1 Introduction

Maize is one of the world’s major staple crops and plays a central role in food, feed, and industrial production, so improving its productivity remains a major agronomic priority. Nitrogen is the mineral nutrient most directly linked to maize growth because it underpins photosynthetic activity, dry matter accumulation, and final grain formation. In maize-based systems, nitrogen fertilization is therefore one of the most important management practices for sustaining yield, but the agronomic value of nitrogen depends not simply on how much is applied, but on how effectively it is matched to crop demand across space and time (Gheith et al., 2022). This is especially important because maize nitrogen uptake is low during early vegetative growth and then rises rapidly, creating clear opportunities to improve performance through better rate, timing, and placement strategies. Across maize production regions, proper nitrogen management can raise grain yield, strengthen nitrogen recovery, and reduce environmental losses, whereas inefficient practices leave a large share of fertilizer outside the plant-soil system of productive use. Reported maize NUE remains low at about one-third globally, reflecting substantial losses through leaching, denitrification, and volatilization (Davies et al., 2020). This inefficiency has both economic and ecological consequences, including wasted fertilizer inputs, soil nitrogen accumulation, water pollution, and broader environmental risk. Recent synthesis and field studies also show that increasing nitrogen rate alone does not guarantee greater yield or efficiency; instead, optimum outcomes often occur within moderate application ranges and depend strongly on management context, including fertilizer timing, placement, and interactions with soil moisture (Jiang et al., 2025). For this reason, optimizing nitrogen management in maize is not only a question of maximizing yield, but also of coordinating nutrient supply with crop uptake to achieve high productivity, high resource-use efficiency, and sustainable production.

However, improper nitrogen application remains common in maize production, and both insufficient and excessive supply can constrain root development, reduce fertilizer-use efficiency, and ultimately limit yield formation. Evidence from field experiments shows that root responses to nitrogen are highly plastic but not linearly positive: nitrogen deficiency suppresses root biomass and accelerates deep-root senescence, while excessive nitrogen can inhibit vertical root expansion, restrict deep soil exploration, and reduce later resource capture. Under drought stress, lowering basal nitrogen from a high level improved root elongation, increased fine-root development in deeper soil, and produced a more favorable root length density distribution, which in turn increased both water use efficiency and nitrogen use efficiency relative to high nitrogen input. Similarly, under alternate partial root-zone irrigation, moderate water supply combined with a reasonable nitrogen rate enhanced total root growth, optimized vertical root distribution, and produced the greatest NUE, whereas unmatched low-water/high-nitrogen conditions impaired root growth and reduced efficiency (Qi and Hu, 2022). Long-term excessive nitrogen application also appears to damage root function even when some root size traits increase, because high nitrogen can lower root activity, nitrate reductase activity, and active absorbing area, thereby weakening post-silking nitrogen uptake and depressing NUE (Ren et al., 2024). Other work likewise shows that optimized rather than excessive nitrogen promotes longer root systems, larger root surface area, and better root distribution, whereas excessive nitrogen can inhibit further yield gain despite continued fertilizer input. At the production scale, high nitrogen rates often increase yield only marginally or not at all while substantially lowering NUE, a pattern observed across studies of nitrogen rate, timing, and irrigation management. These findings indicate that improper nitrogen use does not merely waste fertilizer; it alters root architecture and root physiological performance in ways that weaken the plant’s capacity to acquire nitrogen efficiently from the soil profile.

Against this background, understanding the relationship between nitrogen application methods, root distribution, and nitrogen utilization has become increasingly important for designing more efficient maize fertilization strategies. Roots are the primary organs for water and nutrient acquisition, and their morphology, depth, and spatial arrangement determine how effectively plants intercept soil nitrogen across heterogeneous soil layers. Studies focused on fertilizer placement show that nitrogen application method can directly reshape root distribution. Deep placement of nitrogen at suitable depth promoted greater subsoil root length density and deeper rooting, while a 15% reduced nitrogen rate placed at 15 cm increased grain yield, nitrogen recovery efficiency, agronomic efficiency, and partial factor productivity compared with conventional shallow application. Split nitrogen application also improved root length density and increased the proportion of roots in the 20-60 cm soil layer, thereby enhancing nitrogen absorption efficiency and yield under warming stress. In wide-narrow row systems, changing the horizontal position of strip-applied nitrogen altered the spatial matching between roots and nutrient patches and improved agronomic efficiency under high nitrogen conditions. Layered fertilization further supports this view: compared with single-layer placement, distributing nitrogen across soil depths stimulated both crown and lateral roots, improved nutrient acquisition strategy, and increased yield as well as nitrogen absorption efficiency. Related evidence from subsoiling plus layered nitrogen application and from deeply buried straw combined with medium nitrogen also shows that management practices that increase nitrogen availability in deeper layers can promote deeper root growth, greater nitrogen uptake, and higher partial factor productivity or recovery efficiency. Because NUE is closely related to root morphological traits such as root length, surface area, volume, and vertical distribution, clarifying how different nitrogen application methods regulate root system deployment is essential for explaining differences in nitrogen capture and utilization among management regimes. Therefore, research on different nitrogen application methods should move beyond simple comparisons of yield or fertilizer rate and instead focus on how fertilization patterns modify the spatial coordination between soil nitrogen supply and root growth, thereby determining nitrogen uptake efficiency, post-silking nutrient accumulation, and final grain production. In this context, investigating the effects of different nitrogen application methods on root distribution and nitrogen use efficiency in maize is scientifically important for identifying fertilization practices that simultaneously sustain yield, improve resource efficiency, and reduce environmental risk.

2 Maize Root Distribution and Its Responses to Nitrogen Supply

2.1 Spatial structure and dynamic distribution of maize roots

The spatial structure of the maize root system is hierarchical and strongly stratified in soil, with different root classes contributing differently to soil exploration across depth and direction. Architectural traits such as rooting depth, growth angle, and branching density regulate the vertical and horizontal deployment of roots, thereby influencing access to soil nutrients across heterogeneous layers (Bavasaran et al., 2026). Across development, root characteristic parameters such as root length, surface area, projection area, and volume tend to rise toward silking and then decline thereafter, indicating that maize root distribution is not static but dynamically reorganized with crop growth stage.

Field studies further show that maize roots redistribute in response to stand structure and soil domain occupancy. Under high planting density, root length and biomass per plant decline, lateral root density is reduced, and the root system shifts toward “intra-row contraction and inter-row extension,” while vertical proliferation remains concentrated mainly in the upper 0-36 cm layer. Genotypic comparisons similarly indicate that higher-yielding hybrids under crowding often maintain medium root size, steeper angles, and greater inter-row distribution with lower variability among neighboring plants, suggesting that efficient spatial deployment depends not only on root size but also on coordinated distribution patterns (Shao et al., 2019). The three-dimensional distribution of root length density across soil layers is therefore best understood as a dynamic product of developmental stage, plant competition, and environmental resource gradients rather than as a fixed varietal trait.

2.2 Regulation of maize root growth by soil nitrogen availability

Soil nitrogen availability regulates maize root growth through both morphological and physiological pathways. Under nitrogen deficiency, maize commonly enhances root foraging by increasing total root length, surface area, root volume, and biomass allocation to roots, allowing exploration of a larger soil volume for nutrient capture (Keerthi et al., 2025). At the same time, genotypic evidence shows that nitrogen-efficient materials often express this response not as indiscriminate branching, but as stronger elongation of axial and especially lateral roots across growth layers, including deeper zones, which contributes directly to improved nitrogen foraging and utilization (Figure 1).

Figure 1 Conceptual framework illustrating the nonlinear regulation of maize root growth and spatial nitrogen foraging by soil nitrogen availability. Low nitrogen availability promotes root proliferation and deeper soil exploration, whereas moderate nitrogen availability supports balanced root development and efficient nitrogen acquisition; excessive nitrogen may suppress root morphological development and increase redundant or horizontally biased root growth

The effects of nitrogen availability are nonetheless nonlinear, and excessive nitrogen does not continue to improve root performance. Under coupled water-nitrogen treatments, moderate nitrogen promoted larger root morphological parameters, whereas high nitrogen reduced them and favored more horizontal extension, suggesting that nitrogen imbalance can distort normal vertical development. Consistent with this, field evidence under rain-fed conditions shows that increasing nitrogen rate mainly stimulated root traits in the shallow 0-40 cm layer, but yield and water use efficiency generally did not differ between 240 and 300 kg/ha, indicating that additional nitrogen can promote redundant root growth without proportional agronomic benefit (Zhang et al., 2022). Thus, maize root response to soil nitrogen reflects an optimum range in which nitrogen is sufficient to sustain active root growth but not so excessive that architectural efficiency and resource-use return begin to decline.

2.3 Adaptation and plasticity of roots to spatial heterogeneity of nitrogen

Maize roots exhibit pronounced plasticity in response to spatially heterogeneous nitrogen supply, and this plasticity is a major mechanism by which the crop improves nutrient capture under limiting or patchy conditions. When nitrogen is unevenly distributed, roots proliferate preferentially in nutrient-rich zones, but the response depends on root type, local competition, and whole-plant carbon allocation. Modeling and experimental evidence further indicates that plastic responses such as increased root-to-leaf biomass allocation and reduced axial root emergence can raise root length density in the 15-30 cm layer, where much of the later nitrogen uptake occurs, thereby increasing total nitrogen uptake under low-nitrogen conditions (Lu et al., 2024).

This plasticity also explains why nitrogen placement method can reshape root distribution and ultimately nitrogen use efficiency. Deep or layered fertilizer placement shifts nutrient availability downward, stimulating lateral root proliferation in subsurface layers, whereas shallow placement favors crown root concentration near the surface (Liu et al., 2025). A similar principle appears in intercropping systems, where maize adjusts root length density and root distribution according to local inorganic nitrogen concentration and can substantially increase nitrogen uptake per unit root length during recovery growth. These findings show that maize root plasticity is not merely a stress symptom; it is an adaptive trait that links heterogeneous nitrogen supply with root redistribution, nutrient acquisition, and the final efficiency of fertilizer use. Root distribution in maize is therefore both developmentally programmed and highly plastic. Across the evidence, nitrogen supply affects not only how much root is produced, but where roots are deployed in the soil profile, which is the key link between fertilization method and nitrogen use efficiency.

3 Different Nitrogen Application Methods and Their Effects on Soil Nitrogen Distribution

3.1 Conventional nitrogen application methods: broadcasting, banding, and spot application

Broadcasting remains the simplest and most widely used nitrogen application method, but it usually leaves fertilizer concentrated near the soil surface and therefore more exposed to volatilization, leaching, and early mismatch with maize root uptake. In maize systems, this problem is especially evident when basal nitrogen is broadcast before roots are fully developed, because the fertilizer can be lost before peak crop demand and before lateral roots effectively intercept it (Haque et al., 2026). By contrast, band placement creates a more localized nitrogen-rich zone closer to the developing root system, which generally improves root-fertilizer contact and reduces exposure of fertilizer N to loss pathways. Evidence from isotopic tracing outside maize also supports the same placement principle, showing that banded urea reduced overall nitrogen loss relative to broadcast application while retaining more residual fertilizer N in the soil profile.

The effect of conventional methods on soil nitrogen distribution depends mainly on how concentrated the fertilizer zone is and how far it lies from active roots. Surface broadcasting tends to produce diffuse nitrogen distribution in the upper soil layer, whereas side-banding concentrates nutrients within a narrow strip near the sowing line, and deep banding shifts that enrichment downward into the subsoil (Sharifi et al., 2024). This spatial contrast matters because nitrogen becomes more available when fertilizer is placed at a reasonable horizontal distance and vertical depth relative to root elongation, while placement too far from roots can reduce accessibility and placement too close to seeds can damage emerging tissues. Localized placement methods such as banding or dotting also exploit the tendency of maize roots to proliferate in nutrient-rich patches, thereby strengthening the coupling between soil nitrogen hotspots and root growth. As a result, compared with broadcasting, conventional localized placement methods generally create more stable and functionally useful patterns of soil nitrogen heterogeneity for maize uptake.

3.2 Basal, topdressing, and split nitrogen application

The timing of nitrogen application strongly alters soil nitrogen distribution across the growing season, even when the total nitrogen rate remains unchanged. A single basal application usually creates a large early-season pool of mineral nitrogen that is vulnerable to loss before maize enters its period of rapid uptake, whereas topdressing shifts nitrogen supply closer to the stages when crop demand rises sharply (Jiang et al., 2025). Meta-analytic evidence indicates that topdressing increases the yield benefit of nitrogen application relative to basal fertilization, reflecting better synchronization between soil nitrogen availability and plant demand. In rainfed and coastal maize systems, delaying the first application or reducing the basal share can therefore limit unnecessary early accumulation of available nitrogen in surface soil and improve the efficiency of later nitrogen capture. A lower basal-to-topdressing ratio shows the same tendency, with the 2:8 treatment outperforming 4:6 for yield, nitrogen uptake, and recovery efficiency while requiring a lower effective nitrogen rate.

Split nitrogen application further refines this seasonal redistribution by dividing fertilizer into multiple doses and maintaining available nitrogen during successive critical growth stages rather than creating one short-lived pulse. Across field studies, multi-split topdressing increased grain yield and several NUE indices compared with conventional recommendations, largely because nitrogen availability remained positively aligned with crop uptake at V8 and tasseling (Meena et al., 2021). Under slightly saline coastal conditions, three equal side-dressed splits at 15, 35, and 60 days after sowing produced the highest grain, stover, and root yields, and the advantage was attributed to reduced early nitrogen loss and more continuous nitrogen supply. Under soil warming, split application also increased root length density and improved root distribution in the 20-60 cm layer, indicating that timing can modify not only temporal nitrogen supply but also the effective depth at which roots exploit soil nitrogen. Even so, responses are not completely uniform across environments, and some dryland studies report little advantage or even lower yield with more split applications when water limits nitrogen movement and uptake.

3.3 Deep placement, side-band placement, and precision nitrogen application

Deep placement changes soil nitrogen distribution more directly than timing-based methods because it transfers fertilizer nitrogen from the surface layer into deeper zones that are less exposed to volatilization and more closely aligned with later root activity. In maize, deep band placement commonly increases soil nitrogen availability in subsurface layers while reducing nitrogen concentration in the upper soil profile, which helps lower near-surface losses and promotes more effective nitrogen capture. Field evidence shows that placing nitrogen at 15 cm depth with a 15% reduced application rate increased subsoil root length density, rooting depth, grain yield, and several NUE indices compared with the conventional 5 cm placement at the full rate. Controlled-release urea shows a similar pattern: application depth significantly changed the spatial distribution of soil nitrate-N and total root length, and a 12 cm placement improved NUE while maintaining yield under reduced nitrogen inputs. There is nonetheless an optimum depth range, because very deep placement can move nitrogen beyond the most active root zone and reduce yield, as seen when 35 cm placement lowered performance relative to 25 cm (Wu et al., 2021).

Side-band and precision nitrogen application methods extend the same principle by placing fertilizer where competitive overlap is reduced and where root proliferation can be steered toward nutrient-rich microsites. In wide-narrow row maize, changing nitrogen position altered soil nitrogen heterogeneity and root distribution, and placement on the non-competitive side improved agronomic efficiency and partial factor productivity under high nitrogen supply. Local nitrogen application by banding or dotting likewise increased deep root growth of responsive genotypes, with root length at 40-60 cm increasing by 66-75% and post-silking nitrogen uptake increasing by 16-88%. Layered fertilization represents a more advanced precision strategy because it distributes nitrogen across multiple depths rather than a single band, thereby stimulating both crown and lateral roots while increasing nitrogen absorption efficiency and yield. More broadly, meta-analysis suggests that optimized nitrogen application patterns combining controlled-release and conventional urea can produce the strongest gains in nitrogen uptake, yield, and water use efficiency, supporting precision management as a practical route to better soil nitrogen distribution and higher maize nitrogen-use efficiency. 

4 Effects of Different Nitrogen Application Methods on Maize Root Distribution

4.1 Effects on vertical root distribution and deep root development

Different nitrogen application methods clearly shift the vertical distribution of maize roots by changing where available nitrogen accumulates in the soil profile. Deep placement at 16-24 cm increased nitrate and available phosphorus in the 16-32 cm layer, enhanced lateral root length density and surface area, and raised the proportion of roots in the 8-32 cm layer, whereas shallow 8 cm placement concentrated 32.7%-35.5% of roots in the 0-8 cm layer. A separate field study likewise showed that placing nitrogen at 15 cm depth, even with a 15% lower N rate, increased subsoil root length density and rooting depth compared with conventional 5 cm placement (Ren et al., 2024).

Timing-based methods can also alter vertical root deployment when they improve the match between nitrogen supply and later crop demand. Under split application during warming stress, root length density increased and a larger share of roots occupied the 20-60 cm layer, indicating better exploitation of middle soil layers . Under alternate partial root-zone irrigation, a moderate nitrogen rate combined with moderate water supply promoted deeper roots and optimized vertical root distribution, whereas higher nitrogen did not further improve late-stage root length, surface area, or volume. The depth-dependent response is also visible in reduced-rate controlled-release systems, where fertilization depth significantly changed the spatial distribution of soil nitrate-N and total root length, supporting the view that root deepening depends on where fertilizer N is positioned rather than on total N alone. The depth pattern and treatment contrasts are visualized in a source figure showing how fertilization depth reorganized root distribution and associated root traits.

4.2 Horizontal architecture

Nitrogen application method also regulates horizontal root expansion by changing the position of nutrient-rich patches and the competitive geometry among neighboring plants. In wide-narrow row systems, applying nitrogen on the wide-row side increased total root biomass, total root surface area, and root length relative to narrow-row placement, apparently by pulling growth toward the non-competitive side. The same study interpreted wide-row placement as a “resource pull” that increased the surface area and length of fine roots, showing that horizontal root redistribution is a direct response to localized nutrient position rather than only to nitrogen amount (Zhou et al., 2024).

More broadly, maize root architecture is highly plastic under heterogeneous nitrogen supply, and that plasticity is central to nitrogen acquisition efficiency. Localized nutrient supply produced far stronger root proliferation than broadcast application, increasing root length density by 230%-584% and root respiration by 97%-115%, which indicates that concentrated nutrient patches can reshape overall root system architecture rather than simply enlarging roots uniformly. Not all horizontal expansion is beneficial, however: under water-nitrogen stress, high nitrogen caused lower root density than moderate nitrogen and a tendency toward greater horizontal extension, suggesting an adaptive but less efficient response to nitrogen imbalance.

4.3 Physiological activity

Nitrogen application methods affect not only root placement but also root physiological activity, which is more tightly linked to nitrogen uptake than root size alone. Delaying slow-release fertilizer to the six-leaf stage increased rhizosphere total nitrogen, raised root antioxidant enzyme activities, and increased mean root activity by 20.5% at silking and 23.5% at milking relative to conventional fertilization. Excess nitrogen showed the opposite pattern in a long-term field trial: high N increased root length and surface area but reduced root activity, nitrate reductase activity, and root active absorbing area, which helps explain why NUE failed to improve under excessive N (Ren et al., 2024).

Placement method also modifies rhizosphere processes by changing exudation, microbial activity, and enzyme hotspots around roots. Deep and layered fertilization increased root hair length, root phosphatase activity, and expression of N and P transport genes, indicating that targeted placement can shift the root system toward a more acquisitive physiological strategy. At the rhizosphere scale, maize altered root exudate composition in response to N supply, and N deficiency expanded hydrolase activity hotspots through associated bacterial changes, whereas N addition narrowed the rhizosphere extent of BG and NAG activities by 48% and 39% (Hao et al., 2022). Fertilizer rate also changed exudate chemistry and the rhizosphere microbiome directly, further supporting that nitrogen management influences NUE partly through biologically mediated rhizosphere pathways. Localized nutrient supply fits this same pattern, because it lowered rhizosphere pH, increased acid phosphatase activity, and strengthened coordination between root morphology, physiology, and nutrient uptake. Across these three facets, the evidence shows that nitrogen application methods influence maize NUE mainly by changing where roots grow and how actively they function . Methods that improve spatial matching between fertilizer N and active roots tend to favor deeper or better-targeted root distribution, stronger physiological activity, and more efficient rhizosphere nutrient capture.

5 Effects of Different Nitrogen Application Methods on Nitrogen Use Efficiency and Maize Yield

5.1 Nitrogen use efficiency and its evaluation indicators

Nitrogen use efficiency in maize is not a single trait, but a composite index describing how effectively applied nitrogen is absorbed, retained, and converted into grain or biomass. At the whole-system level, NUE can be expressed as a simple N mass-balance ratio, while at the crop level it is commonly separated into nitrogen uptake efficiency and nitrogen utilization efficiency, which respectively describe the plant’s capacity to acquire N from soil and to convert absorbed N into biomass or yield. This distinction is important because nitrogen management practices can improve one component more than the other, and total NUE only increases when both uptake and utilization are reasonably coordinated (Gheith et al., 2022).

In agronomic studies, the most widely used indicators are agronomic efficiency, recovery efficiency, and partial productivity, because these metrics directly connect fertilizer input with grain output or plant N accumulation. Agronomic efficiency is defined as the increase in grain yield per unit N applied, whereas recovery efficiency represents the increase in aboveground N accumulation per unit N applied. Other studies additionally distinguish physiological efficiency, nitrogen harvest index, and partial factor productivity, which helps reveal whether a treatment mainly improves N capture, internal N remobilization, or final yield formation (Adeoluwa et al., 2022).

5.2 Effects of nitrogen application methods on nitrogen uptake and yield formation in maize

Nitrogen application methods affect maize N uptake and yield formation primarily by changing the timing, depth, and persistence of available nitrogen in relation to crop demand. Synchronizing N supply with maize uptake generally improves both productivity and efficiency, and split or delayed in-season applications often outperform a single early application because they reduce losses before the rapid N-demand phase. Under some environments this effect is large: a three-way split increased grain yield to 12.6-15.7 Mg·ha⁻¹ and improved agronomic and recovery efficiency by about 30% relative to fall or preplant treatments on irrigated coarse-textured soil. The same logic appears in coastal maize, where delaying the first N application and applying three side-dressed splits significantly improved grain, stover, and root yield compared with conventional practice.

Placement and fertilizer type can further improve N accumulation and yield by increasing the fraction of applied N intercepted by roots during later growth. Deep placement with a 15% lower N rate at 15 cm increased late biomass and N accumulation, raising grain yield by 3.9%, recovery efficiency by 66.7%, agronomic efficiency by 38.5%, and partial factor productivity by 22.1% relative to conventional shallow placement (Cheng et al., 2020). Controlled-release and mixed-source strategies show a similar pattern: in the Loess Plateau, slow/controlled-release urea mixed with normal urea gave the largest gains in maize N uptake, yield, and water use efficiency, while in field experiments the controlled-release treatment at 270 kg/ha outperformed the farmer practice of 420 kg/ha in both grain yield and apparent N utilization rate. The yield and NUE tradeoff across N rates is shown clearly in a source table comparing grain yield with agronomic and recovery efficiency under different timing treatments (Figure 2).

5.3 Synergistic relationship between root distribution and nitrogen use efficiency

The improvement in NUE under optimized N management depends strongly on how nitrogen placement reshapes root distribution. Deep placement, local placement, and layered fertilization all increase root proliferation in soil zones where nitrogen remains available later in the season, which promotes sustained N uptake after silking and improves final yield (Chen et al., 2022). This relationship is functionally important because local N application increased post-silking N uptake by 16%-88% in responsive genotypes and increased grain yield by 10%-12% in one genotype, while deeper root length at 40-60 cm was positively associated with yield. Modeling evidence also indicates that maize can combine high yield and high N uptake when root biomass is not excessive but root length density is high at 15-45 cm depth.

Root effects on NUE are not purely morphological; they also depend on root physiological activity and the efficiency of N transport to grain. Recommended rather than excessive N increased post-silking N uptake, NUE, and grain yield by maintaining greater root surface area, root activity, nitrate reductase activity, and root active absorbing area, whereas high N reduced these functional traits even when root length increased. Layered fertilization similarly improved yield and N absorption efficiency by stimulating both crown and lateral roots and shifting the root system toward a more acquisitive nutrient-capture strategy. More broadly, root traits such as root activity, length, surface area, and dry weight are strongly associated with N uptake efficiency, reinforcing that fertilization methods raise NUE most effectively when they improve both the spatial distribution and physiological performance of the root system (Jan et al., 2025). Overall, different nitrogen application methods improve maize yield most efficiently when they increase N recovery rather than simply increasing N rate. The strongest evidence supports methods that better synchronize N supply with crop demand and place nitrogen where active roots can sustain post-silking uptake.

6 Interactions Among Nitrogen Application, Soil, Roots, and Rhizosphere Microorganisms

6.1 Effects of nitrogen application methods on soil nitrogen transformation and availability

Different nitrogen application methods alter soil nitrogen transformation by changing the balance among mineralization, immobilization, nitrification, and nitrate retention. Soil N availability is fundamentally produced by decomposition, mineralization, and immobilization processes, and their net outcome varies with soil type and management. In maize systems, moderate N input under integrated water-fertilizer management increased rhizosphere N fractions across jointing, silking, and grain filling, with nitrate-N and dissolved organic N remaining elevated over time, indicating a more sustained N supply than under zero-N conditions (Huang et al., 2026). This pattern supports the view that N management should be evaluated by its effects on temporal N availability, not only by total N applied.

Placement and accompanying soil management further determine where available N is retained in the soil profile and whether loss pathways are intensified. Layered or deeper fertilization increased nitrate concentration in the 16-32 cm layer and optimized nutrient distribution relative to shallow placement, thereby improving the overlap between soil N supply and active maize roots (Liu et al., 2025). By contrast, excessive N can accelerate loss-prone microbial pathways: high N increased denitrification genes such as nirK and norB in the maize rhizosphere, while other studies show excess N can reduce soil nitrate content together with pH and increase the abundance of denitrification and dissimilatory reduction genes, changes consistent with greater N loss risk.

6.2 Microbial communities

Nitrogen application also reshapes rhizosphere microbial communities, and these microbial shifts feed back on N utilization by changing functional stability, N-cycling capacity, and the abundance of beneficial taxa. In maize, roots recruit specific rhizosphere microorganisms under different N rates, and excessive N can destabilize microbial co-occurrence networks while increasing taxa associated with denitrification and potential N loss. Under integrated maize water-fertilizer management, moderate N increased bacterial richness and shifted community composition toward groups such as Pseudomonadota that are associated with plant growth promotion and N regulation, whereas excessive N did not further improve soil N fractions and tended to weaken efficiency (Huang et al., 2026).

Microbial responses to N are mediated in part by changes in rhizosphere carbon flow and exudation. Long-term N fertilization elevated rhizosphere organic and active carbon, and shifts in bacterial genera were significantly correlated with organic acids in root exudates, indicating that N input modifies microbiomes partly through altered carbon chemistry in the root zone. Experimental N enrichment likewise increased the abundance of rhizosphere genes involved in C, N, P, and S cycling, and these increases were driven mainly by greater plant biomass, organic acids in exudates, and dissolved organic carbon, showing that the microbial effects of fertilization are strongly plant-mediated rather than purely chemical (Liao et al., 2024).

6.3 Root-microbe synergy

Root-microbe synergy promotes nitrogen uptake when N management creates rhizosphere conditions that support both active roots and cooperative microbial functions. Beneficial microorganisms can enhance plant N acquisition by increasing rhizosphere N availability, stimulating root system development, and modulating the expression or activity of nitrate and ammonium uptake pathways (Albornoz and Godoy, 2025). More generally, the rhizosphere is the central interface where plant roots, soil, and microorganisms interact, and N transformation is ultimately a microbially mediated biochemical process whose efficiency depends on habitat conditions such as N source, carbon supply, water, and pH.

Evidence from crop systems shows that this synergy is not abstract but mechanistically linked to root traits and rhizosphere metabolites. In maize under N deprivation, root-derived flavones enriched Oxalobacteraceae, which in turn promoted plant growth and N acquisition, while lateral root development coordinated these reciprocal root-microbiome interactions (Yu et al., 2021). Multi-omics evidence from another crop further shows that host-controlled bacterial taxa can promote lateral root development and explain substantial variation in N uptake, supporting the broader conclusion that root architectural regulation and microbiome assembly jointly determine nutrient acquisition efficiency. Overall, nitrogen application influences maize NUE not only through direct fertilizer supply, but through coordinated effects on soil N transformation, rhizosphere microbial assembly, and root-microbe cooperation. The strongest pattern across the literature is that moderate, well-placed N inputs create the most favorable soil-root-microbe coupling for sustained N availability and efficient uptake.

7 Case Study: Responses of Maize Root Distribution and Nitrogen Use Efficiency to Different Nitrogen Application Methods

7.1 Root distribution and nitrogen use efficiency under broadcasting and banding

Under broadcasting, nitrogen remains concentrated near the soil surface and is often less synchronized with early root access, so part of the applied N can be lost before peak maize uptake. Surface broadcasting was described as simple and common, but it often places nutrients beyond the most effective root zone and increases the risk of leaching, volatilization, and poor recovery (Sharifi et al., 2023). By contrast, side-band and deep-band placement place fertilizer close to emerging roots and increase root-fertilizer contact, which improves plant N status and reduces N loss; in one comparison, root-zone urea placement reduced N losses by 20%-25% relative to surface broadcast. This contrast is reflected in root-zone fertilization systems, where one-time root-zone placement reduced N loss by 11%-24% compared with split surface broadcasting while maintaining a more effective nutrient supply zone around roots.

The agronomic consequences of this spatial difference are substantial. Deep band placement increased leaf and stem N concentration and produced greater root and shoot N accumulation than side banding or broadcasting at moderate-to-high N and P rates, indicating a stronger capacity to sustain nutrient capture through the season. In field maize, one-time root-zone fertilization increased grain yield by 4% and N apparent recovery efficiency by 18% relative to split surface broadcasting, while the optimal N rate could be reduced to 180-225 kg·ha⁻¹, about 14% lower than the broadcast-based system. A second field study likewise found that root-zone fertilization increased total plant N uptake by 7.2%-13.5% and maize yield by 11.5% compared with split surface broadcasting, showing that localized placement improves NUE not only by reducing loss but by increasing the fraction of applied N actually intercepted by the crop (Jiang et al., 2018).

7.2 Split and deep application

Split and deep N application show a stronger root-nitrogen synergy because they improve both the timing and the depth of nutrient availability. In split systems, delayed and side-dressed N better matches maize demand during rapid vegetative growth and reproductive development; in slightly saline coastal soil, three equal side-dressed applications at 15, 35, and 60 DAS produced the highest grain, stover, and root yield, and this benefit was attributed to better synchronization and lower early-season loss. Under soil warming, split N also increased root length density by 8.54%, increased root bleeding rate by 8.57%, and shifted more roots into the 20-60 cm layer, showing that timing can actively reshape root distribution rather than merely supplying more N (Xia et al., 2025). In rainfed Vertisols, skipping basal N and applying 120 kg·ha⁻¹ in two late splits at V8 and VT increased grain yield, agronomic efficiency, partial factor productivity, physiological efficiency, and recovery efficiency over conventional recommendations, again supporting the value of synchronizing N with active uptake stages.

Deep placement complements split timing by moving N into the soil layers where later root proliferation and post-silking uptake are most effective. Placing N at 15 cm depth with a 15% lower rate increased subsoil root length density and rooting depth, then raised grain yield by 3.9%, recovery efficiency by 66.7%, agronomic efficiency by 38.5%, and partial factor productivity by 22.1% compared with conventional shallow placement. Local N application also increased root length at 40-60 cm by 66%-75% in responsive genotypes, and post-silking N uptake rose by 16%-88%, linking deeper localized rooting directly to greater late-season N capture and yield. The depth effect is not unlimited, however: studies comparing multiple fertilization depths found that about 25 cm often outperformed shallower placement, but excessively deep placement reduced root activity or yield, so deep placement works best within an optimum depth window rather than at maximum depth. (Wu et al., 2022)This depth response is illustrated by a field comparison showing how root length distribution shifts under layered versus surface-oriented fertilization.

7.3 Strategy selection

A comprehensive evaluation of optimal N application strategies should therefore weigh root distribution, NUE, yield, N loss, and local soil conditions together rather than treating any single method as universally superior. Layered fertilization performed especially well because it avoided the tradeoff between shallow crown-root concentration and deep lateral-root proliferation: compared with single-layer application, layered N stimulated both root classes and improved yield by 7.8%-24.6% and N absorption efficiency by 11.1%-43.5%. In the North China Plain, subsoiling combined with layered fertilization also increased root length in the 20-40 cm layer, enhanced dry matter and N accumulation, and improved grain yield and N uptake efficiency at 180-240 kg·N·ha^-1, showing that placement strategy and soil physical improvement can act synergistically. These results suggest that the best strategy is often one that improves both nutrient positioning and the soil environment in which roots proliferate.

Rate optimization remains essential because more nitrogen does not necessarily produce better roots or higher NUE. Long-term high N increased root length and surface area but reduced root activity, nitrate reductase activity, and root active absorbing area, so grain yield and NUE did not exceed the recommended N treatment. Similarly, a rate study found that N225 produced the most favorable root distribution and deeper infiltration scale, whereas excessive N inhibited early deep root growth and later root development despite larger external supply (Su et al., 2020). For practical selection, the strongest overall pattern is that maize performs best when N is placed close enough to roots to ensure interception, deep enough to support subsoil foraging and late uptake, and timed or layered to match crop demand under the local soil and climate context.

8 Efficient Nitrogen Management and Sustainable Maize Production

8.1 Optimization of nitrogen placement and application depth based on root distribution

Optimizing nitrogen placement should begin with root distribution, because maize responds strongly to where nitrogen is positioned in the soil profile. Shallow placement at 8 cm mainly stimulated crown roots and concentrated 32.7%-35.5% of roots in the 0-8 cm layer, whereas deeper placement at 16-24 cm increased nitrate and available phosphorus in the 16-32 cm layer and promoted greater lateral root proliferation in deeper soil. This depth differentiation is agronomically important because moderate nitrogen deficiency can increase axial root elongation in maize, showing that root distribution is plastic and can be steered by nutrient location rather than fertilizer amount alone (Xiong et al., 2021). Layered nitrogen placement appears especially effective because it stimulates both crown and lateral roots simultaneously, improving yield and nitrogen absorption efficiency while shifting the root system toward a more acquisitive strategy.

Placement strategy should therefore match fertilizer depth to the most active root zone rather than maximizing surface supply. Reviews of wheat-maize systems indicate that optimized nitrogen strategies work best when N release is aligned with crop demand at critical stages and when irrigation and soil management are coordinated to stabilize nitrogen in the root zone (Zhao et al., 2025). This logic is consistent with broader soil evidence showing that nitrogen availability varies with soil type, climate, depth, and management, so fertilizer placement cannot be separated from the physical environment through which roots proliferate. In practice, the most sustainable root-based strategy is to avoid both very shallow placement, which overconcentrates roots near the surface, and indiscriminate deep placement, and instead use layered or depth-optimized placement that improves overlap between root distribution and subsurface nitrogen supply.

8.2 Integrated water-fertilizer management

Integrated water and fertilizer management improves maize performance by coordinating nitrogen availability with soil moisture, which stabilizes nutrient supply across time and space. Under fertigation in a semi-arid system, moderate nitrogen input of 140 kg/ha maintained higher rhizosphere nitrate-N and dissolved organic N through jointing, silking, and grain filling, and produced the highest grain yield, 25.7% above the zero-N treatment. In the same management context, excessive N did not further raise nitrogen fractions and appeared to reduce nitrogen-use efficiency, indicating that smart management depends more on precise synchronization than on higher fertilizer rates (Huang et al., 2026). A two-year drip-fertigation study similarly found that irrigation-fertilization interactions significantly affected grain N accumulation, and that balanced combinations could simultaneously maintain high yield, economic return, and lower soil nitrate residue.

Smart precision nitrogen application extends this principle by delivering smaller or better-targeted doses close to the root zone. Under subsurface drip irrigation, applying N in five equal root-zone splits saved 25% fertilizer N compared with conventional broadcasting in two splits, while improving nitrogen uptake efficiency and NUE in maize (Gupta et al., 2023). Precision fertilization with drip irrigation also reduced nutrient inputs by 8%-9% without reducing maize growth or grain yield, while increasing nutrient and rainfall use efficiency and net profit. More recent fertigation evidence shows that a moderate combination of irrigation and N can outperform conventional local practice: 47.25 mm irrigation with 240 kg·N·ha⁻¹ reduced water and N inputs by 10.0% and 33.3%, respectively, while increasing yield, irrigation water-use efficiency, and nitrogen partial factor productivity.

8.3 Nitrogen reduction and yield stability

Coordinated nitrogen reduction is most effective when lower N rates are paired with improved fertilizer form, timing, or water management rather than simple fertilizer cuts. In Northwest China, farmer practice at 420 kg·N·ha^-1 carried a clear penalty, with higher nitrate and ammonium residues and greater downward migration into deep soil, whereas optimized strategies using controlled-release N or organic-inorganic combinations maintained high yield with only 270-300 kg·N·ha^-1 and lower leaching risk. In the same system, the controlled-release treatment produced the highest grain yield and higher apparent N utilization than farmer practice, showing that rate reduction can be compatible with yield stability when release pattern and placement are improved. Meta-analysis at the national scale supports this direction: slow- and controlled-release fertilizers, drip irrigation, and subsoiling were among the most effective practices for improving maize yield, water productivity, and partial factor productivity for nitrogen.

Yield stability under reduced N also depends on preserving rhizosphere function and root-microbe cooperation. Moderate N increased bacterial richness and supported a more balanced rhizosphere environment under integrated water-fertilizer management, whereas excessive N destabilized efficiency gains. More generally, beneficial soil microorganisms enhance plant N acquisition by increasing rhizosphere N availability, stimulating root development, and interacting with root N uptake pathways (Albornoz and Godoy, 2025). Under N deprivation, maize can also recruit Oxalobacteraceae through root-derived flavones, improving plant growth and N acquisition, which suggests that sustainable N reduction should be designed to maintain, not suppress, adaptive root-microbe interactions. Overall, sustainable maize production is best supported by nitrogen strategies that place fertilizer where roots can intercept it, synchronize N delivery with water and crop demand, and reduce total input through smarter forms and schedules rather than through uniform cuts alone.

9 Conclusions and Future Perspectives

Different nitrogen application methods influence maize performance mainly by changing the spatial and temporal matching between soil nitrogen supply and root activity. Compared with surface broadcasting, root-zone fertilization, deep banding, layered placement, and well-timed split applications generally improve nitrogen retention in the effective root zone, promote deeper or more balanced root distribution, and increase post-silking nitrogen uptake. Shallow placement tends to concentrate roots near the surface, whereas moderate deep placement commonly stimulates subsoil rooting, lateral root proliferation, and greater root length density in the 20-60 cm layer. These changes are important because maize nitrogen use efficiency depends not only on root size, but also on where roots are distributed and whether active roots remain in contact with available nitrogen during late growth. The agronomic consequence of this root redistribution is usually higher nitrogen recovery efficiency, agronomic efficiency, and more stable grain yield at equal or lower nitrogen rates. Several studies show that deep or localized nitrogen supply can maintain or even increase yield while reducing total nitrogen input, because it improves the overlap between fertilizer nitrogen, active roots, and crop demand. Layered fertilization appears especially promising because it can stimulate both shallow crown roots and deeper lateral roots, avoiding the tradeoff often seen under single-depth placement. At the same time, the evidence consistently shows that simply increasing nitrogen rate is not an effective substitute for better placement or timing. Excessive nitrogen may enlarge root length or surface area, but often suppresses root activity, nitrate reduction, and effective nitrogen absorption, so nitrogen use efficiency stagnates or declines even when fertilizer input rises.

Current research has established the importance of nitrogen placement, depth, and timing, but several limitations still restrict generalization. First, the optimal depth or method is not universal. Some studies identify 12-15 cm as the most efficient placement zone, whereas others find stronger responses near 25 cm, especially in semiarid systems or where deeper soil water is important. This variation indicates that soil texture, rainfall pattern, irrigation regime, tillage condition, and climate strongly modify the response to nitrogen management. As a result, conclusions drawn from a single site or season cannot be directly extended to all maize production regions. A second limitation is that many studies still focus more on morphology than on integrated mechanism. Root length, root density, and root biomass are measured more often than root activity, transporter expression, rhizosphere nitrogen transformation, or long-term nitrogen fate. Genotypic differences are also underexplored, even though local nitrogen-responsive varieties show much stronger deep-root and post-silking uptake responses than less responsive genotypes. In addition, cross-study comparisons remain difficult because nitrogen rates, placement geometry, growth stages, and evaluation indices are not standardized. Evidence from related crop systems suggests that asymmetric nutrient patches, controlled-release fertilizers, and one-time deep placement can strongly alter root-soil interactions, but these mechanisms are still insufficiently tested in maize under diverse field conditions.

Future research should move from single-factor comparisons toward integrated, site-specific nitrogen management frameworks. More work is needed to determine how placement depth, fertilizer form, split timing, irrigation, and tillage should be combined under different soil and climate conditions. Precision nitrogen management should therefore emphasize not only the right rate, but also the right depth, right position, and right growth stage. Moderate deep placement, layered fertilization, and split side-dressing all show strong potential, but their success depends on whether they match root distribution, water availability, and the crop’s peak nitrogen demand. This also means that weather-sensitive management, especially under warming or drought stress, will become increasingly important. Future strategies should also integrate root traits and rhizosphere processes into maize nitrogen management and breeding. High-efficiency systems will likely depend on genotypes with stronger local nitrogen responsiveness, greater deep-root plasticity, and sustained root physiological activity after silking. Combining these traits with controlled-release fertilizers, fertigation, alternate partial root-zone irrigation, subsoiling, or root-zone placement could reduce nitrogen loss while maintaining yield stability. At the same time, precision tools should be developed to diagnose the effective nitrogen zone in soil and to identify the depth at which nitrate availability, root proliferation, and plant uptake are best synchronized. In this sense, the next stage of sustainable maize production is not simply lower nitrogen input, but smarter nitrogen distribution in space and time so that root architecture, rhizosphere function, and crop demand operate as one coordinated system.

Acknowledgments

I extend my sincere gratitude to the anonymous reviewers for their valuable and insightful comments, which have greatly strengthened this paper.

Conflict of Interest Disclosure

The author affirms that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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