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Article

Straw Returning Combined with Application of Sulfur-Coated Urea Improved Rice Yield and Nitrogen Use Efficiency Through Enhancing Carbon and Nitrogen Metabolism

by
Guangxin Zhao
1,
Kaiyu Gao
1,
Ming Gao
1,
Xiaotian Xu
1,
Zeming Li
1,
Xianzhi Yang
1,
Ping Tian
1,
Xiaoshuang Wei
1,
Zhihai Wu
1,2,* and
Meiying Yang
3
1
Faculty of Agronomy, Jilin Agricultural University, Changchun 130118, China
2
Jilin Province Green and High Quality Japonica Rice Engineering Research Center, Jilin Agricultural University, Changchun 130118, China
3
College of Life Science, Jilin Agricultural University, Changchun 130118, China
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(14), 1554; https://doi.org/10.3390/agriculture15141554
Submission received: 5 June 2025 / Revised: 11 July 2025 / Accepted: 17 July 2025 / Published: 19 July 2025
(This article belongs to the Special Issue Effects of Crop Management on Yields)
Browse Figures
Versions Notes

Abstract

Straw returning inhibits tillering at the early stage of rice growth and thus affects grain yield. Sulfur-coated urea (SCU) has been expected to increase nitrogen use efficiency (NUE) and yield, save labor input, and reduce environmental pollution in crop production. Nevertheless, the sulfur coatings of SCU are easy to break and then shorten the nutrient release cycle. Whether there was a complementary effect between straw returning and SCU in NUE and grain yield had remained elusive. To investigate the effects of straw returning combined with the application of SCU on NUE and rice yield, a two-year field experiment was conducted from 2022 to 2023 with three treatments (straw returning combined with conventional urea (SRU), no straw returning combined with SCU (NRS), straw returning combined with SCU (SRS)). We found that straw returning combined with the application of SCU increased rice yield and NUE significantly. Compared with SRU and NRS, SRS treatments significantly increased grain yield by 14.61–16.22%, and 4.14–7.35%, respectively. Higher effective panicle numbers per m2 and grain numbers per panicle were recorded in NRS and SRS treatments than SRU. SRS treatment increased nitrogen recovery efficiency by 79.53% and 22.97%, nitrogen agronomic efficiency by 18.68% and 17.37%, and nitrogen partial factor productivity by 10.51% and 9.81% compared with SRU and NRS treatment, respectively. The enhanced NUE in SRS was driven by higher leaf area index, SPAD value, net photosynthetic rate, carbon metabolic enzyme (RuBP and SPS) activity, nitrogen metabolic enzyme (NR, GS, and GOGAT) activity, sucrose and nitrogen content in leaves, and nitrogen accumulation in plant during grain filling. Moreover, the improved yield in SRS was closely related to superior NUE. In conclusion, straw returning combined with application of SCU boosted grain yield and NUE via enhanced carbon–nitrogen metabolism during the late growth period in rice.

1. Introduction

As a major agricultural producer, China generates approximately one billion tons of crop residues annually, dominated by rice straw, corn stover, wheat straw, and cotton stalks. This output constitutes ~33% of the global agricultural residue production [1]. The traditional approach to straw utilization includes cooking, heating, livestock feeding, and so on in rural areas. However, with the improvement in the rural economy, more and more straw has been replaced by fossil fuels, which impels people to deal with excess straw by incineration. This practice consumes resources unnecessarily and triggers multiple environmental issues [2]. As an effective approach to straw utilization, straw returning not only avoids environmental pollution, but also improves soil fertility, which has been strongly recommended by the government and scientists [3,4]. It is widely recognized that straw returning enhances soil fertility and increases crop yield by improving the soil’s physicochemical and biological properties [5,6,7]. Straw returning in paddy fields can increase levels of organic carbon, nitrogen, phosphorus, potassium, and other nutrients in the soil, while also enhancing rice root activity to promote nitrogen and phosphorus uptake. Moreover, straw returning in paddy fields enhanced nitrogen and phosphorus utilization, significantly boosting grain yield by increasing effective panicles, grains per panicle, seed-setting rate, and 1000-grain weight [8,9,10]. In addition, straw returning can also increase the leaf area index (LAI) and net photosynthetic rate (Pn) of rice, promote the accumulation of dry matter in rice and improve the yield of rice [11]. However, straw decomposition initially consumes soil nitrogen during the early stage of returning, competing with crops and leading to insufficient nitrogen supply for crop growth. Furthermore, the decomposition process releases substantial organic acids, inhibiting rice root growth, reducing the root activity and absorption area. This often results in reduced dry matter accumulation and suppressed rice seedling growth [12,13,14]. It is self-evident that the straw-returning cultivation presents a bright prospect for increasing production.
The application of nitrogen fertilizer is vital to agricultural productivity, exerting a major influence on elevating crop output and guaranteeing food security [15]. At present, urea is extensively utilized in agricultural production. Conventional urea releases nutrients rapidly, making it prone to losses through leaching, surface runoff, and volatilization. This not only diminishes nitrogen fertilizer utilization efficiency but also contributes to multiple environmental problems [16,17]. In addition, the urea requires repeated applications to satisfy crop nitrogen demands throughout the growth cycle, effectively raising labor costs. Recently, slow-release fertilizers have gained significant attention owing to their higher nutrient efficiency, reduced labor requirements, and diminished environmental impact compared to conventional fertilizers [18,19]. Sulfur-coated urea (SCU), a controlled-release nitrogen fertilizer, is extensively utilized in agricultural production systems. It is manufactured by initially coating urea with sulfur, followed by sealing with polymeric microcrystalline wax. Micropores in the sulfur coating enable slow nutrient diffusion, maintaining continuous nutrient availability for crops [16,20]. Moreover, the sulfur coating of SCU can be biodegraded and has less impact on the soil environment compared with other polymers used for urea coating [21]. Relative to conventional urea, SCU significantly improved grain yield and nitrogen use efficiency (NUE) in key staple cereals (maize and wheat), increasing yields by 8.19–28% and NUE by 62–72% [22,23]. Similarly, in rice, SCU elevated LAI, NUE, and nitrogen content in stems and grains compared with conventional urea. Additionally, SCU application increased rice yield by enhancing grain weight per panicle, thereby improving economic returns [24,25]. Furthermore, SCU also reduced environmental pollution by suppressing ammonia volatilization in paddy fields [26]. Despite this, the sulfur coating of SCU is easy to break and then shortens the nutrient release cycle. Also, SCU nutrient release exhibits high variability due to the sensitivity of its sulfur coating to soil properties, including temperature, moisture content, and microbial activity. Due to all these factors, SCU may lead to the explosive release of nutrients in the early stage of crop growth, which not only affects the growth of seedlings in the early stage, but also leads to an insufficient supply of nitrogen fertilizer in the later stage [16,27].
During the initial stage of straw returning, decomposition may consume additional nitrogen, competing with crops and causing insufficient nitrogen supply for crop growth. Conversely, in the later growth stages, straw returning provides sustained nutrient release and buffers soil temperature fluctuations, delaying root senescence. Although SCU enhances rice nitrogen utilization and yield, its unstable coating often causes excessively rapid nutrient liberation in early stages, consequently leading to inadequate nutrient availability during late growth periods. It is not clear whether SCU can alleviate the adverse effects caused by straw returning in the early stage and sufficient nutrient supply in the later stage of straw returning can make up for the insufficient nutrient supply caused by SCU. Here, we investigated the grain yield and NUE of rice (Jinong Da 667) under three treatments (i.e., SRU, NRS, and SRS) using a field experiment conducted two years, which aimed to achieve the following: (1) explore the effects of straw returning combined with application of SCU on NUE and rice yield, and (2) elucidate the underlying mechanisms through integrated analyses of photosynthetic physiology, nutrient uptake, and carbon–nitrogen metabolism. This study would further enrich the cultivation measures and provide theoretical basis for high yield and high efficiency cultivation of rice.

2. Materials and Methods

2.1. Experimental Site and Soil Properties

The experiment was carried out from 2022 to 2023 at the National Crop Variety Validation Characterization Station (44°46′ N, 125°39′ E) of Jilin Agricultural University in Changchun City, Jilin Province. The average air temperature and the cumulative rainfall are shown in Appendix A, Figure A1. The basic physical and chemical properties of 0–20 cm soil layer were as follows: organic matter content 16.55 g·kg−1, alkali hydrolyzed nitrogen content 33.89 mg·kg−1, available phosphorus content 29.42 mg·kg−1, available potassium content 137.09 mg·kg−1, and the pH was 6.70. Soil organic carbon (SOC) content was determined by the K2Cr2O7-H2SO4 oxidation method (external heating). Soil organic matter content was calculated by multiplying SOC by the Van Bemmelen factor (1.724). Alkali-hydrolyzable nitrogen was measured using the alkaline hydrolysis diffusion method. Available phosphorus was extracted via the Olsen method (0.5 M NaHCO3, pH of 8.5). Available potassium was extracted with 1 M ammonium acetate (pH of 7.0) and analyzed by flame photometry [28].

2.2. Experimental Design and Agricultural Practices

A randomized complete block design with three replicates was implemented in the experiment. Four treatments were conducted in 2022 with straw returning combined with conventional urea (SRU), no straw returning combined with SCU (NRS), straw returning combined with SCU (SRS), and zero-fertilizer control (only used to calculate NUE). Three treatments were conducted in 2023 with SRU, NRS, and SRS. The area of each plot was 30 m2. The amount of straw returned to the field was 4500 kg·ha−1. Before returning to the field, the rice harvested in the previous season was threshed and crushed into a length of 5–7 cm by a grinder, and dried and packaged for storage as a returning material. In the experimental season, the crushed straw was evenly spread and then turned into the soil layer of 10–15 cm by rotary tiller. The nutrient profile of the straw is shown in Appendix A, Table A1. All treatments applied 150 kg·ha−1 of N. The SCU (37% N) was applied as basal fertilizer. The conventional urea (46% N) was applied in a ratio of 6:3:1 (basal fertilizer/tillering fertilizer/panicle fertilizer). Additionally, all treatments applied 75 kg·ha−1 of P2O5 (calcium superphosphate) and 75 kg·ha−1 K2O (potassium chloride) as basal fertilizer. Basal fertilizers were rotary-tilled into 10–15 cm soil together with straw, while tillering and panicle fertilizers were surface-applied.
The tested rice variety was Jinong Da 667. The seedlings were raised on 13 April, artificially transplanted on 25 May in 2022, and the seedlings were raised on 6 April, and transplanted on 21 May in 2023. The leaf age of seedlings was 3 leaves and 1 heart, and the transplanting density was 30 cm × 13 cm with 5 plants per hole. The harvest and yield measurements were carried out on 1 October in both years. Diseases, insect pests, and water management were unified according to the requirements of high-yield cultivation.

2.3. Measurement Index and Methods

2.3.1. Tillering Dynamics and LAI

After transplanting, 10 representative rice seedlings with consistent growth were selected as fixed-point seedlings in each treatment. The number of tillers of fixed-point seedlings was examined every 7 days from the beginning of tillering stage until the number of tillers was continuously reduced by 2 times.
Five rice plants were sampled for measuring LAI from each plot at the tillering stage (TS), jointing stage (JS), heading stage (HS), and filling stage (FS). The leaf area of rice was measured according to the leaf length multiplied by width by a constant of 0.75. The leaf area divided by the land area occupied by the plants was used to obtain the LAI, which was the assimilatory surface per unit area of land [29].

2.3.2. Rice Yield

Three survey points were selected for each plot at the mature stage. The effective panicles of 25 hills were investigated at each survey point. Ten representative rice plants were selected according to the average effective panicle number. After natural air drying, the grain number per panicle, 1000-grain weight, and seed-setting rate were measured. In addition, we selected 5 representative points of 1 m2 as the production area to calculate actual yield. The actual yield was only used to calculate the NUE. The water content was measured after drying, and the rice yield was calculated according to the standard water content of 13.5%.

2.3.3. Estimation of NUE Parameters

Nitrogen recovery efficiency (NRE), nitrogen agronomic efficiency (NAE), and nitrogen partial factor productivity (NFP) were calculated according to the formula:
NRE   % = Nitrogen   uptake − Nitrogen   uptake   without   nitrogen   application Fertilizer   nitrogen   ×   100
NAE   kg · kg − 1 = Grain   yield   of   nitrogen   applied − Grain   yield   of   no   nitrogen Fertilizer   nitrogen
NFP   ( kg · kg − 1 ) = Grain   yield   of   nitrogen   applied Fertilizer   nitrogen

2.3.4. SPAD Value and Net Photosynthetic Rate of the Rice Leaves

Measurements of SPAD values and net photosynthetic rate (Pn) were conducted on rice flag leaves during the TS, JS, HS, and FS stages. A SPAD-502 portable chlorophyll meter (Minolta, Osaka, Japan) was employed to determine SPAD values, which serve as an indirect indicator of leaf chlorophyll content [30]. Ten replicate measurements were performed for each parameter.
The Pn of rice flag leaves was measured between 9:00 and 11:00 a.m. on windless, sunny days using a Li-6400 portable photosynthesis system (Li-Cor Inc., Lincoln, NE, USA). Five replicate measurements were conducted for each treatment.

2.3.5. Non-Structural Carbohydrate Content in Rice Leaves

Following dry matter measurement at the filling stage, the samples were crushed, sieved, and then analyzed for soluble sugar, sucrose, and starch contents. Soluble sugar and sucrose contents were determined using anthrone colorimetry [31] and resorcinol colorimetry [32], respectively. Starch content was measured via the dual-wavelength method. Each treatment was repeated three times.

2.3.6. Key Enzyme Activities of Carbon and Nitrogen Metabolism in Rice Leaves

The fully expanded flag leaves were selected during the filling stage, cut into pieces after removing the veins, frozen in liquid nitrogen and ground into powder, and stored in a refrigerator at −80 °C for the determination of enzyme activity. The activities of RuBP carboxylase (Rubisco), sucrose synthase (SS), sucrose phosphate synthase (SPS), glutamine synthetase (GS), glutamate synthase (GOGAT), nitrate reductase (NR), and glutamate dehydrogenase (GDH) were determined according to the manufacturer’s instructions using the kit provided by Beijing Solarbio Technology (Beijing, China). Each treatment was repeated three times.

2.3.7. Plant Organic Carbon and Nitrogen Content

The dry matter from TS to FS was crushed by grinding prototype and passed through 20 mesh sieves. The organic carbon content of the plants was determined by the K2Cr2O7-H2SO4 oxidation method, nitrogen content in different organs of rice was determined by fully automated Kjeldahl analyzer (FOSS-8400, FOSS Analytical A/S, Hillerød, Denmark) after digestion by the H2SO4-H2O2 method [28]. Each treatment was repeated three times. The plant organic carbon and nitrogen accumulation was computed according to the following formula:
Organic carbon accumulation in plant (kg·ha−1) = organic carbon content × dry matter
Nitrogen accumulation in plant (kg·ha−1) = nitrogen content × dry matter

2.4. Statistical Analysis

Microsoft Excel 2021 was used for data collation and processing, and Origin 2022 software was used for drawing. Correlation analysis was performed using IBM SPSS Statistics 27 software, and the least significant difference (LSD) test was performed at the p < 0.05 level.

3. Results

3.1. Effects of Straw Returning Combined with Application of SCU on Tillering Dynamics of Rice

As shown in Figure 1, with the advance of the growth period, the increase in the tiller number in rice exhibited an S-curve. The rapid tillering stage commenced 14 days after transplanting, reaching the maximum tillering stage between 35 and 42 days post-transplanting. Following the maximum tillering stage, tiller numbers in all treatments gradually declined, with a more pronounced decline observed in the straw returning treatments (SRU and SRS). At 56 days after transplanting, tiller numbers under the NRS treatment peaked at 16.7 tillers/hill in 2022 and 18.5 tillers/hill in 2023. These values were 11.33% and 11.45% higher than those under the SRU treatment in 2022 and 2023, respectively, and 4.38% and 4.52% higher than those under the SRS treatment in the corresponding years. When taken together, these results demonstrate that the straw returning treatment will reduce the tiller numbers of rice, but the application of SCU can alleviate the negative impact of straw returning to a certain extent, which provided a basis for ensuring the effective panicle numbers of rice.

3.2. Effects of Straw Returning Combined with Application of SCU on Rice LAI

LAI serves as a key indicator of crop population quality, reflecting photosynthetic capacity and demonstrating strong associations with dry matter accumulation and yield formation. As the growth period progressed, the LAI of all treatments initially increased, peaked, and subsequently declined (Figure 2a,b). Both the LAI of NRS and SRS reached the maximum at the jointing stage, while the LAI of SRU treatment achieved the maximum at the heading stage, which suggested that the application of SCU accelerated the growth of rice leaves. At the tillering stage, the LAI of NRS treatment was the highest, which was 11.91–15.79% and 9.54–9.68% higher than that of SRU and SRS, respectively, indicating that straw returning reduced LAI at the early stage of rice growth. From jointing stage to filling stage, the LAI of SRS treatment was significantly higher than that of SRU and NRS treatments. Based on the above results, straw returning combined with application of SCU can maintain a large leaf area during the late growth period of rice, thus improving the photosynthetic performance and providing sufficient organic matter for grain filling.

3.3. Effects of Straw Returning Combined with Application of SCU on Rice Grain Yield

Straw returning combined with application of SCU significantly increased the grain yield of rice (Table 1). The grain yields in 2023 was obviously higher than those investigated in 2022. Specifically, different treatments had marked effects on grain yield. In both years, SRS consistently produced the highest grain yield compared to other treatments. In 2022, the grain yield of SRS was 9453.83 kg·ha−1, which was 14.61% and 4.14% higher than that of SRU and NRS, respectively. In 2023, the grain yield of SRS treatment (10,459.18 kg·ha−1) was 16.22% and 7.35% higher than that of SRU and NRS treatments, respectively. SRS treatment significantly increased both effective panicles per m2 and grains per panicle relative to SRU, while the seed-setting rate and 1000-grain weight showed no significant differences between the treatments. This indicates SCU application alleviated early-stage adverse effects of straw returning, ultimately increasing grain yield by boosting effective panicles and grains per panicle. Compared with NRS, the seed-setting rate and 1000-grain weight of SRS increased significantly, the number of effective panicles per m2 decreased significantly, but there was no significant difference in the grain number per panicle. These results suggest that ample nutrient supply at the late stage of straw returning could make up for the problem of insufficient nutrient supply caused by SCU and improve grain yield by increasing the seed-setting rate and 1000-grain weight.

3.4. Effect of Straw Returning Combined with Application of SCU on Nitrogen Utilization Rate

Nitrogen recovery efficiency (NRE), nitrogen agronomic efficiency (NAE), and nitrogen partial factor productivity (NFP) represent the degree of nitrogen uptake and utilization by crops from different perspectives. Straw returning combined with the application of SCU significantly increased the NRE, NAE, and NFP. The SRS treatment increased NRE by 79.53% and 22.97%, NAE by 18.68% and 17.37%, and NFP by 10.51% and 9.81% compared with SRU and NRS, respectively (Table 2). These results indicate that the application of SCU can improve the NUE of rice, especially under the condition of straw returning.

3.5. Effects of Straw Returning Combined with Application of SCU on SPAD Value and Net Photosynthetic Rate of Rice Leaves

The SPAD value characterizes the relative content of chlorophyll in leaves, which can embody the photosynthetic capacity of leaves to a certain extent. At the tillering stage, the SPAD value of NRS treatment was the highest, which was 5.83–6.48% and 2.28–3.02% higher than that of SRU and SRS, respectively. At the jointing stage, SPAD values showed no significant differences among treatments. However, at heading and filling stages, SRS treatment significantly outperformed SRU and NRS, with more pronounced differences observed during the filling stage (Figure 3a,b). These results suggest that straw returning combined with the application of SCU could significantly increase the stay-green time of leaves.
Photosynthesis is an important source of substance and energy for rice. The net photosynthetic rate of leaves can characterize the ability to produce and accumulate dry matter in rice. Across all treatments, the net photosynthetic rate in rice leaves exhibited an initial increase followed by a decline during growth progression, peaking at the heading stage (Figure 3c,d). At the tillering stage, the net photosynthetic rate of NRS treatment was the highest, which increased by 1.42–2.68% and 2.17–6.26% compared with SRU and SRS, respectively. Net photosynthetic rates showed no significant intertreatment differences at the jointing stage, whereas during the reproductive phases (heading and filling stages), the SRS treatment achieved the highest net photosynthetic rate. Taken together, these results suggest that straw returning combined with the application of SCU can maintain a higher photosynthetic capacity during the late growth stage by increasing the stay-green time of rice leaves, which established a material foundation for rice filling.

3.6. Effects of Straw Returning Combined with Application of SCU on Dry Matter Accumulation of Rice

Dry matter accumulation is the material basis for yield formation, and directly affects the yield formation process of crops. The dry matter accumulation of NRS treatment was highest in all treatments at the jointing stage, which was 12.38–14.55% and 11.32–14.06% higher than that of SRU and SRS, respectively (Figure 4a,b). After the jointing stage, the dry matter accumulation rose significantly across all treatments, with the SRS regimen exhibiting the most pronounced increase. From the heading stage to mature stage, SRS significantly surpassed SRU and NRS in dry matter accumulation. During the mature stage, the dry matter accumulation of SRS treatment increased by 11.44–13.25% and 7.81–8.63% compared with SRU and NRS, respectively. These results suggest that straw returning treatment could reduce the dry matter accumulation of rice during the early growth stage, but the application of SCU contributed to the dry matter accumulation during the late growth stage, which provided a material foundation for the formation of yield.

3.7. Effects of Straw Returning Combined with Application of SCU on Non-Structural Carbohydrate Content and Carbon Metabolic Enzyme Activity in Rice Leaves

The non-structural carbohydrate content and carbon metabolic enzyme activity in rice leaves during the filling stage exhibited significant disparities among different treatments in 2022, as depicted in Figure 5. The SRS treatment significantly reduced soluble sugars and starch contents, but elevated sucrose levels relative to other treatments (Figure 5a–c). The sucrose content of SRS was 34.63% and 45.52% higher than that of SRU and NRS, respectively. The activities of RuBP carboxylase, sucrose synthase, and sucrose phosphate synthase in all treatments was followed by the sequence: SRS > SRU > NRS (Figure 5d–f). Overall, these results strongly support the notion that straw returning combined with the application of SCU could improve RuBP carboxylase, sucrose synthase, and sucrose phosphate synthase activities during the late growth stage of rice, thereby maintaining a higher photosynthetic rate and synthesizing more sucrose, which fully guarantee the supply of organic matter required for grain filling.

3.8. Effects of Straw Returning Combined with Application of SCU on Nitrogen Absorption and Key Enzyme Activities of Nitrogen Metabolism in Rice Leaves

As shown in Table 3, the nitrogen content of rice leaves, stem sheaths, and panicles decreased with the advance of the growth period. At the tillering stage, the NRS treatment increased the nitrogen content in the stems by 3.24–10.99% and 4.64–13.76% compared with SRU and SRS, respectively. This may be due to straw returning inhibiting the development of rice root system, which affects the absorption and transport of nitrogen in rice. At the heading stage, the nitrogen content of leaves in all treatments was established in the sequence of SRS, NRS, and SRU treatment from high to low. The nitrogen content of the stems was not significantly different in all treatments, and the nitrogen content of the panicles in SRU was significantly higher than that in NRS and SRS. During the filling stage, the nitrogen content of leaves in SRS was significantly higher than that of other treatments, which was 10.38–15.74% and 4.88–11.53% higher than that of SRU and NRS, respectively. The nitrogen content of stem in SRS was the lowest in all treatments, which was 13.50–15.25% and 16.61–19.34% lower than that of SRU and NRS, respectively. There was no significant difference in the nitrogen content of panicles between NRS and SRS, but it was significantly higher than that of SRU. These results demonstrate that straw returning combined with the application of SCU not only significantly increases the nitrogen content of rice leaves at the late growth stage to maintain higher photosynthetic capacity, but also facilitates the transfer of nitrogen from stems to panicles, providing sufficient nitrogen for material transformation in grains.
The key enzyme activities involved in nitrogen metabolism varied significantly in rice leaves during the filling stage across different treatments in 2022 (Figure 6). Across all treatments, the highest activities of nitrate reductase, glutamine synthetase, and glutamate synthase occurred in SRS, followed by SRU, and then NRS (Figure 6a–c). Conversely, glutamate dehydrogenase activity peaked under NRS treatments, with lower levels in SRS and SRU (Figure 6d). These results suggest that straw returning combined with application of SCU could significantly increase the activities of four key enzymes of nitrogen metabolism during the late growth stage of rice, so as to assimilate more nitrogen, which provides a material basis for various metabolism processes.

3.9. Effects of Straw Returning Combined with Application of SCU on Organic Carbon and Nitrogen Accumulation in Rice Plants During Grain Filling Stage

Little difference was found in the accumulation of organic carbon and nitrogen among the various treatments from the tillering stage until heading. However, the SRS treatment stood out during the filling stage by achieving significantly greater accumulation of both elements compared to other treatments. SRS treatment increased the organic carbon accumulation by 13.52–15.15% and 20.89–24.71% (Figure 7a,b) and increased nitrogen accumulation by 5.60–12.56% and 9.12–12.70% (Figure 7c,d) compared with SRU and NRS, respectively. These results indicate that straw returning combined with the application of SCU is beneficial for plants to absorb and utilize more nitrogen at the late growth stage, maintain higher photosynthetic capacity, and accumulate more carbohydrates, which provides a basis for yield formation.

3.10. Correlation Analysis

Correlation analysis revealed the yield associations with the GNP, NRE, NAE, NFP, DMA, NL, and NP as positive, but a negative correlation with the starch (Figure 8). The NUE (NRE, NAE, and NFP) was positively correlated with the LAI, SPAD, DMA, sucrose, NL, and GOGAT, while was negatively correlated with the starch. Furthermore, the NAE and NFP were also positively correlated with the Pn, RuBP, SPS, NR, GS, OCA, and NA, while they were negatively correlated with soluble sugar and NS.

4. Discussion

4.1. Effects of Straw Returning Combined with Application of SCU on Rice Yield and NUE

To enhance rice yields and resource utilization efficiency, scientists and the government have proposed various farming practices. A large number of studies have shown that crop yield and nutrient use efficiency can be improved by using straw returning cultivation mode and applying slow- and controlled-release fertilizers [9,10,25,33]. However, use of a single agricultural management practice or fertilizer input is difficult to fully satisfy the requirements of green and efficient production such as high crop yield and farmland fertility maintenance. Improving cultivation supporting measures has become the main direction for green, high yield, and high efficiency research of crops. The results of our study showed that straw returning combined with the application of SCU could significantly increase rice yield and NUE. Relative to the control without straw returning, straw incorporation significantly reduced tiller density, which in turn reduced effective panicle formation. Conversely, it significantly increased both the seed-setting rate and 1000-grain weight, thereby improving rice yield. Our results are consistent with the previous research [34]. Some studies have also indicated that straw returning did not significantly affect the number of effective panicles, seed-setting rate, or 1000-grain weight, but it increased the grain number per panicle [35]. This apparent discrepancy may be explained by the different rice varieties, temperature and illumination conditions, and the soil physical and chemical properties. The application of slow- and controlled-release fertilizer can significantly increase the effective panicle number per m2 and grain number per panicle of rice, thus improving the yield [25,36]. Here, we found that the application of SCU significantly increased the effective panicle number per m2 and grain number per panicle and increased the yield by 16.22% compared with conventional urea. Our results echo those reported in previous studies. Significant improvement in rice NUE was achieved through both straw returning and the application of slow- and controlled-release fertilizers [9,37,38,39]. We observed that the SRS treatment increased NRE by 79.53% and 22.97%, increased NAE by 18.68% and 17.37%, and increased NFP by 10.51% and 9.81% compared with SRU and NRS, respectively. This result indicates that straw returning combined with the application of SCU can further improve NUE.

4.2. Effects of Straw Returning Combined with Application of SCU on Physiological Characteristics of Rice Leaves

Leaves form an important organ for nitrogen assimilation in crops. The physiological characteristics of leaves are closely related to the photosynthetic performance and dry matter production capacity of crops, which significantly affect the yield formation process. The net photosynthetic rate of crop leaves is significantly positively correlated with the SPAD value and LAI in a certain range [40,41]. During the early stages of rice growth in this study, NRS exhibited significantly greater SPAD values and LAI compared to the other treatments. However, with the advancement in the rice growth period, the SPAD value and LAI of SRS treatment were gradually higher than those of NRS and SRU. In addition, the RUBP carboxylase activity of rice leaves at the filling stage was also significantly higher than that of other treatments. These results suggest that straw returning combined with application of SCU could maintain a high photosynthetic capacity during the late growth stage of rice and provide a sufficient material supply for grain filling. The content of soluble sugar and sucrose in leaves can reflect the supply capacity of crops from source to sink. The higher of the soluble sugar and sucrose content, the more sufficient the photosynthetic products, and the more conducive to grain filling and yield formation in a certain range. Sucrose synthesis is the main carbon metabolism process in photosynthetic organs such as leaves. SS and SPS are the key enzymes of sucrose metabolism; both of them can promote the synthesis of sucrose. Furthermore, SS participates in sucrose translocation from source to sink within plants, as well as its conversion to starch in sink organs [42,43]. This study showed that straw returning combined with the application of SCU treatment could maintain high SS and SPS activities during the late growth stage of rice, promote the synthesis of sucrose in leaves, and facilitate its transport to sink organs such as grains, which provides sufficient organic matter supply for grain filling. NR, GS, GOGAT, and GDH are the main enzymes involved in the nitrogen assimilation of plant. The activity of these enzymes directly affects the rate of nitrogen metabolism in leaves [44,45]. In this study, high NR, GS, GOGAT, and GDH activities were detected in the leaves of the straw returning rice combined with the application of SCU treatment at the late growth stage of rice. This contributes to maintaining a high nitrogen metabolism rate and nitrogen assimilation capacity, thereby increasing the nitrogen content in leaves and panicles, which provides a material basis for grain filling and yield formation.

4.3. Straw Returning Combined with Application of SCU Increased Yield and NUE by Promoting Carbon and Nitrogen Metabolism

Dry matter accumulation and nitrogen uptake/utilization are critical for rice yield formation. [46,47]. In this study, correlation analysis revealed significant positive correlations between yield and dry matter accumulation, leaf nitrogen content, along with panicle nitrogen content, which was aligned with the prior research findings [47,48]. China is the world’s largest consumer of nitrogen fertilizer, accounting for over 30% of global annual consumption. However, crop absorption and utilization rates remain below 50%. This inefficiency represents not only wasteful resource use but also generates multiple adverse environmental impacts [49,50]. Therefore, it is critical and urgent to improve the NUE of crops. The NUE of crops is closely related to the carbon and nitrogen metabolism in crops. The stronger the carbon and nitrogen metabolism ability in crops, the higher the NUE [51]. In addition, carbon metabolism can be enhanced by nitrogen metabolism, thereby supplying the energy and carbon skeletons required for nitrogen assimilation. Sufficient nitrogen availability is essential to boost photosynthesis, augment carbon sources, and facilitate carbon metabolism [52,53]. In this study, correlation analysis showed that NRE, NAE, and NFP were significantly positively correlated with sucrose content, nitrogen content, and GOGAT enzyme activity in leaves. NAE and NFP were also significantly positively correlated with RuBP, SPS, NR, and GS enzyme activity in leaves, as well as organic carbon and nitrogen accumulation in the total plant. Straw returning combined with the application of SCU could maintain high RuBP, SPS, NR, and GS enzyme activities during the late growth stage of rice, which increases the leaf nitrogen content together with organic carbon and nitrogen accumulation in the total plant, thereby improving the NUE. Moreover, the correlation analysis revealed positive correlations between grain yield and NRE, NAE, and NFP. In summary, straw returning combined with the application of SCU improved NUE by promoting carbon and nitrogen metabolism; furthermore, the enhanced NUE can contribute to the formation of yield.

5. Conclusions

This study demonstrates that straw returning combined with application of SCU increased rice yield and NUE significantly. Moreover, a higher SPAD value, LAI, net photosynthetic rate, carbon metabolism enzyme (RuBP, SPS) activity, nitrogen metabolism enzyme (NR, GS, GOGAT) activity, sucrose content, nitrogen content in leaves and panicles, and organic carbon and nitrogen accumulation in the whole plant were observed in the SRS treatment compared with SRU and NRS during grain filling. In conclusion, straw returning combined with the application of SCU improved grain yield and NUE simultaneously by enhancing carbon and nitrogen metabolism during the late growth period in rice. Our study not only provides a novel perspective on the mechanism underlying the synergistic improvement in rice yield and NUE with straw returning combined with the application of SCU, but also proposes a recommendation for a promising cultivation-supporting measure for agricultural production.

Author Contributions

Conceptualization, Z.W. and M.Y.; methodology, G.Z. and X.X.; resources, Z.W.; investigation, M.G. and X.X.; data curation, Z.L.; validation, K.G. and X.Y.; formal analysis, K.G.; visualization, K.G.; writing—original draft preparation, G.Z. and K.G.; writing—review and editing, Z.W., M.Y. and P.T.; supervision, Z.W.; project administration, G.Z. and X.W.; funding acquisition, M.Y. and G.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Jilin Science and Technology Development Plan Project (20240601085RC), and the Scientific Research Project of Jilin Provincial Education Department (JJKH20240436KJ).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. The contents of nitrogen, phosphorus, and potassium in leaves and stems of straws in 2021 and 2022 (g·kg−1).
Table A1. The contents of nitrogen, phosphorus, and potassium in leaves and stems of straws in 2021 and 2022 (g·kg−1).
YearNPK
LeafStemLeafStemLeafStem
20217.54 ± 0.124.63 ± 0.080.09 ± 0.010.10 ± 0.050.84 ± 0.041.32 ± 0.08
20229.43 ± 0.165.79 ± 0.160.12 ± 0.010.17 ± 0.030.93 ± 0.031.45 ± 0.15
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Figure A1. Variations in monthly mean temperature and rainfall over the rice fertility period (2022–2023).
Figure A1. Variations in monthly mean temperature and rainfall over the rice fertility period (2022–2023).
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Figure 1. Effects of straw returning combined with sulfur-coated urea on tillering dynamics of rice. (a), tillering dynamics of rice in 2022; (b), tillering dynamics of rice in 2023; SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; d, days. Data are expressed as the mean ± standard error.
Figure 1. Effects of straw returning combined with sulfur-coated urea on tillering dynamics of rice. (a), tillering dynamics of rice in 2022; (b), tillering dynamics of rice in 2023; SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; d, days. Data are expressed as the mean ± standard error.
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Figure 2. Effects of straw returning combined with sulfur-coated urea on rice leaf area index. (a), rice leaf area index in 2022; (b), rice leaf area index in 2023; SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; JS, jointing stage; HS, heading stage; FS, filling stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
Figure 2. Effects of straw returning combined with sulfur-coated urea on rice leaf area index. (a), rice leaf area index in 2022; (b), rice leaf area index in 2023; SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; JS, jointing stage; HS, heading stage; FS, filling stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
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Figure 3. Effects of straw returning combined with sulfur-coated urea on SPAD value (a,b) and net photosynthetic rate (c,d) of rice leaves. SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; JS, jointing stage; HS, heading stage; FS, filling stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
Figure 3. Effects of straw returning combined with sulfur-coated urea on SPAD value (a,b) and net photosynthetic rate (c,d) of rice leaves. SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; JS, jointing stage; HS, heading stage; FS, filling stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
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Figure 4. Effects of straw returning combined with sulfur-coated urea on dry matter accumulation of rice. (a), dry matter accumulation of rice in 2022; (b), dry matter accumulation of rice in 2023. SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; JS, jointing stage; HS, heading stage; FS, filling stage; MS, mature stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
Figure 4. Effects of straw returning combined with sulfur-coated urea on dry matter accumulation of rice. (a), dry matter accumulation of rice in 2022; (b), dry matter accumulation of rice in 2023. SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; JS, jointing stage; HS, heading stage; FS, filling stage; MS, mature stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
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Figure 5. Effects of straw returning combined with sulfur-coated urea on non-structural carbohydrate content (a–c) and carbon metabolic enzyme activity (d–f) in rice leaves at the filling stage (2022). SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
Figure 5. Effects of straw returning combined with sulfur-coated urea on non-structural carbohydrate content (a–c) and carbon metabolic enzyme activity (d–f) in rice leaves at the filling stage (2022). SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
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Figure 6. Effects of straw returning combined with sulfur-coated urea key activities of nitrogen metabolism in rice leaves at the filling stage (2022). (a), nitrate reductase activity; (b), glutamine synthetase activity; (c), glutamate synthase activity; (d), glutamate dehydrogenase activity; SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
Figure 6. Effects of straw returning combined with sulfur-coated urea key activities of nitrogen metabolism in rice leaves at the filling stage (2022). (a), nitrate reductase activity; (b), glutamine synthetase activity; (c), glutamate synthase activity; (d), glutamate dehydrogenase activity; SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
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Figure 7. Effects of straw returning combined with sulfur-coated urea on organic carbon accumulation (a,b) and nitrogen accumulation (c,d) in rice plants. SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; HS, heading stage; FS, filling stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
Figure 7. Effects of straw returning combined with sulfur-coated urea on organic carbon accumulation (a,b) and nitrogen accumulation (c,d) in rice plants. SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; HS, heading stage; FS, filling stage. Data are expressed as the mean ± standard error. Different letters labeling the bars indicate statistical significance at the p < 0.05 level.
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Figure 8. Correlations between the rice yield, NRE, NAE, NFP, and other indexes. EPN, effective panicle number; GNP, grain number per panicle; SSR, seed-setting rate; TGW, 1000-grain weight; NRE, nitrogen uptake efficiency; NAE, nitrogen agronomic efficiency; NFP, nitrogen partial factor productivity; LAI, leaf area index; SPAD, SPAD value; Pn, net photosynthetic rate; DMA, dry matter accumulation; RuBP, RuBP carboxylase; SS, sucrose synthase; SPS, sucrose phosphate synthase; NL, nitrogen content of rice leaves; NS, nitrogen content of rice stem; NP, nitrogen content of rice panicles; NR, nitrate reductase; GS, glutamine synthetase; GOGAT, glutamate synthetase; GDH, glutamate dehydrogenase; OCA, organic carbon accumulation; NA, nitrogen accumulation.
Figure 8. Correlations between the rice yield, NRE, NAE, NFP, and other indexes. EPN, effective panicle number; GNP, grain number per panicle; SSR, seed-setting rate; TGW, 1000-grain weight; NRE, nitrogen uptake efficiency; NAE, nitrogen agronomic efficiency; NFP, nitrogen partial factor productivity; LAI, leaf area index; SPAD, SPAD value; Pn, net photosynthetic rate; DMA, dry matter accumulation; RuBP, RuBP carboxylase; SS, sucrose synthase; SPS, sucrose phosphate synthase; NL, nitrogen content of rice leaves; NS, nitrogen content of rice stem; NP, nitrogen content of rice panicles; NR, nitrate reductase; GS, glutamine synthetase; GOGAT, glutamate synthetase; GDH, glutamate dehydrogenase; OCA, organic carbon accumulation; NA, nitrogen accumulation.
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Table 1. Effects of different treatments on yield and yield components of rice.
Table 1. Effects of different treatments on yield and yield components of rice.
YearTreatmentEffective Panicle Number per m2Grain Number per PanicleSeed-Setting Rate(%)1000-Grain Weight (g)Grain Yield (kg·ha−1)
2022SRU254.67 ± 4.00 c163.44 ± 4.82 b95.12 ± 1.85 a24.28 ± 0.99 a8248.41 ± 156.45 c
NRS291.00 ± 4.93 a184.11 ± 6.88 a91.75 ± 1.55 b21.32 ± 1.62 b9077.75 ± 97.56 b
SRS276.67 ± 4.93 b180.89 ± 6.81 a95.96 ± 1.35 a23.50 ± 1.5 a9453.83 ± 138.10 a
2023SRU280.67 ± 3.51 c164.11 ± 3.67 b95.52 ± 0.09 a24.07 ± 0.77 a8999.38 ± 15.80 c
NRS303.33 ± 3.21 a185.78 ± 1.54 a92.65 ± 0.60 b21.63 ± 0.91 b9743.27 ± 78.64 b
SRS290.33 ± 5.51 b185.00 ± 4.70 a96.15 ± 0.19 a23.75 ± 0.67 a10,459.18±34.61 a
Notes: SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea. Data are expressed as the mean ± standard error. Different letters indicate statistical significance at the p < 0.05 level within the same column.
Table 2. Effect of straw returning combined with SCU on NUE of rice.
Table 2. Effect of straw returning combined with SCU on NUE of rice.
TreatmentNRE (%)NAE (kg·kg−1)NFP (kg·kg−1)
SRU34.00 ± 1.41 c31.38 ± 0.63 b55.73 ± 0.42 b
NRS49.64 ± 2.72 b31.73 ± 0.74 b56.09 ± 0.60 b
SRS61.04 ± 1.79 a37.24 ± 0.10 a61.59 ± 0.31 a
Notes: SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; NRE, nitrogen uptake efficiency; NAE, nitrogen agronomic efficiency; NFP, nitrogen partial factor productivity. Data are expressed as the mean ± standard error. Different letters indicate statistical significance at the p < 0.05 level within the same column.
Table 3. Effects of straw returning combined with SCU on nitrogen absorption (g·kg−1).
Table 3. Effects of straw returning combined with SCU on nitrogen absorption (g·kg−1).
YearTreatmentTSHSFS
LeafStemLeafStemPanicleLeafStemPanicle
2022SRU33.79 ± 0.09 c14.19 ± 0.09 b16.99 ± 0.09 c5.51 ± 0.09 b9.33 ± 0.09 a8.96 ± 0.16 c5.51 ± 0.09 a6.35 ± 0.09 b
NRS34.25 ± 0.09 b14.65 ± 0.09 a17.45 ± 0.09 b5.97 ± 0.09 a8.96 ± 0.16 ab9.43 ± 0.09 b5.79 ± 0.09 a7.00 ± 0.16 a
SRS35.00 ± 0.16 a14.00 ± 0.16 c18.20 ± 0.16 a5.41 ± 0.09 b8.77 ± 0.09 b9.89 ± 0.09 a4.67 ± 0.09 b6.81 ± 0.09 a
2023SRU34.98 ± 0.23 a13.56 ± 0.25 b18.11 ± 0.12 c5.85 ± 0.23 a10.84 ± 0.22 a9.53 ± 0.15 b6.15 ± 0.13 a6.45 ± 0.12 b
NRS35.61 ± 0.29 a15.05 ± 0.27 a18.54 ± 0.14 b6.36 ± 0.02 a9.22 ± 0.13 b9.89 ± 0.06 b6.38 ± 0.07 a7.64 ± 0.03 a
SRS35.71 ± 0.70 a13.23 ± 0.19 b19.46 ± 0.07 a5.94 ± 0.10 a9.44 ± 0.22 b11.03 ± 0.09 a5.32 ± 0.15 b7.62 ± 0.03 a
Note: SRU, straw returning combined with conventional urea; NRS, no straw returning combined with sulfur-coated urea; SRS, straw returning combined with sulfur-coated urea; TS, tillering stage; HS, heading stage; FS, filling stage. Nitrogen concentration was expressed per kilogram dry matter. Data are expressed as the mean ± standard error. Different letters indicate statistical significance at the p < 0.05 level within the same column.
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MDPI and ACS Style

Zhao, G.; Gao, K.; Gao, M.; Xu, X.; Li, Z.; Yang, X.; Tian, P.; Wei, X.; Wu, Z.; Yang, M. Straw Returning Combined with Application of Sulfur-Coated Urea Improved Rice Yield and Nitrogen Use Efficiency Through Enhancing Carbon and Nitrogen Metabolism. Agriculture 2025, 15, 1554. https://doi.org/10.3390/agriculture15141554

AMA Style

Zhao G, Gao K, Gao M, Xu X, Li Z, Yang X, Tian P, Wei X, Wu Z, Yang M. Straw Returning Combined with Application of Sulfur-Coated Urea Improved Rice Yield and Nitrogen Use Efficiency Through Enhancing Carbon and Nitrogen Metabolism. Agriculture. 2025; 15(14):1554. https://doi.org/10.3390/agriculture15141554

Chicago/Turabian Style

Zhao, Guangxin, Kaiyu Gao, Ming Gao, Xiaotian Xu, Zeming Li, Xianzhi Yang, Ping Tian, Xiaoshuang Wei, Zhihai Wu, and Meiying Yang. 2025. "Straw Returning Combined with Application of Sulfur-Coated Urea Improved Rice Yield and Nitrogen Use Efficiency Through Enhancing Carbon and Nitrogen Metabolism" Agriculture 15, no. 14: 1554. https://doi.org/10.3390/agriculture15141554

APA Style

Zhao, G., Gao, K., Gao, M., Xu, X., Li, Z., Yang, X., Tian, P., Wei, X., Wu, Z., & Yang, M. (2025). Straw Returning Combined with Application of Sulfur-Coated Urea Improved Rice Yield and Nitrogen Use Efficiency Through Enhancing Carbon and Nitrogen Metabolism. Agriculture, 15(14), 1554. https://doi.org/10.3390/agriculture15141554

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