In-Soil Application of NP Mineral Fertilizer as a Method of Improving Nitrogen Yielding Efficiency

This study presents the results of a four-year field experiment assessing the effectiveness of phosphorus application in maize cultivation according to the depth of two-component fertilizer (NP) placement in the soil layer, type of nitrogen fertilizer and date of application. Nitrogen utilization from mineral fertilizer was low—on average, 37.1% during the four years of research. The nitrogen metabolism index, measuring the agricultural and physiological efficiency of nitrogen use, confirmed the significant impact of NP fertilizer placement at 10 and 5 cm as optimal in maize fertilization. The use of nitrogen in maize cultivation before sowing, compared to the application of this component at the phase of 5-6 leaves BBCH 15/16 stage (stage of leaf development with five–six leaves unfolded), significantly increased the agricultural and physiological effectiveness of nitrogen applied in mineral fertilizer. Ammonium nitrate application before sowing the maize, compared to top dressing at the BBCH 15/16 stage, significantly increased nitrogen uptake and utilization from mineral fertilizer. Date of urea fertilizer application to the soil did not have a significant impact on these indicators in maize cultivation.


Introduction
Nitrogen supplied to soil in the form of mineral fertilizers is not fully utilized by crops [1]. While mineral forms of nitrogen are absorbed by plants, leaching out of the soil into groundwater can occur, causing eutrophication [2]. According to Abbasi et al. [3], more than 50% of nitrogen fertilizer applied worldwide is not utilized by plants, and the uptake of this nutrient by maize is approximately 50%. The environmental cost of excess nitrogen application in Europe ranges from 78 to 357 billion USD per year [4]. Nitrogen is an essential nutrient for maize and a key determinant of grain yield formation, in particular due to its role in photosynthesis and other biological processes [5]. In order to reduce the production of excessive amounts of mineral forms of nitrogen in the soil, it is necessary to correctly determine the optimal doses of nitrogen fertilizers, taking into account the physicochemical properties of the soil, the type of nitrogen fertilizer and plant nutritional needs [6]. Achieving an increase in the efficiency of mineral nitrogen fertilizers is not easy, because plants take up nitrogen in the form of nitrate or ammonium ions through the roots from the soil solution [7]. In agricultural practice, the doses of mineral fertilizers, including nitrogen fertilizers, are determined according to the nutritional needs of the plant, without considering the abundance of available nutrients in the soil [1,8]. This causes excessive accumulation of soluble fertilizer components in the soil and increases leaching, which on the one hand loads the natural environment with nitrogen and on the other reduces the effectiveness of fertilizer application [9]. Agriculture is not a closed system and some of the accumulated nitrogen is dispersed into the environment. Hence, rationalizing the use of nitrogen fertilizers in maize cultivation is an important issue for sustainable agriculture, as it can reduce the negative impact of agriculture on the surrounding environment [10][11][12]. Standard broadcast fertilization (superficial) does not always ensure proper plant nutrition, because depending on soil properties, part of the component introduced into the soil in the form of fertilizer will land in places that are beyond the range of crop roots. A much better way to increase nutrient availability is to place fertilizer in close proximity to the seeds [12]. This type of fertilizer application is called row, initial or localized application. This results in a better supply of nutrients to young plants [13], accelerates their vegetation and has a positive effect on grain yield. Initial fertilization can also reduce the nitrogen dose due to better utilization in the year of application [14]. In addition, this method of nitrogen application places the nutrient in a deeper, wetter soil layer, resulting in improved uptake.
In the literature, the effectiveness of initial (row) fertilization was assessed mainly by placing the fertilizer at a distance of 5 cm to the side and 5 cm below the seeds. It was found that the effectiveness of this fertilization method was lower in dry years than in years with a greater sum of precipitation in the growing season [14], with no significant effect of initial (row) fertilization on certain maize morphological traits. Therefore, a comparison of different depths of fertilizer placement in relation to the kernel and soil surface may indicate that a deeper location of the fertilizer in drought conditions, which occurs every year, is advisable [15]. Hence, field studies were conducted at the Department of Agronomy of the Poznań University of Life Sciences to determine the effect of the depth of NP fertilization placement in maize cultivation. The field research hypothesis assumed that the depth of nitrogen application in maize cultivation, the date of nitrogen application and the type of nitrogen fertilizer were of decisive importance in shaping the efficiency indicators of this nutrient application. The purpose of the field experiments was to determine the effect of the depth of two-component mineral fertilizer (NP) placement in the soil layer on the efficiency indices of the applied nitrogen in the cultivation of grain maize. The adopted assumptions were verified on the basis of a four-year field experiment using four depths of NP fertilizer application, two nitrogen fertilizers and two nitrogen dose application dates.

Experimental Field
The field experiment was carried out at the Department of Agronomy of the Poznań University of Life Sciences, on the fields of the Gorzyń Experimental and Educational Unit (52 o 26 N; 16 o 45 E) in the years 2015-2018. The research was carried out for four years in the same scheme in a split-split-plot design with three factors in four field replicates [16]. The following factors were tested: A-1st-order factor-NP fertilizer sowing depth [A1-0 cm (broadcast); A2-5 cm (in rows); A3-10 cm (in rows); A4-15 cm (in rows)]; B-2nd-order factor-type of supplementary nitrogen fertilizer [B1-ammonium nitrate; B2-urea]; C-3rd-order factor-date of supplementary nitrogen fertilization [C1-before sowing; C2-top dressing in the phase of 5-6 leaves (BBCH 15/16 stage)]. The same level of mineral fertilization (30.8 kg P·ha −1 and 107.9 kg K·ha −1 ) was applied in all experimental plots (Table 1). Nitrogen fertilization was balanced to the level of 100 kg N·ha −1 . Ammonium phosphate was applied as row fertilization (27.4 kg N·ha −1 ), and the remaining amount (i.e., 72.6 kg N) was used in accordance with the data presented in Table 1. Fertilization was balanced against phosphorus, which was applied at the whole required dose in the form of ammonium phosphate (18% N and 46% P 2 O 5 ), according to the experimental design under the 1st-order factor. K fertilization was performed before maize sowing in the form of potassium salt (60%). The fertilizer coulters (on plot with initial fertilization) were set 5 cm from the seeds. The application depth of NP fertilizer was according to the 1st-order factor levels. Maize sowing was performed with a precision seeder, with a built-in granular fertilizer applicator ( Table 2). The size of the plot area for harvesting was 14 m 2 . The control object (0 kg N·ha −1 and 0 kg P·ha −1 ) also served for calculating the effectiveness indices of phosphorus utilization in maize cultivation in the experiment. Maize was harvested using a plot combine harvester by Wintersteiger (Table 2). After harvest, grain yield was calculated in terms of dry matter (dt·ha −1 dry matter). Dry grain yield in the control object was as follows: 2015-5.2 t·ha −1 ; 2016-7.4 t·ha −1 ; 2017-6.9 t·ha −1 ; 2018-4.3 t·ha −1 . Nutrient contents (N, P, K, and Mg) in the soil before establishing the experiment were at a medium level, while the pH ranged from 4.5 (2015) to 5.6 (2017). Organic carbon content in the study was from 0.99% C (2018) to 1.07% C (2016).

Meteorological Conditions
Characteristics of climatic conditions that prevailed during the period of field research were based on data from the meteorological station belonging to the Experimental and Educational Department in Gorzyń. Thermal conditions during maize vegetation in the years of research were similar to each other and averaged 15.

Soil Conditions
The morphological structure of the experimental field was typical of the bottom moraine of the North Polish (Baltic) glaciation, the Poznań stadium. The parent materials of the soil were clay or sandy-loam formations. The terrain configuration showed little diversification, the dominant terrain was flat and little undulating. Typologically, the soils of the test field belonged to the black-earth type, a subtype of cambic black-earth, which belong to black earths. According to the international World Reference Base for Soil Resources (WRB), the studied soils should be classified as Phaeozems, and according to the USA Soil Taxonomy, as Mollisols. In terms of soil valuation, the experimental field was classified as class IIIb. Black earths include soils where the direct influence of groundwater or heavy rainfall covers the lower and partly middle parts of the soil profile. In the surface horizons, rainfall and water management dominates, which can be modified to some extent by changing the water properties of the deeper parts of the soil profile.

Assay Methods
In the present study, nitrogen content in grain was assessed using the Kjeldahl method [17] with a KjeltecTM 2200 FOSS device. The use of nitrogen per dose of the mineral fertilizer was calculated using the following formula: where N-use of nitrogen (%), Nf-nitrogen uptake by fertilized plants (kg·ha −1 ), Nc-nitrogen uptake by plants in the control (unfertilized) plot (kg·ha −1 ) and D-nitrogen rate (100 kg·ha −1 ). Agricultural effectiveness was calculated using the following formula: where Ae-agricultural effectiveness (kg dry matter/kg N in fertilisers) GYN-grain yield in the field with a dose of nitrogen (t·ha −1 ) and GY0-grain yield in the field without nitrogen application (t·ha −1 ). Physiological effectiveness was calculated using the following formula: where Pe-physiological effectiveness (kg dry matter·kg N in fertilizers) GYN-grain yield in the field with a dose of nitrogen (t·ha −1 ), GY0-grain yield in the field without nitrogen application (t·ha −1 ), Nf-nitrogen uptake by fertilized plants (kg·ha −1 ) and Nc-nitrogen uptake by plants in the control (unfertilized) plot (kg·ha −1 ).

Statistical Analysis
Statistical analyses such as an analysis of variance (ANOVA) and Tukey's HSD test for comparisons of pairs of means were performed in the study years separately and over the years according to the data model obtained from the split-split-plot experimental designed [16,18]. All calculations were carried out using the STATISTICA 13 software package (2017). Statistical significance was defined at p-value < 0.01 or p-value < 0.05 depending on the source of variation.

Results for Nitrogen (2015-2018)
The data presented in Tables 3 and 4 indicate that variable climatic conditions in the years of the study significantly influenced N uptake and physiological effectiveness of nitrogen utilization by the studied maize variety. The significantly highest nitrogen uptake was recorded in 2016 (138.50 kg·ha −1 ), and the significantly lowest in 2018 (86.87 kg·ha −1 ). In 2015 and 2017, N uptake was similar. Only N utilization and agricultural effectiveness of nitrogen use were not found to be significantly influenced by the year of research. The study focused on selected factor interactions for each variable. The remaining interactions, due to editorial restrictions, have been described only in words.

Nitrogen Uptake and Utilization
The results presented in Tables 3 and 4 indicated a significant impact of NP fertilizer sowing depth (A) on the uptake and utilization of nitrogen by the plants. The use of a depth of at least 5 cm resulted in a significant increase in both N uptake and utilization. The highest means for both traits were obtained for a depth of 5 cm (in rows); however, they did not differ significantly from the means for other fertilizer sowing depths. No significant interaction was found between NP fertilizer sowing depth (A) and the year of research for both traits. Irrespective of the year, the type of supplementary nitrogen fertilizer, (B)-ammonium nitrate or urea did not significantly affect the uptake or utilization of nitrogen by the plants. However, Table 3 indicates that this factor interacted significantly with the year of research (Y) and other factors. The significantly highest mean N uptake for the Y B combination (B1-135.01 and B2-141.99 kg·ha −1 ) was obtained in 2016 ( Table 5). The significantly lowest means were obtained in 2018 compared to other years, and the type of fertilizer did not have a significant impact. In turn, the means of N utilization in the years 2015-2016 were significantly better for the Y B combination than the years 2017-2018, when a clear decrease in N utilization was observed, while in each year separately, the two types of fertilization (B) did not produce significant differences in the means. A significant interaction for both traits was also found between the NP fertilizer sowing depth (A), type of supplementary nitrogen fertilizer (B) and year of the study (Y) ( Table 3). The results listed in Tables 3 and 4 also indicated a significant impact of the date of application of supplementary nitrogen fertilizer (C) on N uptake and utilization by the plants. Additionally, all interactions with this factor were shown to be significant. In particular, it was found ( Table 5) that the significantly highest N uptake by the plants occurred in 2016 (136.35 kg·ha −1 , 140.65 kg·ha −1 ), and considerably the lowest mean N uptake was recorded in 2018 (87.61 kg·ha −1 , 86.14 kg·ha −1 ); the date of application (before sowing or top dressing at the BBCH 15/16 stage) did not play a significant role. For the second trait, the significantly highest mean N utilization was obtained in 2015 for nitrogen application before sowing (45.95%); the mean values decreased in subsequent years. Clearly, the lowest N utilization was recorded in 2018, and the date of application (before sowing or top dressing at the BBCH 15/16 stage) was not significant. The results of the study also showed a significant interaction between the type of nitrogen fertilizer (B), date of supplementary nitrogen fertilization (C) and year of research ( Table 3). The significantly highest mean N uptake was obtained for the Y B C combination in 2016 both for urea before sowing and urea top dressing at the BBCH 15/16 stage. The mean values of this trait increased significantly compared to 2015. Again, significantly lower values of N uptake were recorded in 2017-2018 in comparison to previous years. In turn, the means of N utilization in the years 2015-2016 were significantly better for the Y B combination than in the years 2017-2018, when it was observed that N utilization was clearly decreased. On the other hand, two different types of fertilization (B) did not produce significant differences in the means during each year separately. Considering the B C combination, irrespective of the year (Figure 2), it was found that the mean values of N uptake were similar for each combination of nitrogen fertilization type and nitrogen application date, except when ammonium nitrate was used in top dressing at the BBCH 15/16 stage. Subsequently, the mean value of N uptake decreased significantly (109.81 kg·ha −1 ). Similarly, the mean N utilization for the second trait ( Figure 3) was significantly lower with NP application by top dressing at the BBCH 15/16 stage (33.56%) than with the application before sowing only for ammonium nitrate. For urea, the date of supplementary nitrogen fertilization did not differentiate the mean values of N utilization.

Agricultural Effectiveness of Nitrogen Use
Research has shown (Tables 3-5) that the agricultural effectiveness of nitrogen use depends on the NP fertilizer sowing depth (A), date of supplementary NP fertilization (C) and certain interactions of factors with years and between factors. Table 4 shows that agricultural effectiveness increased significantly with the application of NP fertilizer at a depth (A) of at least 5 cm in rows. Other depths (10 and 15 cm) caused a negligible decrease in efficiency. The influence of factor A (NP fertilizer sowing depth) on the examined trait was independent of the year of the study (interaction with years was insignificant). Apart from the experimental factors, climatic conditions in the study years 2015-2018 did not significantly affect agricultural effectiveness. However, interactions of certain factors or their combinations with the year of research was recorded. A significant interaction between the type of supplementary nitrogen fertilizer and year (Y B) indicated that the highest agricultural effectiveness of nitrogen use was in 2015, with a slight decrease in efficiency recorded in subsequent years. The lowest agricultural effectiveness, compared with previous years, was recorded in 2018 (Table 5). However, the use of ammonium nitrate or urea did not change the effectiveness during individual years. In turn, the significant interaction between the date of supplementary NP fertilization and year (Y C) indicated that the significantly highest agricultural effectiveness of nitrogen use was obtained in 2015, following the application of NP fertilizer before sowing ( Table 5). The significantly lowest agricultural efficiency was recorded in 2018, and the date of fertilization (before sowing or top dressing) was not relevant. Investigating the interaction of Y, B and C, it was found that the use of ammonium nitrate before sowing maize resulted in the significantly highest agricultural efficiency in 2015 compared to other years (except 2017). During each year, the impact of the combination of fertilizer type (ammonium nitrate or urea) and fertilization date (before sowing or top dressing) on the agricultural effectiveness was not significant. Considering the combination of three factors, A, B, C (regardless of the year of research), a significant increase in the agricultural effectiveness was observed when sowing depth of at least 5 cm was applied.

Physiological Effectiveness of Nitrogen Use
The data presented in Tables 3 and 4 demonstrate that changing climatic conditions during the years of the study significantly influenced the physiological efficiency of nitrogen use by this maize variety. The greatest physiological effectiveness in this experiment was recorded in 2017. In the remaining years, the mean for this trait did not differ significantly. In addition, the physiological effectiveness of nitrogen use depended primarily on all interactions of factors with years and between factors. Table 5 shows significant differences in the mean physiological effectiveness for the combination of factors with the years of the study. Considering combination A, it was found that the significantly highest physiological effectiveness was achieved in 2015 compared to other years when NP fertilizer was sown at a depth of 10 cm in rows. During 2017, the highest mean effectiveness was obtained with the use of NP fertilization sowing depths of 5 cm and 15 cm in rows (the difference between the means was insignificant). For combination Y B, the year 2017 again significantly affected the physiological effectiveness. Mean efficiencies for this year and the type of fertilization were the highest compared to other years (B1-60.06 and B2-54.92 kg dm·kg N). However, the type of nitrogen fertilizer (ammonium nitrate or urea) did not affect the physiological effectiveness during each year, except 2018, when the physiological effectiveness was significantly lower after ammonium nitrate application. Considering the combination of the year of research and the date of NP application (Y C), the significantly highest efficiency was again obtained in 2017 compared to the remaining years. Application date of nitrogen fertilizer did not affect the physiological effectiveness in any of the study years.

Discussion
High nitrogen utilization (NUE) from the mineral fertilizer dose is crucial for sustainable maize cultivation [19]. According to Chien et al. [20], the efficiency of nitrogen applied in agricultural crops is relatively low, ranging from 25% to 50%. According to Szulc [21], better understanding the mechanisms of nitrogen application will increase its utilization from the fertilizer dose and reduce the negative impact on the environment. For this purpose, it is very important to synchronize nitrogen application with critical phenological phases of maize demand for this component, and to assess the impact of different nitrogen carriers (fertilizers) on the mechanisms of its application [22][23][24]. Only rigorous cooperation between scientists, decision makers and agricultural advisors as well as the correct implementation of the results can improve NUE in maize production [25]. Nitrogen utilization from mineral fertilizers, as an equivalent of component recovery, is a measure of the effectiveness of its uptake by plants. It is also an assessment indicator in the arable crop production system in terms of both quantity and environmental safety [2]. The value of this indicator depends on soil moisture, temperature, structure and soil volumetric density [26,27]. The worst conditions for the growth and development of maize were recorded in 2018, which was characterized by the lowest total atmospheric precipitation (230 mm) and the highest average daily air temperature (16.6 • C). This situation resulted in the reduction of the nitrogen utilization indices (NA, NU, AE) during maize cultivation. In turn, other authors [27,28] found that too high rainfall during the maize growing season reduced the efficiency of nitrogen application through soil erosion and its leaching into the soil profile. The use of row fertilization is a very good solution in maize cultivation [15,29]. This method of fertilizer application results in a higher concentration of the nutrient. This fertilization method also has an economic and ecological aspect, because lower doses of fertilizers in maize, applied using the initial fertilization method, allow to reduce the costs of fertilization and reduce the risk resulting from their losses to ground and surface waters. Singh et al. [30] claimed that fertilizers placed in soil with high moisture content could be more accessible to plants than those placed shallower or on the surface. Placing a two-component NP mineral fertilizer near maize seeds leads to a higher phosphorus uptake by plants, greater utilization of this component from mineral fertilizer and a higher unit production [12]. The above-mentioned statement was also confirmed in our research, which demonstrated that row application of nitrogen was more effective in maize cultivation compared to broadcast nitrogen application. Fox et al. [31] reported that broadcast application of UAN (urea-ammonium nitrate solutions) resulted in a lower efficiency of nitrogen application than the introduction of UAN into the soil profile. Determining the effectiveness is one of the most important stages in the assessment of fertilization efficiency. The agricultural (agronomic) efficiency expresses the increase in grain yield per unit of applied nitrogen in mineral fertilizer. In the current research, maize fertilized using broadcasting had the lowest agricultural effectiveness (13.31 kg dm·kg N in fertilizers), while maize fertilized in rows at a depth of 5 cm showed the highest agricultural effectiveness (21.50 kg dm·kg N in fertilizers). It should be noted that the application of NP fertilizer in rows, regardless of the application depth, was more effective than broadcast fertilization. The distance of nitrogen fertilization from the plant root affects its uptake. As the distance of the fertilizer from the root system increases, there are fewer roots in a given soil volume, thus the probability that nitrogen will be absorbed by the roots decreases. Therefore, initial (row) fertilization increases soil-fertilizer contact by placing nitrogen in the soil zone with a higher root concentration [12,32]. This in turn increases the effectiveness of this method of sowing a nutrient [1,13]. The date of nitrogen application is a very important problem in maize cultivation [1,33].
In the present study, the entire nitrogen dose of 100 kg N . ha -1 was applied before sowing or divided into pre-sowing and top dressing applied at the BBCH 15/16 stage. It was shown that the application of the entire nitrogen pool before maize sowing significantly increased nitrogen uptake (NU) with grain yield, nitrogen utilization from the fertilizer dose (NU) and the agronomic efficiency of this component (AE) compared to the split dose system. Scharf and Lory [34] showed that nitrogen application before sowing in the autumn was associated with a significant risk of this nutrient loss in the spring, which resulted in reduced yields. In turn, according to Cameron et al. [35], depending on soil and weather conditions, nitrogen application before sowing the plants could result in nitrogen leaching below the crop rooting zone at the beginning of growth, before the period of increased absorption of this nutrient. Therefore, high nitrogen application before sowing provides a high concentration of available nitrogen in the soil profile, before the actual uptake by the plants. The effectiveness of a single dose of nitrogen before sowing decreases with the size of this component dose. On the other hand, nitrogen application during the growing season results in NUE improvement compared to pre-sowing application [36]. Hence, nitrogen application according to crop requirements may contribute to a higher NUE [37]. The use of nitrogen at an early plant growth improves yielding and has a positive effect on vegetative development, while when N is applied significantly later, maize is not able to form generative crop in the maturation period [33]. It should be noted, however, that the cited statement applies only to traditional maize varieties, while the division of nitrogen dose is indicated in the case of "stay-green" hybrids, due to the negative coefficients of nitrogen remobilization [38]. This proves that soil resources are the main source of nitrogen accumulation in the generative growth phase (Nmin). The presented model of the reaction of "stay-green" maize plants should imply a fertilization system with nitrogen and other nutrients. In this type of varieties, only slow-release nitrogen fertilizers should be used. The lack of available nitrogen forms in the soil during the maturation period of "stay-green" maize may limit the yielding of plants in accordance with the law of J. Von Liebig from 1840. According to this rule, the expression of yielding potential of a cultivated plant is not possible with a deficiency of even a single mineral component [38].

Conclusions
Nitrogen accumulation in maize grain clearly depended on the depth of NP fertilization in the soil profile, indicating a slight advantage of the 5-cm depth. The nitrogen metabolism index, i.e., the utilization of this component from mineral fertilizer was at a low level (<38% on average for the years). The value of the index confirmed a significant reaction to the depth of NP fertilization, indicating a depth of 5 cm as preferred in agricultural practice. The nitrogen metabolism index, measuring the agricultural and physiological efficiency of nitrogen use, confirmed the significant impact of NP fertilizer placement at 10 and 5 cm as optimal in maize fertilization. The use of nitrogen in maize cultivation before sowing, compared to the application of this component at the BBCH 15/16 stage, significantly increased the agricultural and physiological effectiveness of nitrogen applied in mineral fertilizer. The application of ammonium nitrate before sowing maize, compared to top dressing at the BBCH 15/16 stage, significantly increased nitrogen uptake and utilization from mineral fertilizer. The date of urea application to the soil did not have a significant impact on these indicators in maize cultivation.

Conflicts of Interest:
The authors declare no conflict of interest.