Comparing Zn speciation and fractionation in selected diagnostic horizons from Khuzestan Province

Soils at the earth crust could be considered as the result of interaction among five soil-forming factors. The strength and weakness of each of these factors causes formation of different soils with different properties and horizons. Previous studies have reported that Iranian agricultural soils show different levels of zinc deficiency. Although the literature review confirms many studies in this regard, the number of studies compared the distribution of the chemical forms of micronutrients and especially zinc after the application of chemical fertilizers in the soils of Khuzestan especially in the common diagnostic horizons of these soils, is limited. Therefore, the purposes of this study were two-fold: (1) to measure and determine the chemical forms of zinc in selected diagnostic horizons (Mollic, Calcic and Salic) of Khuzestan Province, (2) to identify the contribution of some physical and chemical properties of these soils on availability of zinc.

In order to carry out this research, Mollic and Calcic diagnostic horizons were collected from the Karun 3 watershed in the east of Khuzestan province, with thermic and mesic temperature and xeric moisture regimes. However, Salic diagnostic horizon was sampled from the route between Ahvaz to Abadan in the southwest of the province, with hyperthermic temperature regime and an aridic moisture regime. After being transferred to the laboratory, the samples were first air-dried, and after passing through a 2-mm sieve and before carrying out the fractionation and speciation experiments, their physical and chemical characteristics including electrical conductivity (EC), pH, particle size distribution, organic C content, calcium carbonate equivalent, cation exchange capacity (CEC), specific surface area (SSA), and total and DTPA extractable Zn were determined employing common standard procedures (Table 2). Different forms of zinc in the solid phase of the Mollic, Calcic, and Salic soils were extracted using the method described by Tessier et al. (1979). In brief, 5 components including water soluble, exchangeable, carbonates, iron and manganese oxide, organic and residual forms were determined. The concentration of zinc in the extracts obtained from different stages of successive soil extraction was determined using an atomic absorption device. In order to estimate the accuracy of the extraction method described above, the recovery percentage was calculated. To do this, one gram of a certified soil from the state of Montana (ANIST 2711A) was used and the recovery percentage of zinc in our study was 94.5%. In order to determine the chemical forms of zinc in the solution phase of the soils, 100 g of each soil sample was saturated with double distilled water, and then extracted using a vacuum pump and Buchner funnel. The extracts were passed through filter paper and centrifuged at 2500 rpm for 8 minutes, then the values of pH, electrical conductivity, dissolved organic carbon, and dissolved cations and were determined using common laboratory methods described earlier (Table 1). These parameters were considered as input in Visual MINTEQ software to predict zinc species in soil solution (all free and complexed organic and mineral species). The data analysis was conducted considering a factorial design based on a completely randomized design through SAS software (v9.1). The comparison of means was also done using the Tukey’s test at the 5% probability level. Graphs were also drawn using Origin software (v2018).

Considering the critical level of zinc deficiency in the soil (1 mg/kg), all the soils are deficient in terms of available zinc. The studied soils are among alkaline and calcareous soils, but they did not show any differences from each other in terms of soil texture. It is noteworthy that the mollic horizon has a greater cation exchange capacity and specific surface area than the other two soils due to the higher amount of organic matter and the greater abundance of clay contents. The results of zinc fractionation in the studied soils showed that organic, carbonate and oxide forms in the Mollic and Calcic horizons, and carbonate, oxide and exchangable forms in the Salic horizon were the dominant forms of Zn which affects its supply to the plants. The results also showed that Zn2+, Zn(OH)2(aq), Zn-DOM (aq), and Zn (OH)+ species were the dominant species of Zn in soil solution of all diagnostic horizons. The result of Zn mobility factor calculations with the addition of 10 mg/kg Zn(SO4)2 showed lower values of mobility factor in the mollic diagnostic horizon than those of the calcic and salic.