Introduction: Darab region is one of the most important citrus production hubs, especially oranges, in Fars province. Given the frequent reports of iron deficiency chlorosis in citrus grown in calcareous soils of Iran, which reduces fruit production and quality, a better understanding of the mechanisms and kinetics of iron release from soil can provide useful information about its bioavailability and the factors affecting it and help us take the necessary measures to optimize soil conditions to prevent iron deficiency in plants and increase its availability in the soil. The aim of the present study is to investigate the kinetics of iron release in some calcareous soils of orange orchards (Citrus sinensis L.) in the Darab region, southern Iran, using six mathematical kinetic models and to find the effective soil characteristics in the release of iron from the solid phase to the soil solution.

Materials and Methods: 21 Washington Novel orange orchards (Citrus sinensis L. Osbeck) grafted onto orange (Citrus aurantium) rootstock were selected from different locations in Darab region in southeastern Fars Province through field observations and aerial photo analysis. Soil sampling was carried out from the selected tree bases and their shaded areas in four different directions (from 0 to 30 cm depth) in the vicinity of the drippers. Thus, a total of 10 soil samples were selected with different physical and chemical properties. The kinetics of iron release study was carried out using DTPA solution at pH 7.3 as the extractant, which was as follows: 10 g of each of the three soil sample replicates were placed in a 50 ml centrifuge tube and extracted with 20 ml of DTPA solution on a shaker (120 rpm) for 0.083, 0.25, 0.5, 1, 2, 6, 12, 24 and 48 hours at 25 ± 2°C. After shaking, the samples were immediately centrifuged for 10 minutes at 4000 rpm and the supernatant was passed through Whatman filter paper No. 42. The iron concentration in the resulting solution was measured by atomic absorption spectrometry (PG 990, PG Instruments Ltd. UK). The cumulative concentration of released iron (q) was evaluated as a function of time (t) using six different kinetic models. Relatively high values of the coefficient of determination (R2) and low values of the standard error of estimate (SEE) were used as criteria for selecting the best models. Data analysis was performed using SPSS 17.0 software.

Results and Discussion: The pattern of iron release in all soil samples was similar and consisted of a short-term rapid phase (fast phase) and a long-term slow phase (slow phase), so that about 44% (on average) of the total iron released during 48 hours was desorbed in the first two hours. Therefore, this result can confirm the two-stage iron release process from the soil samples. The highest iron release content within 48 hours was observed in soil "A" (17.3 mg kg-1), which could be due to the fact that among the studied soils, the highest organic matter content (7%), cation exchange capacity (2.15 meq 100 g-1 soil) and total iron (7560 mg kg-1) were found in soil A. The power function, parabolic diffusion, and simple Elovich kinetic models had the highest R2 values and the lowest SEE values among the kinetic models, respectively. In the power function kinetic model, the value of the constant “a” ranged from 0.829 to 1.260 with a mean of 1.02, while these values for the constant “b” ranged from 0.23 to 0.30 with a mean of 0.25. The KP constant in the parabolic diffusion kinetic model represents the rate coefficient of iron diffusion (mg Fe kg-1h-0.5) from the solid phase to the soil solution. Among the studied soils, the highest and lowest KP values were for soil A (0.399) and soil B (0.198), respectively.

Conclusion: The soil properties such as organic matter, cation exchange capacity, pH, salinity, percentage of sand, silt and clay (soil texture) were effective on the rate of iron release from calcareous soils, while the total soil iron and equivalent calcium carbonate had no effect on iron release. Soil organic matter and percentage of sand had a positive and significant correlation with the rate of iron release in the soil from the solid phase to the soil solution, while there was a significant negative correlation between the rate of iron release and soil salinity, pH, percentage of silt and clay. Finally, it is suggested that management methods, including creating lighter soil texture by adding sand and reducing salinity (leaching) of calcareous soils under orange cultivation in the Darab region, can significantly contribute to the release of native soil iron from the solid phase to the soil solution, and consequently improve iron nutrition.