of determination and low value of standard error of estimate were considered as criteria for the best fit.

The results of this study showed that the simplified Elovich, parabolic diffusion and power function equations well described the pattern of Zn release from soil as evidenced by higher coefficient of determination and lower values of the standard error of the estimate. The Zn release pattern in all soil samples consisted of a quick stage from the start of the experiment to 240 minutes and a slower stage at subsequent times. The correlation between the coefficients of the kinetic equations of the power function, simplified Elovich and parabolic diffusion showed that there was higher correlations between the coefficients of the equations obtained from the under canopy in comparison with those of out of canopy. Although soil is a major factor influencing vegetation growth and characteristics, plants could, in turn, also affect different soil properties. In addition, the palm root system and the rhizosphere exhibit a complex diversity, which could regulate the plant homeostasis. The interaction of root-microorganism in the rhizosphere can cause many of the physical, chemical and biological properties of the rhizosphere soil that are different from the bulk soil. Important changes in the soil properties of the rhizosphere environment include soil pH, soil oxidation-reduction reactions, soil moisture, and nutrient availability for microorganisms and plant. Root exudates also contribute to enhance of the availability of elements by reducing the pH of the rhizosphere and creating bioavailable Zn complex. Recent studies have shown that root exudates can act as an influential factor in extracting significant values of plant nutrients from calcareous soils. Our results showed that the amount of accumulated Zn released from the under canopy soils was more than the out of canopy soils, and the highest amount of accumulated Zn was released in the soils under canopy of Ghasab cultivar.

Results of present study showed that the values of Zn released from the under canopy soils was higher than interspaces soils, and the highest amount of accumulated Zn was released in the soils under canopy of Ghasab cultivar. Such observations clearly indicate that palm trees are able to supply Zn absorption capacity under their own canopy soils. Therefore, it appears that higher rates of Zn release from the under canopy soils in comparison with interspaces could be attributed to higher root exudates which impact soil properties, microorganism activities and lower pH which, in turn, increase the amounts of metals including of Zn release and bioavailability. Further researches on the effect of palm root system on physical and chemical properties of soil, including organic matter, soil pH, which could cause great impacts on the amount of Zn release are highly recommended.