Wheat is considered the most important grain and one of the vital food products in Iran. After nitrogen, phosphorus is the most important nutrient required by plants and holds a high priority for the growth, yield and quality of plants. However, due to the introduction of phosphorus in various reactions in the soil, a small amount of consumed phosphorus fertilizer is removed by the plant and the rest of it is left in a non-absorbable form in the soil. The efficiency of using phosphorus fertilizers and the availability of this nutrient is considered as a limiting factor for the production of agricultural products in calcareous soils with alkaline reaction of Iran. Since graphene and its oxidized form, with large amounts of active oxygen groups and high specific surface area, have been proposed by many studies as non-toxic and biocompatible materials in the production of compounds with improved efficiency of using nutrient, therefore to increase the efficiency of phosphorus consumption in soil, in this study, phosphorus was loaded on graphene oxide (GO-P). The present study aims to assess the influence of this compound as a source of phosphorus and its mixing with triple superphosphate fertilizer (GO-P-TSP) compared to triple superphosphate soluble fertilizer (TSP) on the amount of water retention of fertilizers in soil and phosphorus concentration in aerial parts of wheat plant.

Methods and Materials:

Graphene oxide was prepared based on the modified Hamers method. Then graphene oxide was adjusted to certain pH and iron sulfate as a source of iron ions was added to the graphene oxide suspension with vigorous stirring. The mixture was stirred for one hour and then centrifuged for 30 minutes. Then the supernatant was removed and the residue of the compound was dry frozen. In the next step, pH was adjusted with sodium hydroxide (NaOH) solution. Then a certain weight of potassium dihydrogen phosphate salt (KH2PO4) was added to the above suspension. The mixture was stirred for one hour and centrifuged for 30 minutes. After centrifugation, the supernatant was removed and the remains of the phosphorus composition based on graphene oxide were dry frozen. Loading tests were performed in three replicates. pH, EC, bulk density, total concentration of phosphorus and iron and X-ray diffraction spectroscopy (EDS) analysis were measured in the sample of phosphorus composition based on graphene oxide. Then three fertilizer formulations were selected, which included (1) triple superphosphate fertilizer, (2) synthesized phosphorus fertilizer based on graphene oxide, and (3) mixing graphene oxide-phosphorus compound with triple superphosphate fertilizer in a ratio of 50:50% phosphorus.To investigate the water retention behavior of fertilizers in the soil, dried samples of the three studied fertilizer formulations was added into a sandy soil completely and weighed. At the same time, dried sandy soil without fertilizer was placed in another beaker as a control.Then each beaker was added distilled water and weighed. The beakers were weighed once every three days at room temperature until they reached constant mass. The water-retention behavior of the soil was calculated.In order to investigate the effect of three fertilizer formulations on phosphorus availability, soil with low amount of phosphorus was selected and physical and chemical properties of the soil sample were measured at a depth of 0-30 cm. A greenhouse experiment on wheat planting was conducted using a randomized complete design with 3 replications. The treatments included three fertilizer formulations at three fertilization levels (10, 15, and 20 mg kg-1) with 3 replications. The control treatment was performed without phosphorus fertilizer. Plants were harvested 72 days after planting, 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. After harvesting, the weight of fresh and dry matter and phosphorus concentration in the soil and aerial parts of the plant were measured. Statistical data were analysed using SAS software (9.4) and the mean values were compared using LSD tests (at 1 and 5% level).

The composition of phosphorus based on graphene oxide (GO-P) in powder form had 35.5% of total P2O5, 31.1% of soluble in water P2O5, 19.6 of total iron and 15.28% of total potassium. The result of EDS analysis confirmed the loading of phosphorus on graphene oxide. The pH of the phosphorus composition based on graphene oxide was 5.8, approximately 2.5 units higher than triple superphosphate fertilizer. The bulk density of the compound (GO-P) was significantly lower than triple superphosphate fertilizer. The EC of the compound (GO-P) was similar to the EC of the triple superphosphate fertilizer. Soil water retention with synthesized phosphorus fertilizer based on graphene oxide (GO-P) was higher than soil (control) and other compounds added to soil. Experimental results showed that the addition of prepared fertilizer formulas (GO-P and GO-P-TSP) increased water retention in the soil for a longer period of time, while in the soil without adding fertilizer and triple superphosphate treatment, respectively, from 10 and 11 days, the absorbed water completely evaporated. Therefore, the combination of soil with GO-P and GO-P-TSP compared to the soil without fertilizer and the combination of soil with triple super phosphate (TSP) fertilizer had better water retention behavior. The greenhouse experiment results of wheat planting showed that all treatments were significant (P<0.01). Among all the treatments and measured levels, the control treatment showed the lowest value. The highest concentration of phosphorus in aerial parts of wheat (0.31%) and in soil after harvesting (9.5 mg kg-1), fresh (10.6 g per pot) and dry weight (2.03 g per pot) of aerial wheat plants were related to the treatment of phosphorus compounds based on graphene oxide at the level of 20 mg kg-1.

The highest concentration of phosphorus in aerial parts of wheat was related to the treatment of phosphorus compound based on graphene oxide at the level of 20 mg kg-1. Therefore, with more research in the future to produce "nutritious plants" in sustainable, efficient and flexible agricultural systems, we can benefit from technologies based on carbon materials.

The availability of phosphorus (P) is a limiting factor for the production of crops due to its reactions with soil components. Furthermore, there are concerns about the depletion of non-renewable global rock phosphate (the main source of P) reserves because of the high demand for P fertilizers. Therefore, it is essential to revisit existing agricultural practices to determine new resource management practices that utilize renewable resources. The application of sewage sludge could be an alternative P source; contrary to inorganic fertilizers, sewage sludge is cheap, contains nutrients, and improves soil quality due to contained organic matter. The total P content of sewage sludge may vary from less than 0.1% to over 14% on a dry solid basis, depending on the nature of the raw sewage being treated and the treatment process under consideration. However, the use of organic P resources can affect the soil chemistry, leading to changes to the P fractions and their quantities. Hence, the objective of this study was to compare the effect of the application of municipal sewage sludge and triple superphosphate on the distribution of soil-P fractions under saline and non-saline conditions.

To investigate the effect of municipal sewage sludge and triple superphosphate on changes in P fractions an incubation experiment was conducted in a completely randomized factorial design with three levels of triple superphosphate (0, 75, and 100 Kg ha-1 which were named T0, T1, and T3, respectively), three levels of municipal sewage sludge (0, 0.25 and 0.5% w/w which were named M0, M1 and M3, respectively), two levels of salinity of irrigation water (2 and 12 dS m−1, which were named saline and non-saline, respectively) and three replicates. The total number of samples was 54. The treated soils were incubated for three months and maintained at field capacity by adding the appropriate amount of saline and non-saline waters. P fractionated to KCl-P (soluble and exchangeable P), NaOH-P (Fe- and Al bound P), HCl-P (Ca-bound P), Res-P (residual P), and organic-P by sequential extraction method. Moreover, P percentage recovery for Olsen-P at each treatment was calculated. P concentration in samples was determined by the molybdate method. Data analysis was performed by MSTAT-C software, and the means were compared at α꞊5% by Duncan test. 

The results showed that although the relative distribution of fractions followed the order of HCl-P < Organic-P < KCl-P < NaOH-P <Res-P, the changes in each fraction were dependent on the type of treatment and fraction. The amounts of KCl-P for application of municipal sewage sludge and fertilizer TSP combined, especially, T2M2 were 3.1 and 2.3 times higher than T0M0 in non-saline and saline conditions, respectively. The same result was obtained for NaOH-P. The combined and separate application of municipal sewage sludge diminished the relative distribution of HCl-P compared with triple superphosphate and control treatments in both salinities. However, the HCl-P in all treatments was more than 57% of the total P, suggesting that most of the soil P was in the carbonate phase. The treatments did not have a considerable impact on Res-P. The relative distribution of Organic-P increased by increasing levels of salinity and municipal sewage sludge. Therefore, it seems that municipal sewage sludge addition along with fertilizer P can reduce the negative effects of salinity and increase soil P availability compared with alone use of P fertilizer through growing the contents of KCl-P, NaOH-P, and organic-P fractions and, consequently, decreasing P entry into HCl-P fraction. Moreover, the application of municipal sewage sludge plus triple superphosphate increased P recovery as Olsen-P compared to a separate application of triple superphosphate which confirmed the advantage of the combined use of these sources.

The findings of this study indicate that the simultaneous application of municipal sewage sludge and triple superphosphate can effectively improve phosphorus (P) availability in saline conditions. This enhancement is attributed to the alteration of the relative distribution of non-stable P fractions, such as KCl-P and NaOH-P, which increase, while stable P fractions like HCl-P decrease. Moreover, the addition of municipal sewage sludge into soils led to a significant increase in organic C as well as the relative distribution of organic-P. Therefore, application of municipal sewage sludge can improve the physico-chemical properties of saline soil along with increase of P availability. Hence, further research on the growth response of halophyte plants as affected by these treatments is recommended.

Nitrogen and phosphorus are essential nutrients in growth of plants. In addition, the type and method of fertilizer application play an important role in the efficient absorption of nutrients and reduce fertilizer use. Therefore, an experiment was conducted in a randomized complete block design with three replications in the Bahar city in 2017: phosphorus fertilizer was three levels (no application, triple superphosphate and diammonium phosphate), nitrogen fertilizer with two levels (placement and no-placement) and potato cultivar with three levels (Agria, Banba and Sante). The highest leaf area index (5.27), crop growth rate (13.22 g.m-2) and total dry matter (582.86 g.m-2) were obtained from diammonium phosphate along with nitrogen placement in by using Agria cultivar. The results also showed that under conditions phosphorus deficiency, nitrogen application method did not affect leaf area index, crop growth rate and total dry matter: However, tuber yield of Agria under phosphorus deficiency increased by nitrogen placement treatment. In this study, the use of diammonium phosphate, compared to triple superphosphate, increased leaf area index, crop growth rate, relative growth rate and total dry matter. The effect of nitrogen placement along with triple superphosphate was higher than that of nitrogen with diammonium phosphate. Chlorophyll index was not affected by nitrogen placement when using diammonium phosphate, whereas, nitrogen placement increased chlorophyll index by 6.5% with triple superphosphate. The results also showed that the tuber yield increased by 25.4% when nitrogen used by placement combined with triple superphosphate fertilizer. As a whole, highest tuber yields of all three cultivars, especially in Agria and Banba, were increased under nitrogen placement treatment along with diammonium phosphate.

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). Because limited studies have been done regarding 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 the 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. To achieve the aim of this study 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 or yellow 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.

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) was 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.

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