Optimizing Nitrogen Use Efficiency and Reducing Nutrient Losses in Maize Using Controlled-Release Coated Fertilizers

Abstract

This study aimed to evaluate the agronomic performance and environmental impact of controlled-release coated fertilizers (CRCFs) in upland maize systems. Specifically, we sought to determine the optimal nitrogen (N) application rate that maximizes nitrogen use efficiency (NUE) and minimizes nutrient runoff, while maintaining yield comparable to conventional fertilization practices. A two-year field experiment (2017–2018) was conducted to assess CRCF formulations composed of urea, MAP, and potassium sulfate encapsulated in LDPE/EVA coatings with talc, humic acid, and starch additives. Treatments included various nitrogen application rates (33–90 kg N ha⁻¹) using CRCF and a conventional NPK fertilizer (150 kg N ha⁻¹). Measurements included fresh ear yield, aboveground biomass, NUE, and concentrations of total N (TN), nitrate N (NO₃⁻–N), and total P (TP) in surface runoff. Statistical analyses were performed using linear and quadratic regression models to determine yield responses and agronomic optimal N rate. CRCF treatments produced yields comparable to or exceeding those of conventional fertilization while using less than half the recommended N input. The modeled agronomic optimum N rate was 88.4 kg N ha⁻¹, which closely matched the maximum observed yield. CRCF application significantly reduced TN, NO₃⁻–N, and TP runoff in 2017 and improved NUE up to 71.2%. Subsurface placement and sigmoidal nutrient release contributed to reduced nutrient losses. CRCFs can maintain maize yield while reducing N input by approximately 40%, aligning with climate-smart agriculture principles. This strategy enhances NUE, reduces environmental risks, and offers economic benefits by enabling single basal application. Further multi-site studies are recommended to validate these findings under diverse agroecological conditions.

1. Introduction

Excessive nitrogen (N) fertilizer application in conventional agriculture has raised significant environmental concerns, including nutrient leaching, greenhouse gas emissions, and chemical accumulation in soils [1,2,3]. A large proportion of nitrogen inputs, particularly in the form of urea, is lost through ammonia volatilization, nitrate leaching, and nitrous oxide emissions, contributing to eutrophication and rising atmospheric greenhouse gas concentrations [4,5,6]. These inefficiencies often result in nitrogen use efficiency (NUE) levels below 50% in many cropping systems.

Controlled-release fertilizers (CRFs) have emerged as promising technologies to improve nutrient uptake while reducing environmental losses. By gradually releasing nutrients for crop uptake in response to environmental factors such as soil temperature and moisture, CRFs aim to synchronize nutrient availability with crop demand and enhance NUE [1,2,3,7,8]. However, the effectiveness of CRFs highly dependent on their release dynamics, which are influenced by coating materials, soil conditions, and fertilizer placement method. Suboptimal release—either delayed under cool or dry conditions or prematurely accelerated under excessive moisture—can lead to poor crop performance and increased nutrient losses [9,10]. Improper fertilizer placement, mismatches with crop uptake periods, and degradation of coatings in biologically active soils further limit their efficiency [11,12]. Moreover, the high cost of CRFs, when not accompanied by consistent yield benefits, limits their adoption in many farming systems [5].

Recent innovations in coating technology, including hybrid organic–inorganic matrices and nano-encapsulation, offer new possibilities for smart nutrient release systems that respond to environmental stimuli [11,13]. For example, polymer and sulfur-based coatings have shown improved synchronization with crop nutrient demand [12], while biodegradable formulations are increasingly favored for their environmental compatibility [13]. In the case of paddy fields, Shoji and Kanno [14] reported an 18.4% increase in NUE when controlled-release fertilizers were applied as side-dressed rather than broadcast topdressing. Building on this, Choi et al. [15] introduced a seedbed fertilization method that delivered nutrients closer to the root zone, allowing a 10% reduction in N input compared to conventional topdressing.

These studies underscore that the most critical factors of CRFs are the application method and the nutrient release pattern, both of which must be carefully tailored to meet the nutrient demands of the crop. As reviewed by Dovzhenko et al. [13], the design of encapsulated fertilizers is intended to synchronize nutrient release kinetics with key crop developmental stages—such as the vegetative and reproductive phases—thereby improving NUE, enhancing crop productivity, and promoting environmental sustainability.

The effectiveness of CRFs is largely determined by the properties of their coating materials, thickness, and formulation components. Synthetic polymers such as low-density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) offer strong mechanical stability and controlled permeability, while biodegradable polymers like starch enhance environmental compatibility. The thickness of the coating plays a key role in regulating nutrient release duration and patterns—thinner coatings allow rapid release suitable for early growth stages, whereas thicker coatings extend release over longer periods, supporting linear, parabolic, and sigmoidal release patterns for crop demand. Additionally, functional additives such as talc improve coating uniformity, humic acid enhances nutrient uptake and soil organic matter, and clay minerals aid in ion exchange and water retention. Together, these components allow CRFs to synchronize nutrient availability with crop growth stages, minimize leaching and volatilization losses, and contribute to more efficient and sustainable nutrient management [5,11].

Although slow- and controlled-release fertilizers are widely used in paddy rice systems across East Asia, their application in upland cereal crops remains limited. Therefore, the objective of this study was to evaluate the agronomic and environmental performance of CRCF under reduced N input conditions in maize cultivation. We hypothesized that subsurface placement of CRCF in the rhizosphere at reduced N application rates would enhance NUE and minimize N losses through surface runoff in upland maize cultivation.

2. Materials and Methods

2.1. Site Description

Field trials were conducted in 2017 and 2018 at the Gyeonggi-do Agricultural Research and Extension Services station, located at 37.22° N, 127.04° E in Korea. Soil samples were collected from the 0–20 cm layer of each plot at the experimental field prior to fertilizer application each year. Soil pH and electrical conductivity (EC) were measured in a 1:5 (w/v) soil-to-water suspension using a pH/EC meter. Total nitrogen (TN) was determined using the Kjeldahl method, available phosphorus (Olsen-P) was extracted with 0.5 M NaHCO₃ and measured by the molybdenum blue method, and exchangeable cations (K⁺, Ca²⁺, Mg²⁺) were extracted with 1 N ammonium acetate and analyzed using atomic absorption spectrophotometry. Soil organic matter (SOM) was measured by the Tyurin wet oxidation method. The experimental soil was classified as loam and characterized by moderate pH and exchangeable cation levels but exhibited elevated available phosphorus (P₂O₅) and relatively low organic matter content (13–14 g kg⁻¹) (

Table 1. Chemical properties of the soils used in the experiment during 2017 and 2018.

Year	Texture	pH (1:5)	OM (g kg−1)	NO3−–N (mg kg−1)	Av. P2O5 (mg kg−1)	Exchangeable Cations (cmolc kg−1)
						K+	Ca2+	Mg2+
2017	Loam	6.4	14	26	497	0.69	6.2	2.2
2018		6.5	13	30	520	0.71	6.3	2.0

OM = organic matter.

).

2.2. Crop Cultivation and Experimental Design

Maize (Zea mays L., cv. Jangsu Heukchal) was transplanted on June 1 and harvested on September 3 in both years, resulting in a uniform 95-day growth cycle. Each treatment plot measured 25.2 m², with a total experimental area of 378 m². Plants were arranged at a spacing of 70 × 25 cm in a randomized complete block design (RCBD) with three replicates.

Seven fertilization treatments were applied as outlined in

Table 2. Amount of mineral fertilizer (N–P₂O₅–K₂O) application in the treatments for maize in 2017 and 2018.

Treatment	Amount of N–P2O5–K2O Application (kg ha−1)
	2017 Year			2018 Year
	N	P2O5	K2O	N	P2O5	K2O
None	0	0	0	0	0	0
STD_NPK	150	30	60	150	30	60
CRCF_T1	33.1	6	8	-	-	-
CRCF_T2	49.7	9	12	-	-	-
CRCF_T3	66	11	16	66	11	16
CRCF_T4	-	-	-	80	14	20
CRCF_T5	-	-	-	90	16	24

Note: STD_NPK = fertilizer recommendation from rural development administration (RDA), Korea.

. The standard treatment (STD_NPK) involved the use of urea, fused phosphate, and potassium chloride, with nitrogen applied in two equal splits: half at transplanting (basal) and half at 30 days after transplanting (DAT). In contrast, controlled-release coated fertilizer (CRCF) was applied entirely as a single basal dose at transplanting by placing the fertilizer 10 cm deep in the root zone. The procedure involved leveling the field, digging transplanting holes to a depth of approximately 10 cm, applying the designated amount of CRCF, placing the seedlings, and covering them with soil.

In the STD_NPK treatment, fertilizers were broadcast on the soil surface and incorporated by tilling to a 10 cm depth, whereas CRCF was placed directly into the root zone. In 2017, the treatments consisted of None, STD_NPK, CRCF_T1, CRCF_T2, and CRCF_T3. In 2018, CRCF_T3, CRCF_T4, and CRCF_T5 were evaluated alongside None and STD_NPK (Table 2).

The recommended nitrogen (N) application rate for maize in upland field is 150 kg ha⁻¹. In the 2017 trial, CRCF was applied at substantially lower rates—ranging from 33 to 66 kg ha⁻¹ of N. Despite the reduced N input, maize yields exhibited a strong linear response to CRCF application, indicating that even low rates of controlled-release N can enhance productivity under appropriate conditions. To further investigate the yield response of maize under controlled-release fertilization, a follow-up field experiment was conducted in 2018. In this trial, CRCF application rates were extended up to 90 kg ha⁻¹ of N to determine the optimal application rate and the response curve under upland maize cultivation. The treatments were designed to test incremental N rates using CRCF with subsurface placement near the rhizosphere, based on the promising results from the previous year. Each treatment was replicated in a randomized complete block design (RCBD) with three replications.

2.3. Details of CRCF Fertilizer Used

The controlled-release coated fertilizer (CRCF) used in this study was developed by NOUSBO Co., Ltd. in collaboration with the Gyeonggi Agricultural Research and Extension Services in Korea. It features a coating matrix composed of both natural substances—such as talc, starch, and humic acid—and synthetic polymers, including low-density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) [16]. This formulation exhibits a sigmoidal nutrient release pattern, characterized by an initial lag phase followed by rapid nutrient release pattern during critical crop growth stage. Such kinetics closely match the nutrient uptake curve of maize (Zea mays L.), thereby reducing N losses from leaching. The CRCF used in this study was specifically optimized for upland cropping systems, with coating thickness and composition adjusted to provide sustained nutrient availability throughout the growing season. Unlike conventional fertilizers that require multiple split applications, this CRCF enables a single basal application placed near the crop rhizosphere.

To deliver nitrogen, phosphorus, and potassium directly to the root zone, urea, monoammonium phosphate (MAP), and potassium sulfate (PS) were used as the core nutrient sources, encapsulated within a coating matrix consisting of organic polymers (low-density polyethylene (LDPE) and ethylene vinyl acetate (EVA)) to regulate flexibility and permeability, mineral fillers (talc and humic acid) to enhance structural stability and provide auxiliary benefits, and a plant-derived starch binder to promote biodegradability and porosity. This formulation was designed to provide a sigmoidal-release type, engineered to delay nutrient release initially and accelerate it in later stages [13,17].

2.4. Measurement of Nitrogen Use Efficiency

At 90 days after transplanting (DAT), aboveground biomass was measured using five randomly selected plants per plot. Plant samples were dried at 70 °C for one week, ground, and analyzed for total nitrogen using an elemental analyzer (VARIO MAX, Elementar, Germany). Nitrogen uptake (N uptake) was calculated using the following equation:

N uptake (kg ha⁻¹) = Aboveground biomass (kg ha⁻¹) × N concentration (%)/100

Plant samples were oven-dried at 70 °C until constant weight, ground, and analyzed for total N content using the Kjeldahl digestion method. The calculated N uptake was used to assess treatment effects on nitrogen use efficiency (NUE). Nitrogen use efficiency (NUE) was determined using the following formula:

$$NUE (\%)={\displaystyle \frac{U_N-U_0}{N_A}}\times 100$$

where, $U_N$ is N uptake in fertilized plots (kg ha⁻¹), $U_0$ is N uptake in unfertilized plots (kg ha⁻¹), and $N_A$ is N applied (kg ha⁻¹).

2.5. Estimation of Optimum Nitrogen Rate

To identify the agronomic optimum N rate for CRCF application, different yield response models were applied to maize ear yield data. A linear model fitted to the 2017 dataset and a quadratic model fitted to the 2018 dataset were used to determine the optimal N rate of CRCF.

2.6. Runoff Collection and Water Quality Analysis

To quantify nutrient runoff, a flow-measuring flume (Tracom Inc., Alpharetta, GA, USA) was installed at the bottom of each experimental plot. During rainfall events, runoff samples (1.5 L) were collected every 2–3 h and stored at 4 °C for no more than 48 h prior to the analysis of total nitrogen (TN), nitrate-N (NO₃⁻–N), and total phosphorus (TP). TN was measured using the alkaline persulfate digestion method with UV spectrophotometric detection (APHA 4500-N C), NO₃⁻–N was determined by the cadmium reduction method (APHA 4500-NO₃⁻ E), and TP was analyzed following acid-persulfate digestion using the ascorbic acid method (APHA 4500-P E). All analyses were conducted in triplicate using a UV-Vis spectrophotometer to ensure data accuracy. Rainfall data were recorded using a magnetic rain gauge (Model TR-525I, Texas Electronics Inc., Dallas, TX, USA). In 2017, four major rainfall events were sampled during the growing season. Water quality analysis focused on four rainfall events that were deemed representative and hydrologically significant (Figure 1).

2.7. Statistical Analysis

All data were subjected to statistical analysis using XLSTAT software (version 2021.3.1). Mean differences among treatments were evaluated using Duncan’s Multiple Range Test (DMRT) at the 5% significance level. For the yield response modeling, linear and quadratic regression models were fitted to determine the agronomic optimum N rate based on fresh ear yield. Model fit was evaluated using the coefficient of determination (R²).

3. Results

3.1. CRCF Response to the Dried Aboveground Biomass, Fresh Ear Yield, and Nitrogen Use Efficiency

The dried aboveground biomass, fresh ear yield, and nitrogen use efficiency (NUE) of maize differed significantly among treatments in both 2017 and 2018 (

Table 3. Effects of controlled-release coated fertilizer (CRCF) and conventional fertilization on aboveground biomass, fresh ear yield, and nitrogen use efficiency (NUE) of maize at 90 days after treatment in 2017 and 2018.

Treatment		2017				2018
	Above- Ground Biomass	Fresh Ear Yield	N Uptake	NUE	Above- Ground Biomass	Fresh Ear Yield	N Uptake	NUE
	(kg ha−1)	(kg ha−1)	(kg ha−1)	(%)	(kg ha−1)	(kg ha−1)	(kg ha−1)	(%)
None	4603c ± 254	4939d ± 117.6	16.7 ± 1.5	-	4951d ± 163	6115c ± 29.3	60.3 ± 4.5	-
STD_NPK	7463a ± 372	6830a ± 199.7	46.3 ± 1.8	20.3c ± 2.5	7736a ± 202	8201ab ± 1089	114.8 ± 8.5	36.4c ± 3.4
CRCF_T1	6583b ± 340	6290c ± 88.9	33.7 ± 0.4	51.4a ± 4.4	-			-
CRCF_T2	7273a ± 206	6487b ± 87.4	39.0 ± 1.4	44.8b ± 0.4	-			-
CRCF_T3	7400a ± 167	6862a ± 47.4	45.5 ± 3.0	43.4b ± 3.4	6698c ± 142	8001b ± 112.5	86.1 ± 5.5	78.1a ± 8.1
CRCF_T4	-			-	7895a ± 240	8680a ± 539.6	117.2 ± 9.6	71.2a ± 8.2
CRCF_T5	-			-	7364b ± 182	8556a ± 198.8	105.9 ± 9.3	47.5b ± 10.1

Note: NUE = nitrogen use efficiency; None = no fertilizer application; STD_NKP = standard fertilization at150 kg N ha⁻¹; CRCF_T1, T2, T3, T4, and T5 indicate N rate at 33.1 kg ha⁻¹, 49.7 kg ha⁻¹, 66 kg N ha⁻¹, 80 kg N ha⁻¹, and 90 kg N ha⁻¹, respectively. Values represent means. Different letters within each column indicate significant differences at p < 0.05.

). In 2017, the highest aboveground biomass was observed in the STD_NPK (7463 kg ha⁻¹), CRCF_T2 (7273 kg ha⁻¹), and CRCF_T3 (7400 kg ha⁻¹) treatments, with no significant differences among them (p < 0.05). CRCF_T1 showed slightly lower biomass (6583 kg ha⁻¹) but was still significantly greater than the unfertilized control (None, 4603 kg ha⁻¹). A similar trend was found for fresh ear yield: CRCF_T3 and STD_NPK recorded the highest yields at 6,862 and 6830 kg ha⁻¹, respectively, followed by CRCF_T2 (6487 kg ha⁻¹) and CRCF_T1 (6290 kg ha⁻¹), all significantly higher than the control (4939 kg ha⁻¹). NUE varied significantly across treatments, with CRCF_T1 achieving the highest value (51.4%), followed by CRCF_T2 (44.8%) and CRCF_T3 (41.0%), while STD_NPK showed the lowest NUE (20.3%).

In 2018, CRCF_T4 (7895 kg ha⁻¹) and STD_NPK (7736 kg ha⁻¹) again produced the highest biomass, followed by CRCF_T5 (7364 kg ha⁻¹) and CRCF_T3 (6698 kg ha⁻¹). The None treatment showed the lowest biomass at 4951 kg ha⁻¹. Fresh ear yield patterns mirrored biomass trends, with CRCF_T4 and CRCF_T5 producing the highest yields at 8,680 and 8556 kg ha⁻¹, respectively, both significantly higher than STD_NPK (8201 kg ha⁻¹). NUE was also highest in CRCF_T4 (71.2%), followed by CRCF_T5 (50.7%) and CRCF_T3 (41.0%), while the lowest NUE was again observed in STD_NPK (36.4%).

3.2. Optimal Nitrogen Rate for CRCF Application in Maize

In the 2017 experiment, fresh ear yield increased in response to CRCF application, with a positive trend observed as CRCF_N input increased. The yield under CRCF treatments approached that of the conventional standard NKP treatment (6830 kg ha⁻¹) at 150 kg N ha⁻¹, despite lower nitrogen input levels. This indicates that CRCF has the potential to maintain or improve yield while reducing N application, supporting the hypothesis that enhanced NUE and yield stabilization can be achieved (Figure 2A). The 2018 data exhibited a quadratic trend with increasing N rates. The yield increased up to a certain N level and then plateaued, reflecting diminishing returns. Several CRCF treatments produced yields comparable to or slightly exceeding that of the STD_NPK treatment at 8201 kg ha⁻¹, despite receiving lower N inputs (Figure 2B).

To estimate the agronomic optimum N rate, a quadratic regression model was fitted to the 2018 yield data using observed values from CRCF_T3, CRCF_T5, and CRCF_T4 treatments. The resulting curve exhibited a typical curvilinear response—the yield increased with N input to a maximum and then declined slightly. The model predicted an optimum N rate of approximately 88.4 kg N ha⁻¹, corresponding to a maximum fresh ear yield of 8676 kg ha⁻¹. This estimate was closely similar with the highest observed yield of 8680 kg ha⁻¹ recorded under CRCF_T4 (90 kg N ha⁻¹), confirming the accuracy of the model in describing the 2018 response. These results indicate that maize yield reached its maximum potential at N rates around 88–90 kg N ha⁻¹ under CRCF application in the upland field, demonstrating that reduced N input with controlled-release fertilizers can achieve high productivity while improving input efficiency.

3.3. Nutrient Concentrations in Runoff

Figure 3 presents the concentrations of total nitrogen (TN), nitrate nitrogen (NO₃⁻–N), and total phosphorus (TP) in runoff water under different fertilization treatments during the 2017 growing seasons. The STD_NPK treatment exhibited the highest nutrient losses in all three parameters (TN, NO₃⁻–N, and TP), highlighting the environmental risks associated with conventional fertilization practice. In contrast, all CRCF treatments (T1–T3) showed significantly reduced concentrations of nutrients in runoff, with clear differences across nitrogen application rates. Among the CRCF treatments, CRCF_T1 (33.1 kg N ha⁻¹) had the lowest concentrations of TN and NO₃⁻–N, while CRCF_T3 (66 kg N ha⁻¹) maintained relatively low runoff losses compared to STD_NPK, despite the higher nitrogen input. TP concentrations were consistently low in CRCF treatments. CRCF treatments significantly reduced nitrogen and phosphorus runoff losses even at increasing nitrogen rates. The results demonstrate that controlled-release fertilizers, even at moderate application rates (up to 66 kg N ha⁻¹), can effectively minimize nutrient leaching compared to conventional fertilization, thereby improving nutrient use efficiency and environmental sustainability.

4. Discussion

4.1. Coating Composition and Nutrient Release Behavior

The effectiveness of CRFs is largely determined by the properties of their coating materials, thickness, and formulation components. Synthetic polymers such as low-density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) offer strong mechanical stability and controlled permeability, while biodegradable polymers like starch enhance environmental compatibility. The thickness of the coating plays a key role in regulating nutrient release duration and patterns—thinner coatings allow rapid release suitable for early growth stages, whereas thicker coatings extend release over longer periods, supporting linear, parabolic, and sigmoidal release patterns for crop demand. Additionally, functional additives such as talc improve coating uniformity, humic acid enhances nutrient uptake and soil organic matter, and clay minerals aid in ion exchange and water retention. Together, these components allow CRFs to synchronize nutrient availability with crop growth stages, minimize leaching and volatilization losses, and contribute to more efficient and sustainable nutrient management [5,11,12].

4.2. Agronomic and Environmental Advantages of CRCF

The results of this study highlight the dual agronomic and environmental advantages of controlled-release coated fertilizers (CRCFs) in upland maize systems. CRCF application at almost half of the recommended N rates produced aboveground biomass and fresh ear yields comparable to those achieved with conventional fertilization at 150 kg N ha⁻¹ (STD_NPK). This underscores the potential of CRCFs to maintain crop productivity while reducing fertilizer input. The superior NUE observed under CRCF treatments is attributed to enhanced nutrient synchrony, whereby N is released gradually in a sigmoidal pattern designed with crop uptake dynamics, thereby reducing the risk of nitrogen loss [5,10,17].

For example, conventional application methods for CRFs, such as surface topdressing and side-band placement, often result in suboptimal NUE, particularly under conditions of variable soil moisture and temperature. These approaches generally fail to ensure close synchrony between nutrient release and root uptake due to spatial and temporal mismatches. Recent studies reinforce this: a two-year field trial in China demonstrated that deep placement of controlled-release urea at approximately 15 cm significantly increased maize grain yield by 14–38% and improved NUE by 5–11% compared to surface application, while also reducing lodging risks [18,19]. Similarly, in North Florida, a two-year evaluation of polymer-coated CRFs showed that even the lowest CRF rate (168 kg N ha⁻¹), when placed near the root zone, produced yields and plant health metrics comparable to or exceeding conventional fertilizer rates (269 kg N ha⁻¹), while reducing late-season NO₃⁻ leaching [20]. In the results of the 2017 year, CRCF treatments (CRCF_T1 through T3) significantly reduced total nitrogen (TN), nitrate nitrogen (NO₃⁻–N), and total phosphorus (TP) runoff compared to the STD_NPK treatment. Conventional surface-applied fertilizers are more susceptible to runoff, especially nitrate, which is highly mobile and environmentally hazardous [21,22]. The observed reduction in total nitrogen (TN), nitrate-N (NO₃⁻–N), and total phosphorus (TP) runoff under CRCF treatments aligns with prior studies employing controlled-release fertilizers in maize and other crops. For instance, He et al. [23] reported that the CRF treatment reduced TN leaching by 27–32% compared to recommended fertilization and farmer practice, with nitrate-N being the main leached form. CRF also increased rice yield by ~20% and improved NUE by up to 19%. The results suggest CRF is an effective strategy for balancing productivity and environmental protection in rice cultivation. These comparisons substantiate the current findings and reinforce the utility of CRCFs in minimizing nutrient transport to surrounding ecosystems. In contrast, subsurface placement of CRCF in the root zone, combined with its sigmoidal release mechanism, effectively minimized nutrient exposure to surface hydrological pathways and reduced off-site contamination risk.

4.3. Yield Response and Optimal N Rate Determination

This study provides compelling evidence that CRCFs can achieve comparable or superior maize yields at significantly lower N input levels than conventional fertilization methods. The linear yield response observed in 2017 and the quadratic trend in 2018 collectively reflect the biological and environmental interactions affecting nutrient uptake and crop performance under CRCF regimes. As shown, the 2017 results indicated a positive linear yield response to increasing CRCF application rates, suggesting that N was not yet a limiting factor within the tested range and that further increases in N supply could continue to support biomass accumulation and ear formation. In contrast, the 2018 quadratic response revealed a saturation point, where additional N did not translate into proportional yield gains—indicative of the law of diminishing returns. This pattern is commonly observed in N response studies and is often attributed to factors such as luxury uptake, nutrient antagonism, or increased nitrogen losses at high application rates [24,25]. The modeled agronomic optimum N rate of approximately 88.4 kg N ha⁻¹, closely matching the maximum observed yield at 90 kg N ha⁻¹, suggests that CRCF can effectively deliver nutrients in synchronization with maize demand. This reinforces the premise that precise nutrient release, when combined with spatially targeted placement, can improve NUE.

4.4. Climate-Smart and Economically Viable Fertilization Strategy

Importantly, the high yields achieved under CRCF treatments at reduced nitrogen rates (approximately 60% of the conventional 150 kg N ha⁻¹) highlight the potential of resource-efficient fertilization strategies that synchronize effectively with climate-smart agriculture principles. This approach not only lowers fertilizer input costs and reduces environmental burdens—such as nitrate leaching and nitrous oxide emissions—but also simplifies field operations by enabling a single basal application, thus minimizing labor and machinery requirements. From a practical perspective, identifying an optimal N rate of approximately 88–90 kg N ha⁻¹ provides a valuable benchmark for farmers and agronomists aiming to integrate CRCF into upland cropping systems.

While the production cost of CRCFs is higher than that of conventional fertilizers due to coating materials and processing, the reduced application rates and improved nitrogen use efficiency (NUE) may offset the initial investment. In particular, the minimized risk of nutrient losses and potential reductions in compliance or remediation costs enhance the economic appeal of CRCFs, especially in regions with environmental regulations or labor shortages. Nevertheless, a formal economic analysis was not conducted in this study. Future investigations should include comprehensive cost-benefit evaluations that consider fertilizer pricing, labor inputs, crop market returns, and environmental impact metrics to fully support the adoption of CRCF technologies under diverse agricultural contexts.

4.5. Comparison with Recent Controlled-Release Technologies

Recent innovations in slow- and controlled-release fertilizer (CRF) technologies include biochar-coated urea, nano-encapsulated nitrogen carriers, and microbial-stabilized N formulations. For instance, Zhou et al. [26] developed a nanocomposite fertilizer combining attapulgite nanoclay, sodium humate, and urea to enhance nitrogen use efficiency (NUE). The formulation reduced urea hydrolysis and allowed a 20% reduction in urea usage while increasing rice grain yield by 11% compared to conventional urea, demonstrating improved nitrogen efficiency and crop performance through nanotechnology-based fertilizer design. Similarly, Jia et al. [27] found that the application of biochar-coated urea (BCU) improved nitrogen use efficiency (NUE) by ~20% compared to conventional urea, primarily by reducing nitrate leaching through slow-release properties and biochar-induced nitrogen adsorption. Although BCU increased ammonia volatilization due to elevated NH₄⁺ levels and soil pH, it enhanced soil nitrogen availability and uptake in oilseed rape. A 20% reduction in nitrogen input with BCU maintained crop biomass while minimizing nitrogen losses and significantly improving NUE. In comparison, the CRCF used in this study—composed of LDPE/EVA polymers, talc, humic acid, and starch—provided a sigmoidal nutrient release pattern tailored to maize uptake curves and allowed efficient subsurface placement at the root zone. This approach enhanced NUE (up to 71.2%) while maintaining yields at reduced N inputs. Unlike nano-fertilizers, CRCF can be manufactured using commercially available coating agents and applied using conventional transplanting equipment, making it a more feasible option for large-scale adoption in upland maize systems. By situating our findings within the broader field of CRF innovations, this study underscores the practical balance CRCF offers between performance, environmental safety, and operational scalability.

4.6. Limitations of CRCF Under Diverse Climatic and Soil Conditions

While CRCF offers numerous agronomic and environmental benefits, its effectiveness can vary significantly depending on climatic and soil conditions. In high-rainfall regions or poorly drained soils, excessive moisture may accelerate coating degradation or enhance nutrient leaching, thereby reducing the intended slow-release effect and increasing environmental risk [16]. In contrast, under arid or semi-arid conditions, insufficient soil moisture can hinder water diffusion through the coating, delaying nutrient release and limiting uptake during early growth stages [28]. Soil temperature is another critical factor: low temperatures reduce diffusion rates and microbial activity, thereby slowing nutrient release from coated fertilizers [29]. Furthermore, soil texture and organic matter content significantly influence CRCF performance. In sandy soils with low cation exchange capacity, nutrients may not be retained efficiently, while in heavy clay soils, slow infiltration and limited root penetration may reduce nutrient accessibility [30]. These findings emphasize the need for region-specific optimization of CRCF formulations, including coating thickness, polymer type, and placement strategies, to align nutrient release dynamics with variable environmental conditions.

5. Conclusions

This study demonstrated that controlled-release coated fertilizers (CRCFs) can sustain maize yield while reducing nitrogen input by up to 40% compared to conventional fertilization. CRCF applied at 88–90 kg N ha⁻¹ achieved yields comparable to the standard 150 kg N ha⁻¹ treatment, with significantly higher nitrogen use efficiency (NUE). The sigmoidal nutrient release pattern and root-zone placement improved synchrony with crop demand and minimized nutrient losses through surface runoff. These results highlight CRCF as a viable strategy for resource-efficient and environmentally sound maize cultivation. Further multi-site and economic assessments are recommended to support broader adoption in upland farming systems.