The evaluation of the effect of coated fertilizers was created by comparing the data within the groups using the treatments with the same fertilizer application system (divided, single, and blends). Each method of fertilization was assigned with a control treatment (the treatments D and S). D served as the control variant for the group with a divided application, and S served as a control for the group with a single application and blends.
2.1. Yield and Oiliness of Rapeseed and N Content in Plant Biomass
The appropriate type of fertilizer and method of fertilization is important for the high yield production of rapeseed. Several studies describe the increase in yield and qualitative parameters of crops after using coated fertilizer application [
38,
39,
40,
41]. Our study showed that the use of coated CAN fertilizers has no negative effect on the yield and qualitative parameters of winter rapeseed. Statistical evaluation of the data shown in
Figure 1 revealed no significant differences between the treatments in the groups with divided application (D, D-opu, D-o) and blends (S, Bl-opu, Bl-o). A significant positive effect was recorded in the group of treatments with a single application of coated CAN fertilizers (opu-CAN-oil-based polyurethane-coated CAN; o-CAN-oil-based polymer-coated CAN) in seed yields and oil contents. Seed yields of this group showed a trend of opu-CAN > o-CAN > CAN with opu-CAN up to 18% higher in comparison to the uncoated CAN. Similar results were recorded in the study by Tang et al. [
42], in which a single basal application of coated nitrogen fertilizers contributed to the increase of the yield and rice quality in comparison to the divided application. A different trend was recorded in the case of the oil content that reached up to 5.5% higher after a single application of oil-coated CAN fertilizer compared to the use of the uncoated CAN fertilizer. The presumption was that the total nitrogen applied in the single application of coated fertilizers was released over a longer period of time and thus was present in the phase of the seed formation confirmed by Tian et al. [
38]. In this study, the increase was recorded by an average of 17.3% after the application of coated fertilizers in rapeseed yield rates compared to the control. This study also proved that lower doses of the total N applied in coated fertilizers contributed to a yield increase of 14.2%, which confirmed their environmental potential in terms of nitrogen release. The study by Lu et al. [
43] showed the positive effects of CRFs application on rapeseed yield manifested in the increase of rapeseed pods from 27 to 32% in comparison to non-coated urea. In comparison to the treatments with coated CAN fertilizers, a single application of the uncoated CAN (treatment S) proved the decline in the parameters of oil production and thousand seed weight (TSW) shown in
Table 1. Similar positive effects of coated CAN fertilizers were proved on yield and qualitative parameters of rapeseed. It can be concluded that o-CAN may be a proper alternative instead of opu-CAN.
The data from
Figure 2 indicate a connection between the yield rates and the nitrogen concentration in aboveground plant biomass. In general, plants can only consume a part of nutrients (in our case nitrogen) from conventional fertilizers, and the rest may be subject to losses to the environment [
44]. This trend is mainly visible in the treatment with the application of conventional uncoated CAN fertilizer in a single dose (S), resulting in a significantly lower concentration of nitrogen in plant biomass in the growth stage of flower bud emergence (
t2) compared to the growth stage of stem elongation (
t1). This decrease indicates that the overdose of quickly released nitrogen in uncoated CAN fertilizer led to N-loss available for direct plant consumption and ultimately caused the lowest yield and oil content. The declining trend in the supply of the available form of N, released from conventional uncoated CAN, during the period and the increased supply of mineral N released from coated CAN is also evident from the assessment of N content in aboveground biomass (
Table 2). The nitrogen content in the plant shows a gradual release of the available forms of this nutrient from the coated CAN that is particularly evident in the group of singly applied fertilizers (S). While the nitrogen content detected in the aboveground mass of rapeseed fertilized with uncoated CAN (S) was detected almost 4 and 2 times higher in the term
t1 compared to the treatments with coated CAN (S-opu, S-o) in the term
t2, the nitrogen content of the treatments fertilized with coated fertilizers was increased. These values show that the oil-coated CAN is able to release nitrogen more rapidly than the oil-based, polyurethane-coated CAN and thus may supply the plant’s demand for this nutrient. Nitrogen contents in plants, treated with coated fertilizers applied in blends with conventional CAN (Bl-opu, Bl-o), can confirm this trend.
The relationship between the optimal nitrogen supply and its impact on the yield and oil content of rapeseed is described in many studies [
45,
46]. A similar trend was recorded in the treatments with coated CAN fertilizers applied in blends with the uncoated CAN fertilizer (Bl-opu, Bl-o). Nitrogen content in plant biomass in the growth stage of stem elongation decreased about 1.3% and 0.9% compared to the uncoated CAN fertilizer applied in a single dose. The N content in plant rapeseed showed the most even N pumping during vegetation in the variant with divided application and a single application of coated CAN fertilizers.
2.2. Mineral Nitrogen Content in the Soil
The release of nitrogen from coated CAN fertilizers significantly affected the dynamic change of the soil mineral N (N
min) content in the growth process of rapeseed. Contents of N
min and its ionic forms (NO
3−, NH
4+) were determined in the soil in three experimental phases (
t1–
t3,). Although, enough of the available nitrogen can be essential for direct plant consumption. The excessive content may inevitably increase its loss in soil [
47]. Average contents of N
min in soil (without differencing into layers), shown in
Table 3, serve as an overview of nitrogen release development in the treatments during the rapeseed vegetation.
One of the important aspects of coated fertilizers is the longevity of nutrient release in sufficient levels for plant uptake. The use of coated CAN fertilizers in each form of the application (D, S, and Bl) has shown a positive effect on N
min release pattern, as can be seen from
Figure 3. The effect was visible, especially in the period between the first (
t1) and the second term (
t2) of soil samples collection that was significantly milder compared to conventional uncoated CAN.
The relatively accelerated release of nitrogen was observed in high N
min concentration after the application of fertilizers (
t1 single application,
t2 divided application) in the treatments with conventional uncoated CAN shown in
Table 3. Rapid release N
min was visible mainly in the single application in which N
min concentration decreased rapidly up to 65.4% between
t1 and
t2 (up to 22 days). Our assumption was that although the part of the soil N
min was obtained from the soil through plant roots, the great contrast in N
min concentration was due to N loss (NH
4+ volatilization and NO
3− leaching) between
t1 and
t2. On the contrary, the data of the soil samples, collected in the harvest time (
t3), showed relatively high levels of N
min in the treatments with divided (especially D-o) and single (S-opu, S-o) application of fertilizers in comparison with conventional CAN treatments. Dynamic of gradual N
min release was most visible after a single application of both coated CAN (S-opu, S-o) with no definite decrease in N
min content in
t3. A single application of oil-based polyurethane-coated CAN fertilizer (S-opu) caused an increase by 14.2% in
t3 in mineral nitrogen content compared to
t2 in soil. These findings corresponded to the data of yield and qualitative parameters (
Figure 1), in which a single application of coated CAN fertilizer (S-opu) proved to be the most effective. The assumption was that the amount of released nitrogen reached sufficient levels for the plant demand in the time of the experiment duration from these treatments, thus leading to the increased nitrogen use efficiency and subsequently to a more positive environmental impact (lower risk of N loss). Our data are consistent with the findings of Xiao et al. [
48], who described that the total N
min content continued gradually to an increase in the top layer of soil on the ninetieth day after the application of coated fertilizers, while high levels were maintained in the middle and bottom layer of soil.
The positive effect of coated CAN fertilizers on N
min content was also visible in the nitrogen distribution between soil layers during the experiment (
Figure 3). The application of conventional uncoated CAN fertilizer (D and S treatment) showed high N
min concentrations mainly in the top and middle layers of the soil right after fertilization. The treatments with coated CAN fertilizers showed that N
min content was, in general, focused mainly on the top layer of the soil during
t1 and
t2. N
min content was evenly distributed between each layer of the soil in the harvest time (
t3). This indicates that both coated CAN fertilizers (opu-CAN and o-CAN) proved a high ability of gradual nitrogen release leading to more efficient nitrogen use by the plant and a reduction in the environmental risk. A gradual N
min release by coated fertilizers was also described in the study by Zheng et al. [
49], who found that the application of coated fertilizers resulted in enhanced N
min concentration in soil, especially during later crop stages.
Considering the placement of the fertilizers (the placement on the soil surface without incorporation to the soil), the highest potential for the NH
4+ volatilization is most likely to be closest to the soil surface [
50]. Ammonium nitrate (used CAN in our experiment), depending on N dose and irrigation, belongs to the conventional nitrogen fertilizers with a high potential of NH
4+ volatilization [
51].
This assumption was confirmed by the data obtained from the top layer of the soil samples (
Figure 4). The data showed the greatest potential for NH
4+ volatilization in the treatments with conventional uncoated CAN (D and S treatments) expressed in significantly high NH
4+ concentrations in
t1 and
t2. Analogous to N
min, the uncoated CAN potential of volatilization was visible between
t1 and
t2, in which the NH
4+ concentration decreased up to 39.8% in soil. Higher NH
4+ concentrations were accountable to the use of conventional uncoated CAN (1/3 of the total N dose) after the application of blend fertilizers (Bl-opu, Bl-o). Similarly, the S variant (a single application of uncoated CAN) was resolved in its rapid release. NH
4+ contents in Bl-opu and Bl-o were detected almost over half lower in
t1 than in the S treatment; therefore, major risks of NH
4+ losses were not found. In addition to the volatilization, a rapid NH
4+ release also presents the risk of the increased concentration of nitrates as an initial component of nitrification in soil and thus increased the risk of NO
3− leaching [
52].
The positive effect of coated fertilizers was expressed by significantly lower NH
4+ concentrations during
t1–
t3 in comparison to conventional uncoated CAN. The data were indirectly consistent with the findings of Xiao et al. [
48], who mentioned that the application of coated fertilizers resulted in lower NH
4+ rates in soil samples in comparison to conventional uncoated nitrogen fertilizer. A gradual NH
4+ release was also expressed by the increase of NH
4+ concentration in the top layer of the soil in
t2. This fact was noticeable in the treatments of D-opu (up to 41.3%), D-o (up to 58.8%), and S-o (up to 29.7%). The treatments with a divided and single application of fertilizers were proved to be the most efficient in terms of the longevity of NH
4+ release. These types of fertilizer applications showed significantly higher NH
4+ contents in
t3 treatments compared to the treatments with conventional uncoated CAN. On the contrary, NH
4+ contents showed no significant difference in the S treatment in Bl-opu and Bl-o. This led to an assumption that all nitrogen contained in coated fertilizers and applied in blends was released during the rapeseed vegetation, predetermining the blend application as the most suitable alternative.
Contents of NO
3− were monitored as the main potential source of N loss in the soil samples due to their high leaching ability. One of the first studies by Liegel and Walsh from 1976 [
53] proved that the application of controlled-release N fertilizers was the most effective technique in sandy irrigated soils with a high risk of nitrate leaching. Preventing the leaching of nitrates presents one of the greatest environmental challenges in terms of nitrogen fertilizer use. The estimation of the potential for N losses due to the NO
3− leaching from the experimental treatments were provided by the isolation of the data from the bottom and middle layers of soil. The data obtained from the middle layer (ML) of the soil (
Figure 5) served for the evaluation of potential NO
3− migration to the lower layers of the soil, which might consequently lead to its leaching into the groundwater. The data obtained from the bottom layer (BL) of the soil (
Figure 6) served to evaluate the potential of nitrates leaching to the groundwater during the rapeseed vegetation and directly after its harvest.
As predicted, significantly, the highest potential for NO
3− leaching was due to rapid nitrogen release from conventional uncoated CAN fertilizers recorded in single or divided CAN application. The potential for NO
3− leaching after uncoated CAN application was possible to confirm from the data of NO
3− concentrations in
t1 and
t2 shown in
Figure 5 and
Figure 6. The NO
3− content of ML and BL was detected over three times higher (>3.3) in the treatment fertilized with a single application of uncoated CAN in
t1 compared to the treatments with coated CAN fertilizers. The data showed that the NO
3− decrease was found up to 73.9% in ML and up to 75.5% in BL in the S treatment between
t1 and
t2. Considering the amount and duration (up to 14 days), it is most likely that nitrates of the uncoated CAN fertilizer were lost due to the nitrate leaching. These findings corresponded with the data by Zhang et al. [
54], who discovered that the rates of the leached nitrates in water samples were detected significantly higher in comparison to coated urea in the treatments with conventional urea.
Identical to N
min and NH
4+, the positive effect of coated CAN fertilizers was recorded in the form of gradual NO
3− release over the course of the whole experiment. Gradual release of nitrates was discovered to be the most visible between
t2 and
t3 in coated fertilizers. The increased NO
3− contents were observed up to 64.7% in ML and up to 119.9% in BL. While the NO
3− amount was decreased in ML and BL in the treatments fertilized with uncoated CAN, the coated CAN fertilizers were able to supply the plants with nitrogen even in the later stages of the development. Compared to the low levels of NO
3− content in the treatments with conventional CAN fertilizers (due to rapid nitrogen release and subsequent N loss). This increase correlated with the data of seed yield and qualitative parameters (
Figure 1) and can be used as a potential supply of available nitrogen for the next crops. The data correlated with the findings of Xiao et al. [
48]. Similar N
min release (especially NO
3−) was proved using oil-based polymer-coated CAN, which can be a proper alternative for oil-based polyurethane-coated CAN. This fact is not suitable for future use due to polyurethane’s lower biodegradability. The positive effect of coated fertilizers on nitrates leaching was recorded in several studies [
55,
56,
57,
58].