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چکیده
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Introduction: The recovery of phosphorus as struvite from treatment plants has attracted researchers' attention due to its potential as a phosphorus fertilizer. Struvite is a white crystalline substance consisting of magnesium, ammonium and phosphorus in equal molar concentrations (MgNH4P04.6H2O). The low water solubility of struvite is thought to limit its agronomic utility as a phosphorus (P) fertilizer compared with highly soluble P fertilizers (triple superphosphate). limited studies have been done regarding to the effect of struvite on the availability of phosphorus in calcareous soils. Therefore, this study examines the effect of struvite replacement with triple superphosphate fertilizer on phosphorus availability in wheat plants in calcareous soils deficient in phosphorus. Methods and Materials: Soil with phosphorus deficiency was collected from 0-30 cm depth under arable lands of Hajjiabad-e Seyyedeh located in Ghorveh Township, Kurdistan Province, Iran. The soil was air-dried and ground to pass through a 2-mm sieve, followed by laboratory analysis to determine its physico-chemical properties. Struvite used in this research was obtained by optimizing the three main factors of sulfuric acid concentration, solid-to-liquid ratio, and time for the leaching process, and the three key factors of Mg/P ratio, N/P ratio and pH for the precipitation process by Response Surface Methodology. The factorial experiment was carried out in the form of a completely randomized design in 3 replications. The factors include the application of different proportions of struvite replaced with triple superphosphate in 6 levels (S0:P0, S0:P100, S25:P75, S50:P50, S75:P25 and S100:P0) and 3 levels of phosphorus (50, 100 and 150 kg/ha) and a total of 54 pots. The application rate for struvite was calculated based on total phosphorus (P2O5) of triple superphosphate. Then 10 wheat seeds were planted in each pot at 2-cm depth which after plant emerging and greening declined to 4 plants in each pot. The pots were randomly moved twice a week during the growth period to eliminate environmental effects. Irrigation and weeding operations were done by hand. Plants (shoots and roots) were harvested 60 days after planting (beginning of flowering), washed with distilled water and dry with tissue paper. The samples were air-dried and then oven dried at 70˚C to a constant weight in a forced air-driven oven. Phosphorus concentrations in plant extracts by the molybdenum vanadate method and Nitrogen concentration in plant was measured by the Kjeldahl method. After harvesting the plants, the soil was immediately air-dried and passed through a 2mm sieve. Then, the amount of phosphorus was determined by Olsen method. The statistical results of the data were analyzed using SAS software and LSD test (at 5% level) was used for comparing the mean values. Results and Discussion: Based on the obtained results, all of the investigated treatments and their interactions were significant at the probability level of one percent (P < 0.01). The comparison of the average effects of different struvite treatments showed that by replacing struvite instead of triple super phosphate fertilizer in all three levels of fertilizer, the highest shoot fresh weight (with an average of 7.79 gr/pot), shoot dry weight (with an average of 1.13 gr/pot). Shoot Nitrogen concentration (with an average of 4.82%) and its uptake (with an average of 5.44 gr/pot) were obtained from the application of S75:P25 150 kg/h superphosphate fertilizer. Also, the results showed that the highest amount of phosphorus concentration and uptake, respectively, with an average of 0.174% and 0.197 gr/pot, was obtained from the application of the S75:P25 treatment with 150 kg TSP/ha, which is compared to the application of the S75:P25 treatment of 50 and 100 kg TSP/ha had an increase equivalent to 26.43, 59.89, 11.49 and 43.14% respectively. The results also showed that the highest amount of soil phosphorus after harvesting the plant with an average of 18.95 mg/kg was obtained from the S100:P0 treatment with 150 kg TSP/ha, which compared to the S100:P0 treatment with 100 and 50 kg TSP/ha with an average of 13.29 and 12.56 mg/kg had an increase equivalent to 29.86 and 33.72%, respectively. Conclusions: In spite of its low solubility, struvite is as effective as highly soluble phosphorus fertilizers for plants. There is still a lack of clarity regarding the mechanisms of struvite dissolution as well as the reasons behind this apparent dichotomy. Therefore, more accurate measurements of pH and EC in substrates, analysis of soil properties and fractionation of phosphorus in soil will enhance our understanding of the use of struvite. Therefore, it is recommended to optimize the timing and application rate of struvite in relation to the demand for different agricultural and garden crops.
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