تاثير هيدروكسيد مضاعف لايه¬اي با آنيون بين لايه¬اي فسفات بر فراهمي فسفر در يك خاك آهكي

The Effect of Layered Double Hydroxide with Phosphate as the Interlayer Anion on the Availability of Phosphorus in a Calcareous Soil

به¬دلیل واكنش فسفر با تركیبات موجود در خاك، فراهمی فسفر به‌عنوان یك فاكتور محدود‌كننده برای تولید فرآورده‌های كشاورزی در سراسر جهان مطرح است. این پژوهش با هدف مقایسه تاثیر تركیب سنتزی هیدروكسید مضاعف لایه¬ای (LDH) با آنیون بین لایه ای فسفات و كود شیمیایی سوپر فسفات تریپل (TSP) بر فراهمی فسفر یك خاك آهكی در طی زمان انجام شد. به این منظور ابتدا Zn-Al LDH با آنیون بین¬لایه¬ای فسفات (P-LDH) به روش تبادل یونی ساخته شد و سپس یك آزمایش انكوباسیون در قالب طرح كاملاً تصادفی با آرایش فاكتوریل دربرگیرنده دو تركیب حاوی فسفر (P-LDH و TSP)، چهار سطح فسفر (صفر (شاهد)، 18، 45 و 90 میلی¬گرم فسفر بر كیلوگرم)، 8 زمان (1، 5، 10، 20، 40، 70، 100 و 150 روز) و سه تكرار انجام شد. در پایان هر زمان مقادیر فسفر و روی قابل دسترس، pH و هدایت الكتریكی (EC) نمونه¬ها اندازه¬گیری گردید. نتایج نشان داد كه كاربرد P-LDH و TSP منجر به افزایش معنی¬دار فسفر قابل دسترس در مقایسه با تیمار شاهد گردید اما روند تغییرات رهاسازی فسفر در این دو منبع با گذشت زمان متفاوت بود. در نمونه¬های P-LDH برخلاف TSP با گذشت زمان فسفر قابل دسترس افزایش یافت تا آن¬جا كه تفاوت میان این دو منبع در هر سه سطح فسفر در زمان 150 روز از نظر آماری معنی¬دار گردید. این امر احتمالا به¬دلیل رهاسازی آهسته فسفر از ساختمان P-LDH و كاهش واكنش فسفر با تركیبات خاك رخ داده است. در اثر انحلال تركیب P-LDH در خاك هم¬زمان با آنیون فسفر كاتیون روی هم وارد محلول خاك شد كه منجر به افزایش قابل توجه روی قابل دسترس خاك در تیمارهای P-LDH گردید. همچنین نتایج نشان داد كه تغییرات pH و EC در هر دو منبع یكسان بوده و تفاوت معنی¬داری میان دو منبع مشاهده نشد. بنابراین به¬نظر می¬رسد كه P-LDH می¬تواند به¬عنوان یك كود كندرهای فسفره سبب افزایش كارایی فسفر گردد اما سطوح بالای این تركیب به¬دلیل بالا بودن مقادیر روی قابل دسترس توصیه نمی¬شوند.

Introduction: After nitrogen, phosphorus is the second most frequently limiting macronutrient for plant growth. It participates in metabolic processes such as photosynthesis, energy transfer and synthesis and breakdown of carbohydrates. H2PO4- and HPO42- are two forms of this element which are present in the soil solution in the available form. Due to phosphorus reactions with soil components (oxy or hydroxides of Fe and Al in acidic soils and Ca2+ and Mg2+ ions in calcareous soils), the availability of this element is a limiting factor for production of agricultural crops in the whole world. To minimize this problem it is needed to improve the recycling of phosphorus and develop new technologies to reduce phosphorus losses and increase its effectiveness. In the recent decades, layered double hydroxides (LDH) have been extremely used as an effective sorbent for organic and inorganic anions sush as phosphate. Furthermore, some studies have suggested that the phosphate form LDH is applicable as a slow release phosphate fertilizer. Therefore, the objective of the present study was to compare the effect of using Zn-Al LDH and triple superphosphate (TSP) as fertilizers on the availability of phosphorus over time in a calcareous soil. Materials and Methods: At the first, nitrate containing Zn-Al LDH (N-LDH) was synthesized by urea hydrolysis method and then ion exchange method was used for the phosphate anions intercalation into N-LDH. In this process, 5.0 g of the N-LDH was suspended in 1000 mL of a solution 0.05 mol/L of K2HPO4. The suspension was kept for 12 h at room temperature (25 °C) under stirring. Afterwards, the material was filtered, washed with distilled water and dried at 70 °C for 18 h. The LDH sample produced by the ion exchange method was nominated as P-LDH. To compare the effects of P-LDH and TSP application on the availability of soil phosphorus, an incubation experiment was carried out using a completely randomized factorial design with two sources of phosphorus (P-LDH and TSP), four levels of phosphorus (0 (control), 18, 45 and 90 mg P kg-1), eight levels of time (1, 5, 10, 20, 40, 70, 100 and 150 days) and three replications. Available phosphorus and zinc, pH and EC of samples were measured at the end of each time period. Available phosphorus was extracted with 0.5 M sodium bicarbonate and phosphorus concentration was determined using the ascorbic acid method. Available zinc content was determined by atomic absorption spectrometry following extraction of the sample by DTPA-TEA method. Also, pH and EC were measured in water (soil/water ratio 1:2). Data analysis was performed by MSTAT-C software, and the means were compared at α꞊5% by Duncan test. Results and Discussion: The results showed that the use of P-LDH and TSP significantly improved available phosphorus compared to control treatment. However, in contrast to TSP, available phosphorus in P-LDH treatments increased with increasing of time, up to significant difference which was observed between the two sources after 150 days. This result is probably due to slow release of phosphorus from P-LDH and reduction of phosphorus reactions with different soil components. Moreover, available zinc was higher for P-LDH treatments than TSP treatments as dissolution of P-LDH may concurrently release zinc ions into the soil solution. It seems that the application of P-LDH not only increased the availability of phosphorus but also improved available zinc. Therefore, due to the zinc deficiency in calcareous soils, P-LDH can be used as a suitable dual purpose fertilizer for these soils. However, the possibility of Zn toxicity risk due to higher level of LDH application in soil is not ruled out. It is worth mentioning that the variation of pH and EC values in P-LDH treatments showed no significant difference compared to TSP tratments. In other words, application of P-LDH increased soil available phosphorus and zinc without any negative effect on soil pH and EC. Conclusions: The results of this study illustrated that the P-LDH probably can be used as a slow release phosphate fertilizer to increase the phosphorus efficiency; however, care should be taken as the high levels of this fertilizer may not be recommended due to the high zinc content. It should be noted that the high levels of phosphorus are not appropriate for all phosphorus fertilizers but in the present study we used the different levels of fertilizers because the behavior of P-LDH was not clear for us.