Assessment of Yield and Nitrogen Utilization of the Mixed CRU and Urea in Wheat–Maize Production in a 5-Year Field Trial

Abstract

To identify the general pattern of impact of the application of the mixed controlled-release urea (CRU) with urea (C-U) on grain yield, plant characteristics, NUE and soil nitrogen contents in wheat–maize production, a 5-year field trial with three release longevity and four ratios of CRU in C-U and common urea alone (U) was carried out in the North China Plain. Results with meta-analysis revealed that C-U had significant effects on grain yield and plant characteristics, NUE and soil NO₃⁻-N contents in wheat–maize production positively with the release longevity of CRU and the ratio of CRU-N in C-U. The application of C-U with 60 d or 90 d CRU for wheat and maize had the best overall effects, while C-U treatment with 30 d CRU had a significant inhibitory effect. For maize, C-U with 30% CRU-N had the largest increase rate on yield, 1000-grain weight, plant height, dry weight and NUE by 5.13%, 1.61%, 3.70%, 11.33%, and 8.63%, respectively, while C-U with 40% CRU-N had the largest reduction soil NO₃⁻-N. For wheat, the application of C-U with 40% CRU-N had a significant effect on yield, sterile spikelet number, and NUE by 4.45%, −9.76%, and −8.04%, respectively. To conclude, the use of the C-U with appropriate release longevity and the ratio of CRU has great potential to proliferate wheat–maize yields and reduce fertilizer loss especially for maize that not only provides an effective generic methodology for agriculture to improve measures but also ensures profitability.

1. Introduction

The winter wheat–summer maize rotation is one of the most important cropping systems on the North China Plain, providing more than 52.4% of wheat and 32.1% of maize production on 25.1% of the cultivated land in China [1]. Nitrogen (N) is the major essential element for plant growth development. Basal nitrogen fertilizer plus topdressing mainly with normal urea N is a traditional fertilization practice in agricultural production. Nevertheless, soil deterioration, low nitrogen use efficiency (NUE), and environmental risks as a consequence of excessive chemical N fertilizer usage are key factors restricting sustainable agriculture. The loss of nitrogen fertilizer in this area is as high as 20–55%, and the nitrogen use efficiency of maize is only 26.1%, which is far lower than the international level of 50% [2,3,4,5]. At present, agriculture has encountered the dual pressures of a reduction in the availability of agricultural workers and an increase in food requirements [4,5]. Topical application with normal urea N increased additional labor for field operations in comparison with a single basal application of all N fertilizer [6]. Hence, exploring the sustainable fertilizer N management to maximize crop production and NUE, minimize environmental risks, and maintain laborsaving fertilizing practices is urgently required in China.

Controlled-release urea (CRU) was designed to meet crop nutrient requirements, reduce labor demands, and increase crop yields [7]. In China, CRU has become the major trend in fertilizer application because of its excellent slow-release performance [8], which has been practiced for many crops [9,10]. However, the wide use of CRU in agriculture is limited due to its more uneconomical price than that of conventional urea. Therefore, instead of employing urea or CRU exclusively, the use of mixed CRU and urea has been advocated in the agricultural industry to acquire the benefits of reduced labor costs and augmented crop yields [4,9,11], which not only is an economical and effective [12] but also benefits the wheat–maize system [13,14]. However, these results are only qualitative, lacking quantitative evaluation of long-term positioning test data, and insufficient discussion on the effect of CRU on release longevity and proportion. Moreover, these studies rarely explored variations in patterns observed in maize and wheat in study periods.

Therefore, in the North China Plain, over five consecutive years, we conducted a trial in one field to optimize the release longevity and the ratio of CRU-N in the mixed CRU and urea to maintain crop yield, increase NUE, and decrease nitrogen leaching. The results demonstrate new fertilizer techniques allowing more sustainable and highly efficient application of fertilizers. Additionally, the meta-analysis method was employed to determine the increase or decrease in the efficiency of C-U on crop yield, plant characteristics, and nitrogen utilization. Revealing variation may help to identify the factors driving the changes in different release longevity and ratios of CRU and to understand the reasons behind the variability in the results. Firstly, the initial objective of the study was to identify the general pattern of the impact of CRU on winter wheat and summer maize crops and to explore variations related to the release longevity and the ratios of CRU in mixed fertilizer. Secondly, we aimed to identify the mechanisms behind the impact of CRU. The results will provide the basis for cleaner production using highly efficient fertilization that is not only more sustainable but also ecologically beneficial.

2. Materials and Methods

2.1. Experimental Site and Materials

The field trials were conducted in winter wheat–summer maize rotation over five years from October 2008 to October 2013 in Taian, Shandong province, China (36.15° N, 117.27° E). The climate in this region is categorized as continental temperate monsoon with an annual average temperature of 12.8 °C, and a mean annual precipitation of 697 mm mainly concentrating in July and August. The soil type is loam, and the physical and chemical properties in the topsoil are as follows: pH 6.9; total organic C, 12.27 g/kg; alkaline nitrogen 82.44 mg/kg; available phosphate 29.40 mg/kg; available potassium 111.70 mg/kg.

The maize variety was Zhengdan958, and the wheat variety was Jimai22. The conventional fertilizers were applied as urea (46.0% N), calcium superphosphate (12% P₂O₅), and potassium chloride (60% K₂O). The CRU incorporated for this study is the polyurethane-coated Urea (42.4% N), and the coating rate is 8.3%.

2.2. Experimental Design and Managements

A randomized block design was implemented to prepare three replicates of each treatment. The three N application modes were no N (CK), common urea alone (U), and the mixed CRU with common urea (C-U). U treatment and C-U treatment were at the same N application rate. Among the C-U treatment, there were three kinds of release longevity of CRU and four ratios of CRU nitrogen (CRU-N) in the mixed CRU and urea: the former including 30 d, 60 d, and 90 d with 30% CRU, and the latter at the ratio of 10%, 20%, 30%, and 40% for wheat and 20%, 25%, 30%, and 40% for maize, all with 60 d CRU. Each plot was 6 m long and 4 m wide in a randomized block arrangement.

The density of maize was 70,000 plants·ha⁻¹, and wheat seeds were sown at 180 kg·ha⁻¹. Fertilizers N, P, and K were applied at the recommended rate of 270 kg N·ha⁻¹, 180 kg P₂O₅·ha⁻¹, and 90 kg K₂O·ha⁻¹ for wheat, and 291 kg N·ha⁻¹, 243.75 kg P₂O₅·ha⁻¹, and 90.75 kg K₂O ha⁻¹ for maize, respectively. In U treatment, the full rate of P and K with 60% of the N was applied as basal fertilizer, and 40% N as topdressing applied at the bellbottom stage in summer maize or at the returning-green period in winter wheat. In C-U treatment, the mixed CRU and urea were all applied as basal fertilizer once before sowing with all phosphorus and potassium fertilizers. Pest and irrigation management refers to local management practices.

2.3. Sampling and Measurement

2.3.1. Grain Yield and Yield Components

The grain yield was measured by harvesting in each plot at crop maturity with 14% moisture content. The numbers of wheat ears in representative plots of 1 m² were surveyed, while the numbers of maize ears in representative plots of 10 m² were surveyed. Grains were threshed and air-dried, and the grain number per ear was measured by counting the grains of 10 plants divided by the number of collected ears. The air-dried grains were used to measure 1000-grain weight.

2.3.2. Plant

Crop plants were sampled for height, stem diameter, and biomass determinations at the crop growth stage: for wheat sampling at the trefoil stage, pre-pre-freezing stage, returning-green period, elongation stage, flowering stage, filling stage, and maturing stage and for maize sampling at the seedling stage, elongation stage, booting stage, tasseling stage, filling stage, and maturing stage. Three plants were sampled randomly in each plot. The height and stem diameter of the plant were measured with a scale. Total above-ground biomass was determined gravimetrically after drying at 65–75 °C for 48 h. The dried samples were milled for analysis of N content. Plant samples were digested using an H₂SO₄–H₂O₂ method [15], and total N was measured using a discontinuous auto-analyzer (SMART 200, SEAL Analytical, Norderstedt, Germany). Nitrogen uptake by the plants was calculated based on plant N and the weights of the plant. NUE was calculated by the Formula (1) [16].

$$\mathrm{NUE}=\frac{N_F-N_0}{F}\times 100\%$$

(1)

where N_F and N₀ denote the seasonal N uptake as measured at the crop growth stage in the fertilized and the control plots (kg N·ha⁻¹), while F gives the seasonal addition rate of N fertilizer (kg N·ha⁻¹).

2.3.3. Soil

A composite soil sample of 3 cores was collected using a soil drill (with the core diameter 3 cm) following harvest at 20 cm intervals from 0 to 100 cm depth sequence in a random zigzag pattern from each plot. Soil NO₃⁻-N and NH₄⁺-N (extracted by 0.01 M CaCl₂) concentrations were determined using the AA3 Auto-analyzer (Bran-Luebbe, Norderstedt, Germany).

2.4. Data Analysis

Considerable variation in absolute values of crop yield, plant characteristics, NUE, and soil nitrogen content with C-U treatments in five years prompted us to use a meta-analytic approach in this study. For each pair of samples, we calculated a reaction ratio of effect size (ES) as the difference between C-U treatment and U treatment, divided by the standard deviation and weighted by the sample size. A negative ES indicates that the effect under study is smaller in C-U treatment than in U treatment.

$$\mathrm{ES}=\ln \left(\frac{{ \overline{\mathrm{X}}}_{\mathrm{treatment}}}{{ \overline{\mathrm{X}}}_{\mathrm{control}}}\right)$$

(2)

$${\mathrm{v}}_{\mathrm{ES}}=\frac{{\mathrm{S}}_1^2}{{\mathrm{n}}_1{ \overline{\mathrm{X}}}_{\mathrm{treatment}}}+\frac{{\mathrm{S}}_2^2}{{\mathrm{n}}_2{ \overline{\mathrm{X}}}_{\mathrm{control}}}$$

(3)

where ${ \overline{\mathrm{X}}}_{\mathrm{treatment}}$ and ${ \overline{\mathrm{X}}}_{\mathrm{control}}$ are mean values of yield or plant characteristics or NUE or soil nitrogen contents in C-U and U treatments, respectively. S₁ and S₂ represent the standard deviation of C-U and U treatments, respectively; n₁ and n₂ represent the number of samples in C-U and U treatments, respectively.

The mean ES and the 95% confidence interval (CI95) of the mean ES were computed and compared using R software with the metafor package [17,18]. C-U treatment was considered to have a statistically significant effect if CI95 of the mean ES did not include zero. We investigated variation among the release longevity or the ratio of CRU-N in C-U treatments, which were considered categorical explanatory variables. All analyses were performed using random effects models.

3. Results

3.1. Effects of C-U on Yield Characteristics

For wheat, the yield, 1000-grain weight, and grain number per ear to all C-U treatments, averaged by the release longevity or the ratio of CRU-N and study years, were with a mean value of 7101.53 kg·ha⁻¹, 43.90 g·plant⁻¹, and 32.67 seeds, compared with U treatment with a mean value of 7059.03 kg·ha⁻¹, 43.98 g·plant⁻¹, and 32.21 seeds, respectively (Figure 1a–c). Meta-analysis revealed that C-U had no significant effect on these yield characteristics (Figure 2a–c). However, the sterile spikelet number drastically reduced with C-U by 3.28% (Figure 2d), which was with a mean value of 2.86 spikelets compared with U treatment with a mean value of 2.97 spikelets (Figure 1d). Effects of C-U on wheat yield characteristics differed among the release longevity of CRU and the ratio of CRU-N:C-U with 30 d CRU significantly diminished the yield, 1000-grain weight, and grain number per ear by 6.29%, 2.16%, and 6.22%, while C-U with 60 d or 90 d CRU increased grain number per ear by 4.23% and 1.86% respectively. Additionally, C-U with 10% CRU-N reduced yield by 7.32%, while C-U with 30% and 40% CRU-N proliferated the yield by 2.12% and 4.45% and depleted the sterile spikelet number by 3.97% and 9.76%, respectively(

Table 1. Effects of the release longevity or the ratio of CRU-N on yield and plant characteristics, NUE, and soil nitrate contents for wheat or maize.

Index	Crop	Variable 1 (p Value)	Variable 2	Estimate	Ci.Lb	Ci.Ub	Percent Change (%)
Yield	maize	CRU release longevity (0.016)	30 d	−0.011	−0.049	0.027	−1.094
			60 d ***	0.04	0.025	0.057	4.081
			90 d ***	0.061	0.028	0.096	6.29
		CRU ratio (0.039)	20%	−0.012	−0.051	0.027	−1.193
			25%	0.026	−0.013	0.066	2.634
			30% ***	0.05	0.032	0.069	5.127
			40% *	0.039	0.003	0.076	3.977
	wheat	CRU release longevity (0.003)	30 d **	−0.065	−0.113	−0.019	−6.293
			60 d	0.011	−0.009	0.032	1.106
			90 d	0.041	−0.003	0.086	4.185
		CRU ratio (<0.0001)	10% ***	−0.076	−0.117	−0.036	−7.318
			20%	−0.003	−0.048	0.041	−0.3
			30% *	0.021	0	0.041	2.122
			40% *	0.044	0.001	0.086	4.446
1000-grain weight	maize	CRU release longevity (0.013)	30 d	−0.006	−0.021	0.008	−0.598
			60 d ***	0.012	0.006	0.018	1.207
			90 d ***	0.023	0.009	0.036	2.327
		CRU ratio (0.0367)	20%	−0.005	−0.019	0.008	−0.499
			25%.	0.012	−0.001	0.026	1.207
			30% ***	0.016	0.009	0.023	1.613
			40%	0.008	−0.005	0.022	0.803
	wheat	CRU release longevity (0.040)	30 d *	−0.022	−0.041	−0.003	−2.156
			60 d	0.005	−0.003	0.014	0.541
			90 d	0.001	−0.018	0.02	0.11
		CRU ratio (0.988)	-	-	-	-	-
Grain number per ear	maize	CRU release longevity (0.016)	30 d	−0.014	−0.039	0.011	−1.371
			60 d ***	0.026	0.014	0.037	2.603
			90 d *	0.028	0.002	0.053	2.788
		CRU ratio (0.441)	-	-	-	-	-
	wheat	CRU release longevity (<0.0001)	30 d ***	−0.064	−0.096	−0.032	−6.218
			60 d *	0.018	0.004	0.033	1.857
			90 d **	0.041	0.011	0.072	4.227
		CRU ratio (0.096)	-	-	-	-	-
Sterile spikelets number	wheat	CRU release longevity (0.055)	-	-	-	-	-
		CRU ratio (0.022)	10%	0.035	−0.029	0.099	3.593
			20%	0.002	−0.071	0.074	0.16
			30% *	−0.041	−0.076	−0.005	−3.969
			40% **	−0.103	−0.17	−0.036	−9.76
Dry weight	maize	CRU release longevity (0.0009)	30 d **	0.043	0.011	0.075	4.373
			60 d ***	0.096	0.082	0.11	10.065
			90 d ***	0.127	0.095	0.158	13.485
		CRU ratio (0.0006)	20% *	0.034	0.003	0.065	3.479
			25% ***	0.091	0.06	0.122	9.549
			30% ***	0.107	0.092	0.123	11.327
			40% ***	0.096	0.065	0.127	10.087
	wheat	CRU release longevity (0.032)	30 d	0.025	−0.003	0.054	2.552
			60 d ***	0.063	0.05	0.075	6.481
			90 d ***	0.074	0.046	0.103	7.681
		CRU ratio (0.087)	-	-	-	-	-
Plant height	maize	CRU release longevity (0.024)	30 d *	0.018	0.002	0.034	1.847
			60 d ***	0.025	0.018	0.032	2.511
			90 d ***	0.047	0.031	0.062	4.77
		CRU ratio (0.005)	20%	0.01	−0.006	0.027	1.045
			25% *	0.018	0.002	0.033	1.776
			30% ***	0.036	0.029	0.044	3.697
			40%	0.015	−0.001	0.031	1.542
	wheat	CRU release longevity (0.028)	30 d	−0.01	−0.028	0.008	−0.995
			60 d ***	0.017	0.009	0.026	1.755
			90 d	0.014	−0.005	0.033	1.42
		CRU ratio (0.134)	-	-	-	-	-
Stem diameter	maize	CRU release longevity (0.027)	30 d *	0.028	0.003	0.052	2.788
			60 d ***	0.063	0.052	0.074	6.481
			90 d ***	0.067	0.043	0.09	6.908
		CRU ratio (0.234)	-	-	-	-
NUE	maize	CRU release longevity (0.022)	30 d	−0.05	−0.139	0.04	−4.849
			60 d **	0.064	0.024	0.104	6.641
			90 d **	0.122	0.033	0.21	12.919
		CRU ratio (0.029)	20%	−0.07	−0.159	0.02	−6.714
			25%	0.057	−0.033	0.146	5.844
			30% ***	0.083	0.039	0.127	8.632
			40%	0.074	−0.016	0.163	7.627
	wheat	CRU release longevity (0.028)	30 d ***	−0.239	−0.317	−0.16	−21.227
			60 d ***	−0.123	−0.157	−0.089	−11.547
			90 d **	−0.126	−0.204	−0.048	−11.865
		CRU ratio (0.006)	10% ***	−0.261	−0.338	−0.184	−22.972
			20%**	−0.123	−0.199	−0.047	−11.574
			30% ***	−0.127	−0.166	−0.089	−11.953
			40% *	−0.084	−0.158	−0.009	−8.038
NH4+-N in soil	maize	CRU release longevity (0.328)	-	-	-	-	-
		CRU ratio (0.338)	-	-	-	-	-
	wheat	CRU release longevity (0.558)	-	-	-	-	-
		CRU ratio (0.779)	-	-	-	-	-
NO3−-N in soil	maize	CRU release longevity (0.008)	30 d *	0.138	0.029	0.248	14.843
			60 d	−0.041	−0.092	0.011	−3.988
			90 d	0.063	−0.049	0.175	6.513
		CRU ratio (0.002)	20%	−0.021	−0.132	0.091	−2.029
			25%	0.047	−0.066	0.16	4.833
			30%	0.045	−0.011	0.1	4.592
			40% ***	−0.202	−0.315	−0.088	−18.25
	wheat	CRU release longevity (0.667)	-	-	-	-	-
		CRU ratio (0.90)	-	-	-	-	-

*, ** and *** represent p < 0.05, p < 0.01, and p < 0.001, respectively.

).

In the case of maize, the yield to all C-U treatments ranged from 6318.78 kg·ha⁻¹ to 9437.78 kg·ha⁻¹, with a mean value of 8244.11 kg·ha⁻¹, in comparison to U treatment with a mean value of 7951.37 kg·ha⁻¹ (Figure 1e). The 1000-grain weight and the grain number per ear to all C-U treatments ranged from 264.13 to 342.93 g and 390.2 to 548.67 seeds, with a mean value of 311.73 g and 455.98 seeds, in comparison to the U treatment with a mean value of 308.73 g and 445.04 seeds, respectively (Figure 1f,g). Meta-analysis demonstrated that the C-U treatments significantly elevated maize’s yield, the 1000-grain weight, and the grain number per ear by 3.80%, 1.12%, and 2.07%, respectively (Figure 2). Effects of C-U on maize yield characteristics varied among the release longevity and the ratio of CRU-N:C-U with 60 d and 90 d CRU having remarkably elevated yield, 1000-grain weight, and grain number per ear by 4.08% and 6.29%, 1.21% and 2.33%, 2.60%, and 2.79%. Furthermore, C-U with 30% CRU-N augmented the yield and 1000-grain weight by 5.13% and 1.61%, and C-U with 40% CRU-N improved the yield by 3.98% (Table 1).

3.2. Effects of C-U on Plant Characteristics

For wheat, the effects of C-U on crop plant characteristics fluctuate among the studied growth stage, for plant height with QM = 40.99, df = 7, p < 0.0001, and for dry weight with QM = 198.94, df = 7, p < 0.0001. The mean value of plant height with C-U in the trefoil stage (3.54 cm), pre-freezing stage (3.56 cm), returning-green period (4.39 cm), elongation stage (15.98 cm), and flowering stage (67.54 cm) exceed ones with U treatment (Figure 3A), while meta-analysis revealed especially in returning-green period and elongation stage, plant height increased considerably (

Table 2. Percent change of crop plant height, dry weight, stem diameter, and NUE of wheat or maize in each crop growth stage under C-U treatment with Meta-analysis (%).

Crop	Crop Growth Stage	Plant Height	Dry Weight	Stem Diameter	NUE
Wheat	Trefoil Stage	0.16	15.49 ***	-	−14.24 **
	Pre-freezing stage	0.08	12.14 ***	-	−33.25 ***
	Returning-green period	5.07 ***	13.84 ***	-	−29.7 ***
	Elongation stage	4.74 ***	11.96 ***	-	−20.9 ***
	Flowering stage	0.33	0.54	-	−4.88
	Filling Stage	−0.45	−1.85	-	−7.96 *
	Maturing stage	1.09	0.53	-	1.91
Maize	Seedling stage	−0.42	15.14 ***	6.62 ***	−27.68 ***
	Elongation stage	5.39 ***	19.96 ***	5.95 ***	−19.99 ***
	Booting stage	5.4 ***	15.81 ***	4.50 ***	35.31 ***
	Tasseling stage	2.19 **	8.25 ***	12.40 ***	15.43 ***
	Filling stage	1.60	7.57 ***	4.41 ***	14.6 ***
	Maturing Stage	1.39 *	3.95 **	5.27 ***	10.48 **

*, ** and *** represent p < 0.05, p < 0.01, and p < 0.001, respectively.

). Although the mean value of the dry weight with C-U in all growth stages exceeds the ones with the U treatment (Figure 3B), dry weight increased remarkably in the trefoil stage, the pre-freezing stage, the returning-green period, and the elongation stage (Table 2). Predominantly, significant increases were revealed in plant height and plant dry weight of wheat by 1.33% and 6.10%, respectively, with C-U in comparison to the U treatment. Furthermore, the effects of C-U on wheat plant characteristics varied among the release longevity, and the ratio of CRU-N:C-U with 60 d CRU had significantly increased the plant height and dry weight by 1.76% and 6.48%, whereas C-U with 90 d CRU significantly proliferated the dry weight by 7.68%. Additionally, C-U with 20%, 30%, and 40% CRU-N elevated the dry weight by 6.23%, 6.52%, and 7.28% notably, while the ratio of CRU-N had no effects on the plant height of wheat (Table 1).

In the case of maize, the effects of C-U on crop plant characteristics varied among the studied growth stage, for plant height with QM = 39.56, df = 6, p < 0.0001, and for dry weight with QM = 87.89, df = 6, p < 0.0001, and for stem diameter with QM = 24.27, df = 6, p = 0.0002. At the seedling stage, elongation stage, booting stage, tasseling stage, filling stage, and maturing stage, a mean value of the dry weight and stem diameter to all C-U treatments were 2.77, 18.65, 60.51,117.65, 116.36, and 151.02 g·plant-1 and 1.32, 2.14, 2.31, 2.35, 2.04. and 2.12 cm. respectively, which all exceeded ones with U treatment. C-U considerably augmented the dry weight and stem diameter at every growth stage. A mean value of plant height with C-U in these growth stages was 43.36, 80.04, 165.03, 271.05, 280.90, and 278.91 cm, in comparison to U treatment with a mean value 43.36, 76.11, 157.69, 265.35, 275.56, and 275.33 cm, respectively, with a remarkable increase at the elongation, booting, and the tasseling stage (Table 2). Comprehensively, C-U treatment also increased the plant height, dry weight, and stem diameter when compared with U treatment by 2.76%, 9.76%, and 6.0%, respectively (Figure 4). Furthermore, C-U with 30 d, 60 d, and 90 d CRU had significantly increased dry weight by 4.37%, 10.07%, and 13.49%, accelerated the plant height by 1.85%, 2.51%, and 4.77% and improved the stem diameter by 2.79%, 6.48%, and 6.91%. Additionally, C-U with 20%, 25%, 30%, or 40% CRU-N elevated the dry weight by 3.48%, 9.55%, 11.33%, and 10.09%, respectively, and C-U with 25% and 30% CRU-N increased the plant height by 1.78% and 3.70%, while the ratio of CRU-N had no effect on stem diameter of maize (Table 1).

3.3. Effects of C-U on Nitrogen Utility Efficiency(NUE)

Meta-analysis showed that C-U treatment drastically reduced the NUE of wheat by 12.99%, while significantly improving the NUE in the case of maize by 5.79% when compared with U treatment (Figure 5). The effects of C-U on NUE differ among growth stage of wheat (QM = 239.27, df = 6, p < 0.0001), and the mean value of NUE in the maturing stage (41.24%) exceeded ones with U treatment, except in the trefoil stage (3.29%), pre-freezing stage (6.96%), returning-green period (9.59%), elongation stage (29.68%), flowering stage (37.04%), and filling Stage (26.24%) (Figure 6A), and Meta revealed that in filling stage, trefoil stage, elongation stage, returning-green period, and pre-freezing stage, C-U significantly decreased NUE (Table 2). Additionally, C-U with 30 d, 60 d, and 90 d CRU had significantly decreased NUE by 21.23%, 11.55%, and 11.87%, and C-U with 10%, 20%, 30%, and 40% CRU-N also had significantly decreased NUE by 22.97%, 11.57%, 11.95%, and 8.04%, respectively (Table 1).

The effects of C-U on NUE varied among the growth stage of maize (QM = 201.30, df = 5, p < 0.0001), and the mean value in booting (20.99%), tasseling (28.85%), filling stage (24.96%), and the maturing stage (34.36%) exceeded the ones with U treatment, except in seedling (1.36%) and elongation stage (8.93%) (Figure 6B). Similarly, the C-U significantly reduced the NUE in the seedling and elongation stages, whereas it proliferated remarkably in the booting, tasseling, filling stage, and the maturing stage. Moreover, the C-U with 60 d and 90 d CRU significantly improved the NUE by 6.64% and 12.92%, whereas C-U with 30% CRU-N enhanced the NUE by 8.63% (Table 1).

3.4. Effects of C-U on Nitrogen Content in the Soil

In wheat season, a mean value of NO₃⁻-N in 0–20 cm (21.44 mg·kg⁻¹) and 80–100 cm (12.66 mg·kg⁻¹) exceeded ones with U treatment, except in 20–40 cm (15.57 mg·kg⁻¹), 40–60 cm (13.17 mg·kg⁻¹), and 60–80 cm (12.64 mg·kg⁻¹) (Figure 7A). A mean value of NH₄⁺-N in 0–20 cm (10.76 mg·kg⁻¹), 20–40 cm (11.36 mg·kg⁻¹), 40–60 cm (9.76 mg·kg⁻¹), and 80–100 cm (9.83 mg·kg⁻¹) exceeded the ones with U treatment, except in 60–80 cm (9.56 mg·kg⁻¹) (Figure 7B). Meta-analysis demonstrated that C-U treatments had no considerable effect on soil NO₃⁻-N and NH₄⁺-N (Figure 5), irrespective of the release longevity or the ratios of CRU (Table 1). However, NO₃⁻-N in 0–20 cm and NH₄⁺-N in 80–100 cm were significantly augmented by 13.61% and 15.17%, respectively, with C-U treatment (Figure 8A,B).

In maize season, a mean value of NO₃⁻-N in 40–60 cm (12.16 mg·kg⁻¹) and 80–100 cm (10.51 mg·kg⁻¹) exceeded ones with U treatment, except in 0–20 cm (14.38 mg·kg⁻¹), 20–40 cm (10.85 mg·kg⁻¹), and 60–80 cm (10.61 mg·kg⁻¹) (Figure 7C). A mean value of NH₄⁺-N in 20–40 cm (12.49 mg·kg⁻¹), 40–60 cm (12.07 mg·kg⁻¹), 60–80 cm (12.46 mg·kg⁻¹), and 80–100 cm (12.10 mg·kg⁻¹) exceeded the ones with U treatment, except in 0–20 cm (12.77 mg·kg⁻¹) (Figure 7D). Meta-analysis also demonstrated that C-U treatments had no effect on soil NO₃⁻-N and NH₄⁺-N (Figure 5), despite soil depth (Figure 8A,B). Meta-analysis revealed that the release longevity and the ratio of CRU had a notable effect on soil NO₃⁻-N: C-U with 30 d of CRU significantly decreasing NO₃⁻-N by 14.84%, and C-U with 40% CRU-N increasing NO₃⁻-N by 18.25% (Table 1).

4. Discussion

Application of fertilizer in accordance with the crop requirements is crucial for efficient yield (Shoji et al., 2001; Geng et al., 2015a). The N demand of most crops has predominantly an obvious S-shaped curve, and more N is often required in the middle of rapid growth [19], expanding from the small bellbottom stage to the heading stage for maize and from the elongation stage to heading stage for wheat. The N release of CRU also forms an obvious S-shape [6,20], indicating that CRU can supply better synchronization for most crop N demand, which not only promotes crop growth and improves dry matter accumulation and N uptake but also prevents premature senescence [21]. Common urea is the instant nitrogen fertilizer utilized all over the world, which can increase soil NO₃⁻-N content rapidly after application into soil in 2 weeks [22]. N application in excessive amounts with inappropriate application methods leads to high N losses through leaching. Especially in maize season with high temperature and concentrated precipitation, topical application of urea N not only obstructs the sole objective of high crop yield, but also induces N waste and descreases the NUE. In addition, CRU promotes crop root activity and augments the absorption area, thereby enhancing nutrient absorption [23]. These changes consecutively improve crop yield and NUE [24]. However, some results revealed that the mixed CRU with or splitting urea N remarkably improved crop yield and N accumulation, but there was no obvious difference between them [20,25,26].

The single basal application of CRU before transplanting could delay and prolong the N release to harvest, due to its slower N release rate [27]. Nevertheless, a single basal application of the mixed CRU with urea could provide N nutrition adequate for panicle differentiation and the grain filling process of the crop [28]. Establishing a reasonable CRU-N proportion in the mixed N fertilizer requires a scientific and flexible fertilizing recommendation [29]. Some reports demonstrated that CRU with a 50% proportion in the mixture can promote maize root growth and enhance the efficient utilization of N by soil microorganisms, and CRU with a 70% proportion plays a vital role in a wheat–maize rotation system, which can potentially be employed to improve the yields, NUE, and net benefit with low N losses [4]. There are differences among results about the ratios of CRU-N in mixed fertilizer. Predominantly, a lot of N for reproduction and vegetative growth in the latter growth stage were indispensable. Zhang et al. analyzed that there was a significant positive correlation between the N release longevity and the positive effect of CRU by implementing the regression method [25]. In this study, 30 d CRU in the mixture was negatively correlated with crop yield, plant characteristics, and NUE, while 60 d or 90 d CRU revealed positive consequences. However, the cost of CRU with 90 d proved to be relatively high, and its release longevity of N is nonfunctional for the fertilizer requirement of summer maize.

CRU governed N release by coating may be more sensitive to climate factors [30]. More precipitation and higher temperatures accelerate nutrient release from CRU [31], especially when the mean annual temperature is > 12 °C and mean annual rainfall is >550 mm [25], while in winter, when the soil temperature is below 0 °C and little N is needed by wheat, the CRU is almost not released into the soil. Additionally, the N release is slow mainly in the early stage of CRU [27], only with 28.7% N released in the first 120 days of the wheat growing season and 37.9% N released in the first 50 days of the corn growing season [20], although the cumulative N release longevity of CRU was 240 days for wheat and 120 days for maize. Most notably, the slow release of N in the early stage would limit the spike number per unit area of wheat, which is a key factor for its grain yield [27,32]. Furthermore, studies proved impacts of SOM on the CRU effect [25]. The higher C/N ratio in soil after maize straw returning would lead to increased competition for nitrogen between soil microorganisms and crops [33]. This increased competition restricting the amount of N released from the soil would cause the N content reduction in crop leaves, leading to a diminished photosynthesis rate [34]. At the same time, the slow release of N from CRU would further accelerate this effect with higher C/N ratios. Therefore, CRU had a poor substitution effect on urea-N for wheat [25], which is in line with this study’s results. Zhang et al. also revealed that when the soil-available N content and the N application rate were both low in wheat season, such as the former < 50 mg kg⁻¹ and the latter < 150 kg ha⁻¹, the positive effect of CRU is weaker than split-urea N [25].

The soil NH₄⁺-N levels in C-U treatment are comparatively proliferated than in U treatment. Some studies also demonstrated that the application of C-U could improve the NH₄⁺-N contents in the topsoil [20]. Notably, although the concentrations of TN and NH₄⁺-N under C-U treatment were higher, the difference among the controlled-release nitrogen fertilizer (CRU-N) was not statistically significant [4]. Those results indicated that the reduction of the conventional urea and the elevation of the proportion of 100% CRU would benefit nitrogen accumulation. Actually, 30% traditional urea in C-U was adequate to restore the nitrogen deficiency [35,36].

Predominantly, C-U contributes to crop yield and NUE, in comparison, to urea. A combination of 30–50% of CRU with urea synchronizes absorption with availability due to a period of increased N availability in soils and is proven to be the best strategy for simultaneously increasing wheat or maize production and NUE and reducing N leaching [7]. However, due to regional climate factors, soil physicochemical factors, and field management measures, the effects of C-U is varied. For crops such as winter wheat with a long growth cycle, CRU may have the risk of insufficient N supply for crop growth [27] and furthermore indicated a twice-split application of CRU to meet the N demand of winter wheat [30]. For crops, such as maize with short growth cycles, a single basal application of C-U may fit their N requirement patterns. Additionally, it is necessary to optimize CRU to reduce costs [37].

5. Conclusions

In comparison to common urea alone at the same N application rate, across five years, C-U (the mixed CRU with urea) had significant effects on grain yield and plant characteristics, NUE, and soil NO₃⁻-N contents in wheat–maize production with meta-analysis. In general, C-U significantly increased yield (3.80%), 1000-grain number (1.12%), grain number per ear (2.07%), plant dry weight (9.76%), plant height (2.76%), stem diameter (6.0%), and NUE (5.79%) for maize, while it decreased sterile spikelet number (3.28%) and NUE (12.22%) and increased plant dry weight (6.10%) and plant height (1.33%) for wheat. These effects were different with the appropriate release longevity of CRU and the ratio of CRU-N in C-U. The application of C-U with 60 d or 90 d CRU for wheat and maize had the best overall effects, while C-U treatment with 30 d CRU had a significant inhibitory effect on grain yield, 1000-grain weight, grain number per ear, and NUE. For maize, the application of C-U with 30% CRU-N had the largest increase rate on yield, 1000-grain weight, plant height, dry weight, and NUE by 5.13%, 1.61%, 3.70%, 11.33%, and 8.63%, respectively, while 40% CRU-N had the largest decrease on soil NO₃⁻-N. For wheat, the application of C-U with 40% CRU-N had noteworthy effects on yield, sterile spikelet number, and NUE by 4.45%, −9.76%, and −8.04%, respectively. Consequently, the use of the C-U has great potential to increase wheat–maize yields and reduce fertilizer loss, especially for maize, which provides an effective generic methodology for agriculture to improve measures and ensure profitability.