Advances in Water and Nitrogen Management for Intercropping Systems: Crop Growth and Soil Environment

Abstract

Intercropping is an eco-friendly, sustainable agricultural model that significantly improves yield stability, nutrient use efficiency, and soil health through spatiotemporal niche complementarity, increases biodiversity, and improves soil health. Water and nitrogen play crucial roles in limiting and regulating efficient resource utilization and ecological sustainability in intercropping systems. Synchronizing water and nitrogen inputs to match crop demands optimizes the spatiotemporal distribution of these resources, alleviates interspecific competition, and promotes mutualistic interactions, which significantly impacts crop growth, yield, and soil environment. This paper reviews the mechanisms of intercropping and water–nitrogen coupling regulation, aligning water and nitrogen supply with crop growth patterns, spatial configuration parameters, irrigation management techniques, and environmental climate change, and explores the response mechanisms of water–nitrogen coupling on crop growth, yield, and soil environmental adaptation. It can provide some references for researchers, extension agents, and policymakers. Research indicates that water–nitrogen coupling can enhance photosynthetic efficiency, promote root development, optimize nutrient uptake, and improve soil water dynamics, nitrogen cycling, and microbial community structures. Intercropping enhances the climate resilience of agricultural systems by leveraging species complementarity for resource utilization, strengthening ecosystem stability, and improving buffering capacity against climate change impacts such as extreme precipitation and temperature fluctuations. Future studies should further elucidate the differential effect of water–nitrogen coupling across regions and climatic conditions, focusing on multidimensional integrated administration strategies. Combining precision agriculture technologies and climate change predictions facilitates the development of more adaptive water–nitrogen coupling models to provide theoretical support and technical guarantees for sustainable agriculture.

1. Introduction

Intercropping refers to the simultaneous cultivation of two or more crop species on the same field with specific row arrangements or spatial configurations [1,2], representing an eco-friendly, sustainable agricultural approach that achieves spatiotemporal niche complementarity [3]. The tall and short crops utilize solar energy in a stratified manner [4], which significantly improves resource utilization efficiency [5] and effectively promotes biodiversity [6,7], concomitantly improving soil health [8]. The stability of this agroecosystem increases its climate resilience while showing promise for carbon sequestration [9,10]. Moreover, intercropping systems reduce agrochemical dependency, which helps sustain yield stability and minimizes the environmental impact [11]. This approach has become essential in the ecological intensification of agriculture. Studies have demonstrated that intercropping can enhance community productivity by optimizing the utilization of plant-available resources and fostering positive interactions, which can ameliorate conditions under high abiotic stress [12].

Water primarily limits efficient resource utilization in intercropping systems in three ways [13]. First, the stratified root systems (e.g., deep-rooted maize with shallow-rooted soybeans) enable vertical water gradient utilization, intercropping enhancing water use efficiency (WUE) by 15–20% compared to monocultures [14]; intercropping can effectively improve WUE by 29.46% [13]. Second, the mitigation of interspecific water competition has been quantitatively validated. Zhang reported that intercropping yields surpassed those of monocultures by 60.38% in arid zones and 56.45% in semi-arid zones [15]. Third, optimized water control synergistically enhances light interception and nutrient uptake efficiency. Silva demonstrated that adjusting planting density and intercropping row spacing improved water productivity by 6.89 kg/m³ [16]. Ultimately, improving system productivity via effective water regulation enhances the soil health benefits of intercropping by mediating root exudate interactions and microbial community structure, serving as a crucial link between the ecological processes above and below the ground [17]. For instance, the increased crop yield and resource use efficiency in maize/soybean intercropping systems can be attributed to aboveground and belowground interspecific interactions, as well as the impacts of root growth and distribution on land and water productivity [18]. Scientific water management is pivotal for achieving synergistic yield enhancement and ecological benefits in intercropping systems.

Nitrogen is the core regulatory factor for achieving both resource efficiency and ecological sustainability in intercropping systems. Legume crops can replace 30–50% of chemical nitrogen fertilizers through symbiotic nitrogen fixation. Tsialtas employed ¹⁵N tracing to show that over 65% of nitrogen was transferred from peas to oats [19]. Research has shown that interspecific nitrogen transfer mechanisms increase the total soil nitrogen and aggregate content in maize–soybean intercropping systems by 9% and 22%, respectively [20], while precision nitrogen control during intercropping reduces the synthetic N input by 17–25% and maintains yield parity [21]. Compared to monoculture, intercropping systems display a 15.8% reduction in nitrate leaching potential (p < 0.05), with soil hydraulic regulation representing the primary factor influencing leaching fluxes [22].

The simultaneous regulation of water and nitrogen plays a particularly significant role in intercropping systems. By optimizing resource allocation among crops, intercropping systems alleviate water and nitrogen resource scarcity to some extent [23]. The rational management of water–nitrogen coupling can significantly promote crop growth and yield [24], alleviate competition among crops, promote mutualistic symbiosis, and further enhance overall system productivity [25], providing a solid foundation for sustainable agricultural development. Efficient resource utilization can be achieved in intercropping systems through optimal water–nitrogen coupling.

This paper systematically elucidated the intercropping system and the mechanisms behind water–nitrogen coupling regulation. It comprehensively reviews and synthesizes the effect of crop combinations, spatial configuration parameters, irrigation management techniques, and environmental climate changes on water–nitrogen regulation in intercropping. Additionally, it summarizes the response mechanisms of water–nitrogen coupling on crop growth and yield in intercropping systems, as well as its impact on soil environment dynamics and adaptation. Figure 1 presents the structural diagram of this paper. Finally, key areas requiring further research are proposed.

2. Intercropping and Water–Nitrogen Coupling Regulation Mechanisms

2.1. Intercropping Mechanisms: Complementarity and Competition

Complementarity and competition among crops constitute the core mechanisms for efficient resource utilization in intercropping systems. Complementarity is primarily reflected in the optimized spatiotemporal allocation of water and nitrogen resources between crops, while competition may constrain microbial nitrogen fixation and phosphorus utilization due to overlapping resource demands [26]. Furthermore, the spatial differentiation between the root systems of different crops can effectively reduce competition for water and nitrogen resources. Shen examined a maize–soybean intercropping system and found that vertical root stratification significantly minimized competition for both water and nitrogen, which enhanced resource use efficiency [27]. The phenological asynchrony of simultaneously cultivated crops further enhances temporal complementarity in resource utilization. In rainfed wheat–maize intercropping systems, wheat exhibits higher water demand during its early growth stages, while maize enters a rapid growth phase after wheat harvest. This staggered resource utilization pattern significantly improves both land use efficiency and total system productivity [28].

However, the competitiveness in intercropping systems cannot be ignored, especially in conditions with limited water and nitrogen resources. Research has shown that the competition among crops during intercropping intensifies significantly in resource-limited conditions, which inhibits the growth of certain crops [29]. Furthermore, the strong competitiveness of wheat in wheat–maize intercropping systems may inhibit maize recovery growth, particularly in nitrogen-deficient conditions, where this competition becomes even more pronounced [30]. Similarly, in dryland farming systems where legumes are intercropped with cereals, competition for water may reduce the nitrogen-fixing capacity of legume root nodules, compromising their ecological functions [31].

The synergistic effect of photosynthesis and root activity also reflects the complementarity of intercropping systems. Research has shown that in maize-based intercropping systems, maize reduces the transpiration rate of associated crops through shading effects, while the associated crops enhance the nitrogen uptake capacity of the maize via root exudates. Likewise, maize and its intercropped companions can utilize light energy in a stratified manner, thereby enhancing the total photosynthetic efficiency of the system [32]. This synergy significantly improves the water and nitrogen use efficiency (NUE) of the system [33]. Root plasticity plays a crucial role in intercropping systems. In resource-limited conditions, intercropped soybeans enhance their capacity to absorb the water and nitrogen in the topsoil by increasing their root length density and expanding their root distribution range, consequently compensating for their competitive disadvantage in aboveground growth [18].

The interspecific interactions in intercropping systems are also significantly influenced by environmental conditions. The intense competition among intercropped species in semi-arid regions can be mitigated effectively by optimizing planting density and spatial arrangement. For instance, adopting ridge–furrow cultivation systems with sorghum–legume intercropping, which rationally configure row spacing and plant spacing, significantly improves soil moisture status, reduces water competition between crops, and simultaneously enhances the 30% WUE in wheat-based systems [34]. In the maize–soybean intercropping system, nitrogen fixation by soybean rhizobia can reduce nitrogen fertilizer input and enhance NUE through nutrient complementarity, while the difference in root depth distribution between maize and soybean enables stratified water uptake, reducing evaporation and competition, thereby improving WUE. In dryland farming systems, the allocation mechanisms of water and nitrogen resources in intercropping systems are further regulated by soil microbial communities. Research indicates that intercropping significantly enhances both the diversity and activity of soil microorganisms. This not only improves the mineralization and transformation capacity of soil nitrogen, but also facilitates complementary nutrient use among crops [35].

2.2. The Regulatory Mechanism Behind Water–Nitrogen Coupling in Intercropping Systems

Effective water–nitrogen coupling optimizes when and where crops access these resources of water and nitrogen to increase efficiency and sustainability during crop production [36]. Research has shown that the rational allocation of water and nitrogen resources can significantly enhance the efficiency of crop photosynthesis and water utilization. The optimization of water–nitrogen distribution regulates root architecture and interspecies interactions in intercropping systems, consequently reducing resource competition while enhancing the complementary effects [37]. Reducing irrigation and applying biochar in faba bean–ryegrass intercropping systems enhances leaf-level WUE by regulating stomatal conductance and transpiration rates, while simultaneously improving biological nitrogen fixation capacity [38]. Similarly, optimizing irrigation scheduling and nitrogen application rates can significantly enhance the crop yield and NUE in maize–soybean intercropping systems, particularly in drought years [39].

The optimization of fertilization and irrigation strategies is critical for water–nitrogen coupling regulation. Research demonstrates that different irrigation regimes and fertilization levels significantly influence crop growth and soil environments [40]. A study on the Loess Plateau investigated apple–maize and apple–soybean intercropping systems exposed to three irrigation approaches (flood irrigation, drip irrigation, and rainfed conditions) and three fertilization levels to assess their effect on soil moisture, nutrient dynamics, and root distribution. The research established optimal water–fertilizer control practices for these intercropping systems [41]. A study employing two wheat varieties intercropped with peas under contrasting irrigation regimes (rainfed vs. drip irrigation) revealed that while different wheat–pea cultivar combinations responded variably to irrigation systems, intercropping consistently improved soil moisture status and increased crop yields [42].

Photosynthesis, nutrient uptake, and soil microbial activity represent the primary regulatory mechanisms behind water–nitrogen coupling during the physiological and ecological processes in crops. Research has found that optimizing the allocation of water and nitrogen resources in intercropping systems significantly improves photosynthetic performance and nitrogen uptake efficiency in crops. A study on the high-efficiency nutrient utilization in Panicum miliaceum L. intercropping systems showed that appropriate nitrogen management increased the net photosynthetic rate and WUE, while simultaneously improving the soil conditions [43]. Another study indicated that water–fertilizer coupling technology improved the fruit quality in an apple–soybean strip intercropping system and significantly enhanced the soil nutrient content, as well as the net crop photosynthetic and transpiration rates [44]. Under water–nitrogen coupling, changes in the community structure of nitrogen-fixing bacteria and other microorganisms enhance nitrogen transformation efficiency, while their secretions promote the decomposition of organic matter, improving nutrient availability [45]. Meanwhile, the elongation of root length and increase in root surface area expand the absorption range, and the development of aerenchyma adapts to water variations, regulating the balance of nutrient uptake [18]. Figure 2 illustrates the interaction process of water–nitrogen coupling mechanisms in intercropping systems.

3. The Effect of Different Conditions on Water–Nitrogen Coupling Regulation in Intercropping Systems

An optimized intercropping community can simultaneously improve the spatial distribution of soil nutrients and enhance nitrogen metabolism efficiency through root interactions [46]. As the primary regulatory factor, water control directly impacts resource transformation efficiency [47]. It has been confirmed that intercropping can effectively reduce NO₃-N leaching by 15.8% [22]. Climate variability imposes multidimensional challenges on intercropping systems, where shifting temperature and precipitation patterns reshape crop phenological rhythms, indirectly regulate nutrient uptake dynamics, and ultimately affect resource use efficiency [48]. Crop combination relationships, spatial configuration parameters, irrigation management techniques, and environmental climate changes are crucial for water–nitrogen regulation in intercropping systems.

3.1. Irrigation Methods and Water Application Rates in Intercropping Systems

Irrigation mode selection significantly influences the synergistic efficiency of water–soil resources in intercropping systems. Targeted water supply strategies can optimize crop growth dynamics by regulating water–fertilizer coupling relationships. Precision irrigation technologies, represented by drip and micro-sprinkler irrigation, effectively suppress unproductive water dissipation and improve the spatial distribution of nutrients. In drip irrigation conditions, appropriate nitrogen levels positively affect root morphology, physiology, and dry matter accumulation in spring wheat [49]. Research showed that combining micro-sprinkler irrigation with foliar nutrient regulation increased the water output efficiency and biomass water productivity in a cotton–wheat rotation system by 46.1% and 5.7%, respectively, compared to flood irrigation [50]. Furthermore, controlled irrigation enhanced the water–fertilizer use efficiency (WFUE).

Irrigation scheduling in intercropping systems necessitates the integrated consideration of crop water requirements and soil water retention characteristics. Trials of fruit-grain intercropping on China’s Loess Plateau demonstrated that drip irrigation significantly improved the soil moisture status (0–60 cm depth) compared to flood irrigation, with the ammonium-N, nitrate-N, and organic matter content increasing by 21–68.4% [51]. This management model maintains superior soil hydro-nutrient conditions during crop transition phases. Subsurface drip irrigation (SSDI) with optimized water–fertilizer coordination reduced the irrigation water usage in cotton–wheat rotations by 43.2% and enhanced system productivity [50]. Research showed that optimized subsurface drip fertigation (SSDF) reduced water usage in cotton–wheat systems by 45.9–58.8% compared to traditional flood irrigation, while increasing economic returns. The intensity of interspecific competition for resources highlighted the role of irrigation in regulating the factors that influence the efficiency of crop combinations [52].

Precision irrigation regulates nitrogen cycling through targeted supply and migration control. Drip irrigation synchronizes nitrogen delivery with water movement, while combining winter green manure cover with summer drip irrigation significantly enhances tomato yield (+62% shoot biomass) and nitrogen uptake efficiency (2.5-fold increase) [53]. Different water supply modes also affect root architecture and nutrient dynamics. Drip irrigation with an optimized application of 92 kg/hm² nitrogen combined with phosphorus and potassium fertilizers yielded the best overall results during the rotation period of an apple–soybean intercropping system [54].

Regulating the irrigation volume directly affects crop growth processes. Its correlation with resource use efficiency is a key issue in water and nitrogen management research for intercropping systems. Scientifically controlled irrigation can both meet the physiological water demand of plants and effectively improve the water–nitrogen coordination efficiency. However, excessive irrigation beyond crop uptake capacity not only leads to water resource wastage, but also accelerates nitrate leaching into deeper soil layers, ultimately reducing the overall system benefits. Studies confirm that formulating precise irrigation schemes requires a comprehensive analysis of crop water consumption patterns, soil water retention characteristics, and regional meteorological factors, optimizing resource allocation by combining multidimensional parameters [39].

Water regulation in arid and semi-arid regions presents greater technical challenges. For example, combining supplemental irrigation with lower nitrogen application levels significantly improved the WUE and NUE in the maize–soybean intercropping system on the Loess Plateau by 12% and 18%, respectively [39]. This case study revealed that moderately increasing the irrigation frequency effectively improved the crop water status in water-stressed conditions, while synchronously reducing nitrogen input helped mitigate nutrient leaching risk. A SSDI trial in a cotton–wheat rotation system demonstrated that adopting a water supply strategy involving 80% of crop evapotranspiration (ETC) saved 43.2% of irrigation water and enhanced crop water productivity (CWP). These findings provide empirical support for simultaneously implementing water-saving irrigation and precision fertilization in water-scarce regions.

Crop growth cycles and dynamic soil moisture variations have a time-dependent impact on irrigation scheduling. Experimental data from sugar beet cultivation demonstrated that the I1N1 treatment (50% ETC irrigation coupled with 30 kg/ha nitrogen application) reduced water and fertilizer utilization while maintaining optimal economic yield and quality parameters [47]. These results indicate that moderate water control during critical growth stages, combined with low-nitrogen control, enhances WUE and improves crop quality traits. Similar patterns were observed in maize–soybean intercropping systems, where the synergistic effect of supplemental irrigation and reduced nitrogen application significantly increased system productivity.

Soil water regulation capacity and climate adaptability are essential for developing irrigation strategies. Case studies conducted in the Loess Plateau demonstrated that supplemental irrigation in low water-retention areas effectively mitigated crop water stress, while coordinated nitrogen reduction minimized nutrient leaching risks. Furthermore, the results of cotton–wheat rotation research revealed that combining moderate deficit irrigation with precision fertilization in water-scarce ecosystems conserved water, improved efficiency, and enhanced yield. This systematic water–nitrogen synergy management approach provides a technical pathway for sustainable agricultural development.

Table 1. Synergistic regulation of water–nitrogen interactions in intercropping systems across different regimes.

Intercropped Species	Cropping System	Climate Conditions	Nitrogen Content	Irrigation System	Optimal Water–Nitrogen Coupling Index	References
Maize–soybean	Sole maize, sole soybean, and maize–soybean intercropping	Semi-humid drought-prone region	The maize received nitrogen at application rates of 240, 180, and 120 kg N/ha, while the soybeans were uniformly treated with 120 kg N/ha	Rainfed and supplemental irrigation (30 mm)	A 25% reduction in the maize nitrogen application (to 180 kg N/ha) enhanced the partial factor productivity of nitrogen (PFPN), while supplemental irrigation increased yields by 17.24–31.16% and improved economic returns.	[39]
Maize–soybean	Sole maize, sole soybean, and maize–soybean intercropping	Semi-arid region	No nitrogen input and conventional farmer nitrogen application rate	Rainfed	The land equivalent ratio (LER) and water equivalent ratio (WER) of the intercropping system were both 1.10. The intercropped crops exhibited temporal complementarity in water utilization, while the nitrogen application rate showed no significant effect.	[18]
Maize–soybean	Sole maize, sole soybean, and maize–soybean intercropping	Semi-humid region	Six nitrogen application rates: 0 kg N/ha, 135 kg N/ha, 216 kg N/ha, 270 kg N/ha, 324 kg N/ha, and 405 kg N/ha	Rainfed, additional 1–2 irrigation events	In deficit irrigation conditions, the recommended nitrogen application rate for the subsequent winter wheat crop was 135 kg N/ha.	[55]
Maize–soybean	Maize–soybean intercropping	Humid region	Three nitrogen application rates: N1 (60 kg N/ha), N2 (120 kg N/ha), and N3 (240 kg N/ha)	Four water regimes: Well-watered (60–70% the field capacity (FC)), mild drought (45–55% FC), moderate drought (30–40% FC), and severe drought (15–25% FC)	The availability of water and nitrogen was used to develop a “spectral reflectance-physiological parameters-productivity” model to predict maize yield and protein content.	[56]
Wheat–maize	Sole wheat, sole maize, and wheat–maize intercropping	Arid region	Both the wheat and maize were subjected to three nitrogen treatments: N0 (0 kg N/ha), N1 (180 kg N/ha), and N2 (300 kg N/ha)	Rainfed	The intercropped wheat exhibited a 5.2–16.9% improvement in WUE, while the maize in the intercropping system showed a 3.2% yield increase under N2 treatment (180 kg N/ha).	[57]
Wheat–maize	Sole wheat, sole maize, and wheat–maize intercropping	Arid region	The experiment included three nitrogen application levels: N0 (control): no nitrogen fertilization, N1: moderate rates of 150 kg N/ha for the wheat and 235 kg N/ha for the maize, N2: high rates of 300 kg N/ha for the wheat and 470 kg N/ha for the maize	Rainfed and supplemental irrigation	Supplemental irrigation increased the wheat yields by 28.7% compared to 30.7% in rainfed conditions, while the intercropped maize showed yield enhancements of 7% and 4% in irrigated conditions at N1 (150 kg N/ha) and N2 (300 kg N/ha) levels, respectively.	[58]
Wheat–maize	Sole wheat, sole maize, and wheat–maize intercropping	Non-irrigated arid region	No nitrogen (N) application (N0), and N application at 150 kg N/ha for the wheat and 235 kg N/ha for the maize (N1)	Rainfed	The wheat in the intercropping system with nitrogen application exhibited a 52.78–70.37% higher yield than the monocultured wheat, while showing a 33.60–52.78% yield increase compared to the no-nitrogen control treatment.	[30]
Wheat–maize	Sole wheat, sole alfalfa, and wheat–maize intercropping	Semi-arid region	Nitrogen application rates: 150 kg N/ha and 235 kg N/ha	Rainfed	From 2019 to 2020, intercropping treatments increased the protein content by an average of 76.43% and 83.72%, respectively, compared to the zero-nitrogen control. Nitrogen application further enhanced the WUE in the intercropping systems by 30.54%.	[59]
Maize–peanut	Sole peanut, sole maize, and maize–peanut intercropping	Semi-arid region	No nitrogen (0 kg/ha), medium nitrogen (100 kg/ha), and high nitrogen (200 kg/ha)	Rainfed	During the microplot experiment, the number of root nodules in intercropped peanuts was 51.6% higher than in the monocropped peanuts. In the field experiment, the nodule number and single-nodule weight in the monocropped peanuts were 48.7% and 58.9% higher than in the intercropped peanuts, respectively.	[60]
Chickpea–wheat	Sole chickpea, sole wheat, and chickpea–wheat intercropping	Semi-arid region	Nitrogen (N) application rates: Low (30 kg N/ha), medium (60 kg N/ha), and high (100 kg N/ha)	Rainfed	A WUE of 0.62 kg/m3 significantly increased the NUE by 1 kg/kg.	[61]
Wheat–chickpea	Sole chickpea, sole wheat, and wheat–chickpea intercropping	Semi-arid and semi-humid regions	N-30, N-60, and N-100 represent nitrogen application rates of 30 kg N/ha, 60 kg N/ha, and 100 kg N/ha, respectively	Rainfed	In N-30 conditions, the wheat chlorophyll content increased by 7.8%, while the highest chickpea protein yield was evident at the N-60 level, with semi-humid regions demonstrating superior WUE.	[62]
Maize–alfalfa	Sole maize, sole alfalfa, and maize–alfalfa intercropping	Semi-arid region	High nitrogen (HN: 150 kg N/ha) and low nitrogen (LN: 90 kg N/ha)	Rainfed	Nitrogen reduction mitigated interspecific competition and significantly increased the LER, reaching a maximum of 1.18.	[63]
Rice–cowpea and rice–adzuki	Sole rice, rice–cowpea, and rice–adzuki intercropping	Humid region	5 t/ha farmyard manure (FYM) inorganic fertilizer (60–30–30 kg/ha NPK) 5 t/ha FYM + 50% inorganic fertilizer (30–15–15 kg/ha NPK)	Rainfed	The crop leaf area showed significant differences among the nitrogen treatments (p < 0.05), with the combined FYM and inorganic fertilizer application achieving optimal yield.	[64]
Apple–maize and apple–soybean	Apple–maize intercropping, apple–soybean intercropping in the same field	Semi-humid region	No fertilization (F0), 375 kg/ha (F1), and 750 kg/ha (F2)	Rainfed, drip irrigation, and flood irrigation, maintained at 50% and 80% of FC, respectively	Drip irrigation increased the yields by 1.6% and 11.8%, respectively, compared to flood irrigation. Both irrigation methods maintained an upper limit of 80% FC with nitrogen application at 412.4 kg/ha.	[41]
Pea–barley	Sole pea, sole barley, and pea–barley intercropping	Four distinct sites in arid regions	The sole barley received 115 kg N/ha, while the sole pea crop was fertilized with 130 kg N/ha. In the intercropping system, the N application rates were adjusted to 28.75 kg N/ha for the barley and 97.5 kg N/ha for the peas	One experimental site received limited growing season irrigation (37 mm), while all other trial sites were maintained in rainfed conditions	The intercropping system achieved the highest NUE of 5.07 kg grain N per kg soil N in high rainfall conditions, while exhibiting peak WUE at 1.40 kg/m3 during low rainfall periods.	[65]
Elymus nutans–M. sativa	Sole Elymus nutans, sole M. sativa, and Elymus nutans–M. sativa intercropping	Arid and semi-arid regions	Soil nitrogen content	The precipitation treatments included: (1) the control (240 mm, simulating local annual rainfall), (2) +50% irrigation (360 mm), and (3) −50% irrigation (120 mm), regulated by an automated rain-out shelter	Compared to the monocultures, the proportion of nitrogen uptake by M. sativa from the soil decreased by 4.5%, 4.2%, and 5.4% under −50%, CK (control), and +50% treatments, respectively, in the intercropping system.	[66]
Saffron–wheat	Sole saffron and saffron–wheat intercropping	Semi-arid region	Fermented cattle manure and chemical urea	40%, 60%, 80%, and 100% of standard crop evapotranspiration (ETc)	It is recommended to adopt a saffron–wheat intercropping system, apply organic cattle manure, and implement an irrigation regime at 60% of standard crop ETc.	[67]
Faba bean–ryegrass	Faba bean–ryegrass intercropping	Humid region	Standard nitrogen application	The experiment compared full irrigation, deficit irrigation, and partial root-zone drying (PRD)	The combination of biochar and deficit irrigation increased the soil water content (SWC) by 17% under high-temperature biochar treatment, while the ryegrass exhibited a 52% higher nitrogen use efficiency (NUE) compared to the faba beans.	[38]
Apple–soybean	Apple–soybean intercropping	Semi-humid region	59.40 kg/ha, 92 kg/ha, and 124.32 kg/ha	60%, 70%, 80%, and 90% FC	A nitrogen application rate of 92 kg/ha and maintaining the soil moisture level at 80% of the FC improved the yield and facilitated optimal water–nitrogen use efficiency.	[54]
Apple–soybean	Apple–soybean intercropping	Semi-humid region	Nitrogen application rates: 59, 92, and 124 kg N/ha	60%, 70%, 80%, and 90% FC	Combining 92 kg N/ha nitrogen application and irrigation at 80% FC enhanced the net photosynthetic rate and transpiration rate of the crop, while alleviating interspecific nutrient competition.	[44]
Apple–maize	Apple–maize intercropping	Semi-humid region	NPK fertilization levels (289–118–118 kg/ha, 412.4–168.8–168.8 kg/ha, and 537–219–219 kg/ha, respectively)	50%, 65%, and 85% FC	Combining an NPK application rate of 144.5–59–59 kg/ha with irrigation at 85% FC significantly enhanced the crop yield and facilitated optimal water–fertilizer use efficiency (WFUE).	[68]
Maize–wheat, maize–soybean, and wheat–soybean	Maize–wheat, maize–soybean, and wheat–soybean intercropping	Semi-arid region	In Experiment 2, the nitrogen application rates were set at N1 (138 kg N/ha) and N2 (276 kg N/ha)	Experiment 3 employed two soil moisture gradients: 40% and 80% of SWC	The increased nitrogen application rates mitigated yield reduction in the intercropping system, while phosphorus addition and enhanced soil water availability collectively decreased both the yield and biomass production.	[69]
Legume–oilseed crop	Sole lentil, sole pea, sole oilseed crop, lentil–oilseed crop intercropping, and pea–oilseed crop intercropping	Semi-arid region	Nitrogen application treatments: Basal application at sowing (50 kg N/ha), topdressing at five weeks after sowing (50 kg N/ha), zero nitrogen control (0 N), and 15N-labeled urea ammonium nitrate (UAN)	Two water regimes: drought (non-irrigated) and irrigation	The %Ndfa increased from 77% (lentil monocrop) to 87% (lentil–oilseed intercrop), and from 66% (pea monocrop) to 76% (pea–oilseed intercrop). However, N fertilization at 50 kg/ha significantly reduced the LER from 1.25 to 1.10 (p = 0.03), indicating lower complementarity. Notably, the yield stability under water stress was 23% higher during intercropping than in the monocultures (p < 0.01)	[70]
Maize–navy bean	Sole maize, sole navy bean, and maize–navy bean intercropping	Semi-humid region	Three nitrogen application rates: 50 kg N/ha, 150 kg N/ha, and 200 kg N/ha	Three water regimes: 200–300 mm, 300–400 mm, and >400 mm	Higher water and nitrogen supply reduced the advantages during intercropping, showing a decline from 28% to 6% for aboveground biomass, a decrease from 40% to 7% for grain yield, a decline from 41% to 0.3% for protein yield, and a decrease from 40% to 9.2% for energy yield.	[24]
Lespedeza davurica–Bothriochloa ischaemum	Lespedeza davurica–Bothriochloa ischaemum mixed cropping	Semi-arid region	Three nitrogen application levels: 0 g/kg, 0.025 g/kg, and 0.025 g N/kg + 0.1 g P/kg	80%, 60%, and 40% FC	The leaf nitrogen concentration reached a minimum at 40% FC.	[71]
Apple–maize and apple–soybean	Apple–maize intercropping and apple-soybean intercropping	Semi-humid region	Three nitrogen application rates: 0 kg N/ha, 375 kg N/ha, and 750 kg N/ha	Two irrigation methods: drip irrigation and flood irrigation. Three water regimes based on the FC: 0% FC (rainfed), 50% FC, and 80% FC	In Years 3 and 4, the optimal treatment combination consisted of drip irrigation, 750 kg N/ha nitrogen application, and irrigation maintained at 80% FC.	[51]
Apple–maize	Apple–maize intercropping	Semi-humid region	Three NPK compound fertilizer application rates: 70%, 100%, and 130% of the recommended standard dose	Three water regimes: 50%, 65%, and 85% of the FC	Combining 50–65% FC irrigation with a 70% fertilizer application rate achieved optimal crop yield, irrigation water use efficiency (IWUE), and partial nutrient factor productivity.	[72]
Winter wheat–saffron	Winter wheat–saffron intercropping	Semi-arid region	Two nitrogen fertilizers: Organic manure (OM) and urea	Four irrigation levels: 40%, 60%, 80%, and 100% of the standard saffron crop ETc	The irrigation regime of 40–60% of the standard saffron crop ETc achieved the highest WUE, while organic fertilizer application enhanced the saffron yield.	[73]
Cotton–wheat	Cotton–wheat intercropping	Semi-arid region	Two nitrogen application rates: 80% and 100% of the recommended N rate	Three subsurface drip irrigation (SSDI) levels: 60%, 80%, and 100% of crop ETc	Combining SSDI with 80% of the crop ETc and 80% of the recommended nitrogen rate increased system productivity by approximately 12.3%.	[50]
Cotton–wheat	Cotton–wheat intercropping	Semi-arid region	Cotton nitrogen treatments: 100% of the recommended N rate (125 kg N/ha) and 125% of the recommended N rate (156 kg N/ha) Wheat NP treatments: 100% of the recommended NP rate (125–62.5 kg NP/ha) and 80% of the recommended NP rate (100–50 kg NP/ha)	Two SSDI depths: 25 ± 2.5 cm and 30 ± 2.5 cm, as well as surface flood irrigation (control)	The optimal system yield and water productivity were achieved with SSDI laterals buried at a depth of 25 ± 2.5 cm and emitters spaced 30 cm apart, combined with cotton receiving 125 kg N/ha and wheat receiving 125–62.5 kg NP/ha.	[52]
Maize–pigeon pea	Sole maize, sole pigeon pea, Maize–pigeon pea intercropping	Humid region	Three nitrogen application rates: 0 kg N/ha, 20 kg N/ha, and 80 kg N/ha	Three water management practices: Ridge tillage, open tillage, and flat tillage	Ridge tillage increased the maize yield by 0.3 t/ha compared to flat tillage, while the recommended fertilizer application further enhanced the yield by 1.60 t/ha.	[74]
Maize–soybean	Maize–soybean intercropping	Semi-arid and semi-humid regions	Three nitrogen application rates: 240 kg N/ha, 180 kg N/ha, and 120 kg N/ha	Two supplemental irrigation levels were applied during the maize jointing stage (soybean flowering stage): 0 mm (rainfed control) and 30 mm	The field trial data were used to calibrate the APSIM model, optimizing supplemental irrigation and nitrogen application rates for the maize–soybean intercropping system.	[75]
Maize–soybean	Maize–soybean intercropping	County-level data in the United States (2008–2020)	County-level crop irrigation water use and nitrogen input data were extracted from specific studies	County-level crop irrigation water use and nitrogen input data were extracted from specific studies	In 1.5 °C (3 °C) warming scenarios, maize irrigation demand increased by 62% (67%), while soybean irrigation decreased by 65% (58%). Concurrently, nitrogen application rates increased by 4% (13%) for maize and 10% (130%) for soybeans.	[76]

provides a comprehensive review of multiple key factors influencing intercropping systems: in addition to different irrigation conditions, different planting conditions and climatic conditions also have a certain impact on intercropping systems.

3.2. The Diversity of Intercropping Systems

The water and nitrogen utilization mechanisms in intercropping systems exhibit multidimensional characteristics and are primarily driven by the interaction between species and spatiotemporal diversity during resource acquisition. Legume–cereal intercropping leverages complementary functional traits, such as symbiotic nitrogen fixation and deep-rooted architectures, to enhance soil nitrogen availability and WUE. This system simultaneously enhances nitrogen bioavailability through biological fixation while reducing reliance on synthetic fertilizers. The spatial configuration and planting density of a crop further regulate the water–nitrogen efficiency. Optimized planting designs establish a balance between light interception and resource competition, significantly increasing system productivity [63]. Studies showed that wheat–chickpea intercropping in low nitrogen conditions enhanced WUE by 15% and NUE by 20%, with particularly pronounced benefits in semi-arid regions [62]. Integrating a limited irrigation and reduced nitrogen (N) strategy into a maize–soybean intercropping system yielded a WUE of 1.65 kg/m³ and a NUE of 45.6 kg/kg, representing an increase of 12% and 18%, respectively, compared to monocultures, while the economic returns rose by 14.31–22.06% [39].

Root system architecture and species diversity form the basis for resource utilization differences. Legumes with shallow root systems preferentially absorb surface nitrogen sources, while gramineous crops with deep root networks exploit water and nutrient resources in deeper soil layers, creating a three-dimensional utilization pattern. For instance, in wheat–maize intercropping, wheat demonstrates strong competitive advantages during the co-growth phase, while maize achieves compensatory growth through precise water and fertilizer control after wheat harvest. This ultimately enhances the total system productivity by 14.8%, with an LER reaching 116% [58]. Moderate nitrogen fertilization increased the leaf area index and growth biomass in a chickpea–durum wheat intercropping system by 25% and 30%, respectively, while the nitrogen uptake was more pronounced during high-rainfall years [61].

Species combinations must strike a balance between resource competition and synergy. Although high-temperature biochar enhances leaf gas exchange in the faba bean–ryegrass system, it concurrently suppresses WUE. However, PRD irrigation mitigates this adverse effect and enhances symbiotic nitrogen fixation efficiency [38]. Legume-associated maize increases grain biomass and yield by 23.5% and 17.2%, respectively, while reducing the rust disease incidence by 34.6% and significantly enhancing the zinc, copper, and iron concentrations. Micronutrient accumulation shows a significant negative correlation with disease suppression [77].

The enhancement of WUE and NUE in intercropping systems relies on the synergistic optimization of planting density and spatial arrangement. By regulating plant spacing, interspecific resource competition can be mitigated for balanced nutrient allocation. When maize is planted at a density of 60,000 plants per hectare in the maize–soybean intercropping system, the soybeans receive more light, increasing the yield by 12% and improving the NUE by 15% [78]. The structural optimization of spatial arrangements has a systematic impact on resource allocation. Compared to row intercropping, the strip intercropping pattern with alternating rows of maize and soybeans demonstrates superior light interception and spatial root distribution. This configuration promotes complementary root architecture, improving the soil nutrient utilization, total system yield, WUE, and NUE by 14%, 8.6%, 10.3%, and 12.7%, respectively [79].

Cropping patterns equally modulate microbial community dynamics and nutrient cycling processes. In the wheat–maize–soybean relay intercropping system, straw incorporation and nitrogen fertilization enhance the soil organic matter content by 16%, while increasing the mean weight diameter and geometric mean diameter of soil aggregates by 6% and 9.1%, respectively, and elevating the microbial activity by 31.1% [80]. This bio-physical coupling effect forms the basis for sustained system enhancement.

In terms of water–fertilizer synergy regulation, the strip intercropping pattern increased the WUE–NUE by 18.5% in limited water conditions (60% ETc) while reducing the nitrogen leaching risk by 39% [55,67]. Intercropping systems demonstrate unique advantages in rainfed environments. The temporal complementarity in water use between maize and soybean results in a WER of 1.1 and an LER of 1.1 [18].

3.3. The Impact of Climatic Conditions on Water–Nitrogen Regulation in Intercropping Systems

Climate change restricts water–nitrogen utilization in intercropping systems, primarily through phenological crop shifts, water supply–demand imbalances, and alterations in dynamic nutrient uptake. Global warming enhances transpiration, which significantly increases crop water consumption, causing a WUE decline. Empirical data revealed that temperature elevations of 1.5 °C and 3 °C increased maize irrigation demand to 162% and 167%, respectively, of the baseline levels, while soybean irrigation required a reduction to 35% and 42% of the standard requirements [76]. This species-specific heterogeneity in water demand underscores the critical role of crop combination optimization in intercropping systems. Spatial–temporal precipitation variability across climate zones exacerbates water supply risks, particularly threatening the efficiency of water–nitrogen synergy in arid regions. Farming practices in the Guanzhong Plain demonstrate that maize–soybean systems require no supplemental irrigation in wet years. However, in drought conditions in SSP245 and SSP585 scenarios, precise irrigation applications of 70 mm at the maize three-leaf stage and 50 mm at the tasseling phase become essential to maintain the yield–WUE equilibrium [75].

Rising temperatures not only alter crop water metabolism but also significantly influence nutrient utilization by regulating soil nitrogen transformation processes. Every 1 °C increase in temperature escalates the intensity of soil nitrogen mineralization by approximately 10%, while exacerbating leaching and gaseous losses [81]. This dual effect is particularly pronounced in intercropping systems with distinct root architectures. The deep root system of maize accesses subsoil nitrogen, while soybeans enrich topsoil nutrients through symbiotic nitrogen fixation. Notably, temperature elevations may reduce soybean nodulation efficiency by 15–22%, directly impacting system nitrogen balance. To address this, it is recommended to apply 100 kg N/ha of basal fertilizer combined with 80 kg N/ha of topdressing for maize in the SSP245 scenario. In high-temperature SSP585 conditions, maize topdressing should be increased to 140 kgN/ha [75].

The impact of variations in precipitation on water–nitrogen coupling efficiency exhibits distinct regional characteristics. Case studies from the Loess Plateau demonstrated that combining supplemental irrigation with lower nitrogen levels increased the WUE and NUE by 12% and 18%, respectively [39]. However, extreme drought events still exacerbated system vulnerability, potentially causing 12–15% yield losses even with implemented irrigation [82]. In this context, adopting SSDI technology increased the water-saving efficiency by 43.2% while improving CWP [50]. Notably, the phenological response of the crop to climate warming exhibits species-specific differences. Higher temperatures accelerated maize tasseling by 7–10 d, while delaying soybean pod formation by 5–8 d. This phenological asynchrony fundamentally alters interspecific nutrient competition dynamics [83]. Strip intercropping and paired-row planting systems demonstrate superior performance in deficit irrigation conditions, increasing the WUE and NUE by 18.5% and 12.8%, respectively [30]. The NUE of intercropping systems peaks in high rainfall conditions, and the WUE reaches its maximum in low rainfall environments, demonstrating soil–climate dependent performance optimization [65].

Figure 3 demonstrates how diverse variables influence water–nitrogen interaction patterns. Based on the comprehensive analysis above, it can be concluded that grass–legume intercropping systems and certain woody plant–grass combinations demonstrate optimal water–nitrogen synergy benefits. For different regions—including humid/semi-humid and arid/semi-arid areas—water–nitrogen management strategies must be tailored according to annual precipitation patterns, specific intercropping combinations, and spatial configurations. Additionally, reference can be made to water–nitrogen management approaches implemented in regions with similar climatic conditions.

4. The Multidimensional Effect of Water–Nitrogen Coupling on Intercropping Systems

4.1. The Response Mechanisms of Crop Growth and Yield Formation

The mechanisms by which water–nitrogen coupling influences crop growth and yield in intercropping systems primarily include photosynthetic efficiency, root development, and dynamic nutrient uptake processes [68]. Research demonstrates that water–nitrogen coupling enhances photosynthetic efficiency and improves crop water and nutrient acquisition capacity by modulating the leaf stomatal conductance (g_s) and transpiration rate (T_r) [84].

Root system development is a critical determinant of crop nutrient acquisition and WUE. Water–nitrogen coupling enhances root architectural plasticity and physiological activity, thereby improving soil resource acquisition. This synergy is significantly more pronounced in maize–soybean intercropping systems [18]. Research reveals that differential vertical root distribution and increased root length density are key mechanisms for enhancing resource use efficiency in intercropping systems.

Nutrient uptake dynamics are critical for crop growth. Water–nitrogen coupling enhances crop nutrient use efficiency by optimizing the nutrient supply and uptake pathways. Nitrogen application significantly increases wheat grain yield and nutrient uptake capacity in dryland wheat–maize intercropping systems, while improving maize growth conditions [30]. This effect demonstrates distinct interspecific complementarity across nitrogen levels, with particularly pronounced positive interactions in the intercropping system in resource-limited conditions [57].

The impact of water–nitrogen coupling on crop growth is also reflected in its regulation of the soil environment. By improving the physicochemical properties and microbial community structure of the soil, water–nitrogen coupling provides a more favorable growth environment for crops. Under semi-arid conditions, the intercropping of cereal–legume crops increased carbon- and nitrogen-acquiring enzyme activities in wheat while improving the soil environment [85]. Maize–peanut intercropping and nitrogen application significantly affected soil fungal richness, species abundance, and diversity, mitigating the negative effects of nitrogen fertilization on the fungal community in peanut soil [86].

The study also indicated that water–nitrogen coupling can further enhance crop nutrient uptake efficiency and growth performance by regulating soil microbial activity and diversity. In a maize–pea intercropping system, delayed nitrogen application significantly reduced greenhouse gas emissions while increasing crop yield by modulating soil microbial biomass and enzyme activity [87]. This microbial-mediated mechanism offers novel insights into applying water–nitrogen coupling in intercropping systems.

Water–nitrogen coupling influences intercropped crop growth and yield through various mechanisms, including enhanced photosynthetic efficiency, stimulated root development, optimized nutrient uptake dynamics, and improved soil environments. The integrated effect of these mechanisms provides both theoretical support and practical guidance for high-efficiency intercropping systems.

4.2. Soil Environment Dynamics and Adaptation Mechanisms

Water–nitrogen coupling has a diverse impact on the soil environment, significantly affecting soil moisture dynamics, nitrogen cycling, and microbial community changes. Soil moisture dynamics substantially influence crop growth and soil conditions. Research has shown that water–nitrogen coupling optimizes the spatiotemporal distribution of soil moisture, which enhances the soil WUE. In the apple–soybean intercropping system of the Loess Plateau, drip fertigation combined with appropriate FC significantly increased soil water content (SWC) and WUE [44]. This optimized water control strategy enhances crop yield and improves soil moisture conditions.

Nitrogen cycling, a critical component of soil nutrient dynamics, is profoundly influenced by water–nitrogen coupling. Research conducted across four distinct soil–climate conditions simultaneously evaluated NUE and WUE in intercropped pea and barley systems. The results showed a positive correlation between the NUE and WUE, with improved WUE and reduced nitrogen leaching [65]. Rational water–nitrogen management can effectively reduce nitrogen loss and enhance the NUE to improve soil nitrogen cycling.

Microbial community changes are critical indicators of soil environmental adaptability. Water–nitrogen coupling enhances soil health by regulating microbial diversity and activity. Combining intercropping with nitrogen fertilization significantly increased the soil microbial diversity, NUE, and soil water retention capacity in a mulberry–alfalfa intercropping system [88]. Incorporating vermicompost as a bio-organic fertilizer during land, water, and nutrient control enhances agronomic NUE, grain protein content, and yield by improving the soil microbial community structure [89]. Maize–peanut intercropping combined with nitrogen application significantly alters soil fungal richness, taxonomic composition, and diversity, effectively counteracting the negative impact of nitrogen fertilization on peanut rhizosphere fungal communities [86]. In a maize–pea intercropping system, delayed nitrogen application technology significantly reduced greenhouse gas emissions while increasing the crop yield by regulating soil microbial biomass and enzyme activity [87]. This microbial mechanism regulation provides novel perspectives for water–nitrogen coupling applications in intercropping systems.

Soil exhibits multifaceted adaptive responses to water–nitrogen coupling. Irrigation and fertilization strategy optimization improves the ability of soils to adapt to the changes induced by water–nitrogen interactions. The combined use of organic cattle manure and appropriate irrigation regimes in saffron–wheat intercropping systems reduced the risk of groundwater nitrate contamination and enhanced soil nutrient use efficiency [67]. This adaptive response increases crop yield and maintains the ecological balance in the soil.

In some regions, planting forage grasses improves soil quality, while water–nitrogen coupling further enhances the water and fertilizer retention capacity of the soil. Research indicates that the mixed cultivation of different grass species can optimize the physical structure and nutrient content of the soil, which improves the water–nitrogen coupling effect [90]. This adaptive strategy provides a new approach for restoring degraded ecosystems.

Overall, water–nitrogen coupling significantly improves the soil environment by influencing soil moisture dynamics, nitrogen cycling, and microbial community changes. It also enhances the adaptive response of the soil to environmental variations. Figure 4 comprehensively demonstrates the multidimensional effects of water–nitrogen coupling on intercropping systems. These studies provide a theoretical foundation and practical guidance for optimizing water–nitrogen management strategies.

5. Research Implications and Future Perspectives

Research on water–nitrogen coupling provides valuable theoretical and practical guidance for crop growth and soil environment management in intercropping systems. Current studies demonstrate that rational regulation of water and nitrogen fertilizer ratios significantly impacts crop growth, yield, and soil health [91]. However, the variability of water–nitrogen coupling effects across different regions and climatic conditions remains incompletely understood, leaving a broad scope for future research. For instance, in arid regions, the role of water–nitrogen coupling in enhancing water-use efficiency and crop growth rates has been confirmed, yet its long-term effects and deeper relationships with soil health require further investigation [92]. Furthermore, the complementarity and competition of water–nitrogen coupling among different crops urgently need to be addressed to optimize water and fertilizer control practices in intercropping systems [68,93,94].

Further research should focus on multidimensional integrated management strategies that combine precision agriculture technologies and climate change predictions to develop more adaptive water–nitrogen coupling models. For instance, formulating customized water–nitrogen application schemes based on the differences in the root characteristics and nitrogen requirements among various crops will help enhance the NUE while reducing its environmental impact [95]. Additionally, precisely regulating water–nitrogen coupling in intercropping systems is essential to enhance crop yield, improve soil fertility, optimize rhizosphere environments, and promote healthy root development [96,97,98]. Additionally, it is necessary to formulate region-specific water–nitrogen management guidelines, which should include conducting long-term trials that simultaneously consider yield and soil biological indicators, as well as researching the economic feasibility for smallholders or water-scarce areas.

Given the increasing challenges posed by global climate change and water scarcity, future research must explore the biological and physical mechanisms underlying water–nitrogen coupling and assess its application potential on a global scale. By considering the correlation between water management, nitrogen fertilization, and soil ecology in an integrated manner, and developing management models that are suitable for different ecosystems, a solid theoretical foundation and technical support can be established for sustainable agricultural production.