Water and Nitrogen Transport in Wheat and Maize: Impacts of Irrigation, Fertilization, and Soil Management

Abstract

Water and nitrogen are fundamental factors for maintaining yield stability and achieving efficient resource utilization in wheat–maize rotation systems. Based on 131 publications indexed in the Web of Science Core Collection from 2010 to 2025, this review systematically synthesizes current knowledge on how irrigation, nitrogen application, and soil management jointly regulate water–nitrogen migration and transformation processes during wheat and maize growth. The results indicate that irrigation practices influence nitrogen transformation and availability by altering the temporal and spatial distribution of soil moisture; optimized nitrogen application strategies align nitrogen release with crop demand at critical growth stages; and the use of soil amendments improves soil physicochemical and biological conditions, thereby enhancing water retention and nitrogen stability. These three management measures exhibit strong complementarity and synergistic effects. Integrating irrigation, fertilization, and soil management can not only improve wheat and maize yields but also harmonize resource use efficiency with ecological sustainability. This review highlights the potential and pathways of integrated management practices for enhancing water and nitrogen use efficiency and ensuring food security, providing theoretical support and practical guidance for developing efficient and sustainable region-specific water–nitrogen management systems.

1. Introduction

1.1. Global Relevance

Global food security remains one of the most urgent challenges facing the international community. Ongoing population growth and the escalating impacts of climate change are driving steady increases in global food demand, amid intensifying competition for limited resources [1,2]. Among major food crops, wheat supplies essential calories and nutrients to approximately one-third of the world’s population, while maize serves not only as a staple food but is also extensively used in animal feed and industrial applications [3,4]. Given their high yields, broad cultivation extent, and dietary importance, both crops are fundamental to global food security [5].

However, the sustainable and efficient production of wheat and maize is constrained by two major factors: water scarcity and low nitrogen use efficiency [6,7]. As key limiting factors for crop growth, the transport and distribution of water and nitrogen within the soil–crop system critically influence productivity and resource use efficiency, while also shaping the environmental impacts of agroecosystems [6,8]. A deeper understanding of water and nitrogen transport processes and their interactions under different agricultural management practices is fundamentally important for improving crop yield and quality, optimizing resource allocation, and reducing non-point source pollution, thereby promoting resource-efficient and environmentally sustainable agricultural development. Although numerous studies have separately examined irrigation, fertilization, or soil management, there remains a lack of systematic and integrative assessments comparing their combined effects on water–nitrogen dynamics. This gap hampers the development of optimized management strategies tailored to specific agroecological contexts. Therefore, this review aims to synthesize and analyze the effects and interactions of irrigation methods, fertilization N strategies, and soil management practices on coupled water–nitrogen transport processes in wheat and maize systems, thereby addressing this knowledge deficiency.

1.2. Literature Search and Selection Criteria

A systematic literature search was conducted in the Web of Science Core Collection, covering the period 2010–2025. Search strings combined controlled vocabulary and free-text terms including “water and nitrogen migration,” “wheat,” “maize,” “irrigation,” “fertilization,” and “soil management.” The initial search yielded 905 records. After removal of duplicates and irrelevant studies, 678 unique records remained for title and abstract screening. Of these, 412 studies were excluded as not directly related to the review topic, leaving 266 articles for full-text assessment. Following these criteria, all selected studies focused on fundamental research or field experiments investigating water–nitrogen transport in wheat–maize systems. Ultimately, 131 peer-reviewed studies were included and synthesized in this review. The selection protocol was designed to ensure transparency and reproducibility in literature identification, screening, and inclusion.

In summary, this review summarizes current global research progress on the impacts of irrigation methods, nitrogen fertilizer application strategies, and soil management practices on water and nitrogen transport in wheat and maize planting systems. By examining how these management practices regulate water and nitrogen allocation, use efficiency, and loss pathways, it elucidates the crucial role of coordinated water and nitrogen management in improving wheat and maize yields, resource use efficiency, and environmental sustainability. Furthermore, this review identifies the limitations of existing research and proposes directions for advancing precision agriculture and sustainable food production.

2. Water and Nitrogen Transport Mechanisms in Soil–Plant Systems

The transport mechanisms of water and nitrogen in soil–plant systems are fundamental to understanding crop uptake dynamics and the water–nitrogen coupling effect [9,10]. These integrated processes collectively determine crop water use efficiency (WUE) (Equation (1)) and nitrogen use efficiency (NUE) (Equation (2)), thereby influencing overall growth and yield [11,12] (Figure 1).

$$WUE={\displaystyle \frac{Y}{ET}}$$

(1)

$$NUE={\displaystyle \frac{Y}{N_{absorbed}}}$$

(2)

where Y represents grain yield (kg·ha⁻¹), ET represents evapotranspiration (m³·ha⁻¹), and N_absorbed represents the total amount of nitrogen absorbed by the plant from the soil (kg·ha⁻¹).

Water movement within the soil–plant system occurs in both vertical and horizontal directions, with gravity-driven vertical transport playing a dominant role in regulating soil moisture distribution and nutrient migration. However, under conditions of water scarcity, lateral diffusion also becomes important, driving water movement along the water potential gradient toward the rhizosphere to sustain continuous root water uptake [13,14]. Crops actively absorb water through their root systems to support physiological development [15,16]. Meanwhile, transpiration from the leaves generates a persistent transpirational pull, which not only maintains root water absorption but also directs soil water movement toward the root zone, thereby stabilizing plant water status [6,17].The mobility of nitrogen species, particularly NO₃⁻ and NH₄⁺, is strongly influenced by soil water dynamics [18]. The high solubility of nitrate enables its rapid downward leaching with infiltrating water, whereas ammonium migrates more slowly, primarily through diffusion and adsorption–desorption processes, with its transport rate and distance largely dependent on soil physicochemical properties [19]. Hence, water availability serves as a key factor controlling nitrogen solubility, mobility, and ultimately, root nitrogen uptake efficiency [20,21]. Under adequate water supply, roots efficiently absorb both water and nitrogen. Furthermore, the transpiration-driven upward flow of water facilitates the internal redistribution and transport of nitrogen within plants, thereby enhancing its utilization in essential metabolic processes such as photosynthesis and protein synthesis [22,23]. In contrast, water deficit restricts root water uptake, significantly reducing nitrogen absorption and utilization. The accompanying decline in transpiration further impedes nitrogen translocation to aboveground tissues, ultimately suppressing crop growth and development [24,25].

Therefore, the water–nitrogen coupling effect manifests as a complex interplay of mutual promotion and constraint during crop growth. Water promotes the dissolution, migration, and transformation of nitrogen fertilizer, making it a key factor in improving crop nitrogen use efficiency (NUE). Conversely, effective nutrient supply can improve water use efficiency (WUE), optimize crop physiological processes, and enhance crop resistance and production potential. Rational fertilization not only increases soil organic matter content, improves soil structure, and enhances soil water and fertilizer retention capacity, but also achieves a coupled regulatory effect of “fertilizer regulating water, water promoting root growth, and roots enhancing drought resistance” by regulating the soil water and fertilizer environment. However, unreasonable water and nitrogen supply reduces resource utilization efficiency, leading to nitrogen leaching, volatilization, and accumulation, ultimately causing environmental pollution. For example, excessive irrigation or nitrogen application increases the risk of nitrogen migration, reduces nitrogen fertilizer use efficiency, and may even lead to groundwater pollution and greenhouse gas emissions. Conversely, insufficient water or nitrogen fertilizer supply limits the effective use of water and nitrogen by crops, reducing crop yield and quality. It is worth noting that quantitative analysis of water–nitrogen coupling effects is usually achieved by combining field observations and model simulations, such as HYDRUS, DSSAT, and DNDC models, which are widely used to simulate water and nitrogen dynamics in the soil–plant–atmosphere continuum [26,27]. The HYDRUS model describes variable saturation flow and nitrogen transport based on the Richards equation and the convection–diffusion equation, thus enabling detailed simulation of soil moisture and solute profiles [28]. The DSSAT model integrates soil moisture balance, crop growth, and nutrient dynamics to assess yield response under different irrigation and fertilization schemes [29], while the DNDC model links soil biogeochemical cycles with nitrogen transformation processes, thus enabling assessment of nitrogen loss and greenhouse gas emissions [30]. These models reveal the intrinsic links between water and nitrogen in transport, transformation, and utilization, providing important tools for a deeper understanding of water–nitrogen coupling mechanisms.

Therefore, the water–nitrogen coupling effect reflects the inseparable and closely interconnected nature of water and nitrogen in agricultural ecosystems. Optimizing this coupling and rationally managing irrigation and fertilization are key strategies for improving water and nitrogen resource use efficiency, reducing resource waste and environmental pollution, and achieving sustainable agricultural development.

3. Effects of Irrigation Modes on Water and Nitrogen Dynamics

The impact of different irrigation methods on soil water and nitrogen distribution, nitrogen leaching losses, crop yield, and water and NUE is one of the central issues in agricultural irrigation research. As a comprehensive regulatory system, irrigation encompasses two aspects: the irrigation method and the irrigation regime. Common irrigation methods, including above-ground irrigation, sprinkler irrigation, drip irrigation, and underground irrigation, modulate the crop growth environment through different water transport mechanisms, thereby affecting the availability, migration, and transformation of water and nitrogen in the soil (Figure 2). Irrigation regimes can be classified into sufficient irrigation (fully meeting crop water requirements) and water-deficit irrigation (including deficit irrigation and water-limited irrigation) based on water supply intensity. Both types further influence the spatial distribution and conversion efficiency of nitrogen by altering soil water movement pathways and the effective range of root water uptake.

3.1. Surface Irrigation

Border irrigation, a traditional surface irrigation technique, remains widely practiced across farmlands in China, particularly in the northern plains. It involves directing water into furrows between ridges, allowing it to advance slowly along the field and infiltrate the soil, thus replenishing crop-available moisture [31]. This method is advantageous under flat topography and sufficient water supply conditions due to its simplicity and ability to irrigate large areas efficiently. Properly designed border irrigation can improve soil structure and water-holding capacity, thereby meeting crop water requirements and promoting yield formation [32]. For example, Yan et al. [31] conducted a two-year traditional border irrigation experiment in the Huang-Huai Plain of China and showed that optimizing the irrigation duration (L40) can maintain suitable soil moisture after wheat flowering, enhance the antioxidant capacity of leaves, delay leaf senescence, promote grain filling, and thus increase the thousand-grain weight. Similarly, Wang et al. [33] observed in a two-year field trial of maize in Heilongjiang Province, China, that side irrigation increased the water content of deep soils. However, the nitrogen concentration in the top and subsurface soils was relatively low, mainly due to nitrogen loss through leaching or denitrification during water infiltration. In addition, Qi et al. [34] demonstrated in a field trial of maize in the Hetao Irrigation District of the upper Yangtze River that excessive nitrogen application (250–300 kg ha⁻¹) under side irrigation conditions did not increase maize yield or nitrogen fertilizer utilization; on the contrary, it led to waterlogging, which in turn aggravated denitrification and nitrogen loss.

Overall, while border irrigation provides rapid water replenishment and is easy to manage, its uneven water distribution and high infiltration rate often increase the risk of nitrogen leaching and resource loss. Consequently, it performs best in regions with relatively abundant water resources but should be integrated with precise fertilization N and water control strategies to improve WUE and NUE while minimizing environmental risks.

Furrow irrigation, a traditional surface irrigation method widely applied in dryland agriculture, delivers water through shallow ditches between crop rows, enabling gravity-driven infiltration into the root zone. Owing to its simple structure, low cost, and adaptability to local field conditions, it is well suited to areas with flat terrain, moderate planting density, and relatively sufficient water supply [35]. However, compared with more uniform systems such as drip or sprinkler irrigation, it often results in pronounced spatial heterogeneity in soil moisture and solute distribution, leading to uneven nitrogen migration and transformation within the soil profile [36]. These characteristics limit its effectiveness in scenarios requiring precise water and nutrient management.

Ridge–furrow irrigation, an improved variant, strengthens the coordinated regulation of soil water and nitrogen in wheat systems. By integrating ridge planting with furrow water delivery, upward capillary movement from furrows to ridges reduces deep percolation and nitrate leaching while maintaining favorable moisture conditions in the tillage layer, thereby enhancing crop N uptake and yield. Lian et al. [37] conducted a two-year wheat experiment in Xinxiang, Henan Province, and showed that under 180 kg·ha⁻¹ nitrogen (70% stabilized urea) and 450 m³·ha⁻¹ irrigation, nitrogen uptake, nitrogen fertilizer utilization rate, and nitrogen harvest index increased by 21.7%, 29.1%, and 2.9%, respectively, relative to traditional management. Irrigation water use efficiency and total water use efficiency increased by 61.0% and 12.3%, while yield, net income, and the benefit–cost ratio increased by 5.2%, 8.3%, and 20.1%, respectively. These results indicate that ridge–furrow irrigation is particularly advantageous in water-limited regions, where reducing N leaching and improving nitrogen use efficiency are priorities.

Irrigation scheduling further determines the efficiency of furrow-based systems. Wu et al. [38] conducted a two-year winter wheat experiment in Luoyang, Henan Province, and found that, compared with every-furrow irrigation at 75 mm during the jointing stage, alternate-furrow irrigation enhanced water storage in the 50–160 cm layer, reduced surface evaporation, and increased water availability from booting to anthesis. These changes improved photosynthesis, post-anthesis biomass accumulation, and grain yield by 14.7–21.9%, with WUE increases of 9.6–21.1%. Concentrating irrigation in alternating furrows promoted deeper infiltration and a more stable moisture supply, whereas full-surface watering caused near-surface water accumulation, greater evaporative losses, and reduced NUE. Thus, alternate-furrow irrigation represents a more efficient scheduling strategy under limited water resources.

In summary, although furrow irrigation has the disadvantages of uneven water distribution and high surface evaporation, which may exacerbate nutrient loss and environmental risks [39], it is still a low-cost and regionally adaptable irrigation method. By adopting techniques such as ridge-furrow irrigation or inter-furrow irrigation, soil moisture retention and crop nutrient absorption can be effectively improved, thereby increasing WUE and NUE while minimizing leaching loss.

3.2. Sprinkler Irrigation

Sprinkler irrigation systems apply pressurized water through atomization to achieve uniform distribution across the crop canopy. This technique provides stable soil moisture throughout key growth stages and creates favorable conditions for nutrient dissolution, migration, and rhizosphere absorption, particularly for nitrogen [40,41]. Compared with traditional surface irrigation, sprinkler irrigation substantially improves water distribution uniformity and overall WUE, while reducing nitrogen leaching risk and enhancing nitrogen fertilizer use efficiency. Numerical modeling results further confirm these benefits. Shahin et al. [42] demonstrated in Fars Province, Iran, using an AquaCrop model calibrated with remotely sensed leaf area index and measured yield data, that sprinkler irrigation improves water productivity (WP) relative to conventional surface irrigation, particularly in normal and wet growing seasons. They also reported that sprinkler irrigation with moderate irrigation efficiency (75%) can increase WP without significantly reducing grain yield.

Field observations corroborate these findings. In a two-year experiment conducted in arid northwestern India, moderate deficit irrigation (80% ETc) under sprinkler systems reduced water use by 17% with only a 5% yield decline, thereby improving WP. A clear interaction between nitrogen rate and irrigation amount was also detected: applying 120 kg·ha⁻¹ N under moderate irrigation maximized both yield and resource use efficiency [43].Nevertheless, in coarse-textured soils or areas with high infiltration capacity, excessive percolation under sprinkler irrigation can still result in nitrate leaching and reduced nitrogen availability, which highlights the need for locally optimized management practices [39].

3.3. Micro-Irrigation

3.3.1. Drip Irrigation

Drip irrigation is a highly efficient and precise water delivery technology that supplies water directly to the crop root zone through a network of emitters and pipes. This approach minimizes surface evaporation and deep percolation losses, allowing accurate, on-demand irrigation and reducing both water waste and nitrogen leaching [44,45]. Compared with conventional surface irrigation, drip irrigation markedly enhances WUE and NUE by maintaining optimal soil moisture and nutrient availability within the rhizosphere [46].

Field experiments have demonstrated the considerable efficiency gains achievable with this method. Si et al. [47] conducted a two-year winter wheat trial in the North China Plain and found that drip irrigation increased maximum WUE by 53.57–56.55% relative to surface irrigation, while conserving irrigation water and significantly improving nitrogen utilization. Similarly, Sandhu et al. [48] reported that drip irrigation increased wheat and maize yields by 25–30%, with corresponding NUE values of 64% and 85%, respectively. These benefits are primarily attributed to the “small and frequent” irrigation pattern, coupled with fertigation, which creates a favorable water–nutrient balance in the root zone and promotes synchronized uptake of both resources.

However, the advantages of drip irrigation depend heavily on precise water and fertilizer management. Inappropriate irrigation schedules or excessive fertilization can lead to localized soil over-saturation, promoting nitrogen migration and loss. Sui et al. [49] observed in the North China Plain that residual soil nitrogen increased with higher N application rates but declined with greater irrigation volumes. When nitrogen input exceeded 190 kg·ha⁻¹ and irrigation was abundant, substantial nitrogen residues remained after the wheat season, heightening the risk of leaching during subsequent rainfall or irrigation events. Although partial uptake of residual N occurs in the wheat–maize system, leaching risks still increased markedly at application rates around 290 kg·ha⁻¹.

Additionally, limited research has addressed nitrogen gaseous losses—particularly ammonia volatilization—under drip irrigation systems. NH₃ volatilization not only lowers fertilizer efficiency but also contributes to atmospheric pollution [50,51]. Therefore, future studies should focus on elucidating the dynamics and drivers of NH₃ and NO_x emissions in drip-irrigated farmlands, with the goal of refining water–fertilizer management strategies that minimize nitrogen loss and support sustainable, low-emission agricultural production.

3.3.2. Micro-Sprinkler Irrigation

Micro-sprinkler irrigation is an advanced irrigation technology that integrates the water-saving benefits of drip irrigation with the uniform water distribution of sprinkler systems. It employs small, low-pressure nozzles that evenly apply fine water droplets to the soil surface near the crop root zone [52]. Compared with conventional sprinkler irrigation, micro-sprinkler systems have a smaller spray radius, finer droplet size, and lower pressure requirements. These features substantially reduce evaporation losses, minimize surface runoff and erosion, and consequently enhance irrigation WUE [53].

Studies have demonstrated that under micro-sprinkler irrigation, optimizing irrigation quotas and nitrogen application timing—such as applying topdressing during the jointing and booting stages—can significantly improve water productivity (WP) and nitrogen partial factor productivity without compromising yield, while reducing nitrogen losses during non-critical growth periods [54]. Maintaining soil moisture at 65–70% of field capacity and synchronizing nitrogen supply with crop demand helps achieve a balanced dynamic of water and nitrogen in the root zone, reducing risks of deep percolation and nitrogen volatilization. Yao et al. [55] reported that in a three-year winter wheat field experiment, combining micro-sprinkler irrigation with a delayed nitrogen application strategy (base fertilizer:topdressing = 5:5) enhanced carbon and nitrogen metabolism during grain filling, increased chlorophyll content and photosynthetic rate, and promoted the activity of nitrogen metabolic enzymes. This improved protein accumulation, thousand-grain weight, and the coordination among yield components, ultimately increasing both yield and grain quality while reducing nitrate nitrogen accumulation in soil and enhancing late-stage WUE and nitrogen uptake efficiency. Li et al. [56] conducted a three-year field trial in the North China Plain to further compare the relationship between conventional irrigation (flood irrigation) and micro-sprinkler irrigation and nitrogen application. Under conventional irrigation conditions, 75.6–81.3% of grain nitrogen came from the reuse of nitrogen before flowering, while under micro-sprinkler irrigation conditions, this proportion decreased to 52.5–60.5%. This is attributed to the continuous and balanced water supply provided by micro-sprinkler irrigation, especially during the grain filling period, which maintained root vigor, enhanced nitrogen uptake capacity, and thus improved WUE, NUE, and yield stability.

Overall, compared to surface irrigation and traditional sprinkler irrigation, micro-sprinkler irrigation significantly improves nitrogen distribution balance and crop nitrogen uptake efficiency. However, its advantages over drip irrigation in improving WUE and NUE largely depend on the irrigation regime, namely, the scientifically designed irrigation time, frequency, and volume based on crop growth stages and environmental conditions. For example, wheat can be irrigated during the jointing, heading, and grain-filling stages, with each irrigation reaching 70–80% of field capacity; while maize can benefit from shallow irrigation of 20–30 mm every 3–5 days, combined with multiple nitrogen applications [57]. Therefore, although micro-sprinkler irrigation shows strong adaptability in optimizing the rhizosphere environment and coordinating water–nitrogen interactions, its full potential can only be realized through comprehensive management of irrigation volume, fertilization timing, and tillage methods.

3.4. Subsurface Irrigation

Subsurface irrigation is an advanced technique that delivers water directly to the active root zone through buried devices such as porous pipes or drip lines. This approach minimizes evaporation losses, conserves water, and maintains an optimized rhizosphere moisture environment [58]. In arid and semi-arid regions, subsurface irrigation substantially reduces surface water evaporation and stabilizes root-zone moisture, thereby improving water and nutrient uptake and enhancing both WUE and NUE [59]. Moreover, the subsurface water supply prevents nitrogen volatilization and surface runoff losses while effectively inhibiting nitrate leaching into deeper soil layers [60].

Among the various forms of subsurface irrigation, subsurface drip irrigation (SDI) and infiltration systems are most common, with SDI widely applied in wheat and maize production. SDI enables precise and quantitative control of water delivery, significantly improving WUE and nitrogen uptake [61]. In wheat–maize rotation systems, SDI combined with the full recommended nitrogen rate can save approximately 55% of irrigation water compared with furrow or flood irrigation, increase total yield by 11.2%, and provide substantial economic benefits, albeit with relatively high installation costs [62]. Integrating SDI with conservation agriculture (CA) further enhances resource-use efficiency. For instance, in rice–wheat rotation systems, appropriate emitter spacing (e.g., 67.5 cm) and installation depth (15 cm), combined with surface residue mulching, can reduce irrigation water use by 48–53% for rice and 42–53% for wheat. At the same time, reducing nitrogen fertilizer by 20% without yield loss—or even with yield improvement—demonstrates the strong potential of this technology for sustainable intensification in South Asian agricultural regions [63].

Field experiments have also shown that across varying irrigation levels (100%, 80%, and 60% of full irrigation) and nitrogen management regimes, SDI requires far less irrigation water than central pivot or furrow systems, while achieving the highest crop water productivity (CWP) and evapotranspiration water-use efficiency (ETWUE), with CWP reaching 3.00 kg·m⁻³. Even under mild water deficit conditions (Ky < 1), yields remain stable, highlighting the system’s buffering capacity and resilience under limited water supply [64].

Nevertheless, several constraints hinder the widespread adoption of subsurface irrigation. High installation and maintenance costs, dependence on soil and water quality, and the need for uniform irrigation and clog-resistant design present operational challenges. Large-scale deployment also demands regular maintenance and monitoring to prevent emitter blockage and performance decline. Consequently, while subsurface irrigation offers remarkable potential for improving WUE and NUE and optimizing root-zone conditions, its large-scale implementation requires integrated advances in irrigation technology, agronomic management, and cost-control strategies to achieve sustainable water use and resilient agricultural production.

In summary, different irrigation methods exhibit distinct trends in water and nitrogen use efficiencies, suitability for soil and climate conditions, leaching risks, and costs. A comparison of these characteristics is presented in

Table 1. Comparison of Irrigation Methods in Terms of Water and Nitrogen Use Efficiency, Suitability, Leaching Risk, and Cost.

Irrigation Method	Trend of Water Use Efficiency (WUE)	Trend of Nitrogen Use Efficiency (NUE)	Suitable Soil/Climate Conditions	Leaching Risk	Cost
Surface Irrigation	Low, prone to evaporation and runoff	Low, nitrogen easily lost with water	Clay to loam soils; regions with relatively sufficient rainfall	High	Low
Sprinkler Irrigation	Medium, better water distribution	Medium, nitrogen in topsoil may leach	Sandy loam to loam; warm dry or semi-arid regions	Medium	Medium
Subsurface Irrigation	High, water supplied directly to root zone, reduced evaporation	High, nitrogen uptake by roots is efficient	Sandy to loam soils; arid or water-scarce regions	Low	High
Drip Irrigation/Micro-irrigation	High, precise water control	High, nitrogen can be supplied locally with water	Suitable for various soils; preferred in arid and semi-arid areas	Low	Medium-High

Note: Trends and characteristics summarized from relevant research studies.

, providing a concise overview to guide selection of appropriate irrigation strategies under varying environmental and resource conditions.

4. Impacts of Fertilization Strategies on Water and Nitrogen Dynamics

In recent years, driven by advances in precision agriculture, integrated water–fertilizer management, and controlled-release fertilizers, the coordinated regulation of water and nitrogen has emerged as a key research focus, offering new pathways for efficient agricultural resource use and environmental protection. Accordingly, this chapter systematically explores the regulatory mechanisms and effects of different fertilization strategies on water–nitrogen dynamics and evaluates their potential to enhance crop yield, optimize water and fertilizer use efficiency, and reduce agricultural nonpoint-source pollution. As shown in Table 2, the type, amount, and timing of nitrogen fertilizer significantly influence the transport processes of water and nitrogen in crops. These fertilization factors not only determine the transformation and spatiotemporal distribution of nitrogen in the soil but also directly affect crop water and nutrient uptake efficiency and the risk of nitrogen loss [65,66]. Improper fertilization practices, such as excessive nitrogen application or a mismatch between fertilization timing and crop demand, can easily lead to nitrogen losses through leaching, denitrification, or ammonia volatilization (Figure 3), resulting in groundwater contamination and greenhouse gas emissions [67]. Conversely, scientifically designed fertilization strategies, particularly when integrated with irrigation management, can effectively improve nitrogen use efficiency, synchronize water and nitrogen availability with crop demand, strengthen their coupling effects, and ultimately promote sustainable and efficient agricultural production [68].

Nitrogen fertilizer type plays a pivotal role in regulating nitrogen migration and transformation within soil–crop systems. Different nitrogen sources exhibit significant differences in solubility, transformation pathways, and environmental fate. Urea, one of the most widely used nitrogen fertilizers, undergoes rapid hydrolysis to NH₄⁺ catalyzed by urease upon application to the soil, and is subsequently converted to NO₃⁻ through nitrification under favorable temperature and moisture conditions. Urea is favored for its high nitrogen content and cost-effectiveness; however, it is prone to NH₃ volatilization under high-temperature or alkaline soil conditions, thereby reducing NUE [69]. For example, Wan et al. [70] found that a single urea application in maize production increased NH₃ volatilization by 43% and ammonia emission factors by 58% compared with split applications, thereby reducing NUE. In contrast, the NO₃⁻ in nitrate-based fertilizers (e.g., ammonium nitrate, potassium nitrate) can be directly absorbed by crops and is fast-acting; however, due to its high mobility, it can readily leach to deeper soil layers during rainfall or irrigation, increasing the risk of nitrogen loss and groundwater contamination. Ammonium-based fertilizers (e.g., NH₄Cl) release large amounts of H⁺ during nitrification, accelerating soil acidification. According to Hao et al. [71], under conventional nitrogen application rates, NH₄Cl induced soil acidification at an annual rate of up to 52.6 k·ha⁻¹, significantly higher than that observed with urea.

Notably, controlled-release nitrogen fertilizers have attracted considerable research interest as a means to address the mismatch between the rapid nitrogen release of conventional fertilizers and crop nitrogen demand. Innovations such as polymer-coated urea (PCU), sulfur-coated urea (SCU), and MgCl₂-modified biochar slow-release fertilizers (MBSRFs) can delay nitrogen release, better synchronizing supply with crop uptake and reducing nitrogen losses during periods of low demand [72]. In winter wheat field trials, split applications of PCU and SCU increased NUE by 6.71–10.33% and yield by 9.96–15.11% [73]. Potted maize experiments demonstrated that NO₃⁻ and NH₄⁺ release rates under MBSRF treatment were 0.40 and 0.67 those of conventional ammonium nitrate, respectively, significantly enhancing plant height, dry biomass, and chlorophyll content [72]. Nevertheless, the high cost of controlled-release nitrogen fertilizers and their dependence on precise management remain major constraints to large-scale adoption.

Fertilizer application rate and timing also exert profound effects on water–nitrogen dynamics and nitrogen losses. While high nitrogen application rates (e.g., >300 kg·N·ha⁻¹) may temporarily boost yields, they can increase residual soil NO₃⁻-N, accelerate leaching into deeper soil layers, and significantly heighten the risk of nitrogen loss under conditions of high water flux—thus posing a serious non-point source pollution risk [74]. Zhang et al. [75] conducted 18 different nitrogen fertilizer treatments (0–765 kg·N·ha⁻¹) on maize in Xinjiang, China. They found that doubling the amount of nitrogen fertilizer applied delayed silking and maturity by 1–2 days, increased leaf area index (LAI), and increased grain moisture content by 1.9–4.0%. This indicates that excessive nitrogen fertilizer can disrupt the natural rhythm of water and nitrogen migration, affecting grain dehydration and yield formation. Furthermore, the application of other nutrients, such as phosphorus, potassium, magnesium, and organic fertilizers (e.g., manure), can indirectly affect water and nitrogen transport by altering soil chemical balance, microbial activity, and nutrient interactions [76,77,78].

Optimizing fertilization timing is critical for synchronizing nitrogen supply with crop demand, improving NUE, and reducing losses. Studies have shown that adopting a split-application strategy in maize production—60% at the V4 stage and 40% at silking—combined with urease inhibitors (e.g., Urea-Limus) can reduce NH₃ volatilization by 70–80%, thus lowering environmental nitrogen emissions without yield penalties [79]. Furthermore, late-season nitrogen supplementation not only increases nitrogen uptake during the silking phase by 2.3–5.5% but also enhances root nitrogen absorption capacity under water stress [80], mitigating yield losses during mid- to late-stage nitrogen deficits.

In summary, nitrogen migration and transformation in the soil-crop system are significantly influenced by the synergistic regulation of nitrogen source type, application rate, and application timing. Different nitrogen sources differ in release kinetics, transformation patterns, and environmental behavior, which determines their alignment with crop nitrogen requirements and potential environmental losses. Excessive application rates or application timing mismatched with crop critical demand periods exacerbate nitrogen volatilization and leaching losses, reduce nitrogen fertilizer use efficiency, and increase pollution risks. Controlled-release fertilizers, by synchronizing nitrogen release with crop needs, provide an effective way to improve nitrogen fertilizer use efficiency and reduce ecological impact.

Table 2. Effects of nitrogen fertilizer type, nitrogen application rate, and fertilization timing on crop water and nitrogen transport.

Crop	Experimental Site and Design Description	Types of Nitrogen Fertilizers	Fertilizer Amount	Fertilization Timing	Impact on Water and Nitrogen Migration	Main Conclusions	Reference
Winter wheat	Jiangsu Province, China—two years, light loam soil	polymer-coated urea (PCU), sulfur-coated urea (SCU)	/	One-time application vs. two-part application (pre-sowing + greening)	Split application increased the inorganic nitrogen content in the soil after greening and improved nitrogen supply during the critical period.	Compared with multiple conventional urea fertilizations, split application of PCU/SCU can increase yield by 9.96–15.11%, improve nitrogen fertilizer use efficiency by 6.71–10.33%, and reduce fertilization labor.	[73]
Winter wheat	Xianyang, Shaanxi Province, China—two years, silt loam	Mixture of ordinary nitrogen fertilizer (ONF) and controlled-release nitrogen fertilizer (CRNF)	192 (N1)/240 (N2) kg·N·ha−1	0%, 30%, 50%, 70%, 100% CRNF replacement ratio	Increased nitrate nitrogen content in the 0–60 cm soil layer from greening to maturity, reduced leaching, promoted nitrogen accumulation and dry matter accumulation during critical periods, and improved nitrogen transport to grains	Using a 70% CRNF blend on top of N2 can increase NUE by 2.8% and yield by 3.0–15.3%.	[81]
Winter wheat	Yangzhou, China—two years, sandy loam	Traditional nitrogen fertilizer/controlled-release urea (CRF-60/CRF-80)	/	One-time application before sowing	Increase the inorganic nitrogen content in the soil in the later period, meet the two nitrogen absorption peaks	One fertilization can meet the nitrogen absorption needs of wheat twice, NUE increased by 9.7–12.1%.	[82]
Winter wheat	Multan, Pakistan—two years, silty clay loam	Urea + farmyard manure	150 kg·N·ha−1	Use throughout the entire growth period	A mixture of 75% urea and 25% farmyard manure will result in nitrogen fixation, while pure urea will lose nitrogen through volatilization and leaching.	With the optimal ratio (75% urea + 25% farmyard manure), the NUE of wheat is 50.37%.	[83]
Maize	Fort Collins, Colorado, USA—two years, clay loam	Blood meal, feather meal, fish fertilizer liquid, blue algae nitrogen fertilizer	/	Fish fertilizer liquid and blue algae fertilizer are applied by drip irrigation four times, and blood meal and feather meal are deeply applied before sowing.	Cyanobacteria nitrogen fertilizer increased leaf stomatal conductance and photosynthetic performance, promoting WUE; salicylic acid and iron contained in the fertilizer were significantly positively correlated with iWUE, fWUE, and stomatal exchange characteristics.	Cyano-fertilizer nitrogen fertilizer has a water use efficiency of 42–51%.	[77]
Maize	Balcarce, Argentina—two years, Argiudolls	Urea, Urea-Limus, Urea-DMPP, CAN	/	Fertilize once (V4) or apply fertilizer in divided doses (60% V4 + 40% silking period)	Split fertilization and the use of Urea-Limus significantly reduced ammonia volatilization (70% to 80%); there was no significant difference in N2O emissions.	Urea-Limus + split-time fertilization can significantly reduce ammonia volatilization and environmental costs without affecting yield.	[79]
Maize	Khuzestan Province, Southwest Iran—pot experiment, 64 days	MgCl2 modified biochar slow-release fertilizer (MBSRF), enriched biochar (EMBC), ammonium nitrate (AN)	/	One-time fertilization (potted plants)	MBSRF slowly releases nitrate/ammonium nitrogen to enhance water retention and nitrogen absorption	The release rate of nitrate and ammonium nitrogen from MBSRF was slower, about 2.5 and 1.5 times lower than that from AN. MBSRF effectively increased plant height, shoot dry weight, root dry weight, chlorophyll content, and leaf area.	[72]
Maize	Shaanxi Province, China—two years, silty clay loam	Urea (U), slow-release fertilizer (SRF), urea + slow-release fertilizer (UNS)	180 kg·N·a−1	Application during the growth period	UNS increased nitrogen absorption in the late stage under water stress (19.1%) and alleviated the effects of water stress. Both SRF and UNS reduced residual NO3−-N and increased NUE and WP.	UNS helps to stabilize yield and increase efficiency under water stress, SRF significantly increases yield under sufficient irrigation conditions, and W3SRF treatment has the highest yield.	[84]
Maize	Lexington, Kentucky, USA—two years, silt loam	Urea ammonium nitrate (UNS)	0–303 kg·N·ha−1	Initial one-time fertilization/split-time fertilization	Split nitrogen application improves the matching of nitrogen supply and crop nitrogen demand in time by supplementing nitrogen during the key growth period, enhances the nitrogen absorption capacity of the root system under water conditions, and thus optimizes the water–nitrogen transfer efficiency.	The strategy of applying fertilizer in stages improves nitrogen agronomic efficiency and yield.	[74]
Maize	Xinjiang, China—three years, sandy soil	urea	0–765 kg·N·ha−1	One-time application before sowing	Nitrogen fertilization promoted the increase in aboveground biomass and LAI, enhanced canopy transpiration and soil water consumption, and to a certain extent accelerated the rate of shallow soil water migration to the root zone.	With increasing nitrogen application, the silking period of maize is delayed by about 1 day, and the maturity period is delayed by 1–2 days. The number of green leaves and leaf area index (LAI) at physiological maturity are increased.	[75]
Maize	Ansai District, Northwest Loess Plateau, Shaanxi Province, China—three years, silt loam	Urea, slow-release nitrogen fertilizer	90, 120, 180, 240, 300 kg·N·ha−1	Base fertilizer 40% N, jointing fertilizer 60% N	The nitrogen absorption during the silking period increased by 2.3–5.5% under slow-release nitrogen fertilizer treatment; the total nitrogen transport volume of slow-release fertilizer was 9.3–22.9% lower than that of urea, but the nitrogen transport efficiency was higher.	Slow-release nitrogen fertilizer plus medium nitrogen application (180 kg N·ha−1) can achieve higher yields and NUE, and is the preferred strategy for sustainable fertilization management.	[80]

Table 2. Effects of nitrogen fertilizer type, nitrogen application rate, and fertilization timing on crop water and nitrogen transport.

Crop	Experimental Site and Design Description	Types of Nitrogen Fertilizers	Fertilizer Amount	Fertilization Timing	Impact on Water and Nitrogen Migration	Main Conclusions	Reference
Winter wheat	Jiangsu Province, China—two years, light loam soil	polymer-coated urea (PCU), sulfur-coated urea (SCU)	/	One-time application vs. two-part application (pre-sowing + greening)	Split application increased the inorganic nitrogen content in the soil after greening and improved nitrogen supply during the critical period.	Compared with multiple conventional urea fertilizations, split application of PCU/SCU can increase yield by 9.96–15.11%, improve nitrogen fertilizer use efficiency by 6.71–10.33%, and reduce fertilization labor.	[73]
Winter wheat	Xianyang, Shaanxi Province, China—two years, silt loam	Mixture of ordinary nitrogen fertilizer (ONF) and controlled-release nitrogen fertilizer (CRNF)	192 (N1)/240 (N2) kg·N·ha−1	0%, 30%, 50%, 70%, 100% CRNF replacement ratio	Increased nitrate nitrogen content in the 0–60 cm soil layer from greening to maturity, reduced leaching, promoted nitrogen accumulation and dry matter accumulation during critical periods, and improved nitrogen transport to grains	Using a 70% CRNF blend on top of N2 can increase NUE by 2.8% and yield by 3.0–15.3%.	[81]
Winter wheat	Yangzhou, China—two years, sandy loam	Traditional nitrogen fertilizer/controlled-release urea (CRF-60/CRF-80)	/	One-time application before sowing	Increase the inorganic nitrogen content in the soil in the later period, meet the two nitrogen absorption peaks	One fertilization can meet the nitrogen absorption needs of wheat twice, NUE increased by 9.7–12.1%.	[82]
Winter wheat	Multan, Pakistan—two years, silty clay loam	Urea + farmyard manure	150 kg·N·ha−1	Use throughout the entire growth period	A mixture of 75% urea and 25% farmyard manure will result in nitrogen fixation, while pure urea will lose nitrogen through volatilization and leaching.	With the optimal ratio (75% urea + 25% farmyard manure), the NUE of wheat is 50.37%.	[83]
Maize	Fort Collins, Colorado, USA—two years, clay loam	Blood meal, feather meal, fish fertilizer liquid, blue algae nitrogen fertilizer	/	Fish fertilizer liquid and blue algae fertilizer are applied by drip irrigation four times, and blood meal and feather meal are deeply applied before sowing.	Cyanobacteria nitrogen fertilizer increased leaf stomatal conductance and photosynthetic performance, promoting WUE; salicylic acid and iron contained in the fertilizer were significantly positively correlated with iWUE, fWUE, and stomatal exchange characteristics.	Cyano-fertilizer nitrogen fertilizer has a water use efficiency of 42–51%.	[77]
Maize	Balcarce, Argentina—two years, Argiudolls	Urea, Urea-Limus, Urea-DMPP, CAN	/	Fertilize once (V4) or apply fertilizer in divided doses (60% V4 + 40% silking period)	Split fertilization and the use of Urea-Limus significantly reduced ammonia volatilization (70% to 80%); there was no significant difference in N2O emissions.	Urea-Limus + split-time fertilization can significantly reduce ammonia volatilization and environmental costs without affecting yield.	[79]
Maize	Khuzestan Province, Southwest Iran—pot experiment, 64 days	MgCl2 modified biochar slow-release fertilizer (MBSRF), enriched biochar (EMBC), ammonium nitrate (AN)	/	One-time fertilization (potted plants)	MBSRF slowly releases nitrate/ammonium nitrogen to enhance water retention and nitrogen absorption	The release rate of nitrate and ammonium nitrogen from MBSRF was slower, about 2.5 and 1.5 times lower than that from AN. MBSRF effectively increased plant height, shoot dry weight, root dry weight, chlorophyll content, and leaf area.	[72]
Maize	Shaanxi Province, China—two years, silty clay loam	Urea (U), slow-release fertilizer (SRF), urea + slow-release fertilizer (UNS)	180 kg·N·a−1	Application during the growth period	UNS increased nitrogen absorption in the late stage under water stress (19.1%) and alleviated the effects of water stress. Both SRF and UNS reduced residual NO3−-N and increased NUE and WP.	UNS helps to stabilize yield and increase efficiency under water stress, SRF significantly increases yield under sufficient irrigation conditions, and W3SRF treatment has the highest yield.	[84]
Maize	Lexington, Kentucky, USA—two years, silt loam	Urea ammonium nitrate (UNS)	0–303 kg·N·ha−1	Initial one-time fertilization/split-time fertilization	Split nitrogen application improves the matching of nitrogen supply and crop nitrogen demand in time by supplementing nitrogen during the key growth period, enhances the nitrogen absorption capacity of the root system under water conditions, and thus optimizes the water–nitrogen transfer efficiency.	The strategy of applying fertilizer in stages improves nitrogen agronomic efficiency and yield.	[74]
Maize	Xinjiang, China—three years, sandy soil	urea	0–765 kg·N·ha−1	One-time application before sowing	Nitrogen fertilization promoted the increase in aboveground biomass and LAI, enhanced canopy transpiration and soil water consumption, and to a certain extent accelerated the rate of shallow soil water migration to the root zone.	With increasing nitrogen application, the silking period of maize is delayed by about 1 day, and the maturity period is delayed by 1–2 days. The number of green leaves and leaf area index (LAI) at physiological maturity are increased.	[75]
Maize	Ansai District, Northwest Loess Plateau, Shaanxi Province, China—three years, silt loam	Urea, slow-release nitrogen fertilizer	90, 120, 180, 240, 300 kg·N·ha−1	Base fertilizer 40% N, jointing fertilizer 60% N	The nitrogen absorption during the silking period increased by 2.3–5.5% under slow-release nitrogen fertilizer treatment; the total nitrogen transport volume of slow-release fertilizer was 9.3–22.9% lower than that of urea, but the nitrogen transport efficiency was higher.	Slow-release nitrogen fertilizer plus medium nitrogen application (180 kg N·ha−1) can achieve higher yields and NUE, and is the preferred strategy for sustainable fertilization management.	[80]

5. Role of Soil Management Practices in Water and Nitrogen Transport

Soil management practices play a critical role in regulating water and nitrogen migration, transformation, and utilization efficiency, and are therefore essential for achieving sustainable agricultural development. In recent years, as understanding of crop water–nitrogen requirements and soil–plant coupling processes has deepened, practices such as straw return, plastic mulching, and biochar amendment have been widely adopted. These measures improve soil structure, increase organic matter content, and enhance pore characteristics and aggregate stability, thereby strengthening soil water-holding capacity, modifying nitrogen dynamics, and promoting root uptake of water and nutrients (Figure 4). Consequently, elucidating the mechanisms through which these management strategies influence water and nitrogen transport pathways has become a key step toward developing efficient water–nitrogen co-utilization systems, improving resource use efficiency, and alleviating environmental pressures.

5.1. Returning Straw to Fields

Numerous studies have shown that returning straw to the field can significantly reduce nitrogen loss, improve soil quality, and increase maize yield [85]. On one hand, straw incorporation improves soil structure and physical–chemical properties by increasing organic matter content, enhancing infiltration and water storage capacity, and strengthening nitrogen retention and slow-release potential, thereby reducing the risk of water-soluble nitrogen migrating with surface runoff (Figure 5) [86,87]. Zhang et al. [88], in a five-year study of a rice–wheat rotation system with different straw incorporation treatments, found that straw incorporation optimized the spatial and temporal distribution of inorganic nitrogen forms in the soil, reduced nitrogen losses during the crop growth period by 2.7–20.9%, and increased yields by 8.7–16.9%.

Regarding the optimization of straw return methods, Liu et al. [89] conducted a field wheat experiment in the Tumuqu Irrigation Area comparing deep plowing return and subsoiling return on topsoil structure and water–nitrogen processes. Both methods lowered bulk density in the 0–45 cm layer, improved water-holding capacity, and significantly increased organic carbon (by 12.76%), total nitrogen (by 25.32%), and available nutrients (e.g., phosphorus and nitrate nitrogen), resulting in second-year yield increases of 13.14% and 11.41%, respectively. Li et al. [90] conducted different combinations of straw and nitrogen fertilizer treatments in a five-year wheat experiment. The results showed that the simultaneous application of straw and nitrogen fertilizer could reduce nitrogen loss by 2.7% to 20.9% and increase wheat yield by 8.7% to 16.9%. Mechanistic analysis suggested that these benefits were concentrated in the rice pre-tillering stage, driven by changes in soil inorganic nitrogen, soil organic carbon, and pH. Additionally, straw return increases cation exchange capacity, thereby enhancing NH₄⁺ adsorption and retention, and improving the spatial–temporal alignment of water and nitrogen.

Notably, straw incorporation also regulates coupled water–nitrogen transformation by modulating soil microbial processes. Li et al. [91] conducted a 16-year field experiment on maize and found that, during straw decomposition, the release of soluble organic carbon enhances microbial activity, stimulates biological fixation of inorganic nitrogen, and mitigates nitrogen accumulation and loss in surface water. Under nitrogen-deficient conditions, intensified microbial competition for nitrogen drives biological nitrogen fixation. Conversely, when nitrogen is sufficient, microorganisms shift from nitrogen to carbon limitation, leading to greater nitrogen retention through abiotic pathways such as mineral-associated fixation and reactions with phenolic compounds. Furthermore, microorganisms can transform NO₃⁻ into more stable nitrogen forms through denitrification or dissimilatory nitrate reduction to ammonium, reducing its mobility and reactivity [90]. Cao et al. [92] reported that during the wheat tillering period, microorganisms reduced active soil nitrogen levels via NH₄⁺ assimilation/fixation and NO₃⁻ microbial reduction, thereby achieving biological regulation of water–nitrogen transformation.

However, the actual effects of straw return depend on factors such as application amount, return method, and crop type. With the increasing mechanization of harvest and straw handling, uneven straw distribution has become a growing challenge. Based on a 12-year wheat–barley and wheat–legume–rapeseed rotation experiment, Flower et al. [93] found that different straw treatment methods (e.g., flat-laying return vs. windrow burning) and machinery operation patterns significantly affected soil properties and yield. In drought years, high straw loads improved water retention and promoted seedling emergence and early growth; however, under high rainfall or low temperatures, straw return—especially wheat straw—could hinder crop growth due to reduced aeration, disease proliferation, and increased nitrogen competition.

Figure 5. Effects of straw return, film mulching, and biochar on nitrogen migration [94,95].

Figure 5. Effects of straw return, film mulching, and biochar on nitrogen migration [94,95].

5.2. Film Mulching

Film mulching effectively reduces water evaporation, stabilizes ground temperature, and regulates nitrogen migration by forming a physical barrier on the soil surface. Particularly in arid and semi-arid regions, this technology can significantly improve soil moisture conditions, increasing both WUE and NUE, providing a more stable water and nitrogen supply for crop growth (Table 3). Among various mulch covering materials, plastic film mulch can significantly improve soil moisture storage capacity and increase surface temperature in the early and middle stages of maize and wheat growth due to its dense structure and strong heat and water retention capacity [96,97]. This in turn promotes nitrogen mineralization and crop nitrogen uptake, thereby increasing yields [98] and reducing ammonia volatilization losses [99]. The mechanism is primarily through suppressing surface water vapor exchange, thereby lowering water evaporation and ammonia volatilization, while maintaining higher soil temperatures to promote nitrogen transformation and uptake. However, prolonged or large-scale PM use in crops such as maize and wheat may increase the risk of deep-layer nitrate (NO₃⁻) leaching, particularly under multi-year application scenarios [100,101].

In contrast, biodegradable mulch films (BMs) have similar initial water and nitrogen retention properties to plastic films (PMs) [97,102]. Notably, BMs are degraded by microorganisms over time, leading to increased soil evaporation, while nitrogen mineralization rates and surface NO₃⁻ accumulation decrease [103]. However, despite their slightly lower water retention capacity, BMs can prevent plastic residue pollution [101] and reduce the risk of leaching due to excessive nitrogen accumulation during later crop growth stages [100].

Some studies report that BM can achieve higher yields and NUE in crops such as maize when irrigation depth (e.g., 22.5 mm) and nitrogen application rates (e.g., 280 kg·ha⁻¹) are properly managed [100,102]. Its main advantage lies in synchronizing nitrogen release with crop demand, thereby improving the nitrogen harvest index. Furthermore, combining mulching with microbial management can yield synergistic effects. For example, Zhu et al. [104]. conducted a field trial of soil mulching (PFM) on the local spring wheat variety “Lunchun 8275” in a typical semi-arid region of the Loess Plateau. The results showed that PFM treatment altered the linear regression coefficients between soil microbial populations and nutrients, and between microbial metabolic activity and nutrients, reaching 0.67 and 0.20, respectively, while these coefficients were −0.24 and −0.37 in the control (CK) group. Furthermore, under PFM conditions, wheat grain yield increased by 19.2%, and water use efficiency improved by 40.7%. Research by Zhao et al. [105]. on the Loess Plateau has confirmed that flat mulching (FP) can stably regulate the soil hydrothermal environment and cultivate a soil bacterial network with closer interspecific relationships and a more stable structure, thereby promoting a 12–16% increase in wheat yield compared to furrow mulching (RP).

It is worth noting that although PM has not shown short-term negative effects on crop yield or soil quality residual materials such as microplastics may pose long-term ecological risks if frequently applied and not adequately removed [101].

Table 3. Effects of different film mulching types on water and nitrogen transport characteristics and regulatory effects of maize and wheat.

Crop	Experimental Site and Design Description	Mulch Type	Water–Nitrogen Transport Characteristics	Main Conclusion	Reference
Maize	Changwu, China—ten years, silt loam	White/black biodegradable film, white/black plastic film	Significant NO3− accumulation in 0–20 cm under NBFM; higher leaching risk; BM degrades in mid-late stage, reducing soil moisture and mineralization	Biodegradable film reduces NO3− accumulation and leaching, improves nitrogen harvest index with no yield loss	[103]
Maize	Inner Mongolia, China—two years, sandy loam	Biodegradable film, plastic film, no mulch	Higher Cmic, Nmic, and enzyme activity in PM mid-stage; BM degradation reduces microbial activity; best water–nitrogen coordination under 22.5 mm irrigation + 280 kg·ha−1·N	BM suitable for 22.5 mm irrigation + 280 N, with superior yield and NUE	[102]
Wheat	Shaanxi Province, China—two years, silt loam	Ridge plastic mulch/no mulch	RP improves soil water storage and mineral N; significantly increases soil temperature in mid-late stage; lower evapotranspiration	RP 180 kg N·ha−1 optimizes source-sink relationship and enhances W-N coordination	[96]
Maize	Shaanxi, China—two years, silt loam	Flat PM, ridge PM, biodegradable mulch, no mulch	PM and BM improve surface water storage, N uptake, and mineral N residue; significantly reduce NH3 and N2O emissions and evapotranspiration	BM reduces emissions while maintaining yield and efficiency, suitable as green alternative	[98]
Maize	Jinju, South Korea—two years, coarse loam	Biodegradable film, plastic film, no mulch	High NO3− accumulation under PM; weaker W-N retention under BM280; lowest NO3− accumulation and leaching, highest NUE under BM160	BM shows better environmental and yield performance under ~200 kg·ha−1·N; model validated	[100]
Maize	Inner Mongolia, China—two years, silt loam	Biodegradable film, plastic film, no mulch	Similar soil water under BM and PM before degradation; surface moisture drops sharply after 40% degradation; increased evaporation and 80 cm flux	BM and PM have similar early-stage dynamics; attention needed to surface moisture post-degradation	[97]
Maize	Shaanxi, China—three years, silty loam	Long-term plastic mulch (33 years)	Mulched plots have higher soil moisture but lower NO3−; P uptake in early stage inhibited by lower pH	No negative impact on yield/water, but soil acidification from long-term mulching requires attention	[101]
Maize	Inner Mongolia, China—two years, silty sandy loam	Straw + plastic mulch, plastic mulch, no mulch	SM increases DNRA gene abundance and microbial N conversion; FM reduces ammonification genes, possibly limiting N availability	SM enhances soil N transformation potential; FM may inhibit some N pathways	[106]
Maize	Liaoning, China—two years, brown soil	Plastic mulch, no mulch	Mulching increases soil temperature and moisture; water droplets capture NH3 reducing loss; slight increase in N2O (not significant)	Plastic mulch reduces NH3 volatilization effectively, providing practical N loss mitigation strategy	[99]

Table 3. Effects of different film mulching types on water and nitrogen transport characteristics and regulatory effects of maize and wheat.

Crop	Experimental Site and Design Description	Mulch Type	Water–Nitrogen Transport Characteristics	Main Conclusion	Reference
Maize	Changwu, China—ten years, silt loam	White/black biodegradable film, white/black plastic film	Significant NO3− accumulation in 0–20 cm under NBFM; higher leaching risk; BM degrades in mid-late stage, reducing soil moisture and mineralization	Biodegradable film reduces NO3− accumulation and leaching, improves nitrogen harvest index with no yield loss	[103]
Maize	Inner Mongolia, China—two years, sandy loam	Biodegradable film, plastic film, no mulch	Higher Cmic, Nmic, and enzyme activity in PM mid-stage; BM degradation reduces microbial activity; best water–nitrogen coordination under 22.5 mm irrigation + 280 kg·ha−1·N	BM suitable for 22.5 mm irrigation + 280 N, with superior yield and NUE	[102]
Wheat	Shaanxi Province, China—two years, silt loam	Ridge plastic mulch/no mulch	RP improves soil water storage and mineral N; significantly increases soil temperature in mid-late stage; lower evapotranspiration	RP 180 kg N·ha−1 optimizes source-sink relationship and enhances W-N coordination	[96]
Maize	Shaanxi, China—two years, silt loam	Flat PM, ridge PM, biodegradable mulch, no mulch	PM and BM improve surface water storage, N uptake, and mineral N residue; significantly reduce NH3 and N2O emissions and evapotranspiration	BM reduces emissions while maintaining yield and efficiency, suitable as green alternative	[98]
Maize	Jinju, South Korea—two years, coarse loam	Biodegradable film, plastic film, no mulch	High NO3− accumulation under PM; weaker W-N retention under BM280; lowest NO3− accumulation and leaching, highest NUE under BM160	BM shows better environmental and yield performance under ~200 kg·ha−1·N; model validated	[100]
Maize	Inner Mongolia, China—two years, silt loam	Biodegradable film, plastic film, no mulch	Similar soil water under BM and PM before degradation; surface moisture drops sharply after 40% degradation; increased evaporation and 80 cm flux	BM and PM have similar early-stage dynamics; attention needed to surface moisture post-degradation	[97]
Maize	Shaanxi, China—three years, silty loam	Long-term plastic mulch (33 years)	Mulched plots have higher soil moisture but lower NO3−; P uptake in early stage inhibited by lower pH	No negative impact on yield/water, but soil acidification from long-term mulching requires attention	[101]
Maize	Inner Mongolia, China—two years, silty sandy loam	Straw + plastic mulch, plastic mulch, no mulch	SM increases DNRA gene abundance and microbial N conversion; FM reduces ammonification genes, possibly limiting N availability	SM enhances soil N transformation potential; FM may inhibit some N pathways	[106]
Maize	Liaoning, China—two years, brown soil	Plastic mulch, no mulch	Mulching increases soil temperature and moisture; water droplets capture NH3 reducing loss; slight increase in N2O (not significant)	Plastic mulch reduces NH3 volatilization effectively, providing practical N loss mitigation strategy	[99]

5.3. Biochar

As a soil amendment, biochar offers considerable potential to regulate water and nitrogen transport and mitigate nitrogen losses [107]. Its high specific surface area and porous structure enable effective adsorption of both water and nitrogen, thereby reducing excessive water infiltration and nitrogen leaching [108]. Furthermore, biochar application can significantly improve soil water retention and aeration, enhancing the soil’s capacity to store water and supply nitrogen. Studies have shown that biochar can reduce nitrogen fertilizer losses via denitrification and volatilization, particularly under high nitrogen input conditions, thereby improving NUE [109]. Biochar is especially promising for wheat and maize cultivation in drought-prone and high-temperature environments, where it can enhance soil water and nitrogen retention and promote healthy crop growth [110]. Multiple studies indicate that biochar not only improves soil water retention but also affects nitrogen transformation pathways and plant uptake, thus synergistically enhancing both WP and nitrogen fertilizer use efficiency. However, the magnitude of these benefits is influenced by environmental conditions, biochar feedstock, application rates, and complementary management practices.

From the perspective of water-use efficiency, studies have shown that biochar can enhance crop WUE by improving soil water retention, reducing evaporative losses, and optimizing water availability in the rhizosphere. A global meta-analysis, for example, reported that biochar application increased WUE by an average of 4.7% [111], with improvements exceeding 40% in sandy soils or arid regions [112,113]. This enhancement is largely attributed to the highly porous structure of biochar, which increases available water capacity (AWC) and overall water-holding capacity, thereby mitigating drought stress [114]. In addition, biochar can reduce saturated hydraulic conductivity and improve slow-release water characteristics, enabling crops to utilize water and nitrogen resources more effectively.

Regarding nitrogen, biochar can regulate nitrogen transport and transformation via multiple mechanisms. Its surface functional groups exhibit strong ammonium adsorption, reducing nitrogen leaching [95,115]. Biochar also promotes the stabilization and gradual release of organic nitrogen, thereby enhancing the soil’s sustainable nitrogen supply [116,117]. Long-term field experiments have shown that biochar increases the ¹⁵N recovery rate of the soil soluble organic nitrogen pool, promotes conversion of inorganic to organic nitrogen, and improves nitrogen retention in the crop–soil system [116]. Furthermore, combining biochar with low to medium nitrogen application rates can enhance nitrogen uptake and utilization, particularly in efficient systems such as mulched drip irrigation or furrow-mulched cropping, leading to concurrent improvements in both WUE and NUE [109,118].

It should be noted that biochars positive effects are most pronounced within an optimal dosage range, while excessive application can negatively impact water and nitrogen utilization. For example, Shuailin et al. [119] found that applying more than 3% apple-branch biochar to loamy clay soils significantly reduced wheat yields due to the high C:N ratio, which can temporarily immobilize nitrogen or alter nutrient balance. Similarly, Gao et al. [120] reported that high-carbon-content biochars, particularly woody types, yielded smaller WUE gains, as excessive stable carbon can reduce nutrient availability. Soil pH compatibility is also critical; applying alkaline biochar to alkaline soils may diminish crop responses to water and nitrogen inputs.

From a coupled-management perspective, multiple studies highlight strong synergistic effects between biochar and irrigation scheduling, fertilization strategies, and straw return practices. For instance, Bai et al. [121] reported that, in a wheat–maize rotation system, combining straw return with biochar allowed a 30% reduction in nitrogen input while maintaining yields and lowering greenhouse gas emissions by over 30%. Likewise, Shuailin et al. [119] demonstrated that, in wheat cultivation under hot and dry conditions, integrating biochar with organic fertilizers significantly improved crop WP, underscoring its potential for resource-limited environments.

5.4. Summary

In summary, straw incorporation, mulching techniques, and biochar amendment are critical soil management strategies that make substantial contributions to enhancing soil water availability and NUE, improving soil physical and chemical properties, and promoting microbially mediated nitrogen transformation and fixation. By regulating root-zone water dynamics, reducing nitrogen leaching and volatilization, and optimizing nutrient availability and uptake, these practices facilitate the synergistic absorption of water and nitrogen by crops, thereby contributing to high-yield, resource-efficient, and environmentally sustainable agriculture.

Nevertheless, the effectiveness of these measures is highly context-dependent and influenced by a combination of factors, including application mode, intensity, crop type, soil characteristics, and climate conditions. For instance, while straw incorporation significantly improves organic matter content and soil buffering capacity, improper use may cause nutrient imbalances or disease buildup. Plastic mulching enhances water and nitrogen retention but may lead to nutrient over-accumulation and the accumulation of microplastics in soil, posing potential environmental and ecological risks. Biochar, with its porous structure and high sorptive capacity, improves soil water and nutrient retention, yet its persistence, long-term stability, and interactions with fertilizer and irrigation regimes remain insufficiently understood.

6. Interaction Effects of Irrigation, Fertilization, and Soil Management

Research on wheat and maize systems has demonstrated that irrigation methods, fertilization schedules, and soil management practices all exert significant influences on water–nitrogen transport and transformation, crop uptake efficiency, and yield formation. However, compared with single management measures, integrated management systems are better suited to the spatiotemporal dynamics of crop water and nutrient requirements, thereby enhancing the overall regulatory capacity of the soil–plant system. Throughout the growth cycle of wheat and maize, water and nitrogen demands exhibit distinct stage-specific patterns, and coordinated regulation can effectively prevent mismatches between supply and demand during critical growth periods. For instance, moderate water control combined with appropriate nitrogen inputs during the seedling stage favors root development, whereas increased water and nitrogen supply during the jointing and grain-filling stages is necessary to support vigorous growth and yield formation. Such integrated strategies enable precise resource allocation and optimize the relationship between physiological responses and yield development.

From a mechanistic perspective, irrigation methods influence soil water distribution and nitrogen migration; fertilization methods determine nitrogen release dynamics and efficiency; and soil management practices improve soil structure, regulate water retention, and modulate nutrient release. These three factors are strongly interdependent. For example, the combination of drip irrigation and controlled-release fertilizer can achieve precise irrigation and fertilization during the growth stages of wheat and maize: drip irrigation ensures accurate water delivery while minimizing losses, and controlled-release fertilizers synchronize nitrogen availability with crop demand, reducing excessive nitrogen supply and subsequent leaching losses [122,123,124,125]. When integrated with biochar, which enhances soil structure and moisture retention, this approach further reduces deep percolation and stabilizes nitrogen availability [126]. The adsorption capacity of biochar can also mitigate nitrogen volatilization and leaching, thereby improving nitrogen use efficiency [127].

The synergy among irrigation, fertilization, and soil management is not limited to physical processes but extends to biological co-regulation, such as enhancing soil microbial activity and modifying the rhizosphere environment. For example, biochar application can promote the proliferation of beneficial microorganisms and enhance microbially mediated nitrogen transformation—regulating nitrification and denitrification—and thereby influence nitrogen availability in wheat and maize rhizospheres [128,129]. Likewise, different fertilizer types and nitrogen application methods can alter rhizosphere ammonia concentrations by modifying nitrogen release rates and transformation pathways, affecting root–microbe interactions and maintaining rhizosphere ecological stability [130,131]. Irrigation management, by influencing soil moisture and oxygen diffusion, also shapes the microbial metabolic environment. Collectively, these interactions form a coupled regulatory network of water–nitrogen–soil–organisms, which underpins system stability and efficient resource utilization [132,133].

In summary, achieving efficient and coordinated utilization of water and nitrogen resources in wheat and maize production systems requires the system-level optimization of irrigation, fertilization, and soil management technologies. These measures exhibit strong complementarity across spatial and temporal scales. Establishing coordinated and locally adapted water–nitrogen regulation models can effectively meet the dynamic water and nutrient demands of wheat and maize, thereby improving overall utilization efficiency. As shown in Figure 6, different management strategies can be coordinated according to soil texture and dominant pathways of nitrogen loss. For sandy soils with low nutrient retention and high leaching risk, drip irrigation allows precise control of water application to reduce deep percolation; combining controlled-release fertilizers with fertigation helps regulate nitrogen release, while biochar application enhances soil nutrient-holding capacity and adsorbs nitrogen. For clayey soils with poor structure and limited aeration, alternate- or furrow-irrigation strategies help prevent compaction and improve aeration; using slow-release fertilizers with deep placement reduces nutrient accumulation near the surface, and straw incorporation improves soil aggregation and the rhizosphere environment. Under conditions where water stress leads to rapid evaporation, drip or subsurface irrigation can minimize non-productive evaporation and maintain soil moisture, controlled-release fertilizers ensure stable nitrogen supply, and plastic mulching effectively suppresses water loss. The coordinated implementation of these measures enables integrated regulation of the water–nitrogen–soil system, thereby achieving high crop productivity alongside environmental sustainability.

7. Conclusions and Prospect

Water and nitrogen are fundamental for sustaining high and stable yields as well as improving resource use efficiency in wheat and maize production systems. Their migration, transformation, and uptake within the soil–plant continuum depend strongly on the coordinated regulation of irrigation patterns, fertilization strategies, and soil management practices. This article synthesizes current research on the mechanisms governing water and nitrogen movement, emphasizing key processes within agricultural ecosystems. It further evaluates how irrigation methods, fertilization regimes, and soil management practices influence water–nitrogen dynamics, and briefly examines the potential interactions among these factors.

Existing studies show that irrigation methods regulate nitrogen migration pathways and availability within the soil profile by altering moisture distribution and aeration. Fertilization strategies affect nitrogen uptake efficiency and the risk of nitrogen loss by determining its release rate, spatial distribution, and alignment with crop demand. Soil management practices (such as straw and biochar incorporation and mulching) can improve soil structure, enhance water and fertilizer retention, and indirectly regulate water and nitrogen migration and crop uptake by altering microenvironmental conditions. These three mechanisms exhibit strong complementarity and synergistic potential; their coordinated application can enhance yield while improving resource-use efficiency and environmental sustainability.

Despite notable progress, several challenges remain. Most existing research focuses on single-factor effects, with limited comprehensive analyses of the interactive influences of irrigation, fertilization, and soil management. Mechanistic understanding and quantitative simulation of the spatiotemporal coupling of water and nitrogen remain insufficient. In addition, studies assessing the applicability of different control measures across diverse regions, climates, and soil conditions are still scarce.

Therefore, future research urgently needs to:

• Elucidate microscale mechanisms of water and nitrogen migration under different management practices to improve understanding of process-level dynamics.
• Strengthen integrated understanding of the crop–rhizosphere–soil system, focusing on interactions between roots, microbes, and soil nutrient–water processes.
• Establish an integrated research framework combining field measurements and model simulations to enhance predictive capacity under multi-factor synergistic controls.
• Develop regionally adaptable management strategies for irrigation, fertilization, and soil fertility, tailored to local cropping conditions, to support resource-conserving and environmentally sustainable agriculture.