1 Introduction

In the context of the circular economy, sustainable agriculture plays a key role in the fight against climate change. Therefore, this work has developed the application of new nitrification and urease inhibitors to avoid nitrogen losses in the form of ammonia to the atmosphere through 4R practices. The objective of this work is to study the effects of different inhibitors and their contribution to the reduction of ammonia emissions which is key strategy to fight climate change, maximizing nutrient absorption in plants. It should be noted that for a better technical evaluation, the same proportions have been used in the fertilization schemes. The circular economy as a concept of major debate and interest throughout the European Union, is a concept that involves economic regeneration based on certain measures of management, integration and reuse of resources in such a way as to reduce emissions of their respective waste [1]. In December 2015, Paris established the key performance indicators Key Performance Indicators (KPIs) in which, by 2030, the industrial sector is expected to have achieved the reduction of CO2 emissions by 60% and by the end of 2050, 100% of this target, i.e., 0% CO2 emissions, should have been achieved [2]. These targets apply mainly to chemical industries such as fertilizer production, fossil fuel energy, production of construction materials, etc. In the fertilizer production sector, the largest source of CO2 emissions is ammonia and nitric acid synthesis plants, since the main raw material for these basic products is natural gas [3].

All nitrogen fertilizers require ammonia as the main raw material and source of nitrogen, therefore, per ton of fertilizer produced, there are also tons of CO2 emitted into the atmosphere at the rate of 1:1 [4]. According to data published by the European fertilizer agency (Fertilizers Europe), agriculture is responsible for 92% of ammonia emissions into the atmosphere, and is therefore a potential source of atmospheric pollution. Sustainable agriculture will be that agriculture whose practices based on 4R: Right source, right place, right rate and right time [5]. This framework describes farm nutrient management practices, and will allow first to reduce CO2 emissions, second, to make a good management of nutrients such as primary as well as secondary, and to allow a better absorption of nutrients for plant metabolization through molecular diffusion from Fick's law shown in Fig. 1.

Fig. 1.
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Fick’s molecular diffusion law

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Where:

〖∂C〗_j: Gradient of nutrients concentration

X: Distance of molecular diffusion

Cx: Local concentration in a certain position

Cs: Output concentration in the root uptake

C0: Fertilizer initial concentration

D: Coefficient diffusivity of the fertilizer

Nitrification inhibitors play a fundamental role in nitrification processes, as well as the urease process. One of the objectives of sustainable agriculture is the fight against climate change, which involves dealing with adverse pedological conditions, water scarcity, arid soils, etc. [6] Dry land represents 41% of the global area available for agriculture and this means feeding more than 35% of the global population [7], this challenge is one of the keys that makes the 4R best practices of smart agriculture important to meet the global food demand [8]. According to data published by the National Institute of Statistics, by 2021 Romania has produced around 14.8 million tons of maize on an area of approximately 2.5 million hectares [9], making Romania the leader in maize production across the European Union as shown in Fig. 2, in terms of quantity and in Fig. 3 y in terms of surface.

Fig. 2.
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Maize and sunflower production in EU,2022*

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Fig. 3.
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Cultivated surface of maize and sunflower in EU,2022*

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*Data collected from the National Institute of Statistic (INS) of Romania and Eurostat.

As can be seen in Fig. 2, the cereal with the highest quotation European continues to be maize, and this is because it is a cereal with a higher productive yield, cultivated in more than 163 countries worldwide and in different pedoclimatic conditions [10]. The countries such as Romania, etc. with high maize production and less cultivated land is due the nature of the agricultural conditions (pedoclimatic factor). The best quality land, provides best performances in yield and nutrients making them leaders in production. The European Union, maize production is experiencing slight fluctuations in production and is becoming more and more remarkable, so the great global demand makes the productive sectors develop relevant technologies for a sustainable agriculture that allows the production, guarantee and food security [11]; for this, it is essential the use and application of nitrification inhibitors, in order to reduce ammonia losses in the atmosphere and increase the use of nutrients, increasing agricultural production with less investment costs [12].

These chemical agents, such as DMPP (3–4 Dimethyl Pyrazole Phosphate), NBPT (N-Butyl Thiosphosphoric Triamide), NPTT (N-propyl Thiosphosphoric Triamide), etc., are the main agents used industrially. These are the main agents used at industrial level. The mechanisms of action of these inhibitors are their effectiveness in the process of transforming nitrogen oxides, nitrites and nitrates into plant available nutrients, and in the case of urease or hydrolysis [13], the solubilization of urea is transformed into ammonia and plant available ammonium as shown in the following chemical reactions (Figs. 4, 5 and 6):

Fig. 4.
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Urea hydrolysis upon nitrogen-based fertilizers

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Fig. 5.
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Nitrification process based on nitrogen fertilizer

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Fig. 6.
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Dimerization of HNO

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Ammonia oxidation reactions:

NH4++ 3/2 O2 → NO2- + 2H++ H2O

NO2-+ 1/2 O2 → NO3-

Both processes, nitrification and hydrolysis or urease, follow the mathematical model of Fick's law of molecular diffusion of nutrients mentioned in the previous chapter [14]. The differentiation between genotypes of cereal and oilseed crops are evident in terms of protein and seed oils. The higher yield of these indicators is directly proportional to soil fertility, weather conditions as well as the fertilization scheme used. The known types of oil crops are rapeseed, sunflower and soybean. Sunflower is an indispensable source of high-quality edible oils for culinary applications [15]; 87% of the world's vegetable oil production comes from sunflower seeds, which demonstrates the importance of this crop and its contribution to the development of agricultural production and world population growth. According to [16], the unrefined oil from this crop has numerous advantages and therapeutic benefits, such as: natural antioxidants, antimicrobial effects, antiinflammatory, antihypertensive and antidiabetic effects. This crop, according to [17], also presents a capacity in terms of adaptation in different agricultural regions, one of the fundamental characteristics it presents is resistance to drought, great resistance to disease and insect attacks and at the same time improves the conditions of the soil by means of a microstructural rearrangement of nutrients for the next crop cycles [18]. The protein content in sunflower seeds is one of the most important indicators, which is directly proportional to the crop genotype, as well as to the agricultural production systems mentioned by [19].

According to FAO data, the oil content of oil sunflower seeds is about 38.50%, while the protein content is about 20%, as confirmed by other experts in agricultural sciences, such as [20] and [8]. Runoff, denitrification, volatilization, leaching are factors that affect nitrogen losses, as well as the increase in nitrogen concentration during planting times. A harmonization between nitrogen fertilizer application and plant nitrogen demand could reduce nitrogen losses and improve nitrogen recovery as well as internal crop efficiencies. Studies by [21] and [22] on the influence of hybrid seed genetics in Indonesia show that the protein content of maize ranges from 8.83%–11.84%, while the oil content ranges from 4.45%–7.17%.

2 Material and Methods

The methodology used for the experimental study is to define the fertilization schemes with their corresponding nitrification-urease inhibitors, fixing an application area in two geographical regions according to the pedoclimatic conditions. The harvested crops, the following indicators will be analyzed: their moisture content, their content of proteins from consumption and total nitrogen, their content of absorbed nitrogen SR ISO 1871:2002, their content of oils based on SR ISO 659:2009. For the measurements of the indicated parameters and object of study, 3 measurements will be taken, taking the average of all of them, 3 samples for each type of crop and batches per geographical area. The measurements will be subject to international agricultural standards. The two difference plots selected to the research where applicate the same fertilization scheme based on base fertilization using complex fertilizer N20:P20+ 0.05% Zn and for cover fertilization, coated urea with different inhibitors were used.

The region characterized by plot L1 presents a flat area, and the predominant soil type for this region is Fluvisol and Gleysol, although there are also other soil types present. While the L2 region is characterized by low plains, and the predominant soil type is Luvisol and Vertisol, although other soil types are also present. For this study, the soil types used in plots L1 and L2 are Luvisol and Fluvisol. These plots are located in the Banat region of Romania, areas well known for the mass production of maize and sunflower. Sunflower and maize seeds were purchased from authorized stores for the sale of agricultural and phytosanitary products. The L1 and L2 plots located in the Western regions of Romania, along the Mures River, present favorable pedoclimatic characteristics that represent mostly the general conditions for planting in Romania. The choice of these two crops is key to be able to analyze the nutrient quality of the crops at the harvest stage. In order to evaluate the performance of both crops with their corresponding nitrification inhibitors, both plots of land were sampled before cultivation, thus analyzing the available nutrient reserves. The parameters monitored were the protein content-quality of the sunflower oil, while in the case of maize it was the protein content. The plot L1 presents a flat area, and the predominant soil type for this region is Fluvisol and Gleysol, although there are also other soil types present. While the L2 region is characterized by low plains, and the predominant soil type is Luvisol and Vertisol, although other soil types are also present.

For this study, the soil types used in plots L1 and L2 are Luvisol and Fluvisol. These plots are located in the Banat region of Romania, areas well known for the mass production of maize and sunflower. Sunflower and maize seeds were purchased from authorized stores for the sale of agricultural and phytosanitary products. The L1 and L2 plots located in the Western regions of Romania, along the Mures River, present favorable pedoclimatic characteristics that represent mostly the general conditions for planting in Romania. The application of solid nitrogen fertilizers coated with nitrification and urease inhibitors has been carried out at Azomures chemical fertilizer plant, located in the city of Targu Mures.

The fertilization schedules used are typical of these crops and are subject to the manual of agricultural practices. The minimum quantities required for these types of crops have been respected in order to achieve the objective of this research.

For the preliminary evaluation of the solid nitrogen fertilizer urea used as the basis of the fertilization scheme, its parameters were chemically analyzed to ensure that it complies with the physicochemical specifications. These parameters were analyzed according to the following standards: For the determination of total nitrogen, it was analyzed according to EN 15478:2009. Biuret determination according to EN 15478:2009 standard. Moisture determination by Karl Fischer method 13466-1:2001. Determination of granulation by SR ISO 2591-1:1998 standard.

Determination of crushing strength of granular fertilizers SR CR 12333:2000. Alkalinity content, expressed as ammonia concentration; determined by STAS 5698-87 and SR ISO 1593:1999, while formaldehyde content, through STAC 408 and urea pH, has been performed through the electrochemical method. The seeds collected at the harvest stage of both crops were analyzed, mainly for moisture content, presenting a result of ϕ ~ 11. For the protein content of both sunflower and maize seeds, it was calculated from the consumption and dependence of total nitrogen based on the method established by standard SR ISO 1871:2002 (“Agri-food products. General guidelines for the determination of nitrogen by the Kjeldhal method”) and for the oil content, applied only to sunflower, the standard method SR ISO 659:2009. (“Oilseeds - Determination of oil content”) was used. On the other hand, for the evaluation of experimental results, descriptive statistics was used, applying Independent Student T-Test and comparison of means.

3 Results

The results obtained for the solid nitrogen fertilizer are shown in Table 1, presented as percent for each measured parameter.

Table 1. Urea physical-chemical parameters result
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As shown in Table 1, the results obtained from the physicochemical analysis of urea show very favorable results that meet the minimum quality conditions in accordance with European Regulation (EC) No. 2003/2003. In the Table 2 the descriptive statistics as mean ± standard deviation (minimum – maximum) is presented for each type of cultivated plots and experimental lots.

Table 2. Descriptive statistic for cultivated plots.
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In the Figs. 7 are presented the box plots for the sunflower protein contain by cultivated plots and lot. It can be observed the visual differences between lots L1 and L2 for sunflower protein content. These differences will be tested, statistically with the Independent Student t test to compare the means of L1 and L2.

Fig. 7.
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Sunflower protein contain by plots

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In the Figs. 8 are presented the box plots for the maize protein contain by cultivated plots and lot. It can be observed, also, the visual range of values differences between lots L1 and L2 for maize protein content. These differences will be tested, statistically with the Independent Student t test to compare the means of L1 and L2.

Fig. 8.
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Maize protein contain by plots

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The results from Table 3 indicate that there are not statistically significant differences in sunflower and also for maize. (p-value > 0.05).

Table 3. The results of the Independent Student T-test
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The results obtained by both the physical-chemical analysis and the statistical evaluation t-test, show that for sunflower, a high amount of proteins is observed in L1 versus L2, which demonstrates that the pedological conditions and the molecular diffusion of nutrients has a greater effect in that plot, so the plant has had greater availability of nutrients and this explains the inhibition of nitrogen losses in the atmosphere through nitrification and urease. Comparing the means obtained in both plots, an increase of ~5.86% more protein was observed in plot L1 compared to L2. Regarding oil content, the means in both plots are not so different, although plot L1 shows better results in oil content, with a 1.7% increase compared to L2. As far as corn is concerned, higher protein content is recorded in L1 versus L2 and according to the statistical results it is also observed that the mean of both plots differs, L1 is still positioned in first place with ~2.4% protein (Fig. 9).

Fig. 9.
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Sunflower oil content

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Regarding oil content, the means in both plots are not so different, although plot L1 shows better results in oil content, with a % increase compared to L2. As far as corn is concerned, higher protein content is recorded in L1 versus L2 and according to the statistical results it is also observed that the mean of both plots differs, L1 is still positioned in first place with ~2.4% protein.

4 Conclusion

The analysis of protein and oil content in corn and sunflower crops have been evaluated in plots L1 and L2. Both crops have an acceptable protein and oil content. Plot L1, being of Fluvisol and Gleysol characteristics, the molecular diffusion of nutrients is much more favored, increasing the effectiveness of nitrification and urease inhibitors. On the other hand, the differences between oil and protein content in plots L1 and L2 are largely due to the slow release of nutrients, thus avoiding nitrogen losses to the atmosphere. The effectiveness of the base and cover fertilization scheme using primary nutrients such as N-P2O5-K2O has been an important factor.

The low efficiency in terms of production and protein content in L2 may be due to the fact that, although the same amount of fertilizers, the same fertilization scheme and nitrification inhibitors have been applied, soil and climatic conditions have influenced the cultivation and germination process, resulting in a delay in the vegetative phase. As for the results obtained for corn, a slight decrease in production and quality of protein content was observed in L2 with respect to L1, although not so relevant; In both plots a stable germination was observed thanks to the effects of zinc oxide (ZnO) as an important catalyst in the flowering stages, harmonizing and favoring the absorption of secondary nutrients such as calcium oxide (CaO), magnesium oxide (MgO), and sulfur trioxide species (SO3-). Nitrification and urease inhibitors are a good alternative for sustainable agriculture, since they prevent nitrogen losses in the atmosphere in the form of ammonia, favoring the availability of nutrients for better plant absorption.