Plasma-modified biodegradable coatings for controlled nitrogen release from urea

Abstract

Slow-release urea based on biodegradable polymers offers a sustainable alternative to conventional formulations, which often causes rapid nitrogen loss and adverse environmental impacts. This study aimed to develop and evaluate novel biodegradable polymer coated urea using polybutylene succinate (PBS), polycaprolactone (PCL), and methylene diphenyl diisocyanate (MDI) as a crosslinker, with or without plasma surface treatment. Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis revealed decreased crystallinity in PCL after MDI crosslinking and plasma treatment, suggesting enhanced polymer interactions and structural modifications within the blend. Nitrogen release dynamics were assessed through a 90-day soil column experiment. Compared with the control, all coated formulations significantly delayed peak of the release of nitrate-nitrogen (NO₃⁻-N) from 30 days to approximately 70 days after precipitation. At day 90, cumulative NO₃⁻-N release was 29.8 mg (PBS + PCL + MDI), 26.5 mg (PBS + PCL + Plasma), and 35.9 mg (MDS), compared with 48.6 mg in control. Total nitrogen concentrations showed similar trends, with the MDS (urea coated with PBS/PCL blend treated with both MDI and plasma) treatment achieving sustained release and a 70-day peak concentration (222.0 mg L⁻¹) versus a 30-day (229.5 mg L⁻¹) in control. Release profiles were best fitted by a three-parameter sigmoidal model, indicating improved control over nutrient availability. These results highlight the potential of plasma-modified biodegradable polymer-coated urea to provide prolonged and tunable nitrogen release, supporting environmentally responsible nutrient management.

Introduction

In the agricultural sector, South Korea ranks among the highest OECD countries in fertilizer consumption, with nitrogen losses from cropland estimated at 40–70%¹. To mitigate these losses, slow-release fertilizers coated with non-degradable materials have been widely adopted. However, extensive use of non-degradable plastics raises environmental concerns, particularly regarding soil and water pollution. Globally, polymer-coated fertilizers—typically formulated with non-biodegradable synthetic resins—have increased steadily in recent years due to the demand for enhanced-efficiency fertilizers².

Urea, with its high nitrogen content (46%) and relatively low cost, is the most widely used nitrogen fertilizer worldwide^1,3. However, once applied to soil, urea is prone to substantial nitrogen losses through ammonia volatilization, nitrate leaching, and surface runoff, and contributes to nitrous oxide (N₂O) emissions. These losses reduce nitrogen use efficiency and pose environmental risks, motivating the development of biodegradable polymer–coated urea as a more sustainable alternative. Biodegradable coatings reduce leaching, limit denitrification and runoff, and improve nitrogen use efficiency⁴. Additionally, such coatings prevent fertilizer overapplication⁵, enhance crop productivity, and improve nutrient uptake efficiency⁶.

Despite these advantages, biodegradable polymer–coated urea remains relatively costly, and nitrogen release patterns vary depending on the chemical composition and structural properties of the coating. Uniform of thin coatings made from truly biodegradable materials are essential for predictable nutrient release and maximizing nitrogen availability⁷. In contrast, conventional polymer-coated urea provides slow nutrient release but raises concerns regarding non-degradability and potential toxicity, which may disrupt soil microbial ecosystems and contribute to long-term pollution⁶. The nutrient release efficiencies and release patterns of conventional chemical fertilizers and biodegradable polymer–coated urea are illustrated in Fig. 1.

Fig. 1.

Proposed nitrogen release patterns for biodegradable polymer–coated urea and conventional chemical fertilizers^8,9. Biodegradable polymer–coated urea is designed to provide a slower, more controlled release profile, reducing early nitrogen loss and extending nutrient availability throughout the crop growth period, whereas conventional chemical fertilizers release nitrogen rapidly, increasing the risk of leaching, volatilization, and runoff.

Biodegradable polymers degrade over time through microbial enzymatic activity. While some are synthesized from renewable resources, their degradability is determined by their chemical composition rather than the source of monomers¹⁰. Common materials include polycaprolactone (PCL), polybutylene succinate (PBS), polylactic acid (PLA), polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), and polyhydroxybutyrate-co-valerate (PHBV), which are used across agriculture, adhesives, and construction^11,12. Polymer blending offers a cost-effective alternative to synthesizing new monomers. Among available blends, PBS and PCL have attracted attention for their processability, mechanical properties, and suitability for coating applications^12–15. However, PBS and PCL are inherently immiscible, often resulting in phase separation and reduced mechanical integrity¹². To overcome this limitation, methylene diphenyl diisocyanate (MDI) has been employed to enhance interfacial interactions, improving adhesion and mechanical strength¹⁶. In terms of biodegradation, PBS/PCL blends degrade by approximately 32.7% after 60 days under composting conditions (typically 50–60 °C), with PBS degrading more rapidly than PCL¹⁷.

Recent studies indicate that biodegradable polymer matrices such as polyhydroxyalkanoates (PHA) and polybutylene adipate-co-terephthalate (PBAT) can provide tunable nitrogen release kinetics, with higher polymer loadings producing slower release rates in both aqueous and soil environments⁸. Similarly, polymer-coated urea often exhibits a characteristic sigmoidal release profile, comprising lag, acceleration, and plateau phases when nutrient release is governed by diffusion through a gradually degrading coating polymer⁹. Building on these insights, PBS/PCL coatings are expected to initially function as low-permeability barriers, followed by gradual pore formation through microbial degradation, ultimately leading to accelerated nitrogen release as urea becomes increasingly exposed. The release profile can be modulated by adjusting the PBS: PCL ratio, leveraging the faster degradation rate of PBS to better synchronize nitrogen availability with crop demand.

Plasma treatment is widely used to modify polymer surfaces without altering bulk properties. In PBS/PCL blends, plasma introduces polar functional groups that increase surface energy, thereby enhancing hydrophilicity, interfacial adhesion, and morphological uniformity. It also increases microscale surface roughness, enhancing mechanical interlocking between phases, reducing immiscibility, and producing more uniform, effective biodegradable coatings. These modifications collectively enhance coating performance and highlight plasma treatment as a valuable approach for preparing high-quality biodegradable polymer coatings.

Therefore, the objective of this study was to investigate the physicochemical characteristics of biodegradable polymer blends and the nitrogen release behavior of PBS/PCL blend–coated urea, with methylene diphenyl diisocyanate (MDI) for crosslinking and plasma surface treatment applied to enhance coating integrity and performance. We hypothesized that the combined application of MDI and plasma treatment would improve the physical properties and morphological uniformity of PBS/PCL coatings, thereby enabling more controlled and sustained nitrogen release profiles that better align with the temporal nutrient demands of crops.

Results and discussion

Characterization of coating polymer blends

The FTIR spectrum of the biodegradable polymer-coated urea has been revised to clearly match each peak with its assignment. Specifically, the absorption bands at 2,940 and 2,863 cm⁻¹ correspond to asymmetric and symmetric C–H stretching, respectively. The prominent band at 1,716 cm⁻¹ is attributed to ester C = O stretching, while the region between 1,293 and 1,160 cm⁻¹ is assigned to C–O–C stretching of ester linkages. In addition, the band at 729 cm⁻¹ is associated with –CH₂ rocking vibrations. These features confirm the presence of methylene groups and ester functionalities, consistent with the structural characteristics of polyesters such as PCL and PBS^18,19. In the PBS/PCL blends crosslinked with MDI, no absorption bands were detected near 2,277 cm^-¹, which corresponds to the isocyanate (N = C = O) group, confirming that MDI reacted completely during crosslinking (Fig. 2a). However, narrow bands appearing at approximately 3,350 cm^-¹ were attributed to N-H stretching vibrations^20,21. This suggests that MDI may have undergone hydrolysis during sample preparation or analysis.

Fig. 2.

(a) FT-IR spectra of neat PBS, neat PCL, and PBS/PCL (9:1) blend with MDI crosslinking and plasma treatment. Characteristic peaks remained largely unchanged following MDI addition and plasma treatment, indicating preservation of core functional groups. (b) XRD patterns of neat biopolymers and coating polymer blends. Peaks at 19.6° and 22.7° correspond to PBS, while peaks at 21.3° and 23.7° correspond to PCL. The observed reduction in peak intensity in the blend after modification suggests decreased crystallinity due to crosslinking and plasma treatment.

The characteristic ester C = O stretching bands (1,720-1,730 cm^-¹) and C-O-C stretching bands (1,170-1,250 cm^-¹), inherent to PBS and PCL, were retained with only minor shifts following MDI crosslinking^20,21. This indicates that the polymer backbone structure was largely preserved after crosslinking.

For PBS/PCL samples treated with plasma (MDS), enhanced absorption intensities were observed in the O–H stretching region (~ 3,433 cm^-¹) and the C = O stretching region (~ 1,716 cm^-¹)^16,22. These increases are attributed to the introduction of polar functional groups such as -OH and -COOH on the polymer surface due to plasma treatment^16,23. Additionally, broadening and slight increases in specific absorption bands further support successful surface modification achieved by plasma exposure.

The characteristic diffraction peaks were observed at 2θ = 19.6°, 21.3°, 22.7°, and 23.7°, consistent with the native crystalline phases of pure PBS and PCL, indicating that solution blending and spinning did not alter their fundamental structures (Fig. 2b). After MDI cross-linking and plasma treatment, additional peaks emerged at 2θ = 26.2° and 29.0°. These new peaks may arise from MDI-related crystalline domains, local lattice distortions induced by cross-linking or plasma activation, or minor crystalline impurities introduced during modification. Although the precise origin requires further investigation, their presence suggests that these treatments influence the crystalline ordering of the polymer blends and may affect their mechanical and barrier properties. Neat PBS exhibited a monoclinic structure with sharp peaks at 19.6° and 22.7°, corresponding to the (020) and (110) planes, respectively^24,25. PCL showed semi-crystalline orthorhombic peaks at 21.3° and 23.7°, assigned to the (110) and (200) planes^26,27. These peaks serve as baselines for assessing structural changes upon blending and modification.

In the PBS/PCL (9:1) blend crosslinked with MDI, diffraction peaks appeared at 19.6°, 22.7°, 26.2°, and 29.0°.

These subtle structural changes indicate successful molecular-level modification without disrupting the overall crystalline framework. The XRD results confirm that while PBS and PCL maintain their fundamental crystalline identities, blending with MDI and plasma treatment effectively modulates crystalline order.

Such reduction in crystallinity is expected to influence mechanical strength, degradation behavior, and nutrient diffusion as all critical factors for the performance of controlled-release fertilizers. These changes suggest increased structural disorder and stronger molecular interactions, contributing to a more compact, amorphous polymer matrix.

Releasing profiles for nitrogen dynamics

The controlled-release behavior of PBS/PCL coated urea contributed to improved nitrogen retention in soil. Unlike conventional urea, which rapidly hydrolyzes to NH₄⁺-N and is prone to volatilization, leaching, and microbial nitrification, the polymer coating moderated nitrogen release, reducing losses under fluctuating moisture and temperature conditions. This controlled release likely enhanced nitrogen use efficiency (NUE), particularly in coarse-textured or well-aerated soils where NH₄⁺-N mobility is high, demonstrating the practical benefit of biodegradable coatings in synchronizing nitrogen availability with crop demand.

Recent studies support the effectiveness of controlled-release fertilizers in mitigating NH₄⁺-N losses. For example, He et al. (2025) reported a significant reduction in NH₄⁺-N loss from coated urea in soil column experiments²⁸. Similarly, Mirbolook et al. (2024) demonstrated that nano-diatomite-enhanced coatings delayed NH₄⁺-N release and improved nitrogen retention in the maize rhizosphere²⁹. In potato cropping systems, Xue et al. (2025) observed that polymer-coated urea reduced NH₄⁺-N leaching by over 30% compared to conventional urea applications³⁰.

In the present study, NH₄⁺-N release dynamics were evaluated using soil column experiments under oxidative conditions. As shown in Fig. 3a, the control (uncoated urea) exhibited a sharp initial increase in NH₄⁺-N concentration, peaking at 254.8 mg L^-¹ on day 5, followed by a steep decline to 0.7 mg L^-¹ by day 45 (Fig. 3a). This pattern reflects the high solubility and rapid release of urea-derived nitrogen. In contrast, all three biodegradable polymer-coated treatments (PBS + PCL + MDI, PBS + PCL + Plasma, and PBS + PCL + MDI + Plasma) showed moderated release profiles, maintaining elevated NH₄⁺-N concentrations well beyond 45 days. Peak concentrations for these treatments were 52.2, 40.1, and 36.5 mg L^-¹, respectively. Moreover, measurable NH₄⁺-N levels persisted in the leachate for up to 85 days, demonstrating prolonged nitrogen availability.

Fig. 3.

NH₄⁺-N concentrations (a) in leachate and sigmoidal curve fitting of cumulative NH₄⁺-N release (b) under different biodegradable polymer-coated urea treatments. A three-parameter sigmoidal model was applied to describe nitrogen release dynamics for each treatment.

Differences in release patterns among the coated formulations were also evident. The plasma-treated coating (PBS + PCL + Plasma) exhibited higher NH₄⁺-N concentrations during the early release phase, likely due to increased surface wettability. Conversely, the MDI-treated coating (PBS + PCL + MDI) showed higher NH₄⁺-N levels during the later phase (after day 45), suggesting enhanced coating durability and delayed nutrient release. The combined treatment (MDS) effectively balanced both early and sustained release phases.

Cumulative release data (Fig. 3b) further corroborates these findings. The control exhibited a near-plateau in cumulative NH₄⁺-N release by day 40, whereas the biodegradable-coated treatments demonstrated extended-release profiles continuing through day 90. Among these, the PBS + PCL + MDI + Plasma (MDS) treatment showed the lowest cumulative NH₄⁺-N release, indicating superior control over nitrogen release.

These results align with previous studies reporting that PBS-based biodegradable coatings can reduce nitrogen release rates from urea³¹. The present study advances this understanding by demonstrating that PBS/PCL-based coatings, when enhanced with MDI crosslinking and plasma treatments, offer significant improvements in NH₄⁺-N retention and release control in upland soils. The observed synergistic effect likely arises from the formation of cross-linked polymer networks facilitated by MDI’s urethane bonding^16,32 combined with plasma-induced surface functionalization^33,34.

Plasma treatment increases surface energy and introduces polar functional groups, while MDI enhances structural stability. Together, these modifications reduce coating permeability and slow nitrogen diffusion⁶, thereby improving release consistency and reducing nitrogen losses.

To evaluate and describe the cumulative NH₄⁺-N release behavior, a three-parameter sigmoid model-commonly used in plant growth and controlled-release kinetics was applied. For all treatments, the cumulative NH₄⁺-N release data exhibited an excellent fit to the sigmoid function, with coefficients of determination (R² ≥ 0.9691), indicating high model accuracy and reliability (Table 1).

Table 1.

Fitted parameters describing cumulative NH₄⁺-N release profiles for different polymer-coated Urea treatments using the three-parameter sigmoid model.

Treatments	Model equations	R 2
Control		0.9978
PBS + PCl + MDI		0.9855
PBS + PCl + Plasma		0.9691
PBS + PCl + MDI + Plasma		0.9893

Nitrate-nitrogen (NO₃⁻-N) accounts for approximately 76–98% of total nitrogen lost via leaching after fertilizer application and represents the primary pathway of nitrogen loss in agricultural systems³⁵. To evaluate the slow-release behavior of NO₃⁻-N under different treatments, both NO₃⁻-N concentrations (Fig. 4a) and cumulative releases (Fig. 4b) were monitored throughout the precipitation period.

Fig. 4.

NO₃⁻-N concentrations (a) and sigmoidal curve fitting of cumulative NO₃⁻-N release (b) under different polymer-coated urea treatments. Sigmoidal curves fitted using a three-parameter sigmoid model are shown to characterize nitrogen release profiles for each treatment.

In the control (uncoated urea), NO₃⁻-N concentrations rapidly increased during the early release phase, peaking at 216.2 mg L⁻¹ on day 30 after precipitation, followed by a gradual decline until day 90. In contrast, urea coated with MDI and/or plasma-modified biodegradable polymers exhibited delayed and moderated NO₃⁻-N release. These treatments maintained relatively low NO₃⁻-N concentrations during the first 30 days, with delayed peaks ranging from 200.0 to 214.7 mg L⁻¹ observed around day 70.

This delayed release profile aligns with previous studies demonstrating that biodegradable polymer coatings can effectively modulate and reduce nitrate leaching^8,36.

Cumulative NO₃^--N release trends further illustrate the prolonged release characteristics of the coated urea formulations. The control exhibited a rapid increase, reaching 27.3 mg by day 50 and plateauing thereafter. In contrast, the PBS + PCL + MDI and PBS + PCL + Plasma (MDS) treatments suppressed NO₃^--N release until approximately day 25, maintaining low cumulative values (1.9–3.7 mg), followed by a gradual increase over time. By day 90, cumulative releases reached 29.8 mg for PBS + PCL + MDI, 26.5 mg for PBS + PCL + Plasma, and 35.9 mg for the combined PBS + PCL + MDI + Plasma (MDS) treatment (Fig. 4b). These results indicate that biodegradable polymer coatings can delay the onset of NO₃^--N release by up to 60 days relative to uncoated urea. The observed shifts in NH₄⁺-N and NO₃⁻-N leaching highlight the influence of coating formulation and degradation behavior on nitrogen dynamics. Specifically, the increased NO₃⁻-N leaching under the PBS + PCL + Plasma (MDS) treatment may enhance nitrate availability for plant uptake, thereby supporting crop growth. However, excessive nitrate mobility also raises the risk of groundwater contamination. These observations emphasize that optimizing biodegradable polymer coatings is essential to improve nitrogen use efficiency while mitigating potential environmental impacts.

Among the treatments, the PBS + PCL + Plasma (MDS) formulation exhibited the most gradual and extended NO₃^--N release pattern, suggesting a synergistic effect of combined MDI and plasma treatments. Pan et al. (2018) incorporated MDI into biodegradable polymer composites via solution blending to produce bulk films with enhanced mechanical strength and stability, thereby delaying degradation. In contrast, our study applies PBS/PCL + MDI as coatings on urea granules, demonstrating that MDI cross-linking likewise improves coating stability and performance¹⁶. The current findings are consistent with these observations, confirming that MDI-treated coatings improve nitrogen retention by enhancing structural durability and reducing early-stage leaching losses.

Changes in cumulative NO₃⁻-N release from biodegradable polymer-coated urea were effectively modeled using a sigmoid curve equation, with fitted parameters (Table 2). This approach is consistent with prior studies on blended biochar pellets, which successfully applied a modified hyperbola model to describe nutrient release in slow-release fertilizers³⁷.

Table 2.

Fitted parameters describing cumulative NO₃⁻-N release profiles for different biodegradable polymer-coated Urea treatments using the three-parameter sigmoid model.

Treatments	Model equations	R 2
Control		0.9970
PBS + PCl + MDI		0.9965
PBS + PCl + Plasma		0.9691
PBS + PCl + MDI + Plasma		0.9955

The cumulative NO₃⁻-N release data across all treatments showed an excellent fit to the sigmoid model (Eq. 1), with coefficients of determination (R²) exceeding 0.9691, indicating strong predictive accuracy. Furthermore, the model demonstrated statistical significance for all treatments, confirming its robustness and suitability for capturing the slow-release behavior of nitrate in soils amended with biodegradable coated urea.

The total nitrogen (TN) concentrations and cumulative TN release in soil columns are treated with biodegradable polymer-coated urea (PBS/PCL) with MDI or plasma treatment (Fig. 5). In the control, TN concentrations peaked sharply between days 5 and 30, reaching 229.5 mg L⁻¹ on day 30, followed by a rapid decline (Fig. 5a). In contrast, treatments with MDI and plasma-modified coatings exhibited delayed TN release, with concentrations rising after day 40 and peaking around day 70 (PBS + PCL + MDI: 226.7 mg L⁻¹, PBS + PCL + Plasma: 207.3 mg L⁻¹, PBS + PCL + MDI + Plasma: 222.0 mg L⁻¹), then gradually decreasing through day 90. The PBS + PCL + MDI treatment showed the highest peak, consistent with previous reports that MDI enhances coating stability and nutrient release control^16,38.

Fig. 5.

TN concentrations (a) and sigmoidal curve fitting of cumulative TN release (b) under different biodegradable polymer-coated urea treatments. Sigmoidal curves fitted using a three-parameter sigmoid model characterize the nitrogen release profiles for each treatment.

Cumulative TN release (Fig. 5b) increased rapidly in the control during the early phase, whereas biodegradable polymer-coated treatments showed slow initial release followed by gradual increases after day 30. No significant differences were observed between plasma and MDI treatments during the first 30 days. However, after day 30, the MDS (PBS + PCL + MDI + Plasma) treatment exhibited the highest cumulative TN release. These results align with prior findings on the effectiveness of biodegradable coatings in modulating nitrogen release³⁹.

The cumulative total nitrogen (TN) releasing patterns from biodegradable polymer-coated urea treatments were effectively described using a sigmoidal model (Eq. 1) (Table 3). Across all treatments, the model demonstrated excellent goodness-of-fit, with coefficients of determination (R²) above 0.9918, indicating high predictive accuracy. These results align well with previous studies that applied similar modeling approaches to characterize nitrogen release kinetics⁴⁰, which reported that the sigmoid model effectively captures the delayed and gradual nutrient release characteristics of biodegradable coatings. The statistical significance observed across all treatments further supports the robustness of the model for simulating TN release dynamics in upland soil conditions, particularly in the soils employing PBS/PCL-based coatings with MDI and plasma enhancements.

Table 3.

Fitted parameters describing cumulative total nitrogen (TN) release profiles for different polymer-coated Urea treatments using the three-parameter sigmoid model.

Treatments	Equations for model application	R 2
Control		0.9981
PBS + PCl + MDI		0.9918
PBS + PCl + Plasma		0.9919
PBS + PCl + MDI + Plasma		0.9935

Estimation of soluble nitrogen release

During the 90-day release period, significant differences (p < 0.05) were observed in the leached amounts of NH₄⁺-N, NO₃⁻-N, and total nitrogen (TN) among the biodegradable polymer-coated urea treatments (Table 4). The MDS treatment reduced maximum NH₄⁺-N leaching by 20.3%, while increasing NO₃⁻–N and total nitrogen (TN) leaching by 21.2% and 6.8%, respectively, compared with the control. The observed nitrate may originate from both the fertilizer and microbial conversion of ammonium in the soil, while the delayed release is likely influenced primarily by the controlled-release effect of the coating. In the MDS (PBS + PCl + MDI + Plasma) treatment, the decrease in NH₄⁺-N coincided with increased NO₃⁻-N leaching, most likely due to enhanced nitrification. The modified coating enables a sustained release of ammonium, providing a continuous substrate for nitrifying microorganisms and thereby promoting its conversion to nitrate. These results highlight the influence of coating formulation and degradation behavior on nitrogen release and mobility, which may be associated with enhanced nitrification and other soil microbial processes in upland soil systems.

Table 4.

Amounts of soluble nitrogen species (NH₄⁺-N, NO₃⁻-N, and total nitrogen) leached under different polymer-coated Urea treatments over the 90-day release period.

Treatments*	Soluble amounts (mg)
	NH4+-N	NO3−-N	TN
Control	15.8	29.6	45.4
PBS + PCl + MDI	14.1	29.8	43.9
PBS + PCl + Plasma	14.4	26.5	40.9
PBS + PCl + MDI + Plasma	12.6	35.9	48.5
F	10.9	21.2	11.7
P-values	0.003	0.001	0.003

^*Significant differences (p < 0.005) were observed among treatments with NH₄⁺-N, NO₃⁻-N, and TN.

Similar trends have been reported in recent studies, where modified slow-release urea formulations-such as biochar-nitrogen complexes-reduced cumulative NH₄⁺-N and NO₃⁻-N leaching within the first 30 days after precipitation compared to conventional urea fertilizers⁴¹. Furthermore, increasing biochar content from 10% to 30% further reduced cumulative NO₃⁻-N leaching, likely due to improved nutrient retention capacity and delayed urea dissolution^42,43. Collectively, these findings emphasize that both polymer composition and supplementary amendments are critical factors in optimizing nitrogen retention and release from coated fertilizers.

These results demonstrate that PBS/PCL-coated urea effectively moderates nitrogen release, enhancing soil NH₄⁺-N retention and potentially improving nitrogen use efficiency. While exact production costs are difficult to estimate at this stage due to variability in scale and processing, the improved efficiency and reduced environmental losses suggest potential long-term economic benefits. Future pilot-scale studies could help clarify the cost-effectiveness of this coating approach.

Conclusions

This study successfully developed biodegradable PBS/PCL coatings, enhanced through MDI crosslinking and plasma surface treatment, for controlled-release urea. Structural and surface analyses revealed that these modifications effectively altered polymer crystallinity and surface characteristics, contributing to improved coating integrity. Soil column experiments demonstrated that coated urea formulations significantly delayed NH₄⁺-N and NO₃⁻-N leaching, extending the overall nitrogen release period by up to 60 days compared with uncoated urea. Nitrogen release kinetics followed a sigmoidal model (R² ≥ 0.96), indicating multi-phase, controlled release behavior. The MDS treatment provided relatively consistent and prolonged availability of NH₄⁺-N, NO₃⁻-N, and TN throughout the experimental period. These results highlight the potential of MDS to effectively regulate nitrogen release, reduce early-stage leaching, extend total nitrogen availability, and improve nutrient retention, demonstrating a sustainable strategy for efficient nitrogen management in agriculture. Overall, the study demonstrates both the practical utility and scientific novelty of integrating cross-linking and plasma treatment into biodegradable polymer coatings for slow-release fertilizers.

Methods

Preparation for coated urea

Methylene diphenyl diisocyanate (MDI) was employed as a crosslinking agent to formulate a biodegradable polymer blend for coated urea. Additionally, plasma treatment was applied as a surface modification technique to activate functional groups and enhance interfacial interactions within the polymer matrix. The combined use of MDI and plasma treatment aimed to improve the coating’s structural integrity, moisture resistance, and microbial degradation resistance, thereby enabling sustained nitrogen release.

The complete process of producing PBS/PCL-coated urea is summarized in Fig. 6. The schematic integrates polymer solution preparation, fluidized bed coating, and drying steps, providing a clear overview of the coating mechanism and film formation on urea granules. The coating solution was prepared by mixing polybutylene succinate (PBS) and polycaprolactone (PCL) at a 9:1 weight ratio. The blend was dissolved in 300 mL of chloroform under magnetic stirring. Subsequently, 10 mL of a 0.01 M MDI solution was added, and the mixture was thoroughly homogenized to ensure even dispersion of the crosslinker (Fig. 6a). Plasma treatment was then applied to the coating solution using a plasma reactor operated at atmospheric pressure (Fig. 6b). Air was used as the working gas and introduced into the reactor at a controlled flow rate of 10 mL·min⁻¹. Plasma discharge was generated by applying an electric voltage of 1.5 kV to the electrode.

Fig. 6.

Schematic mechanism of PBS/PCL-coated urea production. The diagram illustrates polymer (a, b) solution preparation with plasma treatment, (c) application onto urea granules via a fluidized bed coating system, in chamber drying for solvent evaporation, and (d) formation of a uniform biodegradable polymer film.

Urea fertilizer used in this study was obtained from Namhae Chemical Co., Ltd. (South Korea). The biodegradable slow-release coated urea, referred to as MDS, was produced by Bio-technology of Multidisciplinary Sciences Co., located in Wanju-gun, Jeollabuk-do, South Korea.

Each biodegradable polymer solution (3%, w/v) was applied to urea granules using a fluidized bed coating system (Fig. 6c). The ratio of polymer solution to urea was maintained at 3:1 (v/w) to ensure uniform coating distribution. The polymer solution was sprayed onto fluidized urea granules under ambient conditions, followed by in-chamber drying to facilitate solvent evaporation and film formation.

Analysis of FTIR and XRD

Polymer resins, including PBS, PCL, PBS + PCL + MDI, and PBS + PCL subjected to plasma treatment, were dissolved in 3% (w/v) chloroform. The resulting solutions were cast onto aluminum plates and dried under a fume hood to ensure completely solvent evaporation. The dried polymer films were then pulverized using a mechanical grinder to produce fine particles suitable for analysis.

The chemical structures and surface functional groups of the polymers were characterized by Fourier-transform infrared spectroscopy (FT–IR; Nicolet™ iN™10, Thermo Fisher Scientific, Waltham, MA, USA) in the spectral range of 400–4,000 cm⁻¹. Crystallinity and phase composition were further evaluated using high-resolution X-ray diffraction (XRD; D/max-2,500, Rigaku Denki, Japan) with monochromatic Cu Kα radiation (λ = 1.541 Å), a scan speed of 2°/min, a step size of 0.02°, and a scanning range of 5–80°.

Set-up of soil columns

To evaluate the effects of different biodegradable polymer-coated urea formulations on nitrogen release, upland was collected from the experimental field of the Soil Management Division at the National Institute of Agricultural Sciences (NIAS), Korea. The soil was air-dried and sieved through a 2 mm mesh to ensure uniform texture. The physicochemical properties of the soil are summarized in Table 5.

Table 5.

Physicochemical properties of the soil used. Units are shown only in the Central column, except for O.M. (%).

pH (1:5)	EC (dS m−2)	OM (%)	P2O5	SiO2	K	Ca	Mg	Na
		------------------- mg kg−1 ----------------------------
6.6	0.95	1.9	393	187	0.21	6.0	1.2	0.21

The soil pH was 6.6, indicating near-neutral conditions suitable for crop cultivation. The organic matter content was 1.9%, which is lower than the optimal range for upland soils (25–35 g kg⁻¹). Available phosphate concentration was 393 mg kg⁻¹, within the typical range for upland agricultural soils (350–450 mg kg⁻¹). Soil pH and electrical conductivity (EC) were measured in a 1:10 soil-to-deionized water suspension using a pH meter (Thermo Scientific Orion 4-star, Waltham, MA, USA). Total carbon content was determined using an elemental analyzer (Vario Macro Cube, Langenselbold, Germany). Organic matter content was calculated using the Van Bemmelen factor (1.724), which assumes that soil organic matter contains approximately 58% carbon.

For each treatment, 200 g of soil was thoroughly mixed with specified amounts of fused phosphate, potassium chloride, and poultry manure compost. The quantities of these amendments varied depending on the type of biodegradable polymer-coated urea applied. The amended soil was then uniformly packed into individual soil columns (Fig. 7). To minimize leachate clogging caused by fine particles, 5 g of sea sand was evenly placed at the bottom of each column.

Fig. 7.

Diagram of soil column setup for monitoring nitrogen release components.

Nitrogen application rates were based on standard fertilization guidelines for open-field cabbage cultivation (32 kg N per 10 a), as recommended by NIAS⁴⁴. Soil moisture was maintained at approximately 60% of field capacity using a gravimetric weighing method.

Distilled water was added every five days at consistent times to simulate intermittent rainfall over a 90-day experimental period under open laboratory conditions.

To monitor nitrogen release dynamics, 20 mL of leachate was periodically collected from each soil column. Leachate samples were filtered through Whatman No. 2 filter paper and analyzed for ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) concentrations using a Bran-Luebbe Auto Analyzer (Seal Analytical Ltd., Wisconsin, USA). Total nitrogen (TN) in the leachate was defined as the sum of ammonium (NH₄⁺-N) and nitrate (NO₃⁻-N) and was assessed nitrogen leaching from the treatments, excluding any organic nitrogen.

The soil column experiments were conducted using upland sandy loam soil under controlled laboratory conditions (approximately 25–28 ± 2 °C). Four treatments were established as follows: (a) Control (uncoated urea); (b) PBS + PCL + MDI (urea coated with a PBS/PCL blend crosslinked with MDI); (c) PBS + PCL + Plasma (urea coated with PBS/PCL blend followed by plasma treatment); and (d) MDS (PBS + PCL + MDI + Plasma); urea coated with PBS/PCL blend treated with both MDI and plasma (Fig. 6d). Treatments were arranged in a completely randomized design (CRD) with three replications.

Statistical analysis

Statistical analyses were conducted using one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test to compare treatment effects on cumulative nitrogen release and leachate concentrations. All analyses were performed using SAS software (version 12.0, SAS Institute Inc., Cary, NC, USA). Statistical significance was determined at P < 0.001, and standard deviations were calculated to represent variability among replicates.

To evaluate nitrogen release dynamics, a three-parameter sigmoid model was fitted to the cumulative total nitrogen (TN), ammonium nitrogen (NH₄⁺-N), and nitrate nitrogen (NO₃⁻-N) from soil treated with the different biodegradable polymer-coated urea formulations. The model equation used was as follow:

 

1

where, Y (mg) is the cumulative nutrient at time t, a is the difference between maximum and minimum Y, b is the width of the transition, and t₀​ is the time at which 50% of the cumulative nitrogen component release is reached. The model fitting quality was evaluated using the coefficient of determination (R²) and residual sum of squares (RSS) to ensure an accurate representation of nitrogen release trends across treatments.

The three-parameter sigmoid function was selected because it effectively characterizes the release behavior of polymer coated fertilizers, typically exhibiting an initial lag phase, followed by a rapid release phase, and finally a plateau as nutrient availability stabilizes^6,40. This approach reflects the diffusion- and degradation-driven release mechanisms of biodegradable polymer coatings and allows direct comparison of key parameters such as release rate, inflection time, and maximum cumulative release across different treatments.

Author contributions

I. Lee and J.D. Shin designed the experiments. J.Choi conducted the experiments and analyzed the results. I. Lee and J.D. Shin interpreted the results. W. Chung, I. Lee and J.D. Shin wrote the manuscript draft. W. Chung, J.S. Song, I. Lee, and J.D. Shin reviewed the manuscript.

Funding

This work was supported by the Cooperative Research Program for Agricultural Science and Technology Development (Project No. RS-2022-RD010278), Rural Development Administration, Republic of Korea.

Data availability

The data sets generated during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.