The Nitrate-First, Ammonium-Later Strategy in Potato: Implications of Nitrogen Timing, Form, and Soil Transformation

Abstract

Potato nitrogen (N) demand varies with developmental stage rather than remaining uniformly high throughout the season. This review re-examines the “nitrate-first, ammonium-later” strategy by separating total N amount, N-supply timing, N form, and soil N transformation. Current evidence suggests that nitrate is better aligned with pre-tuber initiation because it supports stolon development and tuber set under non-excessive N supply, whereas ammonium-containing nutrition may benefit tuber bulking only when NH₄⁺ persists in the root zone and soil chemical constraints are controlled. Field responses attributed to N form are often shaped by crop N status, genotype × environment × management interactions, nitrification–denitrification dynamics, water regime, soil texture, fertilizer placement, and cultivar. We therefore interpret the strategy as a conditional, stage-oriented framework rather than a universal fertilizer prescription. Integrating NNI-/CNDC-based diagnosis, root-zone monitoring, enhanced-efficiency fertilizers, and soil-process evidence can improve synchronization between N supply and potato demand, supporting yield formation, N-use efficiency, and reduced environmental risk.

1. Introduction

Potato (Solanum tuberosum L.) is one of the world’s major food crops and is highly responsive to nitrogen fertilization. Yet potato N management is inherently complex because the crop does not require uniformly high N availability throughout the entire season [1,2,3,4,5]. Excessive N can stimulate luxuriant vegetative growth, delay stolon-to-tuber transition, and increase environmental risk, whereas insufficient N can restrict canopy establishment and later source activity needed for bulking.

This issue has become more pressing as environmental constraints tighten and commercial potato systems are expected to use N more efficiently [6,7,8,9]. Even so, potato N management is still often discussed in overly simple terms. At least four closely related, but not equivalent, processes need to be separated: total N amount, the timing of N supply during crop development, the physiological effects associated with nitrate and ammonium, and the changes that occur after fertilizer N is transformed in soil. Recent diagnostic approaches further support this separation. The nitrogen nutrition index (NNI) and critical nitrogen dilution curve (CNDC) provide quantitative tools for interpreting crop N status [8,10], N use efficiency components, and the risk of confusing N-form effects with N-supply status. In this context, genotype × environment × management (G × E × M) interactions are central because cultivar, soil type, water regime, and fertilizer placement jointly determine both crop N demand and the persistence of mineral N forms in the root zone. This review therefore treats N form as one interpretive layer within a broader diagnostic framework, rather than as an isolated driver of potato response.

The question has also become more relevant as many potato systems have moved toward fertigation and other forms of tighter input control. In these systems, the decision is no longer only how much N to apply over a season, but when to supply it and which mineral N form is likely to remain available in the root zone. At the same time, concern over nitrate accumulation, groundwater contamination, and declining N-use efficiency has made stage-specific N management more relevant than in earlier fertilizer-response studies [6,7,8,9,11,12,13]. For that reason, discussion of N timing in potato now has to include soil N turnover rather than treating fertilizer scheduling as a separate issue.

Another reason to revisit the topic is that the evidence base is no longer limited to conventional yield trials. Recent work using NNI-based diagnosis, critical N dilution curves, optical sensing, omics approaches, enhanced-efficiency fertilizers, and root-depth analysis has made it possible to interpret potato N responses in a more connected way than before [7,8,14,15,16,17,18,19]. These advances do not replace field agronomy, but they do make it easier to judge whether developmental responses reflect genuine differences between NO₃⁻ and NH₄⁺, the timing of N availability, or the way fertilizer N is transformed after application.

A second point that requires clarification is the characterization of potato as an N-demanding crop. Potatoes certainly need adequate N, but their key requirement is not a continuously high N supply. Rather, the crop depends on efficient N uptake at specific developmental stages. Before and around tuber initiation, a moderate N status helps avoid prolonged vegetative dominance; after tuber set, continued N uptake mainly helps maintain canopy activity and assimilate production for bulking rather than directly driving tuber initiation itself. In this review, accompanying nutrients such as K, P, and Mg are treated as prerequisite conditions for interpreting the N response rather than as co-equal themes; the central question is how stage-specific N management should be understood once major nutritional limitations are not dominant.

With that in mind, this review does not reject the “nitrate-first, ammonium-later” framework, but reconsiders it on firmer terms. The sections that follow examine whether the framework remains physiologically and agronomically meaningful once total N amount, N supply timing, N form, and soil transformation are treated as separate layers of interpretation. Particular attention is paid to why nitrate often appears aligned with the pre-tuber-initiation phase, why ammonium-containing nutrition may become useful after tuber set only under specific soil and management conditions, and where interpretation becomes uncertain in field soils.

The central question, then, is not only whether a nitrate-first, ammonium-later sequence can improve yield formation, but also when that sequence carries real physiological meaning. A fertilizer sequence can only be interpreted as a form effect if the crop is actually exposed to a distinct NO₃⁻-dominated or NH₄⁺-containing root-zone environment for long enough to matter. This point is especially important in potatoes because irrigation pattern, aeration status, and nitrification potential can quickly reshape the mineral N pool in many production soils [20,21,22,23,24].

To integrate these stage-dependent physiological processes with agronomic management factors, we propose a conceptual framework for stage-specific nitrogen management in potato. This framework links developmental transitions with nitrogen form (nitrate vs. ammonium), timing of nitrogen supply, total nitrogen input, and soil nitrogen transformation processes, while accounting for environmental and management constraints that influence nitrogen availability and crop response.

2. Nitrogen Rate and Tuber Formation: A Delicate Balance

The positive response of potato yield to applied N is well established, but the crop’s response to N rate is not linear and should not be interpreted as evidence that potato requires a continuously high N supply [14,25,26,27]. A more accurate view is that potatoes require efficient N acquisition at critical stages. Before tuber initiation, excessive N can maintain shoot growth and delay establishment of a strong tuber sink, while during bulking, inadequate N can shorten canopy duration and reduce assimilate supply to developing tubers.

Seminal controlled-environment studies were important because they showed that tuberization is sensitive to plant N status itself. High and sustained N supply delayed or even inhibited tuber formation, while partial N withdrawal promoted the developmental switch toward tuberization [14,15,28,29]. These studies should not be read as support for severe N deficiency. Rather, they indicate that the crop responds to the timing and intensity of N availability, and that an early stage of moderate N limitation can contribute to sink establishment.

Evidence from greenhouse, pot, and field studies broadly supports the same pattern, although field responses are more complex because soil buffering, rainfall, irrigation, and cultivar traits modify the outcome [20,21,30,31,32,33,34,35,36,37]. High soil mineral N at planting or during early vegetative growth may delay tuber initiation, whereas a moderate N status during the transition to tuberization tends to favor more orderly sink establishment. At the same time, severe N shortage is clearly detrimental because it limits canopy expansion and later source strength. This is why the agronomic problem is not simply one of reducing early N, but of preventing persistent early excess while still sustaining a canopy capable of supporting bulking.

Quantitative threshold studies are especially useful because they connect physiology with management. For example, Ao et al. [38] showed that moderate soil mineral N favored earlier and more prolific tuber initiation, whereas very high mineral N levels inhibited tuber formation. Such evidence supports the idea that early over-supply of N can be counterproductive for setting yield potential. At the same time, these findings should not be generalized beyond their context; threshold values vary with cultivar, soil type, and climate.

Overall, the N-rate literature supports a two-stage view of potato N demand. Around tuber initiation, the crop benefits from avoiding excessive N and from maintaining a moderate N status that allows sink establishment. After tuber set, the role of N shifts toward maintaining above-ground source activity rather than initiating tubers. This stage dependence provides the physiological basis for discussing a “nitrate-first, ammonium-later” strategy, while also showing why N timing and N form should not be treated as the same question.

The same stage-based interpretation also shows why N-rate studies cannot, on their own, settle the question of N form. A treatment that favors tuber set by avoiding excessive early N may appear beneficial whether the crop received nitrate, ammonium, or a mixture, simply because the decisive factor was moderated early N status. By the same logic, a later yield advantage may arise from prolonged canopy function under continued N supply even when the crop was never exposed to a stable NH₄⁺-dominant environment. N-rate studies therefore identify the crop’s sensitivity to early excess and late insufficiency, but they do not isolate the specific role of mineral N form.

This becomes even clearer when results are compared across in vitro, greenhouse, pot, and field experiments. Controlled systems usually produce sharper developmental responses because N supply can be changed abruptly and root-zone composition can be maintained with relatively little interference. Field responses are buffered instead by soil mineralization, water inputs, fertilizer placement, and cultivar-specific growth duration [20,21,30,31,32,33,34,35,36,37]. For that reason, the move from Section 2 to Section 3 is not just structural. Once potato N demand is recognized as stage-dependent, the next issue is whether nitrate and ammonium play different roles at those stages, or whether many apparent form effects are actually timing effects expressed through soil-mediated N availability.

Quantitative interpretation of N-rate responses can be strengthened by linking fertilizer rate to crop N status. NNI-based diagnosis can distinguish deficient, adequate, and excessive N status, and can also help interpret whether observed differences in NUE arise from N uptake, N utilization, or the crop’s position relative to its critical N concentration. This is important for the present framework because an apparent response to nitrate or ammonium may instead reflect whether the crop was below, near, or above its critical N status at a given developmental stage [8].

3. Distinguishing Nitrogen Form Effects from Nitrogen Timing Effects

While N rate determines the overall size of the available N pool, N form adds another layer of complexity. Nitrate and ammonium differ in mobility, rhizosphere effects, assimilation pathways, and signaling properties [39,40,41,42,43]. In principle, therefore, they may influence stolon growth, tuber initiation, canopy maintenance, and root activity in different ways. However, this possibility can only be evaluated correctly if N-form effects are distinguished from three other factors: total N amount, the timing of N delivery, and the composite effect after ammonium is transformed in soil.

This distinction is especially important in field studies. In many agricultural soils, applied ammonium is rapidly nitrified, so the root system is not exposed to a stable NH4⁺-dominated environment for long [7,22,44]. Consequently, an apparent advantage of “late ammonium” may arise from delayed N release, altered soil mineral N distribution, or reduced nitrate accumulation, rather than from an intrinsic physiological preference for ammonium itself. Likewise, an apparent benefit of “early nitrate” may reflect not only the form of N, but also the fact that nitrate is immediately available and better synchronized with early stolon growth. For this reason, classic field comparisons among fertilizer sources are informative but insufficient on their own to isolate true N-form effects.

Controlled studies provide stronger evidence that nitrate and ammonium can play different roles during different developmental phases. Nitrate nutrition has often been associated with stronger stolon branching and a greater potential tuber number, whereas ammonium nutrition supplied after tuber set may support sustained canopy function, N accumulation, and tuber filling under some conditions [23,24,45,46,47,48]. Even so, these findings should be interpreted carefully. The observed responses likely combine direct physiological effects of N form with indirect effects mediated by root-zone pH, ion balance, assimilation cost, and the duration for which a given N form remains dominant in soil.

For that reason, the “nitrate-first, ammonium-later” strategy is better treated as a working hypothesis with several components than as a single undivided treatment effect. One component is an apparent early alignment of nitrate with stolon development and tuber set under moderated N status. Another is a later, conditional role for ammonium-containing nutrition in maintaining productive canopy function after tuber formation, provided that NH₄⁺ persistence and soil chemical constraints are adequately managed. A third is the field reality that performance depends on how long these forms persist before they are transformed. The agronomic interest of the strategy lies in this stage-specific sequence, whereas its scientific value depends on keeping these different causal layers separate. These stage-specific physiological responses are summarized in Table 1.

Table 1. Stage-specific physiological responses to NO₃⁻ and NH₄⁺ in potato.

Physiological Parameter	Nitrogen Form	Pre-Tuberization/Initiation Phase	Tuber Bulking Phase	Maturity/Late Season
Stolon Number and Branching	NO3−	Generally promoted in controlled or closely managed systems. Enhances stolon initiation and branching, establishing a high potential for tuber sites [47,48].	Development stabilized; foundation for bulking already set [43].	Remains stable [43].
	NH4+	Less effective or slow to promote stolon initiation; may result in fewer stolons [47].	Limited direct role; primary stolon number already determined [43].	NR [24,43,47,48]
Stolon/Tuber Diameter	NO3−	Promotes thicker stolons, providing robust structures [47].	Maintains stolon/tuber robustness, supporting expansion [43].	Supports final tuber expansion [43].
	NH4+	Initially less effective but can support thickening later [47].	Supports sustained radial growth and tuber swelling [24,43].	Continues to support dry matter accumulation [24].
Tuber Number	NO3−	Often supports higher tuber set under non-excessive N status [47,48].	Number largely fixed; role of NO3− diminishes [43].	Stable [43].
	NH4+	Often results in lower tuber numbers if supplied alone at this stage [47].	Has minimal effect on increasing tuber number [43].	NR [24,43,47,48]
Canopy Development (Leaf Area Index)	NO3−	Stimulates rapid leaf area expansion, building photosynthetic source capacity [43].	Helps maintain high photosynthetic area [43].	May gradually decline [43].
	NH4+	Slower initial canopy development [43].	May sustain canopy greenness and photosynthetic activity, and may delay senescence [24,43].	Contributes to longer canopy duration [24].
Dry Matter Partitioning	NO3−	Favors allocation to above-ground vegetative growth (source building) [43].	Allocation becomes more balanced [43].	NR [24,43,47,48]
	NH4+	Less efficient for early vegetative structure [43].	Promotes partitioning of photoassimilates to tubers (sink strengthening), enhancing harvest index [24,43].	May contribute to final dry matter accumulation in tubers [24].

Note: Effects are synthesized from key studies on N-form–stage interactions in potato [24,43,47,48]; “NR” indicates that stage-specific effects were not consistently reported in these studies. Most N-form contrasts summarized here derive from controlled, greenhouse, pot, or closely managed field systems. Their transfer to commercial field conditions requires confirmation of root-zone NO₃⁻/NH₄⁺ dynamics, soil pH, and soil chemical constraints. These physiological patterns can be further organized by phenological stage and evidence setting to clarify the strength and context of the supporting evidence (Table 2).

Table 1. Stage-specific physiological responses to NO₃⁻ and NH₄⁺ in potato.

Physiological Parameter	Nitrogen Form	Pre-Tuberization/Initiation Phase	Tuber Bulking Phase	Maturity/Late Season
Stolon Number and Branching	NO3−	Generally promoted in controlled or closely managed systems. Enhances stolon initiation and branching, establishing a high potential for tuber sites [47,48].	Development stabilized; foundation for bulking already set [43].	Remains stable [43].
	NH4+	Less effective or slow to promote stolon initiation; may result in fewer stolons [47].	Limited direct role; primary stolon number already determined [43].	NR [24,43,47,48]
Stolon/Tuber Diameter	NO3−	Promotes thicker stolons, providing robust structures [47].	Maintains stolon/tuber robustness, supporting expansion [43].	Supports final tuber expansion [43].
	NH4+	Initially less effective but can support thickening later [47].	Supports sustained radial growth and tuber swelling [24,43].	Continues to support dry matter accumulation [24].
Tuber Number	NO3−	Often supports higher tuber set under non-excessive N status [47,48].	Number largely fixed; role of NO3− diminishes [43].	Stable [43].
	NH4+	Often results in lower tuber numbers if supplied alone at this stage [47].	Has minimal effect on increasing tuber number [43].	NR [24,43,47,48]
Canopy Development (Leaf Area Index)	NO3−	Stimulates rapid leaf area expansion, building photosynthetic source capacity [43].	Helps maintain high photosynthetic area [43].	May gradually decline [43].
	NH4+	Slower initial canopy development [43].	May sustain canopy greenness and photosynthetic activity, and may delay senescence [24,43].	Contributes to longer canopy duration [24].
Dry Matter Partitioning	NO3−	Favors allocation to above-ground vegetative growth (source building) [43].	Allocation becomes more balanced [43].	NR [24,43,47,48]
	NH4+	Less efficient for early vegetative structure [43].	Promotes partitioning of photoassimilates to tubers (sink strengthening), enhancing harvest index [24,43].	May contribute to final dry matter accumulation in tubers [24].

Note: Effects are synthesized from key studies on N-form–stage interactions in potato [24,43,47,48]; “NR” indicates that stage-specific effects were not consistently reported in these studies. Most N-form contrasts summarized here derive from controlled, greenhouse, pot, or closely managed field systems. Their transfer to commercial field conditions requires confirmation of root-zone NO₃⁻/NH₄⁺ dynamics, soil pH, and soil chemical constraints. These physiological patterns can be further organized by phenological stage and evidence setting to clarify the strength and context of the supporting evidence (Table 2).

Table 2. Evidence map by phenological stage.

Phenological Stage	Evidence Category/Evidence Setting	What Was Studied	Key Takeaway for the Review Framework	Representative Reference(s)
Pre-tuberization/initiation trigger	Classic mechanistic controlled systems	Tuberization response to sustained N supply vs. N withdrawal in aseptic/solution/in vitro systems	Controlled evidence supports that continuous/high N can delay or inhibit tuberization signals, whereas N withdrawal can induce tuber formation (mechanistic basis, not field thresholds).	[14,15,28,29]
Initiation/tuber set	N-form comparison controlled/pot/limited field evidence	NO3− vs. NH4＋ effects on stolon traits and early tuber set	Evidence indicates stage-dependent responses in which NO3− tends to better support stolon development/tuber set at initiation relative to NH4＋-dominant supply.	[47,48]
Bulking	Complementary inputs field/fertigation/EEF studies	Later-season ammonium management, stabilized ammonium supply, and potato-focused management studies	Evidence for the bulking phase is strongest when the discussion shifts from tuber initiation to canopy maintenance and sustained N supply rather than to tuber number.	[6,13,24,43]
Whole season	Deficiency trade-off greenhouse	Physiological response and yield under N deficit (greenhouse)	Severe N deficit constrains canopy development and final yield despite any early developmental shifts, underscoring the need to avoid under-supply.	[30]
Across stages	Soil-process modifiers field and soil-process evidence	Texture effects on mineralization and N availability; rhizosphere responses to N form	Soil texture and rhizosphere processes influence N transformations and expected site suitability for stage-specific N-form management.	[20,21,23]
Across stages	Background nutrition premise	Potato N diagnosis and agronomic interpretation under balanced fertilization	Interpretation of N-form strategies is strongest when major accompanying nutrients are not the primary constraint.	[3,5,26]
Across stages	Genotype/cultivar dimension multi-genotype field or controlled evidence	Cultivar differences in NUE-related traits and responses to N availability	Genotypic variation can shift N demand timing and efficiency traits, supporting cultivar-aware calibration of stage-specific N management.	[8,27,37,49]
Across stages	Authors’ field implementation evidence	Stage-specific NO3− to NH4+ implementation in recent potato studies	Field implementation evidence supports yield/NUE gains and reduced residual N under optimized stage-specific supply, but these results still need interpretation through timing and soil-process effects.	[24,43]

Table 2. Evidence map by phenological stage.

Phenological Stage	Evidence Category/Evidence Setting	What Was Studied	Key Takeaway for the Review Framework	Representative Reference(s)
Pre-tuberization/initiation trigger	Classic mechanistic controlled systems	Tuberization response to sustained N supply vs. N withdrawal in aseptic/solution/in vitro systems	Controlled evidence supports that continuous/high N can delay or inhibit tuberization signals, whereas N withdrawal can induce tuber formation (mechanistic basis, not field thresholds).	[14,15,28,29]
Initiation/tuber set	N-form comparison controlled/pot/limited field evidence	NO3− vs. NH4＋ effects on stolon traits and early tuber set	Evidence indicates stage-dependent responses in which NO3− tends to better support stolon development/tuber set at initiation relative to NH4＋-dominant supply.	[47,48]
Bulking	Complementary inputs field/fertigation/EEF studies	Later-season ammonium management, stabilized ammonium supply, and potato-focused management studies	Evidence for the bulking phase is strongest when the discussion shifts from tuber initiation to canopy maintenance and sustained N supply rather than to tuber number.	[6,13,24,43]
Whole season	Deficiency trade-off greenhouse	Physiological response and yield under N deficit (greenhouse)	Severe N deficit constrains canopy development and final yield despite any early developmental shifts, underscoring the need to avoid under-supply.	[30]
Across stages	Soil-process modifiers field and soil-process evidence	Texture effects on mineralization and N availability; rhizosphere responses to N form	Soil texture and rhizosphere processes influence N transformations and expected site suitability for stage-specific N-form management.	[20,21,23]
Across stages	Background nutrition premise	Potato N diagnosis and agronomic interpretation under balanced fertilization	Interpretation of N-form strategies is strongest when major accompanying nutrients are not the primary constraint.	[3,5,26]
Across stages	Genotype/cultivar dimension multi-genotype field or controlled evidence	Cultivar differences in NUE-related traits and responses to N availability	Genotypic variation can shift N demand timing and efficiency traits, supporting cultivar-aware calibration of stage-specific N management.	[8,27,37,49]
Across stages	Authors’ field implementation evidence	Stage-specific NO3− to NH4+ implementation in recent potato studies	Field implementation evidence supports yield/NUE gains and reduced residual N under optimized stage-specific supply, but these results still need interpretation through timing and soil-process effects.	[24,43]

The available evidence therefore points in the same general direction: nitrate before tuber initiation may favor stolon development and tuber set under adequate but non-excessive N supply, whereas greater ammonium availability after tuber set may help sustain canopy function and tuber filling only when NH₄⁺ persistence, soil pH, and associated cation constraints are managed. Even so, these responses can only be discussed clearly when N amount, N timing, direct N-form effects, and soil transformation are treated separately. Making that distinction does not weaken the “nitrate-first, ammonium-later” concept; it gives it a clearer basis.

4. Physiological Basis of the “Nitrate-First, Ammonium-Later” Framework

Recent studies make it easier to describe the “nitrate-first, ammonium-later” framework in physiological terms. The idea is not that the potato prefers one N form throughout the season. Rather, the role of N changes as development proceeds: before tuber initiation, N status affects the balance between vegetative growth and the stolon-to-tuber transition; after tuber formation, continued N supply mainly supports canopy activity, assimilate production, and tuber bulking. Field, physiological, transcriptomic, and metabolomic studies published in recent years all support this stage-based reading and help explain why nitrate often appears more useful early, whereas ammonium may become useful later when soil conditions are managed closely [16,24,43,50,51,52].

4.1. Source–Sink Dynamics and the Changing Role of Nitrogen

At early developmental stages, excessive N availability promotes leaf and stem expansion and can prolong stolon elongation, thereby delaying the establishment of a strong tuber sink. By contrast, a moderate N status around tuber initiation helps shift growth from prolonged vegetative dominance toward sink establishment. Recent quantitative work on source-sink relationships under different N rates showed that N supply changes not only total biomass accumulation but also the timing of source activity, sink development, and transfer of growth priority toward the tubers [49,51]. This helps explain why the benefit of early nitrate supply is not simply a matter of rapid nutrient availability; it is also related to how early N status conditions the developmental transition into tuberization.

A source-sink perspective also clarifies why early N management can leave such a strong imprint on the final yield structure. During the pre-tuberization phase, the canopy is still being built, and stolons are still determining how many future sinks can form. If available N remains abundant for too long, shoot expansion and stolon elongation continue, and the shift toward tuber set is postponed. In agronomic terms, the result is not only later initiation, but often poorer synchronization among emerging tubers, which can later appear as less uniform bulking and a wider tuber size distribution [1,2,14,15,28,29,30,47,48].

This is also the stage at which nitrate often appears functionally useful, particularly when total N supply remains moderate and does not prolong vegetative dominance. Because nitrate is mobile and readily accessible, it is frequently associated with vigorous early growth and stolon branching, allowing the crop to build a productive source structure without necessarily creating the same root-zone conditions associated with prolonged ammonium supply [39,42,47,48]. The point is not that nitrate should drive maximum vegetative growth, but that it can support early structural development while still allowing a timely shift toward sink establishment when total N remains moderate.

Once tubers are initiated, the role of N changes. At this stage, further yield gain depends less on triggering new tuber formation and more on maintaining active leaves, adequate photosynthesis, and sustained assimilate flow to enlarging tubers. Studies on potato responses to mixed or stage-specific N forms indicate that N form can alter canopy vigor, N metabolism, and stolon or shoot outgrowth patterns, but the agronomic significance of these responses depends strongly on developmental stage [43,50,51]. In this sense, the physiological basis of the framework lies in a shift from regulating developmental transition early to sustaining source persistence later.

After the tuber set, the key yield question changes. The issue is no longer how many sinks can still be initiated, but how effectively the existing sinks can be supplied with assimilates over time. Later-season N management therefore needs to be judged through traits such as canopy duration, photosynthetic persistence, N remobilization, and maintenance of effective leaf area, rather than through tuber number alone [24,43,50,51,53]. Under field conditions, a later advantage of ammonium is most plausible when it helps preserve canopy activity and support sustained assimilate flow during bulking, not when it is assumed to trigger a new developmental phase.

Cultivar differences reinforce this reading. Early- and late-maturing genotypes differ in canopy duration, partitioning behavior, and the length of the interval between tuber set and effective bulking. As a result, the developmental window during which later N remains useful is unlikely to be identical across potato types [27,37,49,51]. This helps explain why form-by-stage responses are not always reproduced consistently across experiments and why cultivar-aware interpretation remains important.

4.2. Hormonal Regulation of Tuber Initiation

Tuber initiation is regulated by carbon status, photoperiod, and hormonal signaling, especially through interactions involving gibberellins, cytokinins, and abscisic acid. N status feeds into this network rather than acting outside it. Recent work showed that mixed N supply can enhance cytokinin accumulation in leaves, strengthen photosynthetic activity, and improve coordination between C and N metabolism in potato, thereby shifting the balance between source activity and sink formation [50]. Transcriptome analyses under contrasting N conditions likewise identified co-expressed genes involved in N transport, hormone biosynthesis, photosynthesis, and energy metabolism, confirming that N availability is tied closely to developmental regulation rather than to growth alone [16].

Seen this way, N effects on tuber initiation are regulatory, not merely nutritional. Nitrogen availability changes the likelihood that the crop remains in a vegetative growth mode or moves toward tuber set under suitable environmental conditions. This point matters because it helps explain why early N supply can alter sink establishment without acting as a simple on-off switch for tuberization.

It also helps avoid an overly simple reading of early nitrate effects. The literature does not support the claim that nitrate, by itself, directly triggers tuberization. A more defensible interpretation is that nitrate often fits the early developmental phase because it supports growth, root activity, and N transport in a way that can still leave room for an orderly shift toward tuber initiation when overall N status is not excessive.

Within this context, nitrate appears better aligned with the pre-tuberization phase because it supports early growth and root activity while allowing the crop to enter tuber initiation under a more orderly N status. This does not mean that nitrate alone determines tuberization, or that ammonium is inherently inhibitory. It means that N form interacts with the plant’s hormonal and metabolic background, and that the same form can have different consequences before and after tuber initiation [16,43,47,48,50].

4.3. Root-Zone Processes and Nitrogen Uptake

The root zone does not experience N as a fertilizer label, but as a changing chemical environment shaped by mineralization, nitrification, transport, irrigation, and root uptake. This is especially relevant to the post-tuberization role of ammonium. The field study by Chen et al. showed that the agronomic value of later ammonium management depended not only on supplying NH₄⁺ itself, but on maintaining an NH₄⁺-containing root-zone environment through frequent, low-dose application so that nitrification did not immediately erase the intended form effect [24]. A companion field study without nitrification inhibitors reached a similar conclusion and showed that supplying nitrate before tuber formation and then maintaining ammonium later through repeated small applications improved plant N accumulation, yield, and NUE under drip irrigation [43]. Soil transformation should therefore be defined explicitly. Applied N is redistributed through mineralization, immobilization, nitrification of NH₄⁺ to NO₃⁻, ammonia volatilization, nitrate leaching, and denitrification under wet or poorly aerated microsites. Denitrification is particularly relevant when high soil moisture, labile organic carbon, and elevated NO₃⁻ coincide, because it can reduce plant-available N and increase N₂O emissions [21,54]. Thus, early nitrate supply should be evaluated not only in relation to stolon development and tuber set, but also in relation to irrigation, aeration, and environmental N-loss risk.

For field interpretation, several root-zone modifiers deserve particular attention. First, soil texture affects both water retention and the timing of mineral N supply, thereby influencing how long applied NH₄⁺ can remain near active roots and how readily NO₃⁻ moves downward in the profile [20,21]. Second, irrigation pattern influences not only mineral N transport but also the frequency with which newly applied fertilizer is redistributed, diluted, or exposed to conditions favoring nitrification. Under frequent fertigation, root-zone chemistry can be managed more deliberately than under large, infrequent applications.

Third, aeration and microsite oxygen status influence whether ammonium persists, is rapidly nitrified, or is associated with other N losses. Potato is commonly grown in systems with alternating wetting and drying, so the root zone can shift quickly between conditions that favor NH₄⁺ retention and those that favor NO₃⁻ production. Fourth, fertilizer placement and application frequency matter because they determine whether mineral N remains close to the active root zone during the period when demand is changing most rapidly. A further modifier is soil chemical buffering. In acidic or weakly buffered soils, ammonium-based sources can acidify the rhizosphere or ridge soil, alter Ca and Mg nutrition, and mobilize Al³⁺ [55]. These effects may override or mask any presumed physiological benefit of NH₄⁺ during bulking. Later ammonium management should therefore be discussed as a conditional tool for controlling N availability, not as a generally superior N form.

These considerations define the boundary of the framework rather than sitting at its margins. If the soil environment is not characterized, conclusions about N form remain partly inferential because the plant response may reflect timing, placement, and soil turnover as much as any direct preference for NO₃⁻ or NH₄⁺. That point is especially important in potato, where the crop is shallow-rooted early, highly responsive to local soil moisture, and often managed in systems where fertilizer can be redistributed quickly within the wetted zone.

The same point explains why results from controlled nutrient solutions cannot be transferred directly to field soils. In controlled systems, the imposed N form can be maintained long enough to test plant responses to NO₃⁻ or NH₄⁺ per se. In the field, mineralization, nitrification, irrigation frequency, and oxygen status rapidly alter the composition of mineral N in the rhizosphere. An ammonium-based treatment may therefore function only as a short NH₄⁺ pulse followed by a largely nitrate phase. Later ammonium should be interpreted as meaningful only when there is evidence that NH₄⁺ remained available in the root zone for a substantial part of bulking, not simply because an ammonium fertilizer was applied [20,21,22,23,24,39,42].

4.4. Additional Considerations

Recent metabolomic evidence further suggests that later-season N responses are closely linked to the plant’s ability to sustain active N metabolism and remobilization. Under contrasting N conditions, potato plants showed marked shifts in amino-acid and carbohydrate pools, while improved recycling of N-containing compounds partly buffered the adverse effects of N limitation [52]. These findings point to later N responses as part of a broader adjustment in source maintenance, remobilization, and sink support rather than as a simple fertilizer-form effect.

Any later physiological advantage of ammonium should therefore be regarded as conditional rather than universal. Ammonium may become useful when it helps maintain leaf N status, delay premature senescence, and support assimilate transfer to enlarging tubers, particularly under split application or stabilized fertilization. Even then, the response remains tied to cultivar traits, canopy duration, and the crop’s shifting carbon economy. Later ammonium is best understood as a way to regulate the temporal pattern of N availability during bulking, not as evidence of an absolute preference for NH₄⁺ over NO₃⁻ [16,24,43,49,50,51,52,53].

5. Agronomic Interpretation and Practical Implications

From a management standpoint, the “nitrate-first, ammonium-later” strategy is best read as an attempt to match N form with the changing role of N during crop development. Before tuber initiation, the practical aim is to provide enough readily available N for early canopy establishment and stolon growth without maintaining an excessively high N status that delays sink establishment. After tuber set, the aim shifts to maintaining canopy activity and supplying assimilates for tuber filling. This change in objective is the agronomic basis for discussing a shift in dominant N form, but it does not mean that timing and form are interchangeable explanations.

In practice, the framework is most useful when it is translated into management objectives for contrasting production systems rather than into a single recipe. Under drip-fertigated systems, for example, small and frequent applications can help separate early nitrate-oriented supply from later ammonium-containing supply more clearly than in systems dominated by a few large applications. In rainfed or weakly controlled systems, by contrast, the intended sequence can easily be blurred by rainfall, redistribution, and rapid nitrification. The usefulness of the framework, therefore, depends not only on fertilizer choice but also on how much control exists over water and root-zone conditions.

The same framework also has different implications across soil environments. In coarse-textured soils or systems with rapid N turnover, large basal applications are especially risky because the crop may receive too much N early, only to face reduced availability later as N moves or is lost [11,12,20,21]. In soils with high nitrification potential, later ammonium becomes meaningful only if split application, fertigation frequency, placement, or stabilization can keep NH₄⁺ in the root zone for a meaningful part of the bulking [22,23,24].

Within that interpretation, nitrate is most useful early because it is immediately available and, in many studies, is more favorable for stolon growth and tuber set. Later in the season, ammonium-containing nutrition—especially when supplied in small doses, stabilized against rapid nitrification, and used where acidification or Ca/Mg/Al constraints are not dominant—may help sustain productive growth without producing the same pattern of nitrate accumulation in the soil profile [24,47,48]. This is the agronomic logic of the strategy. It also fits the broader view that potatoes do not need uniformly high N throughout the season, but a staged supply pattern in which early growth is supported without excessive vegetative prolongation and later canopy persistence is maintained for bulking.

A further practical consideration is that the framework should be discussed only where background nutrition is already adequate. If K, P, or Mg is limiting, apparent differences in N efficiency may instead reflect broader nutrient imbalance rather than the timing or form of N supply itself [3,5,26]. This does not weaken the case for stage-specific N-form management; it simply defines the conditions under which the hypothesis can be tested fairly and interpreted with confidence. The present review, therefore, does not replace balanced fertilization principles, but asks what can be learned about potato yield formation once major accompanying nutritional constraints are no longer dominant.

The nitrate-first, ammonium-later concept should also be treated as an interpretive framework rather than a universal fertilizer prescription. It is most convincing when the crop is not primarily limited by K, P, Mg, or water, when a real difference between early and later root-zone N conditions can be created, and when the goal is to optimize the balance among tuber set, bulking, and N-use efficiency rather than simply maximize vegetative biomass [3,5,11,26,56,57,58,59,60].

By contrast, the framework should not be used to suggest that ammonium is always superior later in the season, or that nitrate is always the best starting form regardless of site context. Where background fertility is poor, water control is limited, or NH₄⁺ persistence is too brief to verify, the observed response often reflects broader resource interactions rather than intrinsic form preference. Stating these limits explicitly strengthens the review because it makes clear when positive field responses can reasonably be interpreted through the timing-form-transformation logic and when they cannot.

5.1. Implications for Interpreting Field Evidence

When field experiments report higher yield under staged nitrate and ammonium application, the outcome should not automatically be attributed to N form alone. It may reflect the combined effects of moderate early N supply, delayed late N release, altered soil mineral N composition, improved synchrony between crop demand and N availability, or differences in the residence time of NH₄⁺ in the root zone.

This distinction is particularly important in northern potato regions, including the Yinshan foothill production zone, where an early-nitrate and later-ammonium pattern is often agronomically useful. Even in such systems, however, the explanation cannot stop at the fertilizer sequence itself. Practical responses depend on soil pH, aeration, irrigation regime, texture, fertilizer placement, and the rate at which applied ammonium is converted to nitrate [20,21,22,23,24].

Recent work on precision N management and enhanced-efficiency fertilizers helps place these field results in context. These studies show that better synchronization of N supply can improve yield, reduce residual nitrate, and strengthen N-use efficiency in potato systems, but they also make clear that management gains are strongly site-specific [6,7,8,9]. For that reason, the “nitrate-first, ammonium-later” framework is more useful as an interpretive and experimental guide than as a fixed schedule detached from local soil and climate.

A related implication is that stage-specific N management increasingly depends on diagnosis rather than on fertilizer source alone. Optical sensing, precision N management, and root-depth-related N capture studies show that the same seasonal N rate can lead to very different outcomes depending on when N becomes available and how effectively the crop can access it. These studies support the practical value of the present framework while also showing that successful implementation requires site-specific calibration with irrigation, soil mineral N monitoring, and cultivar-specific canopy development rather than reliance on a fixed fertilizer schedule [7,8,11,17,18,19]. An additional interpretive layer is crop N status. NNI and CNDC-based approaches [8,10] can help distinguish whether a field response reflects N deficiency, adequate N supply, luxury N uptake, or a true form-related effect. This is particularly important under G × E × M interactions, where genotype, water regime, soil texture, and management determine both N demand and the residence time of mineral N in the root zone. In the present framework, NNI is therefore not treated as an alternative to stage-specific N management, but as a diagnostic tool for testing whether the proposed timing–form sequence has actually been achieved in the crop and soil. Because the proportion of seasonal N uptake occurring during tuber initiation and bulking varies with cultivar, growth duration, soil mineral N supply, and sampling interval, this review does not treat a single percentage as universal. Nevertheless, the tuber initiation-to-bulking interval should be regarded as a major N-demand window for synchronizing N availability with crop uptake.

5.2. Recent Evidence Broadening the Review Base

Recent literature also makes clear that this framework sits within a broader potato N-management context and should not be read only as a fertilizer-sequence rule. Reviews and management studies consistently show that potato N performance depends strongly on the synchronization of water, root-zone N availability, and fertilizer timing, and that this synchronization affects both yield and residual soil nitrate [7,11,12,13,28,61]. The same literature also shows that gains in N-use efficiency are more often achieved through better timing, placement, or controlled release than through simply increasing the seasonal N rate [7,11,12,13].

One useful way to interpret the recent literature is to group it around three converging themes. The first is precision N management and in-season diagnosis. In potato, Sandaña et al. [8] used NNI to evaluate N utilization efficiency and N recovery efficiency across genotypes and N supply levels. Their results support the view that crop N status is essential for interpreting NUE components and that differences in performance should not be attributed to fertilizer source or N form without first considering the crop’s diagnostic N status. This strengthens the present framework by adding a quantitative crop-status layer to the timing–form–transformation interpretation. Although the CNDC study by Soratto et al. [10] was conducted in common bean rather than potato, it provides a useful methodological reference for the present review. Its Bayesian CNDC and NNI framework shows how uncertainty under G × E × M scenarios can be incorporated into crop N diagnosis. Therefore, this evidence is used here as a diagnostic and methodological reference, not as direct evidence for potato N-form preference. Studies using nitrogen nutrition indices, leaf spectroscopy, visible and near-infrared sensing, and other diagnostic approaches consistently show that seasonal N rate alone is a weak descriptor of crop performance unless it is linked to when N becomes available and how crop status changes during the season [7,8,18,19]. This body of work supports the present review because it shifts agronomic interpretation from total input toward temporal synchronization between crop demand and root-zone N supply.

A second theme concerns the coupling of water management, rooting depth, and N capture. Recent studies on irrigation scheduling, root distribution, deep rooting, and water-N interaction indicate that the value of a staged N strategy depends heavily on the crop’s ability to access N both spatially and temporally within the soil profile [11,12,17,28,62,63]. The key point in this literature is not that one fertilizer source is universally superior, but that the developmental benefit of staged N supply is only realized when water regime and root activity allow the crop to intercept the intended mineral N pool.

A third theme concerns enhanced-efficiency fertilizers and environmentally sensitive production systems. Meta-analyses and recent field studies show that improved N performance in potato often comes from management that reduces losses, better matches N release to crop demand, or lowers residual soil nitrate after harvest [6,13,22,64,65,66]. These findings broaden the relevance of the nitrate-first, ammonium-later concept by showing that its practical value lies in synchronization and root-zone control, even when the underlying mechanism involves a mixture of delayed release, stabilized ammonium, improved placement, and site-specific timing.

Recent physiological and diagnostic studies further strengthen the mechanistic side of the argument. Controlled-environment and omics-based work has shown that potato responses to N deficit or altered N status involve changes in canopy growth, source-sink coordination, tuber initiation, organ-specific N metabolism, and root-zone sensing, which fits the developmental interpretation adopted here [14,17,18,30,46,67]. These findings do not, by themselves, prove the “nitrate-first, ammonium-later” concept, but they do support the underlying view that the role of N changes across developmental stages and that root-zone N conditions need to be interpreted dynamically rather than as a fixed fertilizer label [14,17,18,30,46].

Field-scale studies published in 2024–2025 point in much the same direction. Work conducted in groundwater-sensitive environments, with organo-mineral fertilizers, 4R combinations, optical diagnosis of crop N status, and root-depth-related N capture shows that potato productivity and N efficiency are shaped jointly by soil conditions, water regime, cultivar, diagnostic capability, and fertilizer strategy [19,22,56,57,62,63,64,65,66]. This broader body of evidence strengthens the relevance of the framework while also clarifying the conditions under which it can be interpreted with confidence. Recent potato field evidence also shows that N-source effects can be governed by soil chemistry. Yagi et al. [55] compared urea, ammonium sulfate, and calcium nitrate in potato and showed that ammonium sulfate could acidify ridge soil, increase Al³⁺, reduce foliar Ca and Mg, and fail to improve marketable yield in a cultivar-specific manner. Such evidence indicates that soil chemical feedbacks can override or mask any intrinsic physiological preference for NH₄⁺ or NO₃⁻, and it supports interpreting N-form responses through soil pH, base-cation status, and cultivar context.

5.3. Scope and Limits of the Framework

This review therefore retains the “nitrate-first, ammonium-later” framework as its central line of argument, but in a more clearly bounded form. It is most convincing where accompanying nutritional constraints are already well managed, where soil conditions allow at least partial separation of early nitrate and later ammonium effects, and where the discussion remains focused on yield formation rather than on a universal fertilizer prescription. Under those conditions, the strategy offers a useful physiological interpretation of how different N forms may contribute to potato productivity.

The framework is less convincing when very high N rates are used, when the intended NH₄⁺-dominant phase is too brief to verify, or when later yield responses are interpreted as though they arise from N form alone. Field evidence should likewise not be used to argue that the potato is a continuously high-N crop. A more defensible conclusion is that the crop requires a staged N supply pattern in which early N supports establishment and tuber set, whereas later N mainly sustains canopy function during bulking. That distinction is central to both interpretation and recommendation.

Broader agronomic practices define additional boundary conditions for the framework. Previous crops, residual organic fertilizers, tillage, irrigation regime, cover cropping, and biostimulant use can modify mineralization, immobilization, denitrification, root activity, and N uptake efficiency. These factors are not treated here as independent review themes, but they determine whether a staged nitrate/ammonium sequence can be expressed under commercial field conditions. Recognizing these boundary conditions helps prevent the framework from being interpreted as a universal fertilizer prescription. These boundary contexts and their implications for early NO₃⁻ and later NH₄⁺ management are summarized in Table 3.

Table 3. Evidence map by boundary context for stage-specific nitrate vs. ammonium management in potato.

Diagnostic/Mitigation Options	Implication for Later NH4+(Stabilized) Phase	Implication for Early NO3− Phase	Dominant N Pathway or Risk	Context/Boundary Condition	Supporting Refs
Pre-plant–soil mineral N tests; adjust split schedule	Use smaller, better-timed later applications when NH4+ stabilization is feasible	Reduce large one-time basal NO3− inputs; rely more on split supply	Mineralization timing and leaching susceptibility vary with texture	Coarse-textured soils/fast N turnover	[20,21]
Monitor soil NH4+/NO3− dynamics; consider stabilized sources	Late NH4+ should be interpreted through actual soil persistence, not the product label alone	Early nitrate remains useful, but should not be excessive	Applied NH4+ may be converted rapidly after fertilization	High nitrification potential soils	[22,23,24,43]
Use balanced fertilization as the baseline for testing the framework	Interpret later ammonium effects within the same nutritional premise	Interpret early nitrate effects only when K, P, and Mg are not strongly limiting	Apparent N inefficiency may reflect broader nutrient imbalance	Balanced background nutrition	[3,5,26]
Coordinate N strategy with irrigation and stress management	Late-season N should not prolong canopy growth beyond productive bulking	Avoid large early nitrate pools when water control is poor	Moisture and stress alter N cycling and crop demand	Water regime and abiotic stress	[11,12,21,35,36]
Use cultivar-specific calibration where possible	The useful NH4+ window may differ across genotypes	Early NO3− dose and timing may shift with maturity class	Cultivar modifies demand timing and efficiency	Genotype/cultivar differences	[8,27,37,49]
Economic evaluation and site-specific testing are essential	May help preserve NH4+-based supply during bulking under suitable soils	Not central for the early NO3− phase	Potential to extend the practical relevance of later NH4+ nutrition	Use of stabilized ammonium fertilizers	[6,13,24,43]
Emphasize local calibration and field validation.	Use field evidence to test whether intended NH4+ effects are actually maintained in soil, and interpret “trigger” effects cautiously.	Use controlled evidence to explain why early excess N delays tuberization, but not as a basis for field thresholds.	Mechanistic clarity differs from field complexity; some tuberization signals are derived from controlled systems.	Controlled-environment vs. field realism	[14,15,24,28,29,30,46,47,48]
Monitor soil moisture, aeration, NO3−/NH4+ pools; coordinate N application with irrigation	Interpret later NH4+ effects together with nitrification and denitrification risk	Avoid large early NO3− pools when soil moisture is high, or aeration is limited	Denitrification, reduced plant-available N, and N2O emission risk	Wet or poorly aerated soils with elevated NO3− and labile C	[21,54]
Report crop history, organic inputs, pre-plant mineral N, and cover crop management	Recalibrate later NH4+ supply when background N release is high	Adjust early nitrate input according to pre-plant mineral N and expected mineralization	Residual N supply, mineralization–immobilization, and microbial N turnover	Previous crops, residual organic amendments, and cover crops	[64,66,68]
Combine root, canopy, and N-status diagnosis; avoid attributing effects to N form alone	Determine whether improved uptake reflects root activity rather than NH4+ preference	Interpret early N capture through root development as well as N form	Altered root activity, nutrient acquisition, and microbial interactions	Biostimulants or root-promoting inputs	[56,66]

Table 3. Evidence map by boundary context for stage-specific nitrate vs. ammonium management in potato.

Diagnostic/Mitigation Options	Implication for Later NH4+(Stabilized) Phase	Implication for Early NO3− Phase	Dominant N Pathway or Risk	Context/Boundary Condition	Supporting Refs
Pre-plant–soil mineral N tests; adjust split schedule	Use smaller, better-timed later applications when NH4+ stabilization is feasible	Reduce large one-time basal NO3− inputs; rely more on split supply	Mineralization timing and leaching susceptibility vary with texture	Coarse-textured soils/fast N turnover	[20,21]
Monitor soil NH4+/NO3− dynamics; consider stabilized sources	Late NH4+ should be interpreted through actual soil persistence, not the product label alone	Early nitrate remains useful, but should not be excessive	Applied NH4+ may be converted rapidly after fertilization	High nitrification potential soils	[22,23,24,43]
Use balanced fertilization as the baseline for testing the framework	Interpret later ammonium effects within the same nutritional premise	Interpret early nitrate effects only when K, P, and Mg are not strongly limiting	Apparent N inefficiency may reflect broader nutrient imbalance	Balanced background nutrition	[3,5,26]
Coordinate N strategy with irrigation and stress management	Late-season N should not prolong canopy growth beyond productive bulking	Avoid large early nitrate pools when water control is poor	Moisture and stress alter N cycling and crop demand	Water regime and abiotic stress	[11,12,21,35,36]
Use cultivar-specific calibration where possible	The useful NH4+ window may differ across genotypes	Early NO3− dose and timing may shift with maturity class	Cultivar modifies demand timing and efficiency	Genotype/cultivar differences	[8,27,37,49]
Economic evaluation and site-specific testing are essential	May help preserve NH4+-based supply during bulking under suitable soils	Not central for the early NO3− phase	Potential to extend the practical relevance of later NH4+ nutrition	Use of stabilized ammonium fertilizers	[6,13,24,43]
Emphasize local calibration and field validation.	Use field evidence to test whether intended NH4+ effects are actually maintained in soil, and interpret “trigger” effects cautiously.	Use controlled evidence to explain why early excess N delays tuberization, but not as a basis for field thresholds.	Mechanistic clarity differs from field complexity; some tuberization signals are derived from controlled systems.	Controlled-environment vs. field realism	[14,15,24,28,29,30,46,47,48]
Monitor soil moisture, aeration, NO3−/NH4+ pools; coordinate N application with irrigation	Interpret later NH4+ effects together with nitrification and denitrification risk	Avoid large early NO3− pools when soil moisture is high, or aeration is limited	Denitrification, reduced plant-available N, and N2O emission risk	Wet or poorly aerated soils with elevated NO3− and labile C	[21,54]
Report crop history, organic inputs, pre-plant mineral N, and cover crop management	Recalibrate later NH4+ supply when background N release is high	Adjust early nitrate input according to pre-plant mineral N and expected mineralization	Residual N supply, mineralization–immobilization, and microbial N turnover	Previous crops, residual organic amendments, and cover crops	[64,66,68]
Combine root, canopy, and N-status diagnosis; avoid attributing effects to N form alone	Determine whether improved uptake reflects root activity rather than NH4+ preference	Interpret early N capture through root development as well as N form	Altered root activity, nutrient acquisition, and microbial interactions	Biostimulants or root-promoting inputs	[56,66]

6. Conclusions

The main conclusion of this review is not that potatoes require more N throughout the whole season, but that the crop responds to N differently across developmental stages. Around tuber initiation, excessive N can delay the establishment of strong tuber sinks, whereas during bulking, continued N uptake mainly serves to maintain canopy activity and assimilate supply. This developmental shift provides the physiological basis for considering different N forms at different times.

The review also argues that four effects must be distinguished if the “nitrate-first, ammonium-later” strategy is to be evaluated rigorously: total N amount, the timing of N supply, the intrinsic effects of nitrate and ammonium, and the integrated effects generated by soil transformation after fertilization. Once these effects are separated, the framework remains a meaningful but conditional and testable basis for aligning N supply with the changing function of the crop, particularly under conditions of balanced background nutrition, diagnosable crop N status, and at least partial control over root-zone N dynamics. Its value lies primarily in its diagnostic and experimental framework for interpreting field responses, rather than as a universal recommendation for fertilizer source choice.

7. Future Perspectives

Future work should prioritize experimental designs that explicitly separate N timing effects from N-form effects while monitoring soil mineral N dynamics, soil moisture/aeration, denitrification risk, and N₂O emissions, where environmental outcomes are considered. Field studies should no longer classify treatments only by fertilizer product. Instead, they should document the actual NO₃⁻-N and NH₄⁺-N pools before application, after application, and during tuber initiation and bulking, especially in soils with rapid nitrification where the physiological meaning of an “ammonium treatment” may be lost soon after fertilization.

Methodologically, the next generation of experiments should combine developmental, physiological, and soil-process measurements within the same design. Measurements of tuber initiation timing, stolon traits, canopy duration, leaf area persistence, photosynthetic status, and partitioning behavior should be paired with repeated monitoring of soil NO₃⁻-N and NH₄⁺-N pools in the active rooting zone. Without such joint measurements, it remains difficult to determine whether a response arose from a true N-form effect, from altered timing of N availability, or from rapid soil transformation that changed plant exposure after fertilization. Future trials should also report management history more explicitly, including previous crops, residual organic fertilizers, tillage, irrigation regime, cover crop use, and biostimulant application where relevant. These factors may alter mineralization, immobilization, root activity, denitrification, and N uptake efficiency, and therefore should be treated as G × E × M boundary conditions when testing the nitrate-first, ammonium-later framework.

A second priority is to integrate the framework with management technologies that can actually manipulate root-zone N conditions in commercial systems. Precision fertigation, controlled-release products, nitrification inhibitors where appropriate, and optical or tissue-based diagnosis all create opportunities to test stage-specific N targets more rigorously than was possible in earlier fertilizer-comparison studies [6,7,8,13,18,19,24]. Future work should therefore move beyond asking which product performs best on average and instead focus on how early- and late-season N environments can be defined, monitored, and adjusted to match cultivar growth pattern, soil type, and production objective.

Finally, recommendations will be more useful to growers and advisers when they are expressed as stage-specific management targets rather than static fertilizer schedules. Early-season targets should emphasize rapid but not excessive canopy establishment, orderly stolon development, and timely tuber set. Bulking-stage targets should focus on maintaining productive leaf area, sustaining N uptake only to the extent that it supports effective tuber filling, and avoiding residual mineral N accumulation after harvest. Framing future recommendations in this way would connect physiology, agronomy, and environmental stewardship more directly and make the nitrate-first, ammonium-later concept more operational under diverse field conditions.

Additional progress will depend on integrating physiological measurements with field-scale agronomy. Future studies should evaluate tuber initiation, canopy longevity, root-zone mineral N, final and marketable yield, tuber quality, fertilizer recovery, and economic return within the same framework. Multi-environment and multi-site trials are needed to determine when the nitrate-first, ammonium-later pattern is robust and when it is overridden by soil texture, water regime, cultivar, or background fertility [5,7,11,17,18,24,43,46].

Future research should also connect stage-specific N management with broader nutrient stewardship. Recent work on phosphorus efficiency, phosphorus fertilizer economics, potassium management, and integrated nutritional practices indicates that the agronomic value of a staged N strategy is most convincing when accompanying nutritional conditions are also documented and shown to be adequate [58,59,60]. Clearer discrimination among timing effects, form effects, and soil transformation effects will therefore depend on a stronger description of the nutritional background in future trials.

From a practical standpoint, the next step is not merely to compare fertilizer products but to define management targets for each developmental phase of the crop. During early growth, N management should secure canopy establishment without prolonging vegetative dominance or weakening the transition to tuber set. During bulking, the objective should shift toward preserving effective leaf area, maintaining photosynthetic activity, and sustaining N uptake only to the extent required for productive tuber filling. Expressing future recommendations in terms of stage-specific targets for N availability, placement, and application frequency would make the framework more useful for crop advisers and growers, while also encouraging the reporting of economically relevant outcomes such as marketable yield, tuber size distribution, fertilizer recovery, and residual soil mineral N after harvest.