Coupling nitrogen and irrigation under high-retention film mulching regulates plant growth, forage quality, and soil nutrient and carbon dynamics in maize

Abstract

Introduction:

Film mulching, nitrogen input, and irrigation interactively regulate crop productivity and soil carbon sequestration in arid maize systems, yet their combined effects on aboveground–belowground growth, forage quality, and soil carbon fraction dynamics remain insufficiently understood.

Methods:

A two-year field experiment was conducted in the Hexi Corridor of China using two film mulching types, three nitrogen application levels, and two irrigation regimes. We systematically evaluated maize yield, belowground biomass, plant and root traits, forage quality, soil physicochemical properties, and soil organic carbon fractions.

Results:

After two consecutive years, high-retention film increased yield by 9.6% relative to low-retention film, but reduced belowground biomass under equivalent water and fertilizer conditions. Under deficit irrigation, reduced fertilization decreased yield but increased belowground biomass, indicating enhanced carbon allocation to roots under resource limitation. Root traits were more responsive than shoot traits to water–fertilizer coupling, with moderate fertilization promoting greater root length and root volume. Fertilization was the main driver of forage quality and soil nutrient status, whereas irrigation mainly affected acid detergent fiber (ADF), and neutral detergent fiber (NDF), and available nutrient pools. Compared with the conventional control treatment (T7), T4 increased Soil Total Nitrogen (STN) by 4.6%, Total Phosphorus (STP) by 7.6%, and Available Nitrogen (SAN) by 13.7%. High-retention film also promoted soil carbon accumulation, with Soil Organic Carbon, Particulate Organic Carbon (POC), and Mineral-Associated Organic Carbon (MAOC) under T4 increasing by 7.1%, 17.4%, and 4.5%, respectively, relative to T7. Under moderate fertilization, no significant differences were detected between T5 and T7, suggesting that moderate nitrogen reduction under high-retention film can maintain soil fertility and carbon stocks. Redundancy analysis further identified SAN, STP, and MAOC as the major factors driving variation in maize productivity and belowground biomass.

Conclusions:

Overall, high-retention film improved maize yield, soil fertility, and soil carbon accumulation more effectively than low-retention film under comparable water and nitrogen inputs, while moderate urea reduction under high-retention film maintained productivity and soil fertility, highlighting its potential as a resource-efficient strategy for sustainable arid maize production.

1 Introduction

Cropland is among the most essential productive land resources in terrestrial land-use systems, and plays an irreplaceable role in sustaining food supply, supporting forage production, and agricultural sustainability (Potapov et al., 2022; Kopittke et al., 2019). Globally, agricultural land covers approximately 4.8 billion hectares, of which about 1.6 billion hectares are cropland, representing nearly one-third of the total agricultural land area. As the core basis of food production and agroecosystem functioning, cropland not only provides grain, forage, and other agricultural products, but also contributes substantially to soil health maintenance, nutrient recycling, and regional ecological security. Its productivity and soil quality largely determine the carrying capacity, resilience, and long-term stability of agricultural systems (Bagnall et al., 2021). In China, cropland is of particular strategic importance because it underpins national food security and the stable supply of major agricultural products (Liu et al., 2017; Liang et al., 2022). By the end of 2023, China’s cropland area had reached 1.929 billion mu (approximately 128.6 million hectares), and the country continues to implement the strictest cropland protection policies. For decades, China has fed nearly 20% of the world’s population with about 9% of the world’s cropland resources, highlighting the fundamental importance of cropland conservation and quality improvement for ensuring food security and advancing sustainable agricultural development (Turup et al., 2025; Liang et al., 2022).

However, under the combined pressures of climate change and intensifying resource and environmental constraints, cropland systems are facing multiple challenges, including water scarcity, low nutrient use efficiency, and declining soil organic matter, particularly in arid and semi-arid irrigated regions, where the trade-off between maintaining stable crop production and improving soil fertility is becoming increasingly pronounced (Zou et al., 2021; Quan et al., 2024). Irrigation and fertilization are among the most direct management practices for increasing crop yield. Nevertheless, although the conventional high-input management based on excessive irrigation and fertilization can maintain high productivity in the short term, it is often accompanied by greater evaporative losses, enhanced deep percolation, increased risks of nitrogen leaching, and imbalances in soil nutrient pools, thereby weakening the long-term sustainability of cropland systems (Li et al., 2022; Wei et al., 2024). Therefore, optimizing water–fertilizer management by matching fertilizer inputs with water availability is essential for reducing inputs without compromising production efficiency. Such an approach is critical for sustaining crop productivity, improving resource use efficiency, and enhancing soil health in arid cropland systems (Zou et al., 2021; Quan et al., 2024).

In arid cropland production systems, water scarcity and high evaporative demand strongly constrain crop establishment, nutrient acquisition, and yield formation. Film mulching has therefore been widely adopted to regulate the soil hydrothermal environment, reduce non-productive evaporation, and promote early crop growth (Quan et al., 2022; Xiao et al., 2023). Meta-analyses conducted across China have shown that, compared with no mulching, plastic film mulching increases crop yield by approximately 26.0%–39.5% and water use efficiency by 33.0%–37.9% compared with no mulching, with particularly strong benefits under cool, arid, and resource-limited conditions (Huang et al., 2024; Xiao et al., 2023). Similar responses have been reported in maize, where film mulching increased grain yield by 23.9%–36.3% and water use efficiency by 33.8%–39.6% under rainfed conditions (Hossain et al., 2022). These benefits are mainly attributed to improved root-zone water and temperature conditions. By increasing topsoil water retention and nitrate-N availability, film mulching can promote root growth, dry matter accumulation, and plant nitrogen uptake, especially in water-limited agroecosystems (Ma et al., 2018; Quan et al., 2022). Thus, film mulching enhances crop performance not only by conserving soil water, but also by strengthening the coupling between root development, nutrient acquisition, and biomass accumulation. Compared with conventional plastic film, high-retention mulching materials may provide a more stable root-zone hydrothermal environment, thereby improving the effective use of irrigation water and precipitation. Moreover, by altering soil moisture, temperature, salinity, and SOC dynamics, such materials may further regulate soil carbon, nitrogen, and phosphorus transformations and microbially mediated nutrient cycling processes (Quan et al., 2024). Therefore, evaluating the effects of high-retention film mulching on crop growth, resource-use efficiency, and soil nutrient dynamics is essential for developing more sustainable production strategies in arid cropland systems.

The Hexi Corridor is a typical oasis irrigated agricultural region in northwestern China and plays a strategically important role in maize production and seed security. Zhangye, located in the central Hexi Corridor, is one of the major national seed maize production bases in China. Seed maize produced in the Hexi Corridor supplies more than 56% of the seed used for field maize production nationwide, and the artificial oasis croplands in the middle reaches of the Heihe River are recognized as one of the largest seed maize production centers in China (Chen et al., 2023; Yang et al., 2015). However, agricultural production in this region is strongly constrained by water scarcity. The Hexi Corridor is characterized by a typical temperate continental arid climate, with annual precipitation of only about 70–210 mm, whereas annual evaporation reaches 1500–2300 mm, creating a pronounced mismatch between natural water supply and crop water demand (Gu et al., 2017; Yang et al., 2015). In recent decades, groundwater storage in the Hexi Corridor have continued to decline, with an estimated depletion rate of 0.26 cm yr^-¹ from 2003 to 2023, indicating increasing pressure on irrigation-dependent agricultural systems (Zhao et al., 2025). Therefore, maintaining stable seed maize production while reducing water and fertilizer inputs, has become a major challenge for the sustainable agriculture in the Hexi Corridor. Improving resource-use efficiency and preserving preserving cropland quality are thus essential for reconciling regional agricultural productivity with long-term water-resource security (Chen et al., 2023; Zhao et al., 2025).

Previous studies have examined the individual effects of film mulching, irrigation, or nitrogen application on maize yield, water use efficiency, and nitrogen use efficiency (Qin et al., 2015; Li et al., 2020). However, limited attention has been paid to integrated management strategies that combine the replacement of conventional plastic film with high-retention film and reduced nitrogen input, particularly from a multidimensional perspective linking plant growth, biomass accumulation, changes in soil carbon, nitrogen, and phosphorus pools, and soil microbial responses (Qin et al., 2015; Dong et al., 2017; Li et al., 2020). To address this knowledge gap, the present study was conducted in seed maize croplands of the Zhangye oasis irrigated area in the Hexi Corridor. Based on a two-year continuous field experiment, we established a coordinated management regime combining high-retention film mulching with reduced nitrogen application and systematically evaluated maize growth, biomass accumulation, soil physicochemical properties, soil nutrient status and soil carbon fractions. The objectives were to clarify how high-retention film mulching combined with reduced nitrogen input regulates yield formation, resource-use efficiency and soil quality in seed maize systems, and to provide a theoretical basis and technical support for water-saving, fertilizer-reducing and soil fertility-enhancing strategies in the Hexi Corridor and other arid irrigated agricultural regions.

2 Materials and methods

2.1 Study site description

The field experiment was conducted during 2024–2025 in the oasis-irrigated region of Zhangye, Gansu Province, China. The experimental site was located at the Water-Saving Irrigation Experimental Research Center in Zhangye City, in the middle reaches of the Hexi Corridor (100°22′20.2″ E, 38°51′20.0″ N; 1561 m a.s.l.). The region is dominated by alluvial–diluvial plains and has well-developed irrigation infrastructure, making it representative of oasis-irrigated agricultural systems in arid northwestern China. The climate is classified as temperate continental arid, characterized by abundant solar radiation, high evaporative demand, low precipitation, and uneven seasonal rainfall distribution, resulting in a strong dependence of crop production on irrigation. The long-term mean annual precipitation is approximately 130.4 mm, and the mean annual air temperature is 7.3 °C. The site receives approximately 3065.6 h of sunshine annually, with an accumulated temperature above 10 °C of 3085 °C·d, and an annual evaporation of 2002.5 mm (Li and Wang, 2021).

2.2 Experimental design and treatment arrangement

This study was conducted to evaluate water–nitrogen coupling effects on drip-irrigated seed maize under high-retention film mulching in an oasis irrigated-region. A factorial treatment structure was established by combining film mulching type, nitrogen application rate, and irrigation regime. Under otherwise uniform field management, seven treatments were established: T1, high-retention film + high nitrogen + deficit irrigation; T2, high-retention film + medium nitrogen + deficit irrigation; T3, high-retention film + low nitrogen + deficit irrigation; T4, high-retention film + high nitrogen + full irrigation; T5, high-retention film + medium nitrogen + full irrigation; T6, high-retention film + low nitrogen + full irrigation; and T7, low-retention film + high nitrogen + full irrigation. Each treatment had three replicates, and each plot measured 5 m × 4 m. The experiment was arranged in a randomized block design, with a 1 m buffer zone between adjacent plots to minimize water and nutrient movement among treatments.

2.3 Field management practices

2.3.1 Film mulching and sowing

The seed maize hybrid used in this study was Y2515. Sowing was conducted on April 25, 2024, and April 29, 2025, and harvesting was performed on September 17, 2024, and September 18, 2025, respectively. Seed maize was planted using a female–male row arrangement, with a female-to-male row ratio of 6:1. The male parent was sown on May 4, 2024, and May 14, 2025. Row spacing, plant spacing, planting density and sowing depth were 50 cm, 20 cm, 99,000 plants ha^-¹ and 5 cm, respectively. A white high-retention plastic film was used for mulching, 1.0 m in width and 0.01 mm in thickness. All other field management practices, including insect and weed control, followed local conventional practices.

2.3.2 Fertilization regime

All treatments received basal applications of diammonium phosphate (N + P₂₅ > 64%; N–P₂₅–K₂O = 18–46–0) and compound fertilizer. Urea (N ≥ 46%) was used as the nitrogen source for topdressing. Three nitrogen application levels were established: high (H, 450 kg ha^-¹), medium (M, 375 kg ha^-¹), and low (L, 300 kg ha^-¹). Nitrogen was applied at the jointing, grain-filling, and milk stages at a ratio of 4:3:3. Potassium fertilizer was also topdressed at the grain-filling stage at the same rate across all treatments.

2.3.3 Irrigation regime

Subsurface drip irrigation under plastic mulch was adopted. Two irrigation regimes were established: well-watered (WW) and water-deficit (WD). Soil moisture was maintained at 75%–95% of field capacity (θf) under WW and at 55%–75% of field capacity under WD. Irrigation water was supplied from a motor-pumped well, which pressurized the drip irrigation system and delivered water to the field drip lines through PVC pipelines. Irrigation volume was controlled using a water meter, and irrigation was applied whenever the monitored soil moisture content reached the lower threshold of the predefined field-capacity range.

2.4 Measurement of plant traits and sample collection

2.4.1 Assessment of growth traits

Plant sampling and trait measurements were conducted in September 2024 and September 2025 for all treatments. In 2024, only biomass was determined. Four representative plants were selected from each plot using fixed-point or diagonal sampling while avoiding border rows (n = 4). Shoots and roots were collected separately for dry matter determination. In 2025, agronomic traits were additionally recorded. Similarly, four representative plants were selected from each plot to determine plant height, stem diameter, number of fully expanded leaves, and leaf area, and shoot and root samples were collected for determining shoot and root dry weight.

Plant height was measured from the soil surface to the top of the main ear. Stem diameter (cm) was measured at 5 above the ground using a vernier caliper. The number of fully expanded leaves was recorded as the number of functional leaves that had completely unfolded. Leaf area was measured using a leaf area meter (LI-3000C, LI-COR, USA). Shoot samples were first heated at 105 °C for 30 min to stop enzymatic activity and then oven-dried at 65–75 °C to constant weight. Root samples were collected by whole-plant excavation within the 0–40 cm soil layer, thoroughly washed, oven-dried at 65–75 °C to constant weight, and weighed to determine root dry weight (g).

2.4.2 Root morphological traits

Root morphological traits were measured in September 2025. Three representative plants were selected from each plot, and intact root systems were collected by whole-plant excavation within the 0–40 cm soil layer. After carefully washing away adhering soil, root samples were analyzed using a root scanning and image analysis system (Regent Instruments Inc., Canada). The measured traits included root length, projected area, surface area, root volume, and root tip number.

2.4.3 Forage quality analysis

Forage quality was analyzed in September 2025. Representative aboveground samples were collected from each plot avoiding diseased or insect-damaged tissues. Samples were oven-dried at 65–75 °C to constant weight, ground, and passed through a 0.25 mm sieve before analysis. The measured quality parameters included crude protein (CP), ash, acid detergent fiber (ADF), and neutral detergent fiber (NDF). Crude protein was determined using the Kjeldahl method based on total nitrogen concentration and calculated as CP = N × 6.25. Ash content was determined by combusting a known mass of sample in a muffle furnace at 550 °C for 4–6 h, followed by cooling and reweighing. ADF and NDF were determined using the Van Soest detergent fiber method.

2.4.4 Soil sampling and analysis

Soil samples were collected in September 2025. In each plot, soil from the 0–40 cm layer was collected using a five-point composite sampling method and thoroughly mixed to obtain one composite sample per plot. After transport to the laboratory, samples were air-dried, and visible plant residues and stones were removed. The dried soil was then ground and sieved, with 2 mm sieves used for routine physicochemical analyses and 0.149 mm sieves used for elemental analyses.

Soil total carbon was determined using an elemental analyzer by the dry combustion method. Total nitrogen (STN) was measured using the Kjeldahl method. Total phosphorus (STP) was determined by the molybdenum–antimony colorimetric method with spectrophotometric detection. Total potassium (STK) was measured by flame photometry. Available nitrogen (SAN) was determined using the alkaline hydrolysis diffusion method. Available phosphorus (SAP) was measured by the molybdenum–antimony colorimetric method. Available potassium (SAK) was extracted with 1 mol L^-¹ NH₄OAc and determined using flame photometry.

2.5 Statistical analysis

All data are presented as mean ± standard deviation. One-way analysis of variance (ANOVA) was used to evaluate the effects of different treatments on maize yield, belowground biomass, plant traits (plant height, stem diameter, leaf number, and leaf area), root morphological traits (root length, surface area, projected area, volume, and root tip number), forage quality parameters, and soil nutrient characteristics. Two-way ANOVA was further used to test the effects of nitrogen level, and their interaction, and their interaction on maize growth, root traits, forage quality, and soil nutrient characteristics using treatments T1–T6. All statistical analyses were performed using SPSS 26.0 (SPSS Inc., Chicago, IL, USA), and figures were generated using Origin 2024 (OriginLab, Northampton, MA, USA).

For forage quality evaluation, crude protein (CP) was treated as a positive indicator, whereas crude fiber (CF), ash, acid detergent fiber (ADF), and neutral detergent fiber (NDF) were treated as negative indicators. After standardization of the variables, principal component analysis (PCA) was conducted. The first two principal components both had eigenvalues greater than 1 and together explained 75.46% of the total variance, indicating that they captured the main variation in forage quality. The forage quality index (FQI) was calculated as:

where is the variance contribution rate of the k-th principal component, and is the principal component score of sample I on the k-th principal component.

Partial factor productivity of nitrogen (PFPN) was calculated as the ratio of grain yield to nitrogen application rate:

where Y represents grain yield (kg ha^-¹) and N represents nitrogen input (kg ha^-¹).

Irrigation water use efficiency (IWUE) was calculated as:

where: IWUE is irrigation water use efficiency (kg grain m^-³), Y is grain yield (kg ha^-¹), I is seasonal irrigation amount (m³ ha^-¹).

3 Results

Irrigation regime and nitrogen application rate significantly affected maize yield and belowground biomass (P < 0.05). The interaction between irrigation and fertilization significantly affected maize yield in both 2024 and 2025 (P < 0.05), whereas its effect on belowground biomass was significant only in 2025 (P < 0.05, Figure 1). Under the same irrigation and nitrogen conditions, maize yield under the high-retention film treatment (T4) was significantly higher than that under the low-retention film treatment (T7), whereas belowground biomass was significantly lower in T4 than in T7. Under deficit irrigation, maize yield decreased significantly with decreasing nitrogen application rate. Under full irrigation, however, yield was significantly lower under the low nitrogen treatment (L) than under the high nitrogen treatment (H) only in 2024. In both years, belowground biomass increased significantly with decreasing nitrogen application rate under both deficit and full irrigation conditions.

Figure 1

Irrigation significantly influenced maize plant height (PH), stem diameter (SD), leaf number (LN), and leaf area (LA) (P < 0.05), whereas the irrigation × fertilization interaction was significant only for PH and SD (P < 0.05, Figure 2). Under the same irrigation and nitrogen conditions, LA was significantly greater under high-retention film mulching (T4) than under low-retention film mulching (T7), whereas the other shoot traits did not differ significantly between the two film treatments. Under deficit irrigation, PH decreased significantly with decreasing nitrogen application rate, while SD, LN, and LA showed decreasing but non-significant trends (P > 0.05). In contrast, under full irrigation, nitrogen application rate had no significant effect on any measured shoot traits (P > 0.05).

Figure 2

Fertilization had significant effects on CP, ash, ADF, and NDF (P < 0.05), whereas irrigation significantly affected only ADF and NDF (P < 0.05). A significant irrigation × nitrogen interaction was observed for CP, ADF, and NDF (P < 0.05, Figure 3). Under both deficit and full irrigation regimes, CF and ash generally increased with decreasing nitrogen application rate, whereas NDF increased only under the medium nitrogen level. Under the same irrigation and nitrogen conditions, the high-retention film treatment (T4) did not differ significantly from the low-retention film treatment (T7) in CP, ash, ADF, or NDF (P > 0.05).

Figure 3

Fertilization significantly affected root volume (V) only (P < 0.05), whereas irrigation had significant effects on root length (L) and root volume (V) (P < 0.05). The interaction between irrigation and nitrogen significantly affected root traits (P < 0.05, Figure 4). Under deficit irrigation, root morphological traits increased only under the medium nitrogen treatment (M, T2) as nitrogen application rate. Under full irrigation, root length (L), projected area (PA), and root volume (V) increased only under the medium nitrogen treatment (M, T5). Under the same irrigation and nitrogen conditions, the high-retention film treatment (T4) showed significantly greater root length (L) and root volume (V) than the low-retention film treatment (T7) (P < 0.05), whereas no significant differences were observed between T4 and T7 in projected area (PA), surface area (SA), or root tip number (TN).

Figure 4

Fertilization had significant effects on soil nutrients (P < 0.05), whereas irrigation significantly affected only STN, SAN, SAP, and SAK (P < 0.05). No significant irrigation × nitrogen interaction was observed for any soil nutrient variable (Figure 5). Under both deficit and full irrigation, soil nutrient contents generally decreased with decreasing nitrogen application rate. In addition, under the medium nitrogen treatment (M), no significant differences were found in SOC, STN, STP, or STK between deficit and full irrigation, whereas SAN, SAP, and SAK were all significantly higher under full irrigation than under deficit irrigation (P < 0.05). Under the same irrigation and nitrogen, soil nutrient contents in the high-retention film treatment (T4) were consistently and significantly higher than those in the conventional film mulching (T7) (P < 0.05).

Figure 5

Fertilization significantly affected soil carbon fractions and their ratios (P < 0.05), whereas irrigation significantly affected POC, POC: SOC, MAOC: SOC, and POC: MAOC only (P < 0.05). A significant irrigation × nitrogen interaction was observed for POC alone (Figure 6). Under the same irrigation and nitrogen conditions, the high-retention film treatment (T4) showed significantly higher POC and MAOC contents than the conventional film mulching (T7). At the same nitrogen level, POC was significantly higher under full irrigation than under the corresponding deficit irrigation treatment, whereas MAOC did not differ significantly between the two irrigation regimes. Under both irrigation regimes, POC decreased significantly with decreasing nitrogen application rate, while MAOC decreased significantly only under the low nitrogen treatment (T3 and T6) (P < 0.05).

Figure 6

Correlation analysis showed that forage yield was positively correlated with V, PH, SD, LN, LA, STN, STP, STK, SAN, SAP, SAK, and POC, but negatively correlated with ash, ADF, and BGB (P < 0.05, Figure 7). In contrast, BGB showed significant negative correlations with PH, LA, SOC, STN, STP, STK, SAN, SAP, SAK, POC, and forage yield, and significant positive correlations with ash and ADF (P < 0.05).

Figure 7

Principal component analysis (PCA) of forage quality traits showed that the first principal component (PC1) and second principal component (PC2) explained 43.9% and 31.6% of the total variance, respectively, with a cumulative contribution of 75.5% (Figure 8A). Based on the forage quality index (FQI), the treatments were ranked as follows: T4 > T7 > T1 > T6 > T3 > T2 (Figure 8B). Redundancy analysis (RDA) showed that the first ordination axis (RDA1) and second ordination axis (RDA2) explained 54.9% and 22.7% of the total variation, respectively, together accounting for 77.6%. The contribution analysis further indicated that the main soil and root traits associated with aboveground performance were SAN, STP, MAOC, STN, TN, MAOC: SOC, L, SOC, POC: MAOC, and V, with contribution rates of 29.1%, 28.6%, 16.3%, 7.6%, 4.5%, 2.7%, 2.5%, 2.3%, 2.2%, and 1.7%, respectively (Figure 8C).

Figure 8

Nitrogen application and irrigation regime significantly interacted to influence PFPN and IWUE (Figure 9). In both years, the highest PFPN was observed in the low-nitrogen treatment under full irrigation (T6), whereas the highest IWUE occurred in the high-nitrogen treatment under deficit irrigation (T1) (P < 0.05). Under identical irrigation and nitrogen conditions, PFPN and IWUE were higher in T4 than in T7, although the differences were not significant (P > 0.05). Notably, PFPN under moderate nitrogen input (T5) was significantly greater than that under T7. Moreover, under deficit irrigation, IWUE in T1, T2 and T3 was significantly higher than that in T7 (P < 0.05).

Figure 9

4 Discussion

4.1 Effects of film mulching, nitrogen application, and irrigation on aboveground and belowground biomass

Increases in aboveground biomass primarily reflect enhanced assimilate accumulation under improved resource availability (Teixeira et al., 2014; Jin et al., 2022). Under the same fertilization and irrigation conditions, maize yield under high-retention film was 9.6% higher than that under conventional film after two consecutive years of application. Moreover, under full irrigation, yields in the reduced-fertilization treatments (T5 and T6) remained significantly higher than that in T7, indicating that high-retention film improved crop productivity by enhancing the effective use of water and fertilizer through its stronger heat-retention and moisture-conservation effects. This is consistent with previous studies showing that film mulching can increase crop yield and water use efficiency in multi-year field systems (Zhang et al., 2020; Hossain et al., 2022).

Changes in belowground biomass reflected adaptive plant responses to soil water and nutrient availability (Song et al., 2024). Under full irrigation and high fertilization, belowground biomass under high-retention film was significantly lower than that under conventional film, whereas it increased progressively as fertilization decreased and eventually showed no significant difference from T7. This suggests that when nutrient supply becomes limiting, maize allocates more carbon belowground to enhance root foraging capacity and compensate for reduced soil resource availability (Lemaire et al., 2019). The increase in belowground biomass under reduced fertilization may reflect both enhanced root investment for nutrient acquisition and a shift in biomass allocation caused by reduced shoot growth. Under nutrient-limited conditions, maize typically increases root foraging capacity to improve resource uptake (Guo and York, 2019), while reduced shoot growth may further increase the relative contribution of roots to total biomass through altered shoot–root allocation patterns (Pausch and Kuzyakov, 2018).

Water availability was the primary factor limiting maize growth in this arid irrigated system. Accordingly, maize yield under deficit irrigation was significantly lower than that under full irrigation across all fertilization levels. However, after two consecutive years of high-retention film application, yield in the high-fertilization deficit-irrigation treatment (T1) did not differ significantly from that in the conventional-film, high-fertilization, and full-irrigation treatment. This indicates that high-retention film partly alleviated water limitation by improving soil moisture conservation and crop water use efficiency, thereby sustaining relatively high productivity under restricted water supply, This response can be attributed to the improved soil water availability under high-retention film, which supports stomatal conductance, maintains photosynthetic activity, and promotes canopy development, ultimately enhancing biomass accumulation under water-limited conditions (Wang et al., 2011; Zhang et al., 2017).

Roots are responsible for water and nutrient uptake, whereas aboveground traits such as plant height, stem diameter, leaf number, and leaf area determine canopy structure, light interception, and dry matter production. Together, root and shoot traits reflect the coordinated growth strategy of maize under different mulching, irrigation, and fertilization regimes (Chapman et al., 2012; Vetterlein et al., 2022). In this study, irrigation significantly affected all aboveground traits, indicating that water supply dominated vegetative growth and canopy development. The irrigation × fertilization interaction was significant only for plant height and stem diameter, suggesting that these traits were more sensitive to coordinated changes in water and nutrient availability. Under the same irrigation and fertilization conditions, only leaf area was significantly greater in T4 than in T7, indicating that high-retention film mainly promoted canopy expansion by improving the soil hydrothermal environment (Lu et al., 2020; Wu et al., 2024).

Compared with shoot traits, root responses were more strongly expressed in resource-acquisition traits, particularly root length and root volume. The significant irrigation × fertilization interaction for root traits further suggests that belowground growth was more sensitive to water–nitrogen coupling than shoot development. Under both deficit and full irrigation, moderate fertilization (T2 or T5) was more favorable for improving several root traits. Moreover, under the same irrigation and fertilization conditions, root length and root volume were significantly greater in T4 than in T7, indicating that high-retention film enhanced not only leaf expansion aboveground but also root extension and root system development belowground (Lu et al., 2020).

Overall, irrigation was the primary factor governing maize growth, whereas the effect of fertilization depended on adequate water supply and followed an optimization pattern rather than a simple linear increase with input level (Zhang et al., 2021; Ran et al., 2023). Aboveground improvement was mainly expressed as increases in plant height, stem diameter, and leaf area, reflecting enhanced light capture and assimilatory capacity, whereas belowground responses were mainly reflected in greater root length and root volume, indicating stronger soil exploration and resource acquisition. These findings suggest that maize responses to mulching, irrigation, and fertilization result from coordinated regulation between canopy construction and root expansion. High-retention film improved the soil microenvironment, thereby promoting leaf area development and photosynthetic potential aboveground while enhancing root exploration belowground. The stronger root development under moderate fertilization further suggests that appropriate nutrient supply can stimulate belowground carbon allocation and promote coordinated improvement in aboveground and belowground growth (Hirte et al., 2018; Bai et al., 2021). This response can be attributed to improved soil moisture availability under high-retention film, which enhanced canopy development and biomass accumulation (Zhang et al., 2017). In contrast, the increase in belowground biomass under stress conditions reflects an adaptive response through enhanced carbon allocation to root growth for improved resource acquisition (Heinemann et al., 2025; Liu et al., 2025).

The contrasting responses of PFPN and IWUE highlight the importance of coordinated water–nitrogen management under high-retention film mulching. PFPN was highest under the low-nitrogen full-irrigation treatment (T6), indicating improved nitrogen productivity under adequate water supply, whereas IWUE was highest under the high-nitrogen deficit-irrigation treatment (T1), reflecting enhanced irrigation water productivity under limited water conditions. This is consistent with previous studies showing that deficit irrigation can improve maize water productivity by regulating crop water consumption and optimizing water allocation during key growth stages (Zou et al., 2021; Jia et al., 2017). In addition, PFPN and IWUE were generally higher under high-retention film than under conventional film, suggesting that improved soil hydrothermal conditions enhanced the coordination between water availability and nitrogen uptake (Ma et al., 2021; Hou et al., 2023). These results demonstrate that high-retention film combined with moderate nitrogen reduction can support efficient maize production in arid irrigated systems.

4.2 Patterns of forage quality variation and their relationship with plant growth

Forage quality is a key indicator of the feeding value of maize and is jointly regulated by water availability, nutrient supply, and cultivation practices. Crude protein (CP) reflects forage protein value, whereas acid detergent fiber (ADF) and neutral detergent fiber (NDF) indicate fiber composition and digestibility; higher ADF and NDF generally reduce palatability and digestible utilization (Ma et al., 2023). In this study, fertilization significantly affected CP, ash, ADF, and NDF, whereas irrigation significantly affected only ADF and NDF, and the irrigation × fertilization interaction was significant for CP, ADF, and NDF. These results indicate that fertilization was the primary factor regulating maize forage quality, while fiber components were more sensitive to water–nutrient interactions. This is because nitrogen supports chlorophyll formation, amino acid synthesis, and enzyme activity, thereby promoting protein accumulation and improving forage quality and nutrient accumulation (Xu et al., 2012).

Consistent with previous studies, appropriate nitrogen supply increased CP and reduced ADF and NDF, thereby improving forage nutritive value (Ma et al., 2023). By contrast, CF and ash increased as fertilization rate decreased, suggesting that insufficient nutrient input constrained nutrient accumulation and increased the relative proportion of structural and mineral components. The significant irrigation effects on ADF and NDF further indicate that water availability plays an important role in regulating cell wall deposition and tissue maturation. This agrees with earlier studies showing that drought stress can alter forage quality by affecting nutrient accumulation and lignification processes (Rad et al., 2021; Niu et al., 2023).

Notably, NDF increased only under moderate nitrogen supply, suggesting that the response of maize fiber components to fertilization was not linear but had an optimum range. In contrast, under the same fertilization and irrigation conditions, no significant differences were observed between the high-retention film treatment (T4) and the low-retention film treatment (T7) in CP, ash, ADF, or NDF. This indicates that the direct effect of film type on forage quality was limited, and that its contribution was more likely mediated through improved soil conditions and biomass production rather than through direct changes in nutrient composition or fiber structure (Rad et al., 2021).

4.3 Soil nutrient responses and soil–plant feedback mechanisms

Soil nutrients responded clearly to film mulching, fertilization, and irrigation, with fertilization remaining the dominant driver of soil fertility. As fertilization decreased, soil contents levels generally decreased under both deficit and full irrigation, highlighting the critical role of external nutrient inputs in maintaining total nutrient pools and plant-available nutrient availability. This is consistent with previous studies showing that appropriate N, P, and K inputs under film-mulched maize systems enhance nutrient uptake and resource use efficiency (Li et al., 2023). In contrast, irrigation mainly affected STN, SAN, SAP, and SAK, suggesting that water availability regulates soil fertility primarily through its effects on nutrient transport, dissolution, and bioavailability, while exerting limited influence on relatively stable pools such as STP and STK (Quan et al., 2024; Zhang et al., 2025).

Under moderate fertilization, SOC, STN, STP, and STK did not differ significantly between irrigation regimes, whereas SAN, SAP, and SAK were significantly higher under full irrigation, indicating that sufficient water supply mainly promotes the release and maintenance of available nutrients rather than substantially altering total nutrient pools. Under the same fertilization and irrigation conditions, soil nutrient contents were consistently higher under high-retention film than under low-retention film, indicating that high-retention film enhanced nutrient retention by improving the soil hydrothermal environment and reducing evaporative and nutrient losses. This creates more favorable soil conditions for sustained maize growth, consistent with previous reports in mulched maize systems (Li et al., 2023; Zhang et al., 2025).

Fertilization also exerted the strongest influence on soil carbon fractions, whereas irrigation mainly regulated POC and its proportion within SOC, indicating differential responses of labile and stable carbon pools to management. As fertilization decreased, POC decreased significantly under both irrigation regimes, while MAOC decreased significantly only under low nitrogen supply. This suggests that POC is more sensitive to short-term management changes and external carbon inputs, whereas MAOC is relatively stable and responds more slowly to variation in water and nutrient supply (Mustafa et al., 2021; Zhang et al., 2022). Under identical irrigation and fertilization conditions, both POC and MAOC were significantly higher under high-retention film than under low-retention film, indicating that high-retention film promoted not only labile C accumulation but also stable C formation. In addition, POC was significantly higher under full irrigation than under deficit irrigation at the same fertilization level, whereas MAOC did not differ significantly, suggesting that adequate water supply mainly stimulates labile carbon accumulation through enhanced plant growth, root inputs, and microbial turnover, while MAOC formation depends more on sustained carbon inputs and mineral protection processes (Ma et al., 2020; Ding et al., 2022; Zhang et al., 2022).

RDA further showed that SAN, STP, and MAOC were the key variables explaining variation in maize aboveground growth. This indicates that available N determines immediate nutrient supply, total P reflects baseline soil fertility, and MAOC contributes to soil structure, nutrient buffering, and long-term fertility maintenance. STN, SOC, and POC: MAOC also contributed to growth variation, suggesting that total nutrient reserves, soil carbon status, and carbon fraction distribution jointly regulate crop performance. Although root length and root volume contributed less than soil variables, they still had positive effects, indicating that soil resource availability must ultimately be translated into plant uptake through root system expansion. Overall, maize aboveground growth was jointly determined by nutrient availability, soil carbon pool quality, and root resource-acquisition capacity, rather than by any single factor alone (Lynch, 2011; Oldfield et al., 2019).

5 Conclusions

This study showed that film mulching, nitrogen input, and irrigation jointly regulated maize productivity and soil processes, but through distinct pathways. Irrigation primarily controlled biomass production and morphological development, fertilization mainly governed forage quality and soil nutrient status, whereas high-retention film improved system performance by enhancing water retention, stabilizing the soil hydrothermal environment, and increasing water–fertilizer use efficiency. After two consecutive years, high-retention film significantly increased maize yield compared with low-retention film under the same fertilization and irrigation conditions, while also promoting soil nutrient retention and the accumulation of SOC, POC, and MAOC. Under deficit irrigation, reduced fertilization decreased yield but increased belowground biomass, indicating greater carbon allocation to roots under resource limitation. Redundancy analysis identified SAN, STP, and MAOC as the main factors explaining variation in maize productivity and belowground biomass, highlighting the joint roles of nutrient availability and soil carbon dynamics in regulating crop performance. Overall, high-retention film combined with appropriate irrigation improved yield, enhanced soil fertility, promoted soil carbon accumulation, and strengthened water–nitrogen use efficiency in arid maize systems. Moreover, under high-retention film, moderate urea reduction did not reduce maize yield or soil fertility, indicating that this management strategy can support efficient production with lower fertilizer input.

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