Effects of Partial Substitution of Organic Fertilizer for Synthetic N Fertilizer on Yield and N Use Efficiencies in a Semiarid Winter Wheat–Summer Maize Rotation

Abstract

Finding field management techniques that increase crop output while protecting soil sustainability is essential for maintaining a long-term food supply in a changing environment. However, comprehensive evaluation of the effects of nitrogen (N) reduction combined with organic fertilizer on grain yield, N use efficiency (NUE), water use efficiency (WUE), and soil organic carbon (SOC) and total N (TN) contents of winter wheat–summer maize double cropping systems in drought-prone areas remains limited. Therefore, a 3-year field experiment (2018–2021) was conducted in a winter wheat–summer maize double cropping system with five treatments: no N fertilizer (CK), conventional farmer fertilization (CF), recommended fertilization (R), organic N substitution of 20% of the recommended synthetic N (R₂₀), and organic N substitution of 40% of the recommended synthetic N (R₄₀). When results were averaged from 2018 to 2021, R₂₀ had the highest annual grain yield, which increased by 42.15%, 7.69%, 7.58%, and 12.50% compared with CK, CF, R, and R₄₀, respectively. Compared with CF, R₂₀ increased winter wheat and summer maize NAE, NPFP, NUE, and WUE. In addition, the soil organic carbon content of R₂₀ and R₄₀ treatment increased with the increase in years. In conclusion, R₂₀ was considered ideal for improving crop yield, promoting soil fertility, and increasing the fertilizer utilization rate in a semiarid winter wheat–summer maize rotation.

1. Introduction

Fertilization is an effective practice to improve soil fertility and crop yield [1]. One of the most crucial nutrients to boost and sustain crop output globally is nitrogen (N) [2]. Long-term, continuous inputs of N fertilizers have steadily increased crop yields to maintain the global sustainability of farming systems [3]. However, incorrect fertilization can cause major environmental damage, including soil acidification, water eutrophication, and greenhouse gas emissions, as well as a reduction in crop production and soil quality [4,5]. Therefore, scientific and reasonable fertilization is essential for maintaining stable crop yields.

Excessive application of N fertilizer is common in agricultural production. In wheat production, the recommended N rate is between 150 and 250 kg ha⁻¹, but N applications generally exceed 225 kg ha⁻¹ [6]. Approximately 45% of the N applied is not used by crops and is wasted from farmland [7]. The goals of N management are to promote crop growth and productivity while minimizing environmental consequences on other ecosystems [8]. Optimized N fertilizer management can significantly lower residual nitrate levels in soil [9] and increase N efficiency [10]. Increasing the input of organic fertilizer can partially replace and reduce dependence on chemical fertilizers, which is important for reducing chemical fertilizer inputs in farmland ecosystems [11,12,13]. Importantly, using organic fertilizers can also enhance physical and chemical properties of soil [14] and soil quality [15], which are essential for sustainable soil management and clean crop production. Additionally, by raising the proportion of macroaggregates (>0.25 mm), organic fertilizers can improve soil nutrient contents and soil water-holding capacity, which eventually improves yield and WUE [16].

Organic fertilizer applications alone also have problems, such as high dose requirements, inconvenient application, high labor intensity, high transportation costs, and low fertilizer efficiency, and are not the best measures to maintain sustainable crop production under intensive conditions [17,18]. Therefore, integrated crop nutrient management requires the combination of organic and inorganic fertilizers, which can increase the stability of crop production and also significantly improve fertilizer utilization efficiency [19,20]. Organic fertilizer combined with chemical fertilizer has been applied in cereal crop production in recent years, and combinations can effectively improve crop yields [21]. By substituting manure for mineral N in North China’s rotation systems for winter wheat and summer maize, annual yields can be preserved or even increased [22]. In wheat–maize systems of the North China Plain, using organic fertilizer for less than half of a chemical fertilizer application is the recommended alternative [23]. Organic replacement of inorganic N at the same N application rate can effectively increase crop yield [24], improve soil quality [7], and increase WUE [20] and NUE [25].

Although few studies have been conducted on synthetic N reduction combined with organic fertilizer, Banana seedlings benefited from the accumulation of dry matter when N fertilizer was reduced by 20% and organic fertilizer was substituted for 30% of it [26]. However, owing to the great differences between different regions in climatic conditions, planting structures, fertilization methods, and soil properties, regionalized and targeted recommendations for crop nutrient management are of more practical significance for improving food production and ensuring national food security. The semiarid region in the eastern margin of the Loess Plateau in western Henan is one of the main grain-producing areas in China. The area is typically drought-prone and has no irrigation, and the main agricultural production pattern is a winter wheat–summer maize double cropping system [6]. Problems in that system include excessive N fertilizer input and no organic fertilizer application. However, few studies have been conducted to provide an integrated assessment of the yield, N use efficiency, and water use efficiency in the semiarid region. Therefore, the objectives of this study were the following: (1) determine the changes in aboveground dry matter accumulation and N uptake; (2) determine the changes in yields; and (3) determine the changes in NUE and WUE.

2. Materials and Methods

2.1. Experimental Site Description

A field experiment was conducted from 2018 to 2021 at the Goukou Experimental Station (34°49′ N, 112°35′ E, a.s.l. 362 m) in Luoyang City, Henan Province, northern China. The site is in a semiarid region at the east edge of the Loess Plateau and has a temperate continental monsoon climate. Average annual temperature is 13.7 °C, average annual precipitation is 550 to 600 mm, average annual accumulated temperature is approximately 5.05 °C, and average frost-free period is approximately 235 days. Annual precipitation was determined by an automatic weather station at Mengjin Agricultural Experimentation Centre Station (Luoyang, Henan, China), revealing 821.2 mm in 2018, 580.6 mm in 2019, 603.4 mm in 2020, and 1172 mm in 2021, and in 2021, 83.6% of the precipitation occurred from June to September (Figure 1). The soil is a brown soil, and properties of the 0−20-cm soil layer at the start of the experiment were measured by Henan Dry Land Agricultural Engineering Technology Research Center (Luoyang, Henan, China): pH, 8.0; soil organic carbon (SOC), 7.02 g kg⁻¹; total N (TN), 1.05 g kg⁻¹; alkali-hydrolyzed N, 95.2 mg kg⁻¹; available phosphorus (P), 23.5 mg kg⁻¹; and available potassium (K), 123 mg kg⁻¹.

2.2. Experimental Design and Management

The following five treatments were established: (1) no N fertilizer (CK); (2) conventional fertilization by local farmers (CF), with 192 kg ha⁻¹ N, 154.5 kg ha⁻¹ P₂O₅, and 60 kg ha⁻¹ K₂O in wheat and 210 kg ha⁻¹ N, 50.4 kg ha⁻¹ P₂O₅, and 84 kg ha⁻¹ K₂O in maize; (3) recommended fertilization (R), 20% reduction of N fertilizer based on CF, with 153.6 kg ha⁻¹ N in wheat and 168 kg ha⁻¹ N in maize, the recommendation of fertilization is based on previous studies in this area [27]; (4) organic N substituting for the 20% of the recommended N (R₂₀); and (5) organic N substituting for 40% of the recommended N (R₄₀). Nitrogen fertilizer was applied as urea (46.4% N), P fertilizer was applied as calcium superphosphate (12% P₂O₅), and K fertilizer was applied as potassium chloride (50% K₂O). Organic fertilizer was applied as manure solid fertilizer (2.6% N, 1.5% P₂O₅, 2.5% K₂O, and 50% organic matter). All fertilizers were supplied by Luoyang Qihe Ecological Agriculture Technology Co., Ltd. (Luoyang, Henan, China). All treatments received the same amounts of P and K fertilizers. The R, R₂₀, and R₄₀ treatments received the same amounts of N. Organic and P and K fertilizers were applied as basal fertilizer at one time, and chemical fertilizer N was applied 70% as basal fertilizer and 30% at jointing stage in wheat and 40% as basal fertilizer and 60% at the 9–10-leaf spread in maize. Plots were 72 m² (10 m × 7.2 m) and were randomized complete block design with three replicate blocks. Other field management practices were the same as those in a conventional field. The planting system was winter wheat (planted in mid-October and harvested in early June of next year) and summer maize (planted in mid-June and harvested in early October) rotation. Wheat (Triticum aestivum L.) variety “Zhoumai 27” was produced by Zhoukou Academy of Agricultural Sciences (Zhoukou, Henan, China), with seedling density of 30 × 10⁶ plants ha⁻¹; and maize (Zea mays L.) variety “Zhongke I” was produced by Beijing Lianchuang Seed Industry Co., LTD. (Beijing, China), with a planting density 60,000 plants ha⁻¹.

2.3. Data Collection

2.3.1. Dry Matter Accumulation and Nitrogen Uptake

Winter wheat plants were sampled at maturity, containing leaf, stem with sheath, and glumes on spike axis. Summer maize plants were sampled at maturity and divided into stem, leaf, sheath, and ear. Sample preparation for the determination of aboveground biomass (kg ha⁻¹) was achieved by drying the samples at 105 °C for 30 min, followed by drying at 70 °C until constant weight. N concentration was determined by grinding the dried samples, putting them through a 0.25-mm screen, and then N concentration was determined by the semi-micro Kjeldahl method [2]. Nitrogen concentration multiplied with dry weight was used as N uptake. N uptake at maturity was calculated as the sum of N accumulated in each part.

2.3.2. Yield

Data on wheat and maize yields were gathered between 2018 and 2021. Winter wheat grain yield was calculated by taking three randomly chosen subplots (3.0 m²) from each plot. To assess grain yield and other endpoints, each plot served as a repeat (the moisture content was around 12%). Harvesting the three center rows from each plot allowed researchers to examine the grain production for summer maize. In order to quantify grain yield and other endpoints, 30 successive plants per row were picked as a replication when they reached maturity (the moisture content was around 14%).

2.3.3. N Efficiency

N efficiencies were calculated as follows:

$$\mathrm{N} \mathrm{a}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{c} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y} \left(\mathrm{N}\mathrm{A}\mathrm{E}, \mathrm{k}\mathrm{g} {\mathrm{k}\mathrm{g}}^{-1}\right)=\frac{{\mathrm{G}\mathrm{Y}}_{+\mathrm{N}}-{\mathrm{G}\mathrm{Y}}_{\mathrm{N}0}}{\mathrm{N} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}},$$

(1)

$$\mathrm{N} \mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y} \left(\mathrm{N}\mathrm{P}\mathrm{F}\mathrm{P}, \mathrm{k}\mathrm{g} {\mathrm{k}\mathrm{g}}^{-1}\right)=\frac{\mathrm{G}\mathrm{Y}}{\mathrm{N} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}},$$

(2)

$$\mathrm{N} \mathrm{u}\mathrm{s}\mathrm{e} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\mathrm{N}\mathrm{U}\mathrm{E}, \mathrm{\%}\right)=\frac{{\mathrm{N}\mathrm{U}}_{+\mathrm{N}}-{\mathrm{N}\mathrm{U}}_{\mathrm{N}0}}{\mathrm{N} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}\times 100$$

(3)

where GY_+N is grain yield with applied N, GY_N0 is grain yield with no applied N, N U_+N is N uptake at maturity with N, and NU_N0 is N uptake at maturity without N.

2.3.4. Soil Water Status and Water Use Efficiency

From 2018 to 2021, two samples of the 0–100-cm soil profile from each plot were taken at sowing and maturity of wheat and maize in increments of 20 cm using an auger (inner diameter = 4.0 cm). After combining the two soil samples from the same plot, a small amount (40 ± 5.0 g) of the mixture was put in an aluminum box. The soil water content was determined gravimetrically by drying in an oven at 105 °C for 24 h. Evapotranspiration (ET_C) and WUE were calculated as follows:

$${\mathrm{E}\mathrm{T}}_{\mathrm{C}}=\mathrm{P}+\mathrm{I}+\mathrm{U}-\mathrm{S}-\mathrm{D}\pm ∆\mathrm{S}$$

(4)

where ETC is the crop evapotranspiration from sowing to harvest of winter wheat and summer maize, P is the precipitation, I is the net infiltration of irrigation water, U is the upward flow of shallow groundwater into the root zone, S is the surface runoff, D is the downward drainage out of the root zone, and ΔS is the change in stored soil water of the soil profile. When the groundwater depth is greater than 2.5 m, the U value can be ignored. In this experiment, the groundwater depth was 3.5 m, and there was no irrigation, so the I and U values were 0. During the 2021 summer maize growing season, intense rainfall occurred leading to runoff in Henan Province. In that case, D was ignored because the water was pumped out in time to prevent downward drainage in the root zone. In the absence of intense rainfall, S and D were ignored in other years.

$$\mathrm{W}\mathrm{U}\mathrm{E}=\frac{\mathrm{G}\mathrm{Y}}{{\mathrm{E}\mathrm{T}}_{\mathrm{C}}}$$

(5)

where GY is the grain yield and ETc is the evapotranspiration.

2.3.5. Soil Sampling and Analysis

Soil samples were collected from the plow layer (0–20 cm and 20–40 cm) each year in September. Five to six cores were randomly collected in each plot. Soil samples from each plot were mixed thoroughly and air-dried. Soil samples were then sieved through a 0.25-mm screen and stored in sealed plastic jars for analysis. Soil organic carbon (SOC) content was measured by a K₂CrO₇–H₂SO₄ oxidation procedure [28], and TN was determined by a Kjeldahl method [2]. Three replicates were analyzed in each analysis.

2.4. Statistical Analyses

Data were processed using Microsoft Office Excel 2017 (Microsoft Windows, Redmond, DC, USA) and analysis of variance was performed using SPSS v. 21.0 (IBM, New York, NY, USA) and then plotted with Origin 2021 (Origin Lab Corp., Northampton, MA, USA). Statistical methods were performed using one-way analysis of variance (ANOVA) and Duncan’s multiple comparison test (p < 0.05).

3. Results

3.1. Dry Matter Accumulation and Nitrogen Uptake

Nitrogen application significantly increased aboveground DM (dry matter) averaged across the three years (Figure 2a,b). Compared with CK, winter wheat and summer maize aboveground DM increased by 26.86% and 19.89%, respectively. In the third year, aboveground DM increased significantly in R₂₀. Compared with CF, R₂₀ significantly increased winter wheat and summer maize aboveground DM by 12.43% and 4.93%, respectively. Averaged across the three years, N application increased winter wheat N uptake by 45.59%, compared with CK (Figure 2c,d). Nitrogen application also increased summer maize N uptake by 38.22%, compared with CK. In the third year, compared with CF, R₂₀ significantly increased winter wheat and summer maize N uptake by 10.49% and 5.14%, respectively.

3.2. Crop Yield

In the three years of the study, fertilizer application significantly increased yields of winter wheat and summer maize, and grain yields in each year showed different trends among fertilizer treatments (Figure 3). In 2018–2019, winter wheat, summer maize, and annual grain yields were ranked by treatment in the order R ≈ CF ≈ R₂₀ > R₄₀ > CK, with yields not significantly different among R, R₂₀, and CF. In 2019–2020, winter wheat, summer maize, and annual grain yields were ranked R₂₀ > R ≈ CF > R₄₀ > CK, with yields significantly higher in R₂₀ than in CF and R, not significantly different between CF and R, and significantly lower in R₄₀ than in CF. In 2020–2021, winter wheat, summer maize, and annual grain yields were ranked R₂₀ > R₄₀ > CF ≈ R > CK, with grain yields significantly higher in R₂₀ than in CF and R, not significantly different between CF and R, and significantly higher in R₄₀ than in CF.

Averaged across the three years, annual grain yields in CF and R were 15.69 Mg ha⁻¹ and 15.71 Mg ha⁻¹, respectively, with no significant difference, and compared with CK, yields increased by 32.00% (CF) and 32.13% (R). The highest annual grain yield was in R₂₀, with yield increasing by 42.15%, 7.69%, 7.58%, and 12.50% compared with that in CK, CF, R, and R₄₀, respectively. Annual grain yield in R₄₀ increased year to year. In the third year, the yield in R₄₀ was higher than that in CF and R, but the average annual grain yield over three years was lower in R₄₀ than in CF and R by 4.46% and 4.59%, respectively.

3.3. Nitrogen Use Efficiency

Compared with CF, R significantly improved the N use efficiency indicators NAE, NPFP, and NUE (

Table 1. Effects of different fertilization strategies on NAE, NPFP, and NUE of winter wheat and summer maize from 2018 to 2021.

Year	Treatment	Winter Wheat			Summer Maize
		NAE	NPFP	NUE	NAE	NPFP	NUE
2018–2019	CK	—	—	—	—	—	—
	CF	6.62 a	32.15 c	31.22 b	8.88 ab	46.61 c	25.95 c
	R	8.67 a	40.59 a	41.25 a	11.24 a	58.41 a	30.38 b
	R20	7.85 a	39.77 a	41.38 a	10.68 a	57.85 a	37.08 a
	R40	2.79 b	34.71 b	22.09 c	5.20 b	52.36 b	18.36 d
2019–2020	CK	—	—	—	—	—	—
	CF	9.19 b	31.74 c	27.59 b	11.72 c	45.67 d	18.67 b
	R	11.69 b	39.86 b	35.60 b	15.31 b	57.74 b	21.42 b
	R20	17.96 a	46.14 a	59.77 a	20.80 a	63.23 a	29.99 a
	R40	9.07 b	37.25 b	26.88 b	11.38 c	53.81 c	18.67 b
2020–2021	CK	—	—	—	—	—	—
	CF	8.90 c	31.25 c	32.55 c	11.16 c	44.89 c	28.74 b
	R	10.48 b	38.42 b	37.15 c	13.47 bc	55.64 b	32.49 b
	R20	17.47 a	45.42 a	54.04 a	18.40 a	60.56 a	42.11 a
	R40	13.55 b	41.50 b	43.40 b	16.14 ab	58.31 ab	39.14 a
Average	CK	—	—	—	—	—	—
	CF	8.24 c	31.71 d	30.45 b	10.59 c	45.72 d	24.45 c
	R	10.28 b	39.62 b	38.00 b	13.34 b	57.26 b	28.10 b
	R20	14.43 a	43.78 a	51.73 a	16.63 a	60.55 a	36.39 a
	R40	8.47 c	37.82 c	30.79 b	10.90 c	54.83 c	25.39 c
F-values
Y		**	*	ns	**	ns	**
T		**	**	**	**	**	**
Y × T		**	**	*	*	*	**

Note: CK: No nitrogen fertilizer, CF: conventional fertilization by local farmer, R: recommended fertilization, R₂₀: organic N substituting for the 20% of the recommended N, R₄₀: organic N substituting for the 40% of the recommended N. Different lowercase letters within columns indicate significant differences indicated by Duncan’s test at p < 0.05. *, significance at p < 0.05; **, significance at p < 0.01; ns, not significance at p > 0.05.

). Compared with CF, R increased winter wheat NAE, NPFP, and NUE by 17.75% to 30.97%, 22.94% to 26.25%, and 14.13% to 32.13%, respectively. Compared with CF, R also increased summer maize NAE, NPFP, and NUE by 20.74% to 30.57%, 23.94% to 26.43%, and 13.05% to 17.07%, respectively. In the third year, compared with R, R₂₀ significantly increased winter wheat and summer maize NAE by 66.70% and 36.55%, NPFP by 18.22% and 8.84%, and NUE by 45.46% and 29.61%, respectively.

3.4. Soil Moisture and Water Use Efficiency

Soil water content in the 0–100-cm soil profile was measured in different treatments at sowing of winter wheat and summer maize and harvest of summer maize (Figure 4). As soil depth increased, soil water content decreased, but at 100 cm, there was a slight increase. In 2018–2020, at winter wheat sowing, average soil water content was ranked R₂₀ > R₄₀ > R > CF > CK. In 2019–2021, at summer maize sowing, soil water content was generally ranked R₄₀ > R₂₀ > R > CF > CK. In 2019–2021, at summer maize harvest, soil water content was generally ranked R₂₀ > R₄₀ > CF > R > CK.

There was no significant difference in evapotranspiration between winter wheat and summer maize rotation system (Figure 5a,b). Compared with CK, the 3-year average WUE of winter wheat and summer maize increased by 39.27% and 29.73%, respectively, with increases of 35.28% and 19.94% in 2018–2019, 37.42% and 35.17% in 2019–2020, and 47.15% and 37.71% in 2020–2021, respectively (Figure 5c,d). There was no significant difference in WUE of winter wheat and summer maize between CF and R. The best water utilization in the winter wheat–summer maize rotation system was in R₂₀. Compared with CF, the 3-year average WUE of winter wheat and summer maize in R₂₀ increased by 10.87% and 6.49%, respectively.

3.5. Soil Organic Carbon and Total Nitrogen Concentrations and Carbon: Nitrogen Ratio

Concentrations of SOC and TN were higher at 0–20 cm than at 20–40 cm (Figure 6). At 0–20 cm, compared with CK, SOC concentrations under fertilization increased by 17.45% in 2019, 37.99% in 2020, and 44.71% in 2021; TN concentrations under fertilization increased by 4.90% in 2019, 10.39% in 2020, and 15.77% in 2021; and C/N ratios under fertilization increased by 11.68% in 2019, 25.08% in 2020, and 24.22% in 2021. Compared with CF, there was no significant difference in SOC concentration in R₂₀ in 2019, but SOC content increased significantly in R₂₀ by 10.71% in 2020 and 13.03% in 2021. Compared with CF, TN concentration in R₂₀ increased by 1.52% in 2019, 4.06% in 2020, and 7.55% in 2021 and C/N ratio increased by 6.40% in 2020 and 5.13% in 2021. At 20–40 cm, compared with CK, SOC concentrations under fertilization increased by 31.94% in 2019, 40.57% in 2020, and 55.10% in 2021; TN concentrations under fertilization increased by 7.86% in 2019, 17.42% in 2020, and 24.23% in 2021; and C/N ratios increased by 22.35% in 2019, 20.20% in 2020, and 25.35% in 2021. Compared with CF, R₂₀ increased SOC concentrations by 6.06% in 2020, and 8.44% in 2021, with the increase significant, and TN concentrations by 5.19% in 2020 and 10.39% in 2021.

4. Discussion

A crucial component for plant growth and development is nitrogen. In 2018–2021, applications of N fertilizer significantly increased DM accumulation and grain yields of winter wheat and summer maize. In this study, DM accumulation was similar in R₂₀ and R in 2018–2019 but was significantly higher in R₂₀ than in R in 2019–2020 and 2020–2021. Similar to DM accumulation, grain yield in R₂₀ was similar to that in CF and R in 2018–2019 but was significantly higher than that in R in 2019–2020 and 2020–2021, with the 3-year average yield significantly higher in R₂₀. Zhou et al. [29] reported that compared to recommended fertilizing plots, the yield for wheat improved with treatments that included organic fertilizer by 26.4% to 44.6% and for maize by 12.5% to 40.8%. In this study, grain yield in R₄₀ increased year to year, and it was lower than that in R in 2018–2019 and 2019–2020 but was higher in 2020–2021, with the 3-year average grain yield lower than that in R. Due to the introduction of a large amount of organic matter, nitrogen may be adsorbed by organisms. In this case, available nitrogen may have been reduced due to the development of soil microorganisms breaking down the original organic matter. Xin et al. [23] reported that when compost completely replaced all mineral fertilizers, wheat yield consistently decreased and maize yield decreased within the first few years. Those results demonstrate that in order to attain high crop yields on the North China Plain, organic fertilizer cannot entirely replace chemical fertilizer. A very large (over 30-fold) increase in the concentration of water-soluble phenolic compounds under the influence of fertilization with manure was obtained in a study [30]. The phenolic compounds released during the decomposition of soil organic matter help to reduce the biological activity of soil [31]. Many studies demonstrate that optimizing fertilizer application improves N uptake by crops, maximizes N fertilizer efficiency, and increases N effectiveness, resulting in increases in yields. A successful N management method that has been used on a variety of crops is combined application of organic and chemical fertilizers. In this study, R₂₀ improved crop total N uptake and NUE. The benefits of combining organic and chemical fertilizers on maize yield and NUE might be due to increases in post-silking N uptake and DM accumulation [32]. In order to sustain high yields, suitable organic replacement under advised N circumstances is a good strategy.

The main element limiting crop output in dryland farming is water [33]. Increasing WUE is a major objective of sustainable agricultural development in some agricultural producing areas with inadequate irrigation [34]. In this study, combined application of organic and chemical fertilizers increased soil water content, but there was no significant difference in the total evapotranspiration loss of soil water among treatments. Previous studies also show that organic fertilizers increase grain yield without affecting ET, thereby increasing WUE [35,36,37]. The outcomes may be attributable to organic fertilizer enhancing the effectiveness of precipitation storage [38] and alleviating soil water depletion [36] or to reduced soil water evaporation and transpiration caused by straw returned to the field after harvests of crops. In a previous study, organic fertilizer increased water use efficiency (WUE) by 41% when synthetic fertilizer was absent and by 11% to 13% when synthetic fertilizer was applied at a rate of 150–250 kg ha⁻¹ [16]. In addition, to increase the yield and WUE, regular soil management for wheat should concentrate on the combined use of manures and inorganic fertilizer [39]. Increases in WUE under organic fertilizer treatment are due to increases in photosynthesis and decreases in transpiration (Tr) and stomatal conductance (Gs) [37].

Soil organic carbon is an important factor in increasing crop productivity and improving soil structure and water retention [40]. During the 3-year experiment, SOC and TN contents in R topsoil did not significantly decrease compared with those in CF. Crops require less N during the early stages of growth, and excessive fertilization results in significant soil nitrogen loss (41% to 51% of the N intake) [41]. In this study, R₂₀ increased SOC and TN in 2020 and 2021, consistent with conclusions reported previously [2]. The increase in SOC concentrations was attributed to the addition of organic manure and return of roots and stubble crop residues [42]. The increase in TN concentration in R₂₀ might be due to the slow release of N from the organic fertilizer, which can reduce soil N losses due to leaching or denitrification, thus enabling increased N utilization by plants [26]. It is also possible that development of an extensive root system helped to increase N levels [43]. Combining the use of chemical and organic fertilizers improves soil fertility by encouraging nutrient release from the organic fertilizer and reducing nutrient losses [44,45,46], but also improves the microecological environment by keeping the C/N ratio at an optimal level [47]. Furthermore, due of the high soil carbon sequestration capacity and ecosystem service values, applying optimized doses of organic fertilizers combined with rational mineral fertilization can be a successful strategy for creating sustainable farmlands [48].

5. Conclusions

Averaged across the three years, the effects of different treatments on winter wheat and summer maize were similar; N management was optimized with a 20% reduction in N input that did not sacrifice grain yields. Compared with the recommended fertilization, the R₂₀ treatment significantly increased DM accumulation and yield, improved N uptake and NUE, and increased soil water content and WUE. The R₂₀ treatment also significantly increased SOC and TN concentrations at both 0–20 cm and 20–40 cm. Nitrogen reduction combined with 20% organic fertilizer not only reduced the application rate of chemical fertilizer, but also greatly improved yield while maintaining clean, environmentally friendly, and sustainable agricultural production.