عنوان مقاله [English]
Introduction: Focusing on population and industrial centers in the different areas has caused the formation of a huge volume of sewage which their treatment (refining) has led to produces a larger volume of sludge so that without any attention to find out the best disposal way, has created various environmental difficulties. One of the economic and efficient ways to use sewage sludge is utilizing them in agriculture. Sewage sludge, due to its large amount of nutrients and organic matters and being less expensive, nowadays it is widely used as a fertilizer and/or amendment of soil physical and chemical properties. In the present study, the effect of different levels of industrial sewage sludge application on the bioavailability and distribution of various forms of iron in spinach plant, Shahr Reza cultivar was evaluated.
Materials and Methods: Soil was collected from 0-30 cm depth around Aq Qala industrial town. After air drying, the soil was passed a 2 mm sieve and its physical and chemical properties measured. For this purpose, a completely randomized design with different levels of industrial wastewater sludge (0, 22.5, 45, 90, 180 and 360 ton/ha) was conducted in the form of a pot experiment with 4 replicates in greenhouse of research center of Gorgan Agricultural and Natural Resources Research. The experimental units were pots of 7 kg soil. Then, 20 seeds per pot were planted at 2 cm depth of soil and after two weeks of emergence, the number of shrubs was reduced to 5 plants per pot. Irrigation and weeding were done manually. After the end of the growth period (90 days), the plant samples were digested by dry-digesting method. The concentration of iron in the samples was determined by atomic absorption device (AAS-Unicam-919). Immediately after harvesting, the soil of pots was air-dried and passed through a 2 mm sieve. Then the amount of soil extractable iron by DTPA was read by the atomic absorption device. In addition, Tessier et al. (1979) method was used to investigate the distribution of different forms of iron after sludge application. Statistical analysis of the test data was performed using SAS software and LSD test (5%) was used to compare the means. Also, Graphs were plotted using Excel software.
Results and Discussion: The results showed that concentration of iron in the soil and aerial parts of spinach was significantly affected by the sewage sludge application. The highest dry weight of the spinach aerial part with an average of 5.89 g/plant was related to 90 tons/ha sludge treatment and the lowest with an average of 1.07 g/plant was related to the control treatment. Although the application of sludge at high levels such as 180 and 360 t. ha-1 led to a reduction spinach yield, this seems to be due to sludge effects such as the presence of heavy metals, disturbance of soil C/N equilibrium, and especially increase soil salinity. The concentration of iron in spinach aerial parts increased linearly in 22.5, 45, 90, 180, 360 ton/ha sludge treatments, which had an increase of 3.19, 5.16, 12.04, 26.06 and 52.76 percent compared to the control treatment, respectively, while the highest amount of iron uptake with an average of 767.47 mg/kg dry matter obtained in 90 ton/ha sludge treatment and the lowest amount with an average of 124.43 mg/kg dry matter was related to control treatment. The average of iron uptake in the aerial parts of spinach in 180 and 360 t/ha sludge treatments, respectively, was 735.93 and 341.08 mg/kg dry matter, which was 4.28% and 125% lower than 90 t/ha treatment, respectively. The results also showed that application of 360 t/ha sewage sludge increased 73.48% DTPA extractable iron compared to the control treatment. The results of iron fractionation showed that the amounts of all iron components in the soil (except iron and manganese oxides) increased by application of sewage sludge. In the control soil, different iron components were as follows: exchangeable> carbonate> organic> iron and manganese oxides> residual. By applying treatments, the order did not change except that after application of treatments, in all components of iron (except iron and manganese oxides) showed an increase in the amount of each component compared to the control treatment. The correlation between the chemical forms of iron with DTPA-extractable iron and iron concentration of spinach showed that between iron concentration in spinach and extractable iron with DTPA (r= 0.93), exchangeable iron (r= 0.95) and iron bound to the organic fraction (r= 0.98) had a significant and positive correlation at the 1% level and with iron bond to iron and manganese oxides (r= 0.98) had a significant and negative correlation at 1% level which may indicate the role of these forms of iron (DTPA-extractable iron, exchangeable iron and organic-bound iron) in the nutrition of spinach (Spinacia oleracea L.). Increased Fe concentration in plant organs by sewage sludge application may be due to increased concentration of elements in soil and consequently higher uptake of this element by plant.
Conclusion: In general, it can be concluded that the application of sewage sludge in high amounts of 180 and 360 ton/ha with the effect on physical and chemical properties of soils such as disturbance of nutrient balance and increasing soil salinity causes a decrease in plant dry weight and the ability of iron uptake in the spinach plant. However, sewage sludge application provides nutrients for plant growth and reduces the economic cost of disposal. However, it is recommended to determine the threshold of toxicity and salinity before application of sewage sludge depending on soil, plant and environmental conditions. More extensive research is also needed to standardize the method and relative amounts of use of this organic amendment.