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Article

Controlled-Release Urea Coordinates Maize Physiology with Soil Nitrogen Retention: Balancing High Yield and Environmental Sustainability

by
Wei Yan
1,2,
Meng Huang
1,
Huiqing Yang
3,
Zhonghua Wang
1,2,
Shujuan Sun
3,
Yinshan Xie
1,
Jinbian Sun
1,
Qiong Li
2,
Bo Liu
1,
Chengcheng Gao
2,
Yanfang Xue
2,* and
Kaichang Liu
1,*
1
State Key Laboratory of Nutrient Use and Management, Shandong Academy of Agricultural Sciences, Jinan 250100, China
2
National Engineering Research Center of Wheat and Maize, Maize Research Institute, Shandong Academy of Agricultural Sciences, Jinan 250100, China
3
Binzhou Agricultural Technology Promotion Center, Binzhou 256600, China
*
Authors to whom correspondence should be addressed.
Agriculture 2025, 15(16), 1778; https://doi.org/10.3390/agriculture15161778
Submission received: 16 July 2025 / Revised: 13 August 2025 / Accepted: 16 August 2025 / Published: 19 August 2025
(This article belongs to the Special Issue Technologies for Achieving High-Yield Maize of High Nutritional Quality)
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Abstract

Controlled-release urea (CRU) can improve nitrogen (N) use efficiency and yield, but comprehensive evaluations of its agronomic, physiological, and environmental impacts remain limited. Through a two-year field experiment comparing three CRU types with conventional urea at five N rates (0-280 kg N ha−1), we demonstrate that CRU at 180 kg N ha−1 maintained high maize yields (13.9 Mg ha−1) while improving N use efficiency, with thermosetting polymer-coated samples (TCU) showing superior performance. There was a significant increase in the net photosynthetic rate by 7.9–32.7% and intercellular CO2 concentration by 20.6–40.0% under CRU treatments during the silking and milking stages. The CRU treatments also sustained optimal levels of hormones, N metabolism enzymes, and sucrase and urease activities. Compared to common urea, life cycle assessment indicates that CRU has achieved a 47.5% reduction in reactive N losses and an 18.7% decrease in greenhouse gas emissions. Economically, CRU outperformed common urea, with TCU providing the highest net benefit through yield stability and labor savings. These findings establish TCU at 180 kg N ha−1 as an optimal strategy of maize production in the North China Plain, balancing productivity, profitability, and environmental protection.

1. Introduction

Maize (Zea mays L.) is one of the primary food crops and plays a crucial role in ensuring food security in China [1]. To achieve high yields, excessive nitrogen (N) fertilizer exceeding crop demand is frequently applied in the intensive crop production of the North China Plain (NCP) [2]. This practice not only elevates production costs but also triggers significant environmental degradation, including soil acidification, reduced microbial diversity [3,4], and increased reactive N (Nr) pollution [5]. A fundamental challenge driving these issues is the temporal mismatch between the rapid N release of conventional urea and the phased N demand of maize [6], leading to both agronomic inefficiency and ecological damage.
Controlled-release urea (CRU), employing polymer coatings (PSCU and PCU) or thermosetting materials (TCU) [7,8], offers a promising solution by synchronizing N availability with critical maize growth stages. Notably, CRU can maintain yields even at application rates 30% lower than conventional urea [9], often enhancing plant growth vigor [10]. More importantly, CRU provides a more continuous N supply, moderating soil urease activity compared to the sharp fluctuations induced by conventional urea [11], thereby potentially enhancing soil N retention. This capacity of CRU to coordinate maize N physiology with soil N retention processes is central to its benefits. However, determining the optimal CRU rate for yield maximization in the NCP and elucidating the underlying physiological mechanisms, particularly regarding hormonal regulation and source–sink relationships, remain key research gaps.
On the NCP, intensive precipitation combined with excessive urea application commonly leads to substantial nitrate (NO3−-N) leaching, reducing nitrogen use efficiency (NUE) and threatening groundwater quality [12,13]. CRU has demonstrated the potential to mitigate multiple Nr loss pathways, including reduced NO3−-N leaching [14], NH3 volatilization [15], and N2O emissions [16], supporting its recommendation for environmental risk reduction [17]. Nevertheless, reported effectiveness can vary [18,19], and crucially, most studies focus on single loss pathways. A comprehensive assessment of integrated Nr losses and greenhouse gas (GHG) emissions across the soil–plant–atmosphere continuum under CRU management is lacking. Life cycle assessment (LCA) is a valuable tool for identifying optimal strategies that minimize environmental impacts, yet its application to evaluate CRU’s holistic environmental performance requires further investigation to inform policy and practice.
Therefore, to bridge these knowledge gaps and advance the pathway for balancing high maize yield and environmental sustainability, a consecutive two-year field experiment was conducted on the NCP. This study compared common urea with three types of CRUs applied at three N rates. The specific objectives were (1) to investigate the responses of photosynthetic characteristics, plant hormone dynamics, N metabolism enzymes, and soil enzyme activity to different N sources and rates, elucidating their synergistic effects and physiological coordination in enhancing yield and NUE, and (2) to comprehensively evaluate Nr losses (leaching, volatilization, and denitrification) and GHG emissions, alongside economic benefits, to establish optimal N management strategies that balance agronomic productivity with environmental sustainability.

2. Materials and Methods

2.1. Experiment Site

The field experiment was conducted during the growing seasons of 2019 and 2020 (mid-June to early October) at the Zhangqiu Experimental Station of the Shandong Academy of Agricultural Sciences in Jinan City, Shandong Province, China (36°43′ N, 117°32′ E). The experimental area is situated in a warm temperate, sub-humid continental monsoon zone with cold winters and hot summers. It is a representative site for agricultural production in the NCP, with a double cropping system of winter wheat–summer maize. The daily mean temperature, solar radiation, and precipitation during the two growing seasons are presented in Figure 1. Accumulated growing degree days with a base temperature of 10 °C during the two growing seasons were 1649 °C in 2019 and 1571 °C in 2020, and total precipitation was 755 mm in 2019 and 461 mm in 2020. In addition, 50 mm of irrigation was applied after sowing each year. The characteristics for the initial topsoil layer (0–20 cm) were as follows: soil pH (1:2.5 w/v in water) 8.0, organic matter content 13.8 g kg−1, total soil N 0.62 g kg−1, available phosphorus (Olsen P) 35.0 mg kg−1, and available potassium (NH4OAc-K) 117.0 mg kg−1.

2.2. Experimental Design and Crop Management

The maize cultivar used in this study was Denghai605. All treatments had an equal plant density of 75,000 plants ha−1, with a row spacing of 60 cm and a plant spacing of 22.2 cm. This study used a completely random design with three replicates (33.6 m2 plot−1). Five N application rates, as urea, were used as follows: 0, 126, 180, 234, and 280 kg N ha−1, recorded as N0, N126, N180, N234, and N280, respectively (Table 1). Generally, N180 was the optimal N rate based on local yield levels and expert recommendations. N126 and N234 were representative of 70% and 130% of the optimal rate (N180), respectively. The N application was split into two, with 40% of the total N being applied as the base fertilizer while the remaining 60% was split-applied as the topdressing at the 10-leaf stage. The N280 treatment was set to correspond with local farmers’ N management, with a preplanting application of 187 kg N ha−1 and a topdressing of 93 kg N ha−1 at the 10-leaf stage. Based on the established optimal N rate (180 kg N ha−1), three kinds of CRU, including PSCU (≥35% N), TCU (≥44% N), and PCU (≥43% N), were applied, with a one-off application as the basal fertilizer. The three used CRUs had a release longevity of two months and were provided by Kingenta Ecological Engineering Group Co., Ltd. (Linshu, Shandong, China). In addition, 60 kg of P2O5 ha−1 (in the form of superphosphate) and 90 kg of K2O ha−1 (in the form of potassium sulfate) were applied. All the fertilizers were broadcast and then incorporated into the upper 0–15 cm of the soil by rotary tillage before sowing. After the fertilizers were applied, the field plots were immediately irrigated. The irrigation rate was based on the soil water content.

2.3. Measurements and Calculation of Evaluation Indicators

2.3.1. Plant and Soil Sampling

At maturity, two rows of plants in the middle of each plot were harvested to determine the grain yield (at 14% moisture). Sub-samples of three-plant samples were collected and separated into grain and straw samples. All samples were oven-dried at 75 °C to determine the dry weight. Plant samples were ground with a stainless steel grinder (RT-02B, Taiwan, China). The N concentration in samples was determined using a CN analyzer (vario Macro cube, Elementar, Hanau, Germany). During the harvest period, the 0–20 cm soil layer was used to analyze chemical properties. The chemical characteristics were as follows: soil pH (1:2.5 w/v in water), organic matter content, total soil N, available phosphorus (Olsen P), and available potassium (NH4OAc-K) [20].

2.3.2. Measurements of Physiological Characteristics and Soil Enzyme Activities

Photosynthetic parameters, including the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (gs), and intercellular CO2 concentration (Ci), were measured using a portable LI-6400 photosynthesis system (LI-COR Inc., Lincoln, NE, United States) at 09:00–11:00 on a fine day. The ear leaves at the silking and milking stages were selected for measurement in 2020. The photosynthetically active radiation, temperature, and CO2 concentration during measurement collection were set at 600 µmol m−2 s−1, 25 °C, and 380 µmol mol−1, respectively. In addition, leaf samples were taken randomly at the silking and milking stages for each treatment in 2020 to determine the contents of different hormones, including salicylic acid (SA), gibberellic acid (GA3), indoleacetic acid (IAA), and abscisic acid (ABA). Four ear leaves from each treatment were collected and kept at −20 °C until their biochemical analyses were performed. Analyses were performed using the Plant Hormone Quantifying Service (IBMCP-UPV) in a Thermo Scientific™ Q Exactive™ Hybrid Quadrupole-Orbitrap Mass Spectrometer (LC-MS/MS HR) (Thermo Fisher Scientific, Bremen, Germany). Frozen samples taken at silking were used to determine nitrate reductase (NR), glutamine synthetase (GS), glutamate oxaloacetate transaminase (GOT), and glutamate pyruvate transaminase (GPT) activity using enzyme immunoassay kits [21]. Soil samples were collected from a depth of 0–20 cm at maturity, and were kept at −20 °C until analyses were performed. Soil urease was determined using the sodium phenol–sodium hypochlorite colorimetric method [22]. The activity of soil sucrase was determined using the 3,5-dinitrosalicylic acid colorimetric method [23]. The activity of soil catalase (CAT) was determined using the KMnO4 titration method [23].

2.3.3. N Release Rate of CRU

The fertilizer bag-burying method was adopted to determine the N release rate of CRU [21]. For each CRU type (PSCU, TCU, and PCU), 10.0 g samples were sealed in 100-mesh nylon bags (12 × 8 cm) and buried at a depth of 15 cm at sowing. Quadruplicate bags per treatment were retrieved at 10-day intervals. The loss of weight was assumed as the release rate of CRU.

2.3.4. Reactive Nitrogen Losses and GHG Emission Calculations

The Nr losses (kg N ha−1) were divided into three sources: N leaching, NH3 volatilization, and N2O emission. For the N fertilizer application period, the total Nr losses were estimated [8,24]. To ensure the accuracy of empirical models, the peer-reviewed publications were restricted to the NCP. The Nr losses from common urea and CRU were calculated as follows:
N leaching = 18.9 + 0.35 × N rate (Urea)
N leaching = 25.0 + 0.19 × N rate (CRU)
N2O emission = 0.50 + 0.0117 × N rate (Urea)
N2O emission = 0.14 + 0.0076 × N rate (CRU)
NH3 volatilization =5.3 + 0.088 × N rate (Urea)
NH3 volatilization =5.3 + 0.043 × N rate (CRU)
Nr losses = N leaching+N2O emission + NH3 volatilization
where N rate represents the N application rates of common urea and CRU, respectively. Urea represents N losses from conventional urea fertilizer through N leaching, NH3 volatilization, and N2O emission. CRU represents N losses from CRU fertilizer through N leaching, NH3 volatilization, and N2O emission.
Total GHG emissions, including CO2 and N2O during the whole life cycle of crop production, consisted of three components: those during N fertilizer application, including direct and indirect N2O emissions, which can be calculated based on the empirical Nr losses model mentioned above; those during N fertilizer production and transportation; and those during the production and transportation of P and K fertilizers and pesticides to the farm gate, and diesel fuel use in farming operations such as sowing, tillage, and harvesting [17,24]. The 100-year global warming potential of N2O is 273 times the intensity of CO2 on a mass basis [25]. The GHG emissions were calculated using the following equations:
GHG emissions = GHGN + Total N2O × 44/28 × 273+ GHGothers
Total N2O = N2O direct + 1% × NH3 volatilization + 0.75% × N leaching
GHGN represents agricultural N fertilizer input. GHGothers represents the GHG emission potential per kg of agricultural material input produced and transported, as shown in Table S1 [26,27,28,29,30,31]. The application rates of input agricultural materials are shown in Table S2. Total N2O represents the total N2O loss from the direct and indirect pathways, where indirect N2O emissions were estimated as the sum of 1% of the NH3 volatilization and 0.75% of the N leaching.
System boundaries were defined as the periods of the life cycle from the production of inputs (such as fertilizers and pesticides), the delivery of the inputs to the farm gates, farming operations, and the crop harvesting period. The Nr losses and the intensity of GHG emissions were calculated as Nr losses and GHG emissions per million grams of grain production, expressed as kg N Mg−1 and kg CO2 eq Mg−1.

2.3.5. Evaluation of Economic Benefits

Economic benefits were evaluated throughout the maize growing season based on crop production and gross returns gained from selling maize. All material and machinery costs were taken into account, and all input costs were determined according to local market prices. Material costs included the cost of seed, fertilizers, pesticides, herbicides, and irrigation. The ecological costs were calculated with the following equations [24]:
Ecological costs = CGHG + Ceu + Cacid = (CO2 × 0.0204) + (1.12 × NO3 + 0.24 × NH3 + 0.0018 × N) + (1.87 × NH3 + 0.021 × N)
where CGHG, Ceu, and Cacid are the costs of GHG emission damage, water eutrophication damage, and soil acidification damage [32,33]. CO2 emissions accounted for the total GHG emissions associated with the production, transportation, and application of N fertilizers. In 2008, the market price of CO2 was USD 0.0204 kg−1. The costs of mitigating the eutrophication impacts were USD 1.12 kg−1 for NO3 and USD 0.24 kg−1 for NH3, respectively. Additionally, during the N fertilizer application process, the cost of addressing soil acidification damage caused by NH3 was USD 1.87 kg−1. The costs associated with eutrophication and soil acidification damages from producing one kilogram of N fertilizer were estimated at USD 0.0018 and 0.021 kg−1, respectively.

2.4. Statistical Analysis

The data for 2019 and 2020 were subjected to an analysis of variance (ANOVA) using the General Linear Model procedure (GLM) in SPSS (version 21.0). Maize grain yield response curves to the N application rate were generated in SPSS. Three response models (quadratic, quadratic with plateau, and linear with plateau) were evaluated, and the linear-with-plateau model produced the best fit. Differences were compared using Duncan’s multiple range tests (DMRT) at the 0.05 probability level.

3. Results

3.1. N Release Curves of Controlled-Release Urea

The N cumulative release curve of CRU exhibited an “inverted-L” shape (Figure 2). More than 40.0% of N was released in the first 30 d after being buried in soil, followed by a period of constant release, and ended with a reduced N-release stage. The total N released until the harvest stage was more than 80% for TCU and PCU in both years, but was only 73.0% in 2019 and 69.9% in 2020 for PSCU (Figure 2). The N release rate of PSCU before the six-leaf stage (V6) was faster than that of TCU and PCU, and slower than that of TCU and PCU after the silking stage.

3.2. Maize Yield Response to N Management

When N fertilizer was applied in the form of urea, the responses of the maize grain yield to increased N application rates were similar in 2019 and 2020, and a linear-with-plateau model fit the data well (Figure 3A,B). The minimum N rates required to achieve the maximum grain yield were 221 kg N ha−1 in 2019 and 216 kg N ha−1 in 2020. The calculated maximum grain yield reached 10.73 Mg ha−1 in 2019 and 13.30 Mg ha−1 in 2020. The actual grain yield peaked with N234 treatment (10.97 Mg ha−1 in 2019 and 13.39 Mg ha−1 in 2020), and was very close to the calculated maximum grain yield.
Furthermore, compared with no N application, the grain yield was significantly increased by 36.0% and 38.0% with N234 treatment in 2019 and 2020, respectively. However, when further increasing the supply of N from N234 to N280, the grain yield was not significantly increased in either year (Figure 3C,D). Compared with N234 to N280, the three reduced-rate CRU treatments, with an N application rate of 180 kg N ha−1 (PSCU, TCU, and PCU), showed no significant effects on grain yield. Among the three types of CRUs, the grain yield was the highest with TCU treatment, measuring 10.7 Mg ha−1 and 13.9 Mg ha−1 in 2019 and 2020, respectively.

3.3. Maize N Uptake and Response of Economic Benefits to N Management

The NUE and PFPN significantly decreased with the increase in N rates, from an average of 180 kg N ha−1 to 280 kg N ha−1 (Table 2). In comparison to PSCU at 180 kg N ha−1, TCU and PCU improved NUE by 10.3% and 6.4%, respectively, while PFPN increased by 5.8% and 1.4%. Compared to the N0 treatment, the net profit significantly increased by 55.8% and 44.8% with N234 and N280 treatments in 2019 and 2020, respectively (Table 3). In comparison to conventional urea, the net profits of the three CRU fertilizers (PSCU, TCU, and PCU) increased by 0–10.0%, 9.9–14.4%, and 1.2–5.3% across both years, respectively. Among all treatments, TCU exhibited the highest net profit when compared to both common urea and CRU fertilizer treatments.

3.4. Response of Physiological Characteristics to N Management

3.4.1. Leaf Photosynthetic Parameters

In comparison with no N supply, the N234 and N280 treatments led to a significant augmentation in Ci and Tr during the silking stage, with average increments of 95.8% and 113.2% in 2020, respectively (Figure 4). Compared with the N234 and N280 treatments, the TCU treatment increased Pn by 7.9% (Figure 4A), and the PCU treatment increased Ci by 27.2% (Figure 4B). During the milking period, relative to the N0 treatment, the application of common urea has a relatively smaller impact on photosynthetic indicators. In comparison with the N234 and N280 treatments, the PCU treatment increased Pn and Ci by 32.7% and 20.6%, respectively, and the TCU treatment enhanced Ci by 35.4%.

3.4.2. Enzyme Activity

Different N fertilizer treatments can significantly affect NR, GOT, and GPT activities in ear leaves (Figure 5). In comparison with no N supply, the N234 and N280 treatments led to a significant augmentation in NR and GPT activities in ear leaves, with average increments of 6.3% and 22.4% in 2020, respectively. However, the three CRU treatments had a less pronounced effect on these enzyme activities compared to N234 and N280 in 2020.

3.4.3. Hormones

In comparison with no N supply, the N234 and N280 treatments led to an increase in SA during the silking stage, with average increments of 71.2% and 22.6% in 2020, respectively (Figure 6A). In contrast to N234 and N280, the GA3 levels associated with the three CRU fertilizers (PSCU, TCU, and PCU) increased by 22.4%, 27.2%, and 14.0%, respectively (Figure 6B). During the milking period, relative to the treatment N0, common urea exhibited a relatively minor effect on hormone content. However, when compared to the N234 and N280 treatments, TCU application significantly enhanced GA3 content by 17.9% in 2020. The IAA levels for PSCU, TCU, and PCU increased by 32.1%, 22.4%, and 25.7%, respectively (Figure 6C). Furthermore, when comparing PSCU, TCU, and PCU treatments with N234 and N280 applications, the ABA content increased by 37.8%, 30.9%, and 12.2%.

3.5. Soil Chemical Properties and the Response of Enzyme Activities to N Management

Compared to the N0 treatment, both N234 and N280 treatments exhibited a relatively minor impact on soil enzyme activity in 2020 (Figure 7). In contrast, TCU increased sucrase activity by 10.8% when compared to N234 and N280 (Figure 7A). The three CRU treatments enhanced urease activity by 18.3%, 35.8%, and 13.3%, respectively. Notably, CAT activity remained unaffected by N fertilization, irrespective of the nitrogen source or application rate.
Soil pH experienced a significant decrease of 3.6% in the N234 and PCU treatments; other treatments also showed a decreasing trend but without statistical significance in 2020 (Table 4). When compared to the N0 treatment, the N234 and N280 applications significantly increased alkali-hydrolyzed N content by 57.6% and 40.4%, respectively. Additionally, relative to N234 and N280, the alkali-hydrolyzed N content exhibited only minor variations with the three CRUs applied. In terms of the impact of soil organic matter from common urea application versus no N supply, it was observed that the effect of common urea was comparatively smaller. Furthermore, CRUs significantly enhanced the soil’s organic matter content by 23.7% when compared to common urea applications. However, no significant differences were observed in the effects of the three CRUs on soil organic matter.

3.6. Reactive N Losses and GHG Emissions Affected by N Management

Compared to the N0 treatment, the N234 and N280 treatments significantly increased Nr losses by 4.3 and 5.1 times in both years, respectively (Figure 8A). When compared to the N234 and N280 treatments, CRUs effectively reduced Nr losses by 47.5% across PSCU, TCU, and PCU applications. However, no significant differences were observed among the three CRU treatments regarding Nr losses. In fertilized plots, Nr losses primarily stemmed from nitrogen fertilizer application, with leaching being the largest contributor (77–80%), followed by NH3 volatilization (18–21%). N2O emissions accounted for a very small percentage (<2%) of N losses.
In comparison to the N0 treatment, GHG emissions were significantly elevated by 1.2 times and 1.6 times for the N234 and N280 treatments in both years, respectively (Figure 8B). When compared to the N234 and N280 treatments, CRUs effectively reduced GHG emissions by an average of 18.7% across PSCU, TCU, and PCU applications. However, no significant differences were observed among the three CRU treatments regarding GHG emissions. The production and transportation of nitrogen fertilizer accounted for a substantial portion of GHG emissions (29–33%), followed by other sources contributing between 25% and 80%, while N fertilizer application contributed between 20% and 43% within fertilized plots. Under the N0 treatment condition, other sources emerged as the predominant contributor to GHG emissions, representing approximately 80%.

3.7. Reactive N Losses and Intensity of GHG Emissions Affected by N Management

Compared to the N0 treatment, the N234 and N280 treatments significantly increased the intensity of Nr losses by 2.7 and 3.5 times in both years, respectively (Figure 9A). In comparison to the N234 and N280 treatments, CRUs notably reduced the intensity of Nr losses by 45.5% across PSCU, TCU, and PCU treatments. However, no significant differences were observed in the intensity of Nr losses among the three CRU treatments.
Compared to the N0 treatment, the N234 and N280 treatments significantly increased the intensity of GHG emissions by 1.0 and 1.3 times in both years, respectively (Figure 9). In comparison to the N234 and N280 treatments, CRUs notably reduced the intensity of GHG emissions by 27.0% across PSCU, TCU, and PCU treatments. However, no significant differences were observed in the intensity of GHG emissions among the three CRU treatments. The slight improvement in the intensity of GHG emissions under TCU was attributed to the decreased GHG emissions and the increased grain yield.

4. Discussion

4.1. Effects of CRU and Common Urea on Maize Yield and N Uptake

The maize grain yield under common urea application followed a linear-plateau response, reaching a critical threshold at 234 kg N ha−1 (N234) (Figure 3). This pattern exemplifies the “law of diminishing returns” in agronomy, where physiological and edaphic constraints limit yield gains beyond an optimal N input [34], likely reflecting the crop’s maximum capacity to convert absorbed N into grain biomass. Crucially, CRU treatments achieved yields comparable to common urea while reducing N application rates by 12–19% (180 kg N ha−1 vs. 207–222 kg N ha−1 plateau rates, as reported in prior NCP studies) [35,36]. Notably, the TCU treatment yielded the highest results out of all treatments. This superior performance in NUE for CRU compared to common urea (Table 2) demonstrates CRU’s capacity to better coordinate N supply with crop demand.
CRU exhibits an initial rapid release within the first month post-application, providing sufficient N during the seedling stage when uptake demand begins to rise while minimizing early-season losses through controlled availability [37]. The superior cumulative N release of TCU and PCU (>80% by harvest) compared to PSCU (70–73%) suggests better synchronization of nutrient supply with crop demand. This enhanced synchronization, particularly sustained N availability during critical post-silking grain filling, is essential for yield optimization [4]. The incomplete release from PSCU likely resulted in insufficient late-season N availability, constraining its yield potential despite adequate early supply [35,38]. TCU’s release kinetics appear optimal for coordinating nutrient supply with the maize N uptake pattern.

4.2. Response of Physiological and Soil Enzyme Plant Activities to N Management

CRU application significantly enhanced photosynthetic efficiency during the reproductive stages. The significant increase in Pn by 7.9–32.7% and Ci by 20.6–40.0% under three types of CRUs during the silking and milking stages (Figure 5) highlights the critical role of a synchronized N supply in sustaining carbon assimilation [35,39]. Unlike common urea, which suffers from rapid hydrolysis and early-season N losses, CRU’s biphasic release profile ensures sustained N availability during reproductive phases [40], maintaining chlorophyll content and Rubisco activity and increasing light energy conversion efficiency [10]. Consequently, elevated Pn increased photoassimilate production, while optimized N delivery during grain filling strengthened sink capacity, synergistically enhancing both NUE and yield [41,42].
At the optimal N rate (180 kg N ha−1), CRU elicited coordinated improvements in both N assimilation and hormonal regulation, surpassing common urea [43]. The application of CRU demonstrated dual physiological benefits by simultaneously enhancing key N metabolism enzymes (NR, GPT, and GOT) and maintaining elevated hormone levels (GA3, IAA, and ABA) (Figure 5 and Figure 6). The stable N supply from CRU ensured continuous NR activity for efficient N reduction; stable transaminase function for uninterrupted amino acid and protein synthesis, which is vital for growth [44,45]; and sustained substrate availability, stimulating GA3 biosynthesis. GA3 promotes cell division and elongation and can upregulate photosynthetic genes [46], further enhancing carbon fixation. The observed hormonal synergy (elevated GA3 with maintained ABA) provides a physiological mechanism for the consistent yield and NUE benefits across both study years. These results confirm that CRU effectively coordinates whole-plant physiology, optimizing N assimilation, hormone dynamics, and ultimately source–sink relationships.
Soil enzyme activities (sucrase and urease) serve as key indicators of microbial functional capacity and nutrient cycling [47]. CRU treatments, particularly TCU, maintained significantly higher sucrase and urease activities compared to common urea (Figure 7), consistent with meta-analyses [48]. This enhancement is attributed to CRU’s slow-release characteristics [49], which provide a more stable N substrate flow. TCU may offer superior stability and temperature resistance, ensuring steadier release, and its coating properties might foster microenvironments conducive to enzyme–substrate interactions [50]. Elevated enzyme activities promote nutrient cycling and transformation. Furthermore, the associated improvement in soil organic matter content benefits soil structure, enhancing water/air infiltration and retention, thus creating a favorable environment for both soil microorganisms and plant roots [48,51]. CRU coordinates soil N retention and microbial function to foster a healthier rhizosphere, ultimately supporting improved plant growth, yield, and NUE.

4.3. Reactive N Losses and Response of GHG Emissions to N Management

Using established empirical models [8,17] and LCA methods [52], the total Nr losses and intensity were estimated. CRU application significantly mitigated total Nr losses by 47.5% compared to common urea, with no significant differences observed among the three CRU types. This substantial reduction stemmed primarily from diminished N leaching (27.1%) and NH3 volatilization (39.4%), aligning with findings by Chen et al. [19]. Polymer/sulfur coatings delay urea dissolution, reducing sudden spikes in soil NH4+/NH3 concentrations that drive volatilization [8]. Moreover, a gradual N supply matches crop uptake, minimizing NO3− accumulation and leaching risks [9,53]. Although TCU optimized N availability for uptake (yielding the highest result), all CRUs were equally effective in preventing Nr losses. Under field conditions, environmental factors (temperature, humidity, and soil pH) may diminish characteristic differences among coating materials. Crucially, all CRU coatings delay urea hydrolysis, resulting in NH4+ release rates generally lower than both crop uptake and soil adsorption capacities [54], leading to significantly reduced NH3 emissions irrespective of coating type [55,56]. This demonstrates CRU’s inherent capacity to coordinate N release with soil retention processes and plant uptake, minimizing environmental losses.
CRU production and transportation emitted more GHGs than common urea (an additional 0.72 kg CO2 eq per kg N) [8]. However, during the field application phase, CRU significantly reduced direct N2O emissions. This reduction was substantial enough to fully offset the higher production/transport emissions, resulting in lower total GHG emissions and a lower emission intensity for CRU at the optimal rate (Figure 6 and Figure 7). When combined with similar or higher grain yields, CRU achieved comparable or lower GHG emissions per unit yield [57]. The results confirm that N fertilizer production and application are major GHG sources, underscoring the importance of reducing N rates to minimize the carbon footprint [27]. CRU’s ability to maintain yields with lower N rates is therefore key to its net GHG mitigation benefit.
Although direct microbial community analysis was not conducted, the observed N dynamics strongly suggest that CRU’s controlled release significantly modulated the function of the soil microbiome. The gradual N release pattern, which is particularly characteristic of TCU, likely suppressed Nitrosomonas-dominated ammonia-oxidizing bacteria (AOB) communities [58]. This is evidenced by the 47.5% reduction in N2O emissions compared to urea and aligns with metagenomic studies showing that moderate NH4+ availability favors K-strategist Nitrosospira over r-strategist Nitrosomonas [58]. Concurrently, the 35.8% increase in urease activity under TCU (Figure 7) indicates stimulation of ureolytic microbes (e.g., Bacillus subtilis and Pseudomonas stutzeri), which are known for high-affinity urease systems under moderate N conditions [50]. These inferred microbial shifts, driven by CRU’s coordinated N release, contribute to enhanced NUE by promoting organic N mineralization while reducing gaseous and leaching losses.
Economically, CRU adoption, particularly TCU, offered compelling advantages despite a 20–40% higher initial cost than urea. TCU generated 12.6% greater net profits, primarily through (1) maintaining yield stability (13.9 Mg ha−1) with 12–19% less N input and (2) reducing labor requirements by 50% due to a single basal application. This reduction in labor is especially significant given China’s rapid agricultural urbanization, where rural labor costs have risen by 8.3% annually since 2015. When coupled with emerging carbon credit schemes for N2O mitigation, these trends position TCU as an ecologically and economically superior N management strategy for intensive maize systems, effectively coordinating productivity, environmental protection, and farm profitability.

5. Conclusions

This study demonstrates that CRU, particularly TCU, established an ecologically optimized N management regime for maize production in the NCP. The major findings were that (1) CRU achieved maize yields comparable to common urea while reducing application rates by 12–19% (180 kg N ha−1), primarily through enhanced photosynthetic efficiency, as well as hormone levels (GA3, IAA, and ABA), optimizing source–sink relationships; (2) the CRU system improved NUE through higher soil urease activity and the activity of key N assimilation enzymes (NR, GPT, and GOT); and (3) CRU reduced Nr losses and GHG emissions while improving the soil’s organic matter content, fostering a more stable soil–plant N cycle. These improvements were achieved while maintaining 9.9–14.4% higher net profits compared to conventional urea, primarily through yield stability and labor savings from a single application. The thermosetting polymer coating of TCU proved particularly effective, demonstrating optimal N release kinetics that balanced crop demand with environmental protection. These findings position TCU as a cornerstone technology for the sustainable intensification of maize production systems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture15161778/s1, Table S1. GHG emission factors for the production and transportation of various agricultural inputs in the winter wheat production system. Table S2. Mean application rate of various agricultural inputs in 2019–2020 in the maize production system.

Author Contributions

Conceptualization, H.Y. and S.S.; methodology, M.H.; software, Z.W.; validation, Y.X. (Yinshan Xie), Q.L. and C.G.; formal analysis, J.S.; investigation, M.H.; resources, M.H.; data curation, B.L.; writing—original draft preparation, W.Y.; writing—review and editing, Y.X. (Yanfang Xue); visualization, K.L.; supervision, K.L.; project administration, Y.X. (Yanfang Xue); funding acquisition, Y.X. (Yanfang Xue). All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Science and Technology Cooperation Projects between Shandong Province and Israel (2023KJHZ001), the National Key Research and Development Program of China (2021YFD1901003, 2017YFD0301005), the Shandong Provincial Natural Science Foundation (ZR2021QC115), the Innovative Talent Introduction and Innovation Project of Shandong Academy of Agricultural Sciences (CXGC2025C02), the Agricultural Science and Technology Research Project of Bureau of Agriculture and Rural Affairs of Jinan (GG202501), and the Project of Science and Technology commissioners in Shandong Province (2022DXAL0125).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Daily mean temperature, solar radiation, and precipitation during the maize growing seasons in 2019 (A) and 2020 (B) at the Longshan site of the Shandong Academy of Agricultural Sciences (36°43′ N, 117°32′ E), Shandong Province, China.
Figure 1. Daily mean temperature, solar radiation, and precipitation during the maize growing seasons in 2019 (A) and 2020 (B) at the Longshan site of the Shandong Academy of Agricultural Sciences (36°43′ N, 117°32′ E), Shandong Province, China.
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Figure 2. N release rate and accumulative release rate of CRU under field conditions in 2019 (A) and 2020 (B). PSCU-interval, the time interval release rate of polymer-sulfur-coated urea; TCU-interval, the time interval release rate of thermosetting-polymer-coated urea; PCU-interval, the time interval release rate of polymer-coated urea; PSCU-accumulative, the accumulative release rate of polymer-sulfur-coated urea; TCU-accumulative, the accumulative release rate of thermosetting-polymer-coated urea; PCU-accumulative, the accumulative release rate of polymer-coated urea. R1, silking stage; R3, milking stage; R5, dent stage; R6, maturity stage.
Figure 2. N release rate and accumulative release rate of CRU under field conditions in 2019 (A) and 2020 (B). PSCU-interval, the time interval release rate of polymer-sulfur-coated urea; TCU-interval, the time interval release rate of thermosetting-polymer-coated urea; PCU-interval, the time interval release rate of polymer-coated urea; PSCU-accumulative, the accumulative release rate of polymer-sulfur-coated urea; TCU-accumulative, the accumulative release rate of thermosetting-polymer-coated urea; PCU-accumulative, the accumulative release rate of polymer-coated urea. R1, silking stage; R3, milking stage; R5, dent stage; R6, maturity stage.
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Figure 3. Maize grain yield under different N treatments in 2019 and 2020. Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05), ** significant at the 0.01 probability level, *** significant at the 0.001 probability level. (A) Relationship between the N application rate and grain yield in 2019; (B) relationship between the N application rate and grain yield in 2020; (C) maize grain yield under different N treatments in 2019; (D) maize grain yield under different N treatments in 2020.
Figure 3. Maize grain yield under different N treatments in 2019 and 2020. Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05), ** significant at the 0.01 probability level, *** significant at the 0.001 probability level. (A) Relationship between the N application rate and grain yield in 2019; (B) relationship between the N application rate and grain yield in 2020; (C) maize grain yield under different N treatments in 2019; (D) maize grain yield under different N treatments in 2020.
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Figure 4. Leaf photosynthetic parameters, including Pn (A), Ci (B), gs (C), and Tr (D), of newly developed leaves under different N treatments in 2020. Error bars represent the standard error of the mean (n = 4). Significant differences at p < 0.05 are shown with different letters. Pn, photosynthetic rate; Ci, intercellular CO2 concentration; gs, conductance to H2O; Tr, transpiration rate.
Figure 4. Leaf photosynthetic parameters, including Pn (A), Ci (B), gs (C), and Tr (D), of newly developed leaves under different N treatments in 2020. Error bars represent the standard error of the mean (n = 4). Significant differences at p < 0.05 are shown with different letters. Pn, photosynthetic rate; Ci, intercellular CO2 concentration; gs, conductance to H2O; Tr, transpiration rate.
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Figure 5. Enzyme activities related to nitrogen metabolism in ear leaves, including NR (A), GS (B), GOT (C), and GPT (D), under different N treatments in 2020. Error bars represent the standard error of the mean (n = 4). Significant differences at p < 0.05 are shown with different letters. NR, nitrate reductase; GS, glutamine synthetase; GOT, glutamate oxaloacetate transaminase; GPT, glutamate pyruvate transaminase.
Figure 5. Enzyme activities related to nitrogen metabolism in ear leaves, including NR (A), GS (B), GOT (C), and GPT (D), under different N treatments in 2020. Error bars represent the standard error of the mean (n = 4). Significant differences at p < 0.05 are shown with different letters. NR, nitrate reductase; GS, glutamine synthetase; GOT, glutamate oxaloacetate transaminase; GPT, glutamate pyruvate transaminase.
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Figure 6. Hormone content comparison between the applied fertilizer treatments (CRU and common urea) and the control (no fertilizer applied) in the field experiment in 2020. The quantified hormones were salicylic acid (SA), gibberellic acid (GA3), indoleacetic acid (IAA), and abscisic acid (ABA). (A) SA content under N treatments at different growth stages in 2020; (B) GA content under N treatments at different growth stages in 2020; (C) IAA content under N treatments at different growth stages in 2020; (D) ABA content under N treatments at different growth stages in 2020. Error bars represent the standard error of the mean (n = 4). Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
Figure 6. Hormone content comparison between the applied fertilizer treatments (CRU and common urea) and the control (no fertilizer applied) in the field experiment in 2020. The quantified hormones were salicylic acid (SA), gibberellic acid (GA3), indoleacetic acid (IAA), and abscisic acid (ABA). (A) SA content under N treatments at different growth stages in 2020; (B) GA content under N treatments at different growth stages in 2020; (C) IAA content under N treatments at different growth stages in 2020; (D) ABA content under N treatments at different growth stages in 2020. Error bars represent the standard error of the mean (n = 4). Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
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Figure 7. Soil enzyme activities under different N treatments in 2020. (A) Sucrase activity under N treatments at silking stages in 2020; (B) CAT activity under N treatments at silking stages in 2020; (C) urease activity under N treatments at silking stages in 2020. Error bars represent the standard error of the mean (n = 4). Significant differences at p < 0.05 are shown with different letters.
Figure 7. Soil enzyme activities under different N treatments in 2020. (A) Sucrase activity under N treatments at silking stages in 2020; (B) CAT activity under N treatments at silking stages in 2020; (C) urease activity under N treatments at silking stages in 2020. Error bars represent the standard error of the mean (n = 4). Significant differences at p < 0.05 are shown with different letters.
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Figure 8. Reactive N losses and GHG emissions in maize production as affected by N treatments. (A) Nr losses include N2O emissions, N leaching, and NH3 volatilization; (B) GHG emissions include those from N fertilizer application, N fertilizer production and transportation, and other sources (phosphorus and potassium fertilizer, crop management). Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
Figure 8. Reactive N losses and GHG emissions in maize production as affected by N treatments. (A) Nr losses include N2O emissions, N leaching, and NH3 volatilization; (B) GHG emissions include those from N fertilizer application, N fertilizer production and transportation, and other sources (phosphorus and potassium fertilizer, crop management). Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
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Figure 9. Reactive N losses and GHG emissions intensity in maize production as affected by N treatments in 2019 (A,C) and 2020 (B,D). (A,C) Intensity of Nr losses includes N2O emissions, N leaching, NH3 volatilization, and N runoff. (B,D) Intensity of GHG emissions includes those from N fertilizer application, N fertilizer production and transportation, and other sources (phosphorus and potassium fertilizer, crop management). Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
Figure 9. Reactive N losses and GHG emissions intensity in maize production as affected by N treatments in 2019 (A,C) and 2020 (B,D). (A,C) Intensity of Nr losses includes N2O emissions, N leaching, NH3 volatilization, and N runoff. (B,D) Intensity of GHG emissions includes those from N fertilizer application, N fertilizer production and transportation, and other sources (phosphorus and potassium fertilizer, crop management). Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
Agriculture 15 01778 g009
Table 1. Description of the N treatments used in this study.
Table 1. Description of the N treatments used in this study.
TreatmentN SourceTotal N Rate
(kg N ha−1)		Basal N RateTopdressing
at 10-Leaf Stage
(kg N ha−1)
Common Urea
(kg N ha−1)		CRU
(kg N ha−1)
N0−0000
N126Common urea12650.4−75.6
N180Common urea18072−108
N234Common urea23493.6−140.4
N280Common urea280186.7−93.3
PSCUCRU180−1800
TCUCRU180−1800
PCUCRU180−1800
Table 2. Aboveground N uptake of maize and economic benefits under different N treatments in 2019–2020.
Table 2. Aboveground N uptake of maize and economic benefits under different N treatments in 2019–2020.
YearN TreatmentGNCSNCGNUSNUTotal N
Uptake		NHIPFPNNUE
(g kg–1)(g kg−1)(kg ha−1)(kg ha−1)(kg ha−1)(%)(kg kg−1)(%)
2019N010.58 c6.93 b70.1 b45.4 b115.4 b60.9 a––
N23413.32 ab8.42 a125.4 a70.5 a195.9 a64.1 a46.9 b34.4 ab
N28013.07 ab8.81 a117.7 a76.8 a194.5 a60.8 a37.4 c28.2 b
PSCU13.04 ab8.72 a114.3 a77.7 a192.0 a59.9 a56.5 a42.5 ab
TCU12.98 c8.96 a119.8 a84.9 a204.6 a58.5 a59.6 a49.6 a
PCU14.06 a8.50 a121.8 a75.4 a197.2 a61.8 a56.0 a45.4 ab
2020N09.30 b6.90 c77.2 b48.6 b125.7 b61.4 a––
N23413.46 a11.49 a152.1 a132.7 a284.8 a53.7 a56.2 b72.4 ab
N28014.13 a10.50 ab161.0 a120.6 a281.6 a57.0 a47.2 c59.3 b
PSCU13.70 a9.90 ab154.1 a111.2 a265.2 a58.4 a72.7 a83.2 ab
TCU13.60 a9.47 b162.7 a108.3 a271.1 a60.0 a77.2 a86.5 a
PCU13.67 a9.73 b159.6 a114.5 a274.1 a58.4 a75.4 a88.1 a
Source of variance
Year (Y)ns*******************
N treatment (N)***************ns*****
N*Y*nsnsnsnsnsnsns
GNC, grain N concentration; SNC, straw N concentration; GNU, grain N uptake; SNU, straw N uptake; NHI, N harvest index; PFPN, partial factor productivity from applied nitrogen; NUE, N use efficiency. Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05). ns, non-significant at the 0.05 probability level; * significant at the 0.05 probability level; ** significant at the 0.01 probability level, *** significant at the 0.001 probability level.
Table 3. Annual total revenue, cost, and net profit of corn production with different N management treatments in 2019–2020.
Table 3. Annual total revenue, cost, and net profit of corn production with different N management treatments in 2019–2020.
YearTreatmentTotal RevenueFertilizer CostTop Dressing Cost Other CostsEcological CostsNet Profit
(USD ha−1)
2019N02526.0 125.4 0.0 628.5 81.0 1062.5 b
N2343612.4 260.2 47.0 628.5 308.8 1739.4 a
N2803447.3 286.7 47.0 628.5 353.6 1503.0 a
PSCU3315.5 298.0 0.0 628.5 199.7 1560.8 a
TCU3531.7 293.4 0.0 628.5 199.7 1781.5 a
PCU3349.9 253.0 0.0 628.5 199.7 1640.3 a
2020N03176.6 125.4 0.0 628.5 81.0 1713.2 b
N2344406.7 260.2 47.0 628.5 308.8 2533.7 a
N2804347.5 286.7 47.0 628.5 353.6 2403.1 a
PSCU4468.9 298.0 0.0 628.5 199.7 2714.2 a
TCU4572.9 293.4 0.0 628.5 199.7 2822.7 a
PCU4309.7 253.0 0.0 628.5 199.7 2600.0 a
Other costs include machinery, irrigation, pesticide, and seed costs. Net profit = total revenue—fertilizer cost—top dressing cost—other costs—ecological costs. Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
Table 4. Soil chemical properties under different N treatments in 2020.
Table 4. Soil chemical properties under different N treatments in 2020.
N TreatmentpHTotal N Total PTotal KAlkaline HydrolysisAvailable P Available KOrganic Matter
(g kg−1)(g kg−1)(g kg−1)of N (mg kg−1)(mg kg−1)(mg kg−1)(g kg−1)
N07.80 a0.94 a0.73 a25.65 a47.02 b34.58 a170.33 a18.24 b
N2347.52 b1.07 a0.78 a26.43 a74.11 a31.42 a162.00 a17.63 b
N2807.54 ab0.90 a0.83 a26.00 a66.00 a35.08 a141.33 a18.78 b
PSCU7.55 ab1.11 a0.85 a26.60 a71.82 a46.50 a151.33 a21.47 a
TCU7.66 ab1.17 a0.90 a26.93 a68.06 a40.92 a142.33 a23.15 a
PCU7.52 b1.13 a0.78 a26.31 a58.49 ab49.75 a133.33 a22.92 a
Different letters in each column within a given treatment indicate that the means differ significantly (p < 0.05).
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MDPI and ACS Style

Yan, W.; Huang, M.; Yang, H.; Wang, Z.; Sun, S.; Xie, Y.; Sun, J.; Li, Q.; Liu, B.; Gao, C.; et al. Controlled-Release Urea Coordinates Maize Physiology with Soil Nitrogen Retention: Balancing High Yield and Environmental Sustainability. Agriculture 2025, 15, 1778. https://doi.org/10.3390/agriculture15161778

AMA Style

Yan W, Huang M, Yang H, Wang Z, Sun S, Xie Y, Sun J, Li Q, Liu B, Gao C, et al. Controlled-Release Urea Coordinates Maize Physiology with Soil Nitrogen Retention: Balancing High Yield and Environmental Sustainability. Agriculture. 2025; 15(16):1778. https://doi.org/10.3390/agriculture15161778

Chicago/Turabian Style

Yan, Wei, Meng Huang, Huiqing Yang, Zhonghua Wang, Shujuan Sun, Yinshan Xie, Jinbian Sun, Qiong Li, Bo Liu, Chengcheng Gao, and et al. 2025. "Controlled-Release Urea Coordinates Maize Physiology with Soil Nitrogen Retention: Balancing High Yield and Environmental Sustainability" Agriculture 15, no. 16: 1778. https://doi.org/10.3390/agriculture15161778

APA Style

Yan, W., Huang, M., Yang, H., Wang, Z., Sun, S., Xie, Y., Sun, J., Li, Q., Liu, B., Gao, C., Xue, Y., & Liu, K. (2025). Controlled-Release Urea Coordinates Maize Physiology with Soil Nitrogen Retention: Balancing High Yield and Environmental Sustainability. Agriculture, 15(16), 1778. https://doi.org/10.3390/agriculture15161778

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