1 Introduction

Given the ongoing global population growth, which is currently nearing eight billion and expected to exceed nine billion by 2050, it is undeniable that there will be a substantial increase in future food demands (United Nations 2019). Wheat, the second most-produced cereal grain after maize, surpasses all other crops combined in global trade. In 2022, the world’s total wheat production reached 781 million tons. China, India, and Russia stand out as the top three individual wheat producers globally, collectively contributing to approximately 40% of the total global wheat production. Turkey also holds a notable position among the world’s top 10 wheat-producing countries, with a production of 20.5 million tons (Filipenco 2023).

The adoption of intensive fertilizer-based cultivation practices for wheat, particularly during and after the green revolution, has played a pivotal role in augmenting yield levels and ensuring food security. However, these practices have also been associated with detrimental environmental consequences. In light of concerns regarding resource sustainability and environmental welfare, there is a pressing need to redirect farming priorities towards responsible production systems. This shift aims to meet the escalating food demands of a growing population while operating within the constraints of available resources (Praveen and Aditya 2022).

Phosphorus (P) enters the soil through the weathering of P-containing rocks and minerals like apatite over time. Additionally, organic materials such as decomposing plants, animal residues, and organic waste also contribute P to the soil. In agriculture, P fertilizers are commonly applied to plants, which can be inorganic, like superphosphate, triple superphosphate, and diammonium phosphate, or organic in nature (Bindraban et al. 2020). Fertilizers are indispensable components for agricultural practices to meet the increasing food demand of the growing population, as they contribute to the provision of essential nutrients required for crop growth and the attainment of substantial yields. The predominant source of nutrients supplied to agriculture primarily relies on the utilization of commercial chemical fertilizers.

In the year 2021, world agriculture used 109 million tonnes of N, 46 million tonnes of P and 40 million tonnes of K fertilizers, with an average application rate of 112 kg of nutrients (N + P2O5 + K2O) per hectare of arable land (FAO 2023). Total N, P and K consumption of Turkey in 2022 were 1.6 million ton, 0.60 million ton and 0.13 milion ton respectively (Anonymous 2023). Wheat, as a crop, exhibits substantial utilization of N and P in terms of fertilizer nutrients. It holds the position of being the primary consumer of both N and P among all agricultural crops. In specific terms, wheat production accounts for 18.2% of the total global N consumption, while its contribution to P consumption is estimated at 14.6% (Praveen and Aditya 2022). The current chemical fertilizers have remarkably low effectiveness (Blackwell et al. 2019). Nevertheless, excessive utilization of fertilizers can engender environmental complications, including water contamination and the release of greenhouse gases, while also imposing significant financial burdens on farmers (Chien et al. 2009). Additionally, imbalances in plant nutrients arise as a result of excessive N and P fertilizer use. Unidirectional fertilization leads to problems such as iron (Fe) and zinc (Zn) deficiencies in plants (Cakmak et al. 2010).

Consequently, it is imperative for farmers to adopt efficient fertilizer usage strategies to mitigate wastage, curtail environmental repercussions, and simultaneously sustain elevated crop yields. While the raw materials for nitrogen fertilizers are currently considered an inexhaustible resource, numerous sources indicate that the raw materials for P fertilizers will become depleted in the near future (Blackwell et al. 2019).

In recent years, innovative studies have focused on the development of more effective fertilizers that can cause less harm to the environment, taking into account the increasing fertilizer demand. Various fertilizers such as polymer or sulphur-coated urea, urease inhibitors, and urea formaldehyde have been developed and proven effective in enhancing the efficiency of N-based fertilizers (Lawrencia et al. 2021). On the other hand, research aimed at improving the chemical efficiency of P-based fertilizers is scarce (Taskin and Gunes 2023). Recently, studies have been conducted on nano hydroxyapatite (nHAP) as an alternative to conventional P-based fertilizers. These studies have reported the potential of nHAP as an alternative sources (Lin et al. 2021; Miranda-Villagómez et al. 2019; Taşkın et al. 2018; Xiong et al. 2018). Furthermore, there have been studies conducted on the doping of hydroxyapatite with urea as a means to enhance P use efficiency while simultaneously providing plants with N (Elhassani et al. 2019; Kottegoda et al. 2017; Pradhan et al. 2021). However, these studies have all been conducted under greenhouse conditions. The effects of nHAP and its derivatives have not been determined under field conditions.

Enhancing the efficiency of fertilizers used in crop production is an important issue to address both economically and in terms of effectively utilizing fertilizer resources and minimizing their negative environmental impacts. Research has demonstrated that applying biochar can enhance productivity across various plant species (Inal et al. 2015; Liu et al. 2022; Xi et al. 2020; Zhang et al. 2017). However, the energy-intensive production process of biochar and its high pH resulting from combustion restrict its direct application in alkaline soils (Taskin et al. 2019). Thus, employing biochar to coat fertilizer granules could prove to be a more efficient strategy than using it in large quantities directly. This study aims to improve the efficacy of diammonium phosphate (DAP) and urea fertilizers by incorporating biochar coatings. Additionally, several researchers have highlighted nHAP as a potent source of phosphorus fertilizer (Taşkın et al. 2018; Taskin and Gunes 2023). Since nHAP contains only P, a fertilizer containing both N and P would be more readily accepted by farmers. Therefore, in this study, a composite fertilizer was obtained by doping nHAP fertilizer with urea. This study seeks to develop highly efficient, ecological, and innovative basal and top fertilizers for wheat cultivation.The research employs two distinct approaches to enhance the effectiveness of P and N fertilizers: 1) utilizing biochar-coated urea fertilizer as a top-dressing in combination with nHAP-based basal fertilizer doped with urea (UnHAP), and 2) employing biochar-coated diammonium phosphate (BDAP) and urea (BUrea) as both basal and top-dressing fertilizers for wheat plants under natural rainfed field conditions for two consecutive years.

2 Materials and Methods

2.1 Soil Properties of the Experimental Area

The experiments were carried out over a two-year period, spanning from 2018–2019 to 2019–2020, under rainfed conditions at various locations within the Haymana Research and Application Farm of the Faculty of Agriculture at Ankara University. All soil analyses have been conducted according to Page and Keeney (1982). The results of some physical and chemical analyses of soil samples taken before planting, representing the experimental area, were presented in Table 1. According to the soil analysis results, in the first year, the experimental soil was clay texture, slightly alkaline, non-saline, moderately calcareous, and deficient in organic matter, N, P, Fe, Zn, and manganese (Mn). However, copper (Cu), K, calcium (Ca), and magnesium (Mg) concentrations were found to be excessive. In the second year, the experimental soil was clayey loam texture, slightly alkaline, highly calcareous, with low concentrations of organic matter, N and P, moderate levels of Fe concentration, very low concentration of Zn, sufficient Cu concentration, and excessive concentrations of K, Ca, and Mg.

Table 1 Some soil properties of experiment area
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2.2 Meteorological Data for the Experimental Area

The dominant climate in Haymana district of Ankara is continental. In the first year of the experiment, the lowest average temperature was recorded in January (-0.8 °C), while the highest average temperature was observed in August (20.7 °C). In the second year, the lowest average temperature occurred in January (-2.1 °C), while the highest average temperature was recorded in July (23.1 °C). According to the long-term average precipitation, the highest rainfall was measured in May (43.7 mm), while the lowest rainfall was recorded in August (7.8 mm), and the average annual precipitation amount was 296.2 mm. In the first year of the experiment, the highest rainfall was recorded in December (54.4 mm), while the lowest rainfall occurred in May (3.8 mm). When compared to the long-term average precipitation, it was determined that the annual rainfall amount measured in the first year, 315 mm, was 18.8 mm higher than the long-term average. In the second year, the lowest rainfall was observed in July–August (0.6–0 mm), and the highest rainfall occurred in May (47.2 mm). The total annual precipitation of 248.9 mm was 47.3 mm lower than the long-term average (Table S1).

2.3 Preparation Urea-Enriched Hydroxyapatite

Hydroxyapatite was obtained as reported by Liu and Lal (2014). Firstly, solutions of 0.6 M 500 mL phosphoric acid (H3PO4) and 1 M 500 mL calcium hydroxide (Ca(OH)2) were prepared. Then, the H3PO4 solution was placed in a separation funnel and dropped onto the calcium hydroxide solution at a rate of one drop per second. These solutions at room temperature were mixed at a speed of 700 rpm using a magnetic stirrer. The pH value of the mixed solution was adjusted to 12 using 10 N sodium hydroxide (NaOH). The solution was transferred to a cylinder, and the solid part was allowed to settle. The clear portion on top was decanted, and the remaining precipitates were removed by washing with distilled water for 5 times. The obtained hydroxyapatite was spread on a clean tray and dried at 100 °C for 12 h. It was then ground to a thickness of 0.5 mm and stored in glass jars for subsequent use. The resulting powdered hydroxyapatite (nHAP, 26.8% P2O5) was enriched with urea solution to achieve a ratio close to the nitrogen and phosphorus content in DAP (18:46:0) fertilizer. For this purpose, 24 g of urea fertilizer was added to 100 g of nHAP. To ensure granule stability during this process, ground rice husk was added at a rate of 10%. All materials used were mechanically mixed until a homogeneous pulp-like consistency was achieved. The mixture in a pulp-like consistency was passed through a pressurized system to obtain pellets measuring 3–5 cm in length. To facilitate application with a plot seeder, the fertilizer material was converted into granules. The urea-enriched hydroxyapatite (UnHAP) fertilizer in granule form was subjected to P and N analysis, revealing N content of 8.10% and P2O5 content of 20.0%.

2.4 Preparation of Biochar and Biochar Coated DAP and Urea Fertilizer

Rice husk was first ground in a stainless steel grinder and sieved through a 0.5 mm mesh. The sieved material was then subjected to the pyrolysis process, following the method described by Gunes et al. (2015). Rice husk biochar was obtained using a slow pyrolysis method (300 °C, 3 h). In the process of coating DAP fertilizer with biochar derived from rice husk, dextrin, which is water-soluble and exhibits thickening and binding properties, was used. A 0.3% dextrin solution was employed to enable the adherence of biochar to the DAP fertilizer. For this process, 100 g of DAP fertilizer was weighed and 25 g of biochar was added to it. To ensure homogeneous coating, the dextrin solution was applied while continuously shaking the container containing the fertilizer by hand. As a result, the DAP fertilizer was coated with biochar. The coated fertilizers were left to dry under laboratory conditions. Once dried, the coated fertilizers were sieved through a 2 mm mesh to remove any remaining biochar that did not adhere to the fertilizer. The amount of biochar used for coating the DAP fertilizer was determined based on the increase in weight. The levels of total N, total P2O5, and water-soluble P2O5 in the biochar-coated DAP fertilizer (BDAP) were determined as 16.5%, 41.5%, and 39.7%, respectively.

To coat the urea granules with rice husk biochar, 100 g of urea was weighed. Subsequently, to enhance adhesion, a 0.3% dextrin solution was sprayed onto the urea granules, followed by an immediate mixture with 25 g of rice husk biochar. The mixture was left to dry under laboratory conditions. To ensure a homogeneous coating, the dextrin solution was continuously agitated while being applied to the container. As a result, the urea fertilizer was coated with biochar. Once dried, the coated fertilizers were sieved through a 2 mm mesh to remove any biochar that did not adhere to the fertilizer. The obtained biochar-coated urea fertilizer was found to have a total nitrogen content of 42.2%

2.5 Characterization of nHAP, UnHAP and Biochar

The surface morphology of the synthesized nHAP was determined using a Quanta 250 FEG Scanning Electron Microscope (SEM) with versatile high-performance low-vacuum tungsten electron source and three imaging modes (high vacuum, low vacuum, ESEM) capable of measurements up to 20 mm Hg pressure, and an acceleration voltage range of 200 V to 30 kV.

To determine the morphology of the nHAP samples after urea dopping and biochar samples, the Scanning Electron Microscopy (SEM) was used. A Tescan brand Mira-Tima model device with 30 keV voltage and 9.99 nA current was used for SEM analyses. The images were taken in depth viewing with different working distance (WD), field of view (FoV), spot size and magnification.

Powder X-Ray Diffractometer (XRD) analysis carried on for the characterization of the processed (UnHAP) and unprocessed hydroxyapatite (nHAP) and rice husk biochar. XRD analysis were carried on using an Inel Equinox 1000 diffractometer machine which was equipped with Co tube at a wavelength (λ) of 1.788970 Å. The voltage and current are 30 kV and 30 Ma, respectively. The estimated 2θ range is between 0–120° and the estimated step 0.030° s−1. The raw data were smoothed and the baseline was corrected. The American Standard Testing Material (ASTM) cards were used for the data evaluation.

2.6 Field Experiments

The experiments were set up in a randomized complete block design with 5 replications, consisting of two separate experiments. In experiment I, UnHAP fertilizer was applied as the basal fertilizer, while in Experiment II, BDAP fertilizers were applied at the specified rates. The Tosunbey bread wheat variety was sown in plots measuring 5.00 m in length and 1.20 m in width, with a row spacing of 15 cm and 6 rows per plot. The seeding rate was 220 kg ha−1 using a plot seeder.

Based on soil analysis results and considering the conditions of Central Anatolia, the recommended P fertilizer application for wheat cultivation under dry conditions is 80 kg P2O5 ha−1. In the experiments, a phosphorus rate of 80 kg P2O5 ha−1 was applied with DAP fertilizer at the time of sowing, which served as the reference treatment. Urea dopped nano hydroxyapatite (UnHAP) and biochar-coated DAP (BDAP) fertilizers were applied at rates equal to or lower than the reference treatment (80, 60 and 40 kg P2O5 ha−1) at the time of sowing. For topdressing, all plots received 150 kg N ha−1 of urea and BUrea. Top-dressing was carried out in April during the stem elongation period in accordance with the suitability of the experiment location.

The first-year planting took place on November 7, 2018, followed by top dressing at tillering (March 22) and plant sampling prior to heading (on May 22) periods. Sample collection for yield components occurred before harvesting of plants (on July 8), and the harvest took place on August 6, 2019. Similarly, in the second year, the same procedures were conducted on October 28, 2019, March 23, May 20, July 20, and August 8, 2020, respectively.

Urea and BUrea fertilizers were manually applied to the plots, distributed homogeneously at a rate of 150 kg N ha−1. The experimental plots were treated with Resital Duo at a rate of 800 mL ha−1 to control weeds.

2.7 Plant Sampling

Plant samples were collected in 10 replicates at the onset of heading and representing the entire plant for each experimental plot. The samples were transferred to the laboratory in paper bags and dried in a forced-air drying cabinet at 65 °C until a constant weight was reached after being passed through tap water twice and distilled water twice. The dried plant samples were ground using a mill with a steel inner chamber to prepare them for analysis.

To determine the yield components, sampling was performed by cutting and collecting the plants within 0.25 m2 frames randomly placed in the plots, just above the soil surface using pruning shears. The samples were bundled and transferred to the laboratory, where they were weighed using a precision balance to determine the combined weight of stems and grains Finally; the harvest of all plots (6.0 m2) were carried out using a combine harvester, and grain samples were transported to the laboratory and weighed using a precision balance to determine grain yield.

2.8 Plant Analysis

Harvested plant parts were dried and ground i ≤ nto fine powder, and sieved with a 200 µm diameter. Plant samples were digested with 65% HNO3 (Merck) using a microwave digestion system (Berghof-MWS-2, 24 units). Total P concentration was determined with the vanadomolybdophosphoric acid colorimetric method (Barton 1948), and total Fe and Zn concentrations were determined by ICP-OES (Varian 720-ES ICP-OES) according to Temminghoff and Houba (2004). Total N concentration was determined by Dumas method (Rutherford et al. 2008). The wheat flour with the code 1567a (NIST) was used as the standard reference material.

2.9 Statistics

The significance of the treatments was determined via one-way ANOVA using MINITAB 17 (Minitab Inc, USA, 2014). The significant differences among treatment means were calculated via Duncan’s multiple range test (p ≤ 0.05) using MSTAT-C (Michigan State Univ, 1991) and descriptive statistics (± SE).

3 Results

3.1 Surface Morpholohy of nHAP, UnHAP and Biochar

The size and surface morphology of the synthesized hydroxyapatite were presented in Fig. 1. The particle size of hydroxyapatite was varied between 10–35 nm. Upon examining their sizes and shapes, it can be observed that hydroxyapatite predominantly consists of rod-shaped nanometric particles that tend to aggregate together. SEM images of nHAP and UnHAP at a scale of 200 µm were provided in Fig. 2. Evaluating the morphologies based on the SEM images of nHAP, it is evident that the particle sizes are different and irregular. In contrast, in the images of UnHAP, the particle sizes appear to be close to each other and regular. These two images indicate that urea molecules has entered the pores of nHAP (Fig. 2). The particle morphology and surface porosity of rice husk biochar was revealed through the SEM images provided in Fig. 3. In Fig. 3, the SEM images depict the presence of irregular pores on the surface of the biochar, which arise from the physical structure of rice husk and the emission of volatile substances during thermal decomposition. Additionally, the biochar produced from rice husk exhibits a rough and diverse structure with various shapes.

Fig. 1
figure 1

Scanning Electron Microscope (SEM) micrograph of nano hydroxyapatite (nHAP) at 200 nm

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Fig. 2
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Scanning Electron Microscope (SEM) micrograph of nano hydroxyapatite (nHAP) and urea dopped nano hydroxyapatite (UnHAP) at 200 μm

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Fig. 3
figure 3

Scanning Electron Microscope (SEM) micrograph of rice husk biochar

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The XRD pattern of the synthesized nHAP and UnHAP were provided in Fig. 4. Upon examining the XRD pattern of nHAP, peaks indicating the formation of HAP are observed. The peaks identified in the graph are found to be consistent with the characteristic peaks of hydroxyapatite present in the database (Inorganic Crystal Structure Database ICSD-171549), specifically at positions 25.9°, 31.8°, 32.2°, and 32.9°. When examining the XRD pattern of UnHAP, it can be observed that the identified peaks are largely in agreement with the peaks in the XRD pattern of nHAP. However, in the graph corresponding to nHAP, there is a single peak, whereas in the graph corresponding to UnHAP, the presence of two peaks is detected. The difference in the peak observed at approximately 22° 2θ value is attributed to the interactions between urea and nHAP nanoparticles, indicating the incorporation of urea into the nHAP structure. The observed peak positions at 25.9°, 31.8°, 32.2°, and 32.9° 2θ values in the XRD graph of urea-enriched nHAP correspond to the crystallographic planes of the nHAP’s XRD graph.

Fig. 4
figure 4

The compared X-Ray Diffraction (XRD) patterns of processed (UnHAP) and unprocessed hydroxyapatite (nHAP)

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The XRD diffractograms of biochars which are obtained from rice husk were seen in the Fig. 5. The data reflect the amorphous carbon. Upon graphical evaluation of the structure, it has been determined that the peaks identified around 22° 2θ are consistent with the characteristic XRD patterns of rice husk biochar as given in the literature. The broad peak observed at this diffraction angle originates from the presence of amorphous silica and carbon in the biochar samples.

Fig. 5
figure 5

X-Ray Diffraction (XRD) patterns of biochar obtained from rice husk

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3.2 Yield and Yield Components of Wheat Plants (Experiment I)

The effects of hydroxyapatite doped with urea (UnHAP), urea and urea coated with biochar (BUrea) fertilizers on grain yield and yield components of wheat are presented in Table 2. The effects of decreasing amounts (80, 60, and 40 kg P2O5 ha−1) of UnHAP fertilizer applied as an equivalent and alternative to the reference base fertilizer (DAP 80 kg P2O5 ha−1), as well as BUrea fertilizer applied from the top dressing, on grain yield, yield components, and protein content of the grain, were found statistically significant in two consecutive years. As expected, the lowest grain yield was obtained from the unfertilized plots (control), with 3477 kg ha−1 in the first year and 1796 kg ha−1 in the second year. The highest grain yields were achieved in the DAP + BUrea treatments in the first year, and in both DAP + BUrea and DAP + Urea treatments in the second year. Although UnHAP and BUrea treatments resulted in lower grain yields compared compared to DAP + BUrea treatments, there was no significant decrease in grain yield, particularly in the second year, even the UnHAP level was reduced to 40 kg P2O5 ha−1 among the applied P doses (Table 2).

Table 2 The effect of basal application of urea-doped nano-hydroxyapatite (UnHAP) and top-dressing urea and biochar-coated urea (BUrea) fertilizers on wheat’s grain yield, biological yield, 1000 grain weight and protein content
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The biological yield was at its lowest level in the control plots for both years. In the first year, high biological yield was obtained from the DAP + urea and DAP + BUrea treatments, while equivalent biological yield was obtained from the 40 kg P2O5 ha−1 UnHAP + Urea and 60 kg P2O5 ha−1 UnHAP + BUrea treatments. In the second year, except for the lowest dosage application of UnHAP + BUrea, all treatments resulted in biological yields equivalent to the reference treatment (Table 2).

In both years, the wheat grown in unfertilized plots had the lowest thousand grain weight and protein content. In addition to this, thousand grain weight obtained from the application of 60 kg P2O5 ha−1 UnHAP + Urea and 80 kg P2O5 ha−1 UnHAP + BUrea belonged to the same group as the thousand grain weight obtained from the control plots in the first year. Although the highest thousand grain weight was obtained from the application of 80 kg P2O5 ha−1 UnHAP + Urea and 60 kg P2O5 ha−1UnHAP + BUrea, there was no significant difference observed among the treatments when compared to the reference. In the second year, all treatments had higher grain weight than the control plots without any noticeable difference among the treatments. In both years, all treatments had higher protein content than the control plots. In other words, there was no decrease in protein content associated with decreasing UnHAP doses (Table 2).

3.3 Nitrogen, Phosphorus, Iron and Zinc Concentrations of Wheat Plants (Experiment I)

 In the first year, the total N concentration of plants in the unfertilized and DAP + Urea-treated plots was lower than in all other treatments. The UnHAP + Urea and UnHAP + BUrea treatments, applied at decreasing doses (80, 60, and 40 kg P2O5 ha−1), resulted in the highest N concentration in plants. In the second year, once again, the lowest N concentration was obtained from the unfertilized plots, followed by the reference treatment (DAP + Urea) and 80 kg P2O5 ha−1 DAP + BUrea. The other treatments provided higher N concentrations compared to these three treatments (Table 3).

Table 3 The effect of basal application of urea-doped nano-hydroxyapatite (UnHAP) and top-dressing urea and biochar-coated urea (BUrea) fertilizers on nitrogen (N), phosphorus (P), iron (Fe) and zinc (Zn) concentrations of wheat plant prior to heading stage
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In both years, the P concentration of plants grown in unfertilized plots was the lowest. In the first year, regardless of the fertilizer treatments, all plants had higher P concentrations compared to the control. In the second year. Even the application rate decreased to 40 kg P2O5 ha−1, there was no significant decrease in the P concentration of the plants (Table 3).

In the first year, there was no difference in terms of the concentration of iron in plants based on the applications. However, in the second year, there were increases in the iron concentrations of the plants depending on the fertilizer applications compared to the unfertilized plot. These increases were significant in the treatments of DAP + Urea, DAP + BUrea, 80 kg P2O5 ha−1 UnHAP + BUrea, and 60 kg P2O5 ha−1 UnHAP + BUrea, compared to the control (Table 3).

The concentration of Zn in plants was lowest in the control, DAP + Urea, and DAP + BUrea treatments in both years. In the first year, the highest Zn concentrations were obtained from the treatments of 40 kg P2O5 ha−1 UnHAP + Urea, 60 kg ha−1 UnHAP + BUrea, 80 kg P2O5 ha−1 UnHAP + Urea and 80 kg P2O5 ha−1 UnHAP + BUrea. In the second year, the highest Zn concentrations were obtained from the treatments of 60 and 40 kg P2O5 ha−1 UnHAP + Urea, as well as 60 kg P2O5 ha−1 UnHAP + BUrea (Table 3).

3.4 Iron and Zinc Concentrations of Wheat Grains (Experiment I)

In both years, the treatments did not have a significant effect on the grain Fe concentration (Table 4). In the first year, the grain Zn concentration was lowest in the control, DAP + Urea, and DAP + BUrea treatments. According to these treatments, the grain Zn concentrations significantly increased in the 80 kg P2O5 ha−1 UnHAP + Urea, 80 and 40 kg P2O5 ha−1 UnHAP + BUrea treatments. However, in the second year, the treatments did not have an effect on the grain Zn concentrations (Table 4).

Table 4 The effect of basal application of urea-doped nano-hydroxyapatite (UnHAP) and top-dressing urea and biochar-coated urea (BUrea) fertilizers on iron (Fe) and zinc (Zn) concentrations of wheat grain
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3.5 The Effect of Biochar Coated DAP (BDAP), Urea and Biochar Coated Urea (BUrea) Fertilizers on Yield and Yield Components (Experiment II)

In both years, the grain yield and biological yield of plants grown in unfertilized plots (control) were lower than those of other treatments (Table 5). In the first year, similar grain yields were obtained from all treatments except for the application of 60 kg P2O5 ha−1 BDAP + Urea. In other words, even if the application rate dropped to 40 kg P2O5 ha−1 in BDAP + Urea and BDAP + BUrea treatments, there was no loss in yield. In the second year, the highest grain yield was obtained from the DAP + Urea, 80 kg P2O5 ha−1 BDAP + Urea, DAP + BUrea, and 80 kg P2O5 ha−1 BDAP + BUrea applications. In the first year, the highest biological yield was obtained from the application of 40 kg P2O5 ha−1 BDAP + Urea. In the second year, even in all treatments, including the reduced BDAP applications down to 40 kg P2O5 ha−1, higher biological yields were obtained than from the unfertilized plot. In other words, even though the application rate was reduced by up to 40 kg P2O5 ha−1, the BDAP treatments still achieved a biological yield comparable to the 80 kg P2O5 ha−1 treatments (Table 5).

Table 5 The effect of basal application of biochar coated diammonium phosphate (BDAP) and top-dressing urea and biochar-coated urea (BUrea) fertilizers wheat's grain yield, biological yield, 1000 grain weight, and protein content
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In the first year, there was no significant difference observed in the thousand grain weights among the treatments. However, in the second year, the thousand grain weight was higher in all treatments compared to the plants grown in the unfertilized plot. In all basal and top fertilizer applications, the protein content of the grains was higher compared to those obtained from the grains of plants grown in the unfertilized plot (Table 5).

3.6 Nitrogen, Phosphorus, İron and Zinc Concentrations of Wheat Plants (Experiment II)

In the first year, the N concentration of the DAP + BUrea application was higher in compared to control plots and no significant differences observed among the treatments except 80 kg P2O5 ha−1 BDAP + BUrea. In the second year, there were no significant differences in the N concentrations of the plants (Table 6).

Table 6 The effect of basal application of biochar coated diammonium phosphate (BDAP) and top-dressing urea and biochar-coated urea (BUrea) fertilizers on nitrogen (N), phosphorus (P), iron (Fe) and zinc (Zn) concentrations of wheat plant prior to heading stage
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In both years, the P concentration of the plants were higher in all treatments compared to the control. Even reducing the P rate from 80 to 40 kg P2O5 ha−1 did not cause a decrease in the plants’ P concentrations except 40 kg ha−1 BDAP + BUrea (Table 6).

In the first year, there were slight variations in the Fe concentrations of the plants among the different applications, and the highest Fe concentration was obtained from the 60 and 40 kg P2O5 ha−1 BDAP + BUrea applications compared to the control. In the second year, all treatments provided higher Fe concentrations compared to the control (Table 6). There were significant differences among the treatments in terms of the Zn concentrations of the plants. The highest Zn concentration, compared to the control, was obtained from the 60 kg P2O5 ha−1 BDAP + BUrea treatment. This was followed by the DAP + Urea, 40 kg P2O5 ha−1 BDAP + Urea, and DAP + BUrea treatments. In the second year, the highest Zn concentration, compared to the control, was obtained from the 60 kg P2O5 ha−1 BDAP + BUrea treatment (Table 6).

3.7 Iron and Zinc Concentrations of Wheat Grains (Experiment II)

The treatments did not have a significant effect on the Fe and Zn concentrations of the grains (Table 7).

Table 7 The effect of basal application of biochar coated diammonium phosphate (BDAP) and top-dressing urea and biochar-coated urea (BUrea) fertilizers on iron (Fe) and zinc (Zn) concentrations of wheat grain
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4 Discussion

Research focused on improving the effectiveness of N and P fertilizers with low efficiency are important for the preservation of the environment and natural resources, the sustainability of agricultural production, and reducing input costs of chemical fertilizers, which are one of the most crucial inputs in agricultural production. The study compared the application of a reference fertilizer consisting of 80 kg P2O5 ha−1 DAP + 150 kg N ha−1 urea with reduced doses (80, 60, and 40 kg P2O5 ha−1) of UnHAP + 150 kg N ha−1 urea and UnHAP + 150 kg N ha−1 BUrea. In the first year, higher rainfall was received, resulting in higher grain yields compared to the second year. Although higher grain yields were obtained compared to the reference treatments, significantly higher yields were achieved when the UnHAP rate was reduced to 40 kg P2O5 ha−1. Reducing the P dose from 80 to 40 kg P2O5 ha−1 in the UnHAP + Urea and UnHAP + BUrea treatments did not lead to a significant loss in grain yield, particularly in the second year. In addition, during the first year, the DAP + Urea and DAP + BUrea treatments yielded a high biological yield, while the 40 kg P2O5 ha−1 UnHAP + Urea and 60 kg P2O5 ha−1 UnHAP + BUrea treatments produced an equivalent biological yield. In the second year, all treatments, except for the lowest dosage application of UnHAP + BUrea, resulted in biological yields comparable to the reference treatment. These results demonstrate that UnHAP can be used at lower doses compared to conventional fertilizers. Previous studies have shown that nano-hydroxyapatite (nHAP) fertilizer is either equivalent to or more effective than conventional fertilizers (Sajadinia et al. 2021; Samavini et al. 2018; Taşkın et al. 2018). It has also been reported that urea-doped nano-hydroxyapatite is effective (Kottegoda et al. 2017; Pohshna and Mailapalli 2022). Nano-fertilizers, with a molecular diameter of less than 100 nm, can be readily taken up by plants. Additionally, due to their structural properties, their reaction with the soil is lower and exhibit lower levels of fixation (Bindraban et al. 2015; Kopittke et al. 2019). Therefore, the ability to use nano-fertilizers at lower levels compared to conventional fertilizers presents a significant advantage. Upon evaluation of grain weight and grain protein content, it is evident that the application of UnHAP fertilizer and BUrea not only had no detrimental effects but also resulted in improved outcomes compared to the reference and control treatments.

It is important to understand not only the impact of the applied fertilizers on yield and quality parameters but also how effectively plants can utilize the nutrients present in these fertilizers and the effects they have on other elements. In both years, it was observed that UnHAP and BUrea applications had equivalent or more positive effects on the total N and P concentrations in plants compared to the control and reference treatments. Furthermore, particularly in the second year, positive effects were obtained on the Fe concentration in plants with the UnHAP + BUrea application.

The reference fertilizer application led to significant reductions in Zn concentration in plants. In contrast, it was evident that plants fertilized with UnHAP and BUrea had much better zinc nutrition. Phosphorus fertilizers can induce Zn deficiency in plants through a process known as P-Zn antagonism. When plants uptake an excess of P, it can interfere with the absorption and translocation of zinc. This interference occurs at both the root level, where high P concentrations can limit Zn uptake, and within the plant, where P competes with Zn binding sites in various cellular structures. Furthermore, the application of P fertilizers may alter the soil’s pH, creating conditions that reduce the solubility and availability of zinc. This reduced solubility makes it harder for plants to absorb Zn from the soil, exacerbating the deficiency. The antagonistic effects of P fertilizers on micronutrients, especially Zn, are well-known (Sánchez‐Rodríguez et al. 2017), whereas the absence of such antagonism in the new generation UnHAP fertilizer is a crucial finding and a significant advantage. Phosphorus fertilizers have been found to have a negative effect on the concentration of micronutrients in grains. In our study, particularly in the first year, similar to the plant grain Zn concentration, plants fertilized with UnHAP and BUrea exhibited higher Zn concentrations. This result can be considered a significant advantage, especially in addressing the widespread issue of Zn deficiency in humans. It is well known that substantial efforts are being made to increase grain Zn concentration in staple crops (Cakmak et al. 2017).

In the second part of the study, the effects of biochar-coated DAP and urea fertilizers were investigated alongside regular DAP and urea fertilizers. Numerous articles have been published on the effects of biochar on plants, and research in this field continues to be conducted (Gunes et al. 2015; Liu et al. 2022; Xi et al. 2020). Indeed, it has been reported that biochar applications can alleviate heavy metal toxicity, especially in acidic soils (Chen et al. 2018; Ibrahim et al. 2019), and enhance the availability of P and other nutrients in soils (Gul and Whalen 2016; Li et al. 2017). However, it should be noted that the effectiveness of biochar applications can vary depending on soil characteristics. Furthermore, due to the high energy requirements for biochar production, its practical use is not yet economically viable, which limits its widespread adoption. Additionally, the alkaline nature of biochar suggests that it should not be used without modification in alkaline soils. In our study, as biochar was solely used as a coating material, it was anticipated that many of the barriers to its utilization would be overcome. Biochar-coated fertilizers are a novel approach in agricultural practices, combining the benefits of biochar with those of traditional fertilizers. Biochar-coated fertilizers are fertilizers where the granules or pellets of traditional fertilizers are coated with a layer of biochar. The biochar coating serves as a matrix that slows down the release of nutrients from the fertilizer, leading to a more sustained nutrient supply to plants over time (An et al. 2021).

Although the highest grain yield of wheat was obtained from the reference treatment (80 kg P2O5 ha−1 DAP + Urea), there was no significant decrease in grain yield even when the base fertilizer application rate was reduced to 40 kg ha−1 in the first year. In the second year, the BDAP + Urea and BDAP + BUrea treatments, applied at equivalent amounts to the DAP + Urea treatment at 80 kg P2O5 ha−1, exhibited similar effects. Similar to our results, Pogorzelski et al. (2020) reported that biochar coated fertilizer increased P recovery and P use efficiency compared to conventional fertilizer in clay soil by reducing the contact time of P with clays and also preventing its precipitation. The biological yield was highest at the 40 kg P2O5ha−1 BDAP + Urea application rate in the first year, but there was no significant difference among treatments in the second year. In other words, reducing the base fertilizer application rate to 40 kg P2O5 ha−1 did not result in any negative effects. Prior to this study, there were no existing studies on the application of biochar-coated DAP, urea, or similar treatments. This study determined, for the first time, that the coating of biochar on DAP and urea fertilizers enhances their effectiveness. The fertilizer applications did not have a negative impact on the thousand grain weight. Furthermore, the protein content of the grains increased in all treatments compared to control.

Despite the reduction in the base fertilizer application rate, there was no significant decrease in the N and P concentrations of plants compared to the reference treatment. The Zn concentrations of plants varied depending on the treatments and the year. In the first year, the highest plant Zn concentration was achieved with the application of 60 kg P2O5 ha−1 BDAP + BUrea. However, no significant differences in Zn concentration were observed among the treatments in the second year. The direct application of biochar has been previously reported to have effects on the N, P and other nutrient elements in plants (Purakayastha et al. 2019). However, the effects of biochar coating on N, P, and other nutrient elements were reported for the first time in this study.

5 Conclusion

Working on nanotechnology and biochar coating technologies is emerging as an important strategy to enhance the effectiveness of conventional fertilizers. Urea-dopped nano hydroxyapatite, biochar-coated diammonium phosphate and biochar-coated urea fertilizers could function as alternatives to conventional fertilizers. These new generation fertilizers can be applied at rates lower than those of conventional fertilizers, thereby promoting fertilizer economy and ensuring sustainable use of raw materials in the fertilizer industry. Additionally, their use can help reduce the negative environmental impacts of fertilizers and alleviate the antagonistic pressures on micronutrients caused by excessive fertilizer application due to their lower efficacy. It should be noted that the long-term and residual effects of these novel ecological fertilizers, which have not been determined in this study, may be higher than those of conventional fertilizers. This aspect should be considered in terms of sustainable agriculture and the efficient utilization of resources. The evaluation of fertilizer application strategies has been conducted, considering the effectiveness of urea-dopped nano hydroxyapatite, biochar-coated diammonium phosphate and biochar-coated urea as demonstrated in the study. By implementing these strategies, farmers and policymakers can contribute to sustainable wheat cultivation practices, enhance crop productivity, and improve overall fertilizer use efficiency, taking into consideration environmental conservation and long-term soil health.