Optimizing Lucerne Productivity and Resource Efficiency in China’s Yellow River Irrigated Region: Synergistic Effects of Ridge-Film Mulching and Controlled-Release Nitrogen Fertilization

Abstract

To address low productivity and water constraints in lucerne fields of China’s Gansu Yellow River Irrigation Region, this study optimized lucerne (Medicago sativa L.) cultivation through synergistic planting nitrogen regimes. A two-year field trial (2021–2022) evaluated three systems: ridge-furrow with ordinary mulch (PM), ridge-furrow with biodegradable mulch (BM), and conventional flat planting (FP), under four controlled-release N rates (0, 80, 160, 240 kg ha⁻¹). Multidimensional assessments included growth dynamics, dry matter yield, forage quality (crude protein [CP], acid/neutral detergent fiber [ADF/NDF], relative feed value [RFV]), and resource efficiency metrics (water use efficiency [WUE], irrigation WUE [IWUE], partial factor productivity of N [PFPN], agronomic N use efficiency [ANUE]). The results showed the following: (1) Compared with conventional flat planting, ridge planting with film mulching significantly promoted lucerne growth, with ordinary plastic film providing a stronger effect than biodegradable film. Plant height and stem diameter exhibited a quadratic response to elevated nitrogen (N) application rates under identical planting patterns, peaking at intermediate N levels before declining with further increases. (2) Ridge planting with both ordinary plastic film and biodegradable film combined with an appropriate N rate improved lucerne yield and quality. In particular, the PMN2 treatment reached the highest value of yield (14,600 kg ha⁻¹), CP (19.19%) and RFV (124.18), and the lowest value of ADF (29.63%) and NDF (48.86%), and all of them were significantly better than the other treatments (p < 0.05). (3) WUE, IWUE, PFPN, and ANUE followed the pattern PM > BM > FP. With increasing N application rates, WUE, IWUE, and ANUE initially rose and then declined, peaking under N2, whereas PFPN showed a decreasing trend and reached its maximum under N1. Principal component analysis revealed that ridge planting with ordinary plastic film combined with 160 kg·ha⁻¹ N (PMN2) optimized lucerne performance, achieving balanced improvements in yield, forage quality, and water–nitrogen use efficiency. This regimen is recommended as the optimal strategy for lucerne cultivation in the Gansu Yellow River Irrigation Region and analogous ecoregions.

1. Introduction

Lucerne (Medicago sativa L.), a perennial high-quality leguminous forage, is known as the “King of Forage” because of its strong adaptability, high nutritional value and high grass yield [1,2]. Lucerne is widely planted around the world, especially in arid and semi-arid areas. Its developed root system and abundant leaves significantly increase the surface coverage, effectively reduce soil erosion and desertification of pastures, and also play an important role in the enhancement of soil fertility [3,4]. In recent years, China has implemented the policy of grain-to-feed conversion and encouraged the planting of high-quality pasture, and the planted area of lucerne has continued to expand, but its output and quality are still far lower than those of developed countries such as Europe and the United States [5]. Although lucerne has strong drought tolerance, its water consumption is large [6]. The main production areas of lucerne in China are mostly located in arid and semi-arid regions, where water scarcity and soil infertility seriously restrict the development of the pasture industry [7]. To address this issue, our research team previously explored the effects of conventional urea application in combination with flat crop planting (FP) or ridge planting with ordinary plastic film mulching (PM) on lucerne establishment year production performance [7]. However, three key gaps remain: (1) the long-term dynamics of perennial lucerne under multiple years of continuous cropping are still unclear; (2) the potential of controlled release nitrogen fertilizer (CRNF) to harmonize nutrient supply with crop demand has not yet been assessed; and (3) the feasibility of biodegradable mulches as an alternative to conventional plastic mulches still needs further validation. Therefore, in light of the resource characteristics of arid and semiarid areas, this study extends previous work by introducing CRNF and ridging biodegradable film. Our goal is to explore a reasonable lucerne planting and nitrogen (N) application strategy, to create a suitable growing environment, to promote the efficient use of lucerne water and fertilizer resources and increase yield and quality, and to promote the green and sustainable development of animal husbandry.

Moisture is an important external stress factor for plant life activities [8], and reasonable planting methods can reduce soil moisture evaporation and plant transpiration, thus improving crop production performance [9,10]. Ridge mulching technology reduces the ineffective evaporation of soil moisture through mulching, improves soil water retention capacity, and at the same time, improves soil temperature and day–night temperature difference, changing from “passive drought resistance” to “active drought resistance” [11,12,13]. Studies have shown that ridge mulching technology in dryland winter wheat planting showed good rainwater harvesting and moisture retention effect, and significantly increased the number of tillers, the number of spikes and the biomass of wheat [14]. In addition, ridge mulching can increase soil temperature by increasing surface area and solar radiation absorption through changing microtopography [15,16]. In European studies on fodder maize, mulching was found to significantly increase crop yield and improve quality [17]. However, the widespread use of plastic film has led to the accumulation of residual film in the soil, which degrades soil physical and chemical properties and nutrient status, resulting in “white pollution” [18]. To address this issue, new eco-friendly mulching materials, such as liquid films and biodegradable films, have been introduced. It has been found that liquid mulch improves soil permeability and water retention capacity by enhancing the aggregation of soil particles [19]. However, compared with traditional plastic mulch, biodegradable mulch still has a certain gap in the effect of yield increase [20]. Therefore, this study will evaluate the potential application of biodegradable mulch in lucerne cultivation to balance the agricultural benefits with environmental sustainability.

Nitrogen is an important nutrient element for plant growth and development, directly affecting photosynthetic efficiency, growth and metabolism [21]. Studies have shown that nitrogen fertilization can significantly increase lucerne plant height and yield and improve nutritional quality by reducing acid detergent fiber and neutral detergent fiber content and increasing crude protein and crude fat content [22]. Controlled release nitrogen fertilizer (CRNF) can further improve nitrogen use efficiency (NUE) and reduce environmental pollution by slowly releasing nutrients that match crop uptake patterns compared to conventional urea [23]. In clay loam soils of eastern Canada, moderate application of controlled-release N fertilizer increased corn yields compared to pure urea [24]. In a study on ryegrass (Lolium perenne L.) lawn, controlled-release N fertilizer significantly reduced N leaching and improved N fertilizer use efficiency [25]. Therefore, it is of great significance to explore the high yield, high quality and high efficiency grassland management model of lucerne by combining controlled release N fertilizer and ridge mulching technology.

Currently, ridge mulching technology is widely used in grain crops [26] and cash crops [27,28], but there are fewer studies for perennial forage grasses such as lucerne, especially the research on different materials of ridge mulching to regulate soil moisture conditions and coupled with controlled-release nitrogen fertilizers is not yet common. Gansu Yellow River Irrigation Region is an important area for lucerne cultivation in China, where the annual precipitation is lower than the national average, and water resources are scarce and inefficiently utilized; however, the area has good irrigation facilities, large diurnal temperature difference, and abundant light and heat resources, which are suitable for lucerne cultivation [29]. In view of this, this study was conducted on lucerne in the Gansu Yellow River Irrigation Region, aiming (1) to clarify the coupling relationship between the use of ordinary mulch on ridges, biodegradable mulch on ridges and flat planting without mulching, and controlled-release nitrogen application; (2) to quantify the multi-year effects of these integrated measures on lucerne growth, yield, quality and water and nitrogen use efficiency; and (3) to obtain the optimal planting and nitrogen management strategies to realize lucerne’s high yield, high quality, and high efficiency and reduce the environmental impacts. This will provide a technical reference for the sustainable cultivation of lucerne in the Gansu Yellow River Irrigation Region and similar arid ecosystems.

2. Materials and Methods

2.1. Overview of the Experimental Site

Field experiments were conducted during the growing seasons (April–October) of 2021 and 2022 at the Gansu Jingtai Chuan Electric Lifting Irrigation Water Resource Utilization Center Irrigation Experiment Station (37°23′ N, 104°08′ E; altitude 2028 m). The experimental site is characterized by an arid climate with long-term averages of 191.6 mm annual precipitation, 2761 mm evaporation, 2652 h sunshine duration, 6180 MJ·m⁻² solar radiation, 8.5 °C mean air temperature, and 191 frost-free days.

The soil is classified as sandy loam with a bulk density of 1.45 g·cm⁻³, and the field capacity is 24.1% (on a mass basis). The topsoil (0–20 cm) properties were as follows: pH 8.11, organic matter 6.09 g·kg⁻¹, total nitrogen 1.62 g·kg⁻¹, total phosphorus 1.32 g·kg⁻¹, total potassium 34.03 g·kg⁻¹, available nitrogen 74.51 mg·kg⁻¹, available phosphorus 26.31 mg·kg⁻¹, and available potassium 173 mg·kg⁻¹. According to the soil nutrient grading standards of the Second Soil Census of China, soil organic matter content and phosphorus content are poor, while nitrogen content is moderate, and potassium content is rich [30]. During the experimental periods, the total growing-season precipitation reached 147.43 mm (2021) and 170.46 mm (2022), with mean temperatures of 19.07 °C and 18.77 °C, respectively (Figure 1).

2.2. Experimental Design

The experiment was arranged in a completely randomized block design with two factors: (1) planting patterns—flat planting without mulching (FP), ridge planting with biodegradable film mulching (BM), and ridge planting with ordinary plastic film mulching (PM); and (2) controlled-release nitrogen (N) fertilizer rates—0 (N0), 80 (N1), 160 (N2), and 240 (N3) kg N ha⁻¹ (

Table 1. Experimental design.

Treatment	Cultivation Pattern	Nitrogen Level (kg·ha−1)
FPN0	Traditional flat planting (FP)	0 (N0)
FPN1		80 (N1)
FPN2		160 (N2)
FPN3		240 (N3)
BMN0	Ridge-furrow planting with biodegradable film (BM)	0 (N0)
BMN1		80 (N1)
BMN2		160 (N2)
BMN3		240 (N3)
PMN0	Ridge-furrow planting with ordinary plastic film (PM)	0 (N0)
PMN1		80 (N1)
PMN2		160 (N2)
PMN3		240 (N3)

). The test lucerne variety is “Longdong lucerne”, which has strong drought resistance, small and thick leaves, and low stomatal density, which helps to reduce water loss. This variety is able to maintain good growth under the condition of lack of soil moisture and has high yield and nutritional value. It is adapted to semi-arid climatic conditions and is suitable for cultivation within an altitude of 500–3500 m. Each treatment combination was replicated three times, resulting in 36 experimental plots (3 planting patterns × 4 N rates × 3 replicates). Individual plots measured 5.5 m × 7.8 m (42.9 m²). The ordinary mulch film used for the experiment was made of low-density polyethylene (LDPE) with a thickness of 0.008 mm and a light transmittance of 85–90%, while the biodegradable mulch film for ridges was made of starch-based composite liquid, and the thickness of the film was 0.005–0.008 mm after spraying into the film, with a light transmittance of 70–75%, and a disintegration rate of ≥90% within 60 d. The thickness of the film was 0.005–0.008 mm, and the light transmittance of the film was 70–75%. The width of both types of films was 100 cm. Before sowing we inoculated lucerne seeds by soaking them in a solution containing highly effective rhizobacteria (Sinorhizobia meliloti). The seeds were sown in April 2021, the land was leveled 10 d before sowing, and the seeds were manually sown in strips at a rate of 22.5 kg·ha⁻¹ (1.0 to 1.2 × 10⁷ seeds per hectare). After lucerne germination, the germination rate of lucerne was estimated by randomly selecting a one square meter area in each treatment. After averaging all the estimated values, the germination rate of lucerne reached more than 80%, which met the requirements of the experiment. In the PM and BM treatments, furrows were dug and ridges formed prior to sowing, and lucerne was planted along the ridge edges and in the furrows, with four total rows in the furrows at 20 cm spacing. In FP plots, the row spacing was 30 cm (Figure 2).

The integrated water and fertilizer drip irrigation system was designed. In PM and BM plots, drip lines were spaced 40 cm apart, while in FP plots, they were spaced 60 cm apart. All emitters across treatments were standardized with a nominal discharge rate of 2 L·h⁻¹. Installation of shut-off valves and 0.001 m³ metering water meters in the main pipes of the irrigation network system in order to realize precise control of irrigation water quantity. The nitrogen fertilizer applied was a controlled-release nitrogen fertilizer (N, P₂O₅, and K₂O mass percentages of 30%, 4%, and 6%, respectively) produced by Shandong Kingenta Ecological Engineering Group Co., Ltd. (Linyi, China), at a N application rate of 160 kg·ha^–1. Phosphorus was supplied as superphosphate (40.0 kg·ha^–1; 16% P₂O₅), and potassium as potassium sulfate (19.2 kg·ha^–1; 50% K₂O). All fertilizers were applied once before the first regrowth each year, and other field management practices followed the standard local procedures for lucerne fields. Lucerne was mowed at the initial flowering stage of each cut; two cuts were mowed in 2021, the establishment year, on 16 July and 5 October, and three cuts were mowed in 2022, on 29 May, 29 July, and 13 September, respectively.

2.3. Measurement Indicators and Calculation Methods

2.3.1. Plant Height and Stem Diameter (cm)

At each harvest event, ten representative lucerne plants with uniform growth and intact stems were randomly selected per plot. Plant height was measured as the vertical distance from the soil surface to the apical meristem of the primary stem using a graduated steel tape (1 mm accuracy). Stem diameter was determined at 5 cm above the soil surface on the primary stem with a digital caliper (±0.01 mm accuracy).

2.3.2. Yield and Quality

(1) Yield (Y, kg·ha^–1)

Lucerne yield was determined using a 1 m × 1 m quadrat sampling method. At each harvest, homogeneous quadrats were selected within plots, and plants were cut 5 cm above ground using field scissors. After manual removal of non-target species, fresh weight was immediately recorded. Samples were oven-dried at 105 °C for 30 min (enzyme inactivation) followed by 75 °C until constant weight (<0.5% mass variation between consecutive weighings). Dry matter yield per hectare (kg·ha⁻¹) was calculated using the area conversion factor (quadrat/plot ratio). Three biological replicates were performed for statistical reliability.

(2) Quality

The harvested samples were oven-dried at 75 °C, ground through a 40-mesh sieve (0.4 mm), and used for quality analysis. Crude protein (CP) content was determined by the Kjeldahl method using a fully automated nitrogen analyzer (Kjeltec™ 8400, FOSS, Höganäs, Sweden), with results expressed on a dry matter basis. Acid detergent fiber (ADF) and neutral detergent fiber (NDF) were analyzed according to the Van Soest method [31] using a semi-automated fiber analyzer (F800, Ankom Technology, Fairport, New York, NY, USA). All measurements were performed in triplicate.

The relative feed value (RFV) was calculated as follows:

$$\mathit{RFV}=\left(120/{\mathit{V}}_{\mathit{NDF}}\right) \times \left(88.9 - 0.779\times {\mathit{V}}_{\mathit{ADF}}\right)/1.29$$

(1)

In the formula, V_NDF is neutral detergent fiber content (%), and V_ADF is acid detergent fiber content (%).

2.3.3. Water and Nitrogen Use Efficiency

(1) Water Consumption (ET, mm)

Evapotranspiration (ET) was calculated using the water balance method.

$$ET=10{\displaystyle \sum _{i=1}^n{\gamma }_i{H}_i({\theta }_{i1}-{\theta }_{i2})+I+P+K}-R-D$$

(2)

In the formula,

i is the index of the soil layer;

n is the total number of soil layers;

γ_i is the bulk density of the i-th soil layer (g cm^–3);

H_i is the thickness of the i-th soil layer (cm);

θ_i1 and θ_i2 represent the soil moisture content (%) at the initial and final stages of the time period for the i-th soil layer, respectively, calculated as the mass percentage relative to dry-soil mass;

I is the irrigation amount (mm);

P is the precipitation (mm);

K is the groundwater recharge (mm);

R is the surface runoff (mm);

D is the deep percolation (mm).

Given that the experimental field is flat, the groundwater table is relatively deep, and each individual rainfall event is small; K, R, and D were considered negligible.

(2) Water Use Efficiency (WUE, kg·ha^–1·mm^–1)

$$WUE=Y/ET$$

(3)

Here, Y is the lucerne yield (kg·ha^–1), and ET is the amount of water consumed (mm).

(3) Irrigation Water Use Efficiency (IWUE, kg·ha^–1·mm^–1)

$$IWUE=Y/I$$

(4)

(4) Partial Factor Productivity of Nitrogen (PFPN, kg kg^–1)

$$PFPN=Y/F$$

(5)

Here, F is rate of N application (kg·ha^–1).

(5) Agronomic Nitrogen Use Efficiency (ANUE, kg kg^–1)

$$ANUE=\left(Y_{NPK}-Y_{PK}\right)/F$$

(6)

Here, Y_NPK is the total annual lucerne yield under N-fertilized treatment (kg·ha^–1); Y_PK is the total annual yield of lucerne in no N application treatment (kg·ha^–1).

2.4. Data Analysis

Data organization was conducted using Microsoft Excel 2019. Statistical analyses were performed with IBM SPSS Statistics 26: (1) Plant height, stem diameter, yield, and water–nitrogen use efficiency across treatments were analyzed by one-way ANOVA with Duncan’s multiple-range test (p < 0.05). (2) Two-way ANOVA (p < 0.05) was employed to evaluate the main effects of planting patterns and nitrogen application levels, along with their interaction effects. Data visualization was implemented using Origin 9.0.

3. Results

3.1. Growth Performance of Lucerne in Response to Planting and Nitrogen Application Patterns

3.1.1. Plant Height

Both planting pattern and nitrogen (N) application level significantly influenced lucerne plant height (p < 0.05), while their interaction effect was not significant (Figure 3). With successive harvests, plant height showed an overall decreasing trend, and in the corresponding harvests, plant height in 2022 was significantly greater than in 2021. Under the same N application level, plant height followed the order PM > BM > FP. Specifically, compared with FP, PM increased plant height by 11.38%, 12.81%, 15.30%, and 13.13% under N0, N1, N2, and N3, respectively; BM increased it by 3.8%, 4.49%, 4.14%, and 2.40%. Within each planting pattern, plant height ranked as N2 > N3 > N1 > N0. Under the N2 level, average plant height exceeded that of N0, N1, and N3 by 9.91–13.38%, 5.02–7.67%, and 3.59–3.79%, respectively. Among all treatments, PMN2 achieved the highest plant height, averaging 3.79–26.28% greater than other treatments.

3.1.2. Stem Diameter

Two-way ANOVA revealed significant main effects of cultivation patterns and nitrogen levels on lucerne stem diameter (p < 0.01), with no significant interaction effect (Figure 4). A progressive decline in stem diameter was observed with increasing cutting frequency across both experimental years (2021–2022). Under equivalent nitrogen inputs, stem diameter followed PM > BM > FP. Compared to FP, PM enhanced stem diameter by 23.08%, 24.15%, 34.01%, and 30.22% under N0-N3 gradients, while BM showed respective increases of 6.04%, 6.44%, 11.31%, and 11.30%. Stem diameter exhibited a quadratic response to increasing nitrogen rates, peaking at N2 (160 kg ha⁻¹) with 17.40–27.83%, 8.64–17.27%, and 5.00–8.05% increases compared to N0, N1, and N3, respectively. The PMN2 treatment demonstrated maximum stem diameter enhancement, surpassing other treatments by 8.05–57.33%. This highlights the critical role of nitrogen-cultivation coordination in lucerne morphology regulation.

3.2. Yield and Quality of Lucerne in Response to Planting Nitrogen Application Patterns

3.2.1. Yield

Two-way ANOVA revealed a significant interaction effect between planting patterns and nitrogen application rates on lucerne annual yield (p < 0.05;

Table 2. Effects of nitrogen application patterns on lucerne yield (kg ha⁻¹).

Treatment		2021			2022
Planting Pattern	Nitrogen Level	First Cut	Second Cut	Total Yield	First Cut	Second Cut	Third Cut	Total Yield
PM	N0	4990 c	2370 bc	7370 c	10,400 c	5460 de	2300 c	18,100 c
	N1	5490 b	2480 ab	7940 b	11,400 a	5900 b	2350 bc	19,700 b
	N2	5930 a	2540 a	8500 a	12,000 a	6270 a	2430 a	20,700 a
	N3	5710 ab	2430 abc	8140 b	11,200 ab	6170 a	2380 ab	19,700 b
BM	N0	4430 f	1940 e	6370 gh	9290 ef	4800 g	1940 f	16,200 f
	N1	4730 cde	2250 d	6980 de	9520 cdef	5420 de	2090 de	17,200 e
	N2	4820 cd	2350 cd	7170 cd	10,500 bc	5640 c	2150 d	18,300 c
	N3	4540 def	2240 d	6780 ef	9880 cde	5530 cd	2100 de	17,500 d
FP	N0	4380 f	1770 f	6150 h	8890 f	4710 g	1880 f	15,500 g
	N1	4460 ef	1840 ef	6300 gh	9440 def	5070 f	1880 f	16,400 f
	N2	4640 def	1950 e	6590 fg	10,200 cd	5360 e	2050 e	17,600 d
	N3	4670 def	1920 e	6590 fg	9770 cde	5150 f	1920 f	16,900 e
P		**	**	**	**	**	**	**
N		**	**	**	**	**	**	**
P × N		**	**	*	ns	*	*	*

FP (flat planting without mulching), PM (ridge-furrow with ordinary mulch), and BM (ridge-furrow with biodegradable mulch) represent three cultivation patterns; N0, N1, N2, and N3 correspond to nitrogen application rates of 0, 80, 160, and 240 kg ha⁻¹, respectively. Different lowercase letters indicate significant differences among treatments (p < 0.05, Duncan’s test). P denotes the main effect of planting pattern, N represents the main effect of nitrogen level, and P × N indicates their interaction. Symbols *, **, and ns denote significance at p < 0.05, p < 0.01, and non-significance (p > 0.05), respectively.

), indicating that nitrogen fertilizer effects vary with different planting methods. Specifically, under ridge-covered ordinary film (PM), the optimal nitrogen application level (N2) synergistically enhanced yield, with annual yield significantly higher than other treatments. In contrast, excessive nitrogen application (N3) in flat planting (FP) and ridge-covered biodegradable film (BM) systems led to yield reduction (p < 0.05). From the main effects perspective, planting patterns significantly influenced yield (p < 0.01). Under equal nitrogen amounts, the PM system increased yield by 20.29% and 4.82% compared to FP and BM, respectively. Nitrogen application rates exhibited a single-peak response curve, with the N2 level reaching peak yield, increasing by 13.72–17.40%, 4.01–6.32%, and 1.89–4.01% compared to N0, N1, and N3 levels. Additionally, yield showed a systematic decline with increasing harvest times, but the PM-N2 combination maintained optimal performance across multiple harvests and total annual yield.

3.2.2. Quality

Two-way ANOVA revealed highly significant main effects of planting patterns and nitrogen application rates on lucerne quality parameters (CP, ADF, NDF, RFV) (p < 0.01, Figure 5), with significant interaction effects on CP, ADF, and RFV (p < 0.05). Under the same planting pattern, increasing nitrogen application (N0–N3) resulted in single-peak curves for CP and RFV (peak at N2), while ADF and NDF showed valley trends (lowest at N2). At fixed nitrogen levels, PM and BM systems demonstrated significantly higher CP and RFV compared to FP, with significantly lower ADF and NDF. The PMN2 treatment (ridge-covered film + 160 kg·ha⁻¹) achieved the highest CP (19.19%) and RFV (124.18), with the lowest ADF (29.63%) and NDF (48.86%), significantly outperforming all other treatments (p < 0.05). The results indicate that synergistic regulation of planting patterns and nitrogen fertilization can significantly optimize lucerne quality, with PMN2 representing the optimal combination.

3.3. Lucerne Water and Nitrogen Utilization in Response to Planting Nitrogen Application Patterns

The interaction between planting pattern and nitrogen application on WUE, PFPN, and ANUE was significant (p < 0.05), and planting pattern and nitrogen application level highly significantly affected lucerne WUE, PFPN, and ANUE (p < 0.01) (

Table 3. Lucerne water and nitrogen utilization in response to different cropping nitrogen application patterns.

Cultivation Pattern (P)	Nitrogen Level (N)	WUE (kg·ha–1·mm–1)	IWUE (kg·ha–1·mm–1)	PFPN (kg·kg–1)	ANUE (kg·kg–1)
PM	N0	2.73 ± 0.78 b	3.51 ± 0.31 c
	N1	2.99 ± 1.25 a	3.81 ± 0.11 b	172.69 ± 0.18 a	4.54 ± 0.65 b
	N2	3.05 ± 0.61 a	4.02 ± 0.66 a	91.10 ± 1.41 d	5.44 ± 0.27 a
	N3	2.95 ± 0.36 a	3.84 ± 0.32 b	58.00 ± 0.33 f	1.96 ± 0.45 ef
BM	N0	1.98 ± 0.32 de	3.08 ± 0.66 e
	N1	2.09 ± 1.19 d	3.33 ± 1.3 d	151.10 ± 6.43 b	3.81 ± 0.50 bc
	N2	2.24 ± 0.20 c	3.50 ± 0.26 c	79.51 ± 0.58 e	4.04 ± 0.45 b
	N3	1.96 ± 0.62 e	3.34 ± 0.72 d	50.61 ± 1.16 g	1.58 ± 0.24 f
FP	N0	1.39 ± 0.47 h	2.98 ± 0.59 f
	N1	1.60 ± 0.25 g	3.12 ± 0.95 e	141.83 ± 4.14 c	2.65 ± 0.68 de
	N2	1.80 ± 0.1 f	3.32 ± 0.08 d	75.73 ± 0.22 e	3.25 ± 0.22 cd
	N3	1.70 ± 0.2 fg	3.24 ± 0.37 d	49.03 ± 0.65 g	1.50 ± 0.15 f
P		**	**	**	**
N		**	**	**	**
P × N		**	ns	**	**

FP (flat planting without mulching), PM (ridge-furrow with ordinary mulch), and BM (ridge-furrow with biodegradable mulch) represent three cultivation patterns; N0, N1, N2, and N3 correspond to nitrogen application rates of 0, 80, 160, and 240 kg ha⁻¹, respectively. Different lowercase letters indicate significant differences among treatments (p < 0.05, Duncan’s test). P denotes the main effect of planting pattern, N represents the main effect of nitrogen level, and P × N indicates their interaction. Symbols ** and ns denote significance at p < 0.01 and non-significance (p > 0.05), respectively.

). At the same nitrogen application level, lucerne WUE, IWUE, PFPN, and ANUE showed PM > BM > FP; WUE and IWUE increased by an average of 80.59%, 19.9%, and 27.43%, 4.66% for PM and BM, respectively, compared with FP. The main effect of nitrogen application showed that the WUE, IWUE, and ANUE of lucerne showed a trend of increasing and then decreasing with increasing nitrogen application, and reached the maximum value under N2 conditions (p < 0.05), whereas the PFPN decreased linearly with increasing nitrogen, and the N1 was significantly higher than the N2 and N3 (p < 0.05). In conclusion, the PM mode combined with the N2 level could synergistically improve the efficiency of water nitrogen utilization.

3.4. Comprehensive Evaluation

3.4.1. Correlation Analysis

Pearson correlation analysis (Figure 6) revealed significant interparameter correlations among agronomic traits (plant height, stem diameter, yield), forage quality indices (ADF, NDF, CP, RFV), water–nitrogen use efficiencies (WUE, IWUE), and nitrogen response parameters (PFPN, ANUE) under contrasting cultivation-nitrogen regimes. The correlation coefficients between most of the indicators are above 0.500 or below −0.500, suggesting strong positive and negative correlations between indicators. Plant height exhibits a strong positive correlation with stem diameter (r = 0.73) and yield (r = 0.70), while ADF shows a strong negative correlation with CP (r = −0.62) and RFV (r = −0.72). Such correlations indicate that increases in some indicators are associated with proportional increases in others, whereas increases in some indicators are linked to decreases in others. Therefore, principal component analysis (PCA) was used for comprehensive evaluation of these variables.

3.4.2. Principal Component Analysis

As shown in the PCA results (Figure 7a), the variance contribution rates of Principal Component 1 (PC1) and Principal Component 2 (PC2) were 79.7% and 12.2%, respectively, with a cumulative variance contribution rate of 91.9%. This suggests that the two principal components explain the majority of the variation in forage-related indicators. ADF and NDF exhibited significant negative correlations with the other indicators (Figure 7a). The comprehensive scores of each treatment were ranked (Figure 7b), revealing that, among the three planting patterns, PM had the highest comprehensive score; among the four nitrogen application levels, N2 had the highest comprehensive score; and among all treatments, PMN2 achieved the highest comprehensive score, indicating it was the optimal planting and nitrogen application strategy.

4. Discussion

4.1. Synergistic Promotion Effects of Different Cropping Patterns and Nitrogen Application on Lucerne Growth

In agricultural ecosystems, the combination of ridge-furrow tillage and ridge mulching techniques has been proven to effectively regulate precipitation distribution, optimize soil microenvironments, and promote crop growth [32]. Specifically, mulching reduces soil surface evaporation, increases soil moisture content in the furrows, and creates a more favorable water environment for crop roots. The insulating properties of the mulch stabilize soil temperature fluctuations in the furrows, providing a stable microclimate for root activity. Additionally, ridge-furrow structures help reduce nutrient runoff losses and enhance nutrient use efficiency [33]. This study investigated the effects of this technique on lucerne growth and further analyzed the regulatory role of different nitrogen application rates.

The results showed that at the same nitrogen application level, compared with FP, PM increased plant height and stem diameter by 11.38–15.30% and 23.08–34.01%, respectively, while BM increased them by 2.40–4.45% and 6.04–11.31%, respectively. This result is similar to the findings of Beres et al. [34] in Canada, who found that covering biodegradable film promotes the growth and development of silage maize and consequently increases its plant height compared to no film. Similarly, Yu et al. [35] in the alpine region of Gannan found that ridge-furrow mulching significantly increased lucerne stem diameter (2.70 mm) compared to conventional planting (1.30 mm), with plant height increasing by 1.36 and 2.13 times compared to ridge planting and conventional planting, respectively. Nitrogen application rate exerted significant regulatory effects on lucerne growth architecture. Moderate nitrogen supply enhanced root hydraulic conductivity and dehydrogenase activity, thereby improving rhizosphere nutrient interception and mass flow-driven nutrient acquisition, which jointly promoted morphological development including plant height and stem diameter [36]. Lu et al. [37] documented in coastal saline-alkali soils that lucerne plant height followed a unimodal curve peaking at 225 kg ha⁻¹ N input. In our study, it was found that plant height and stem diameter showed similar secondary responses to the nitrogen application gradient (0–240 kg ha⁻¹), but the optimal threshold was moved down to 160 kg ha⁻¹ (N2). The primary reason for this difference may be the significantly lower annual precipitation (about 150 mm) in the experimental site of this study compared to 1042.2 mm in Lu et al.’s study. Under such arid conditions, moderate nitrogen application significantly improved lucerne’s water and nutrient use efficiency. Additionally, the soil in this study contained relatively high nitrogen levels, and excessive nitrogen application may have caused nutrient imbalances, ultimately inhibiting lucerne growth. In conclusion, the combination of ridge-furrow mulching technology and an appropriate nitrogen application rate can effectively improve the soil microenvironment and promote lucerne growth and development.

4.2. Effect of Cropping Pattern and Nitrogen Application on Yield and Quality of Lucerne

Forage yield, as a key productivity indicator of the grassland ecosystem, comprehensively reflects the crop’s adaptation to environmental stress and resource use efficiency [35]. The formation of lucerne hay yield is driven by transpiration throughout its growth cycle and is closely related to plant growth status, soil moisture conditions, and light-temperature environments [38]. Ridge-furrow mulching improves yield by altering soil moisture content, soil temperature, soil nutrients, wind speed, and solar radiation within the furrows [39,40]. Our study found that both ridge-furrow planting with ordinary plastic film (PM) and biodegradable film (BM) produced significantly higher yields than flat planting without mulching (FP). Farrell et al. [17] in Northern Ireland found that mulched conditions significantly increased fodder maize yields compared to open conditions, aligning with our findings. It is worth noting that the three harvests of lucerne in this study followed a decreasing yield gradient: first cut > second cut > third cut. This could be attributed to the physical deterioration of ordinary plastic film and the progressive degradation of biodegradable film over the growing season (60–70 days to initiate degradation). Biodegradable films show comparable rainwater and moisture retention effects as ordinary films during the first growing season, but their biodegradable characteristics avoid the risk of residual pollution from agricultural films and make them more friendly to the environment [41]. Our study found that lucerne yield showed an increasing and then decreasing trend with increasing N application, and excessive N application was not favorable to lucerne yield increase, which is consistent with Oliveira et al.’s [42] finding in South America that lucerne yield was not promoted by annual N application of 450 kg ha⁻¹. Therefore, it is significant to balance the nutritional effects between two nitrogen sources, exogenous nitrogen application and autochthonous rhizomatous nitrogen fixation in lucerne fields. In addition, the increase in N application to 160 kg ha⁻¹ did significantly increase lucerne yield in this study, but the increase in yield was relatively small compared to 80 kg ha⁻¹, and therefore, the economic benefits of further increasing the N application rate may be limited. Meanwhile, high N doses may lead to environmental pollution, including nitrogen loss and greenhouse gas emissions, etc. We will further explore the combined effects of different N application rates on lucerne yield and environmental impacts in future studies to provide more comprehensive and scientific fertilization recommendations.

The optimization of forage quality depends on increasing crude protein (CP) content while effectively regulating fiber components (ADF, NDF) [43]. This two-year field experiment revealed that, compared to FP, PM and BM combined with an appropriate nitrogen application rate significantly improved lucerne quality. In particular, the PMN2 treatment had the highest values of CP and RFV, and the lowest values of ADF and NDF, and all of them were significantly better than the other treatments (p < 0.05). These findings align with the meta-analysis by Yin et al. [44], which demonstrated that nitrogen application significantly increased CP content by an average of 7.3% while reducing ADF and NDF contents by 5.60% and 3.00%, respectively, compared to non-fertilized controls. Additionally, this study found that the effect of nitrogen application on quality indicators exhibited a threshold effect: With the rise of nitrogen application, the CP content and RFV rose at first and then declined. In contrast, the ADF and NDF contents showed a downward trend initially and subsequently rose. This dose–response mechanism can be explained as follows: Moderate nitrogen supply activates key enzymes such as glutamine synthetase, thereby promoting nitrogen assimilation and protein synthesis [45]. The enhanced soil moisture retention in mulched systems inhibits cell wall lignification, reducing structural carbohydrate accumulation [46]. During the early growth stage, when root nodules are not yet fully capable of nitrogen fixation, exogenous nitrogen effectively compensates for plant nitrogen demand [47]. In addition, the lucerne RFVs derived in this study show that the lucerne is a medium quality feed, and therefore more suitable for feeding animal groups such as calves aged 12–18 months, beef cows and their calves, as well as dairy cows during dry milking period, which have moderate RFV requirements for lucerne, and which are able to efficiently utilize the nutritive value of lucerne to promote their healthy growth [48].

Although PM has shown significant advantages in improving lucerne yield and quality, its potential environmental impacts cannot be ignored. The widespread use of ordinary films may lead to microplastic contamination, which can penetrate into the soil and crops, posing a threat to ecosystems and food security. In contrast, the use of BM can effectively reduce residual pollution from agricultural films and provide technical support for sustainable agricultural management. However, the high cost of BM may be an obstacle to their large-scale dissemination. Therefore, environmental benefits and economic costs need to be considered comprehensively when choosing cultivation patterns. In addition, besides nitrogen fertilizer application, phosphorus and potassium fertilizers also have irreplaceable roles in lucerne growth and development, and subsequently we also plan to study the effects of synergistic effects of different fertilizers on lucerne production performance.

4.3. Improvement Effects of Optimized Cropping Pattern and Precision Nitrogen Application on Water and Nitrogen Use Efficiency of Lucerne

Appropriate planting and nitrogen application patterns play a significant role in improving lucerne’s water and nitrogen use efficiency. WUE reflects the efficiency of water transformation into energy during crop physiological processes, while IWUE measures the contribution of irrigation water to dry matter accumulation [49,50]. PFPN indicates the fertilizer cost required to produce a given amount of product, while ANUE is a more precise indicator for evaluating the yield-increasing benefits of nitrogen fertilizer [51,52]. Ridge-furrow mulching effectively captures precipitation, enhances soil water and nitrogen availability, promotes plant growth and nitrogen uptake, and improves water use efficiency. Huo et al. [53] conducted a study on lucerne in a semi-arid region and found that, compared with FP, PM and BM increased average WUE by 35% and 31%, respectively. This study further confirmed these findings. Under the same nitrogen application conditions, compared with FP, PM and BM treatments increased WUE, IWUE, PFPN and ANUE over two growing seasons. The nitrogen application rate also had a significant impact on lucerne’s water and nitrogen use efficiency. Feng et al. [54] reported that WUE initially increased and then decreased with increasing nitrogen levels, with 80 kg·ha^–1 nitrogen being more beneficial for root water and nutrient uptake than 120 kg·ha^–1. Similarly, in Pakistan, Abbasi et al. [55] found that the highest nitrogen fertilizer use efficiency in maize was achieved at 90 kg·ha^–1 of nitrogen application and it showed a decreasing trend with the increase in nitrogen fertilizer application. This study demonstrated that as nitrogen application increased, WUE, IWUE, and ANUE initially increased and then decreased, reaching their highest values under N2 (160 kg·ha^–1), while PFPN showed a decreasing trend, peaking at N1 (80 kg·ha^–1). The results indicate that although nitrogen application at 80 kg·ha^–1 achieved the highest PFPN, its WUE, IWUE, and ANUE were significantly lower than those at 160 kg·ha^–1. This may be because the experimental site had poor soil fertility, and nitrogen limitation had a stronger influence on lucerne growth than planting patterns. Moderate nitrogen application significantly boosted yield, but further increases in nitrogen led to a slight decline in yield response, causing PFPN to decrease accordingly. Overall, the highest water and nitrogen use efficiency was observed under the PMN2 treatment (ridge-furrow planting with ordinary plastic film + 160 kg·ha^–1 nitrogen application). This result highlights the importance of selecting an appropriate planting pattern and nitrogen application rate in practical agricultural production to achieve efficient utilization of water and nitrogen resources while maximizing crop yield.

In large-scale lucerne cultivation, mechanized management is a necessary means to achieve efficient production. As the level of intelligence and mechanization matures, in order to solve the problems of mechanized harvesting and film management, it is necessary to develop lucerne harvesters equipped with precision cutting devices and automatic row-alignment functions, so that they can be effectively adapted to the topography of ridges and reduce the interference with the soil and crops. The difficulty of mechanized operation can be further reduced by optimizing the design of the irrigation system (rational layout) and adjusting the time of harvesting operation (e.g., when the soil is dry after irrigation). Second, to avoid damage to the film from mechanical operations, it is recommended that the equipment be adjusted prior to harvesting to ensure smooth operation, or that more efficient and environmentally friendly biodegradable films be developed. Film recycling can be carried out by a combination of mechanization (collecting most of the film debris) and manual assistance (supplemental processing of the parts that are difficult to mechanically recycle), while encouraging the research, development and promotion of more efficient agricultural film recycling machinery and technology.

5. Conclusions

This study demonstrated that the ridge-mulching system significantly increased lucerne productivity compared to conventional flat planting (FP), with ordinary mulch (PM) being superior to biodegradable mulch (BM) in terms of synergistic effects. At 160 kg ha⁻¹ (N2), lucerne plant height and stem thickness reached the peak, and the PMN2 treatment (ridge-furrow ordinary mulch + 160 kg ha⁻¹ N) achieved the optimization of morphology and yield-quality, with a significant increase in yield, crude protein, and relative feeding value, and simultaneous decrease in acidic and neutral detergent fiber fractions, and optimal water and nitrogen use efficiency, which is a suitable planting and nitrogen application mode for lucerne production in the Gansu Yellow River Irrigation Region and analogous ecoregions.

However, this study has geographical limitations, it does not quantify the long-term ecological risks of PM-treated microplastics and the impact of BM degradation products, and the suitability of ridge mulching with mechanized harvesting needs to be optimized. In the future, we plan to validate the adaptability to multiple climatic zones and develop equipment adapted to ridge and furrow harvesting in order to minimize the mechanical damage to the mulch caused by multiple mowing. In addition, policy guidance is needed to gradually shift to biodegradable film mulching in order to balance production efficiency and environmental sustainability.