1 Introduction

Yardlong bean ( Vigna unguiculata subsp. sesquipedalis ), a vegetable form of cowpea, is an important legume crop because of its nutritional value, consumer preference, and multifunctional role in food and farming systems. Cowpea more broadly is recognized as a major staple and multipurpose legume used for food, fodder, and soil fertility improvement, particularly in Africa, Asia, and other tropical and subtropical regions (Soares et al., 2024). In India, vegetable cowpea or yardlong bean holds a particularly important place in vegetable production, yet rising demand and limited cultivated land require greater productivity from improved agronomic management (Ramanjineyulu et al., 2025). Cowpea-based systems are also valued for ecological services, including nitrogen fixation, weed suppression, soil cover, moisture retention, and compatibility with intercrops and rotations, making them useful for sustainable intensification. Despite these advantages, cowpea productivity remains constrained by declining soil fertility, degraded lands, drought, salinity, pests, poor management, and limited access to fertilizers or other inputs. Nitrogen and phosphorus are repeatedly identified as among the most limiting nutrients in smallholder systems, while low fertilizer use and poor nutrient replenishment often leave yields well below potential. This makes nutrient management a central production issue. Although cowpea can contribute biologically fixed nitrogen to the system, the amount fixed and the agronomic benefits realized depend strongly on soil condition, genotype, inoculation effectiveness, and balanced nutrient supply (Omomowo and Babalola, 2021). Studies on integrated nutrient management further show that combining mineral and organic nutrient sources can improve nodulation, chlorophyll status, growth, yield, and soil health more consistently than sole reliance on either source. In yardlong bean specifically, integrated nutrient-source combinations, including mineral fertilizer with biochar, have produced strong gains in vegetative growth, pod traits, and marketable yield under semi-arid conditions, indicating that fertilizer strategy is a major determinant of productivity.

Nitrogen is central to cowpea and yardlong bean growth because it directly supports chlorophyll formation, protein synthesis, photosynthetic metabolism, canopy development, and reproductive sink formation (Pardhi et al., 2021). Even in legumes, early crop growth often depends on soil or fertilizer nitrogen before symbiotic fixation becomes fully functional, because cotyledon reserves are rapidly exhausted and seedlings can experience temporary nitrogen deficiency. For this reason, starter nitrogen frequently enhances early vegetative vigor, root activity, branching, peduncle formation, and eventual pod and seed production (Ndor and Faringoro, 2020). Experimental evidence shows that nitrogen supply can increase plant height, branch number, leaf number, nodulation at moderate rates, and grain yield, especially when combined with effective inoculation. Nitrogen nutrition also interacts strongly with other physiological processes. Bradyrhizobium inoculation has been shown to increase leaf area, net photosynthesis, water-use efficiency, shoot nitrogen content, and nitrogen derived from the atmosphere, demonstrating that nitrogen acquisition is closely linked with assimilation efficiency and biomass production. At the same time, excessive or poorly timed nitrogen can suppress nodulation, reducing the contribution of biological fixation and lowering overall efficiency. Nitrogen responses are therefore not simply a matter of applying more fertilizer; they reflect a dynamic balance among mineral nitrogen supply, rhizobial symbiosis, phosphorus availability, soil chemical condition, and environmental stress. This is evident under drought, where reduced stomatal conductance, photosynthesis, leaf pigments, and carbohydrate transport impair nodulation, biological nitrogen fixation, dry matter accumulation, and yield. Likewise, low phosphorus restricts nodulation and nitrogenase activity, whereas phosphorus fertilization can strengthen fixation, plant nitrogen accumulation, grain yield, and grain protein content.

Research on nitrogen use efficiency (NUE) in cowpea and related legumes has increasingly shifted from simple fertilizer-rate comparisons toward integrated frameworks that consider nutrient source, timing, genotype, microbial symbiosis, soil constraints, and environmental sustainability. Broad agronomic evidence shows that high NUE requires site-specific and balanced nitrogen management, because uniform fertilizer strategies perform poorly across variable soils, climates, crops, and farming systems, and excessive nitrogen use can degrade soil health and reduce long-term efficiency. In cowpea, promising approaches include integrated nutrient management, liming in acidic soils, targeted starter nitrogen, rhizobial inoculation, and combined inoculation with phosphorus, all of which can improve nutrient uptake, biomass partitioning, nodulation, yield, or profitability depending on local constraints. Genetic progress is also beginning to inform NUE improvement: substantial variability exists among cowpea genotypes for nodulation efficiency and related biological nitrogen fixation traits, and recent association mapping has identified markers and candidate genes that could accelerate breeding for high nitrogen-fixing cultivars. However, major scientific challenges remain. Field responses to nitrogen are often inconsistent across environments, with trade-offs among yield, nodulation, and grain nitrogen concentration, and with strong interactions among fertilizer source, application timing, rainfall, and soil fertility status. Bioinoculant performance also varies because of strain-host specificity, limited inoculant availability, inoculant instability, and mismatch with soil conditions. More broadly, legume-based systems improve nutrient use efficiency and can reduce dependence on inorganic nitrogen, but adoption still depends on identifying regionally appropriate species, input combinations, and management packages. These gaps are especially relevant for yardlong bean, for which direct NUE evidence remains more limited than for grain cowpea. Therefore, evaluating nitrogen use efficiency in yardlong bean under different fertilization strategies is scientifically important for identifying nutrient-management options that sustain yield, improve nitrogen recovery and utilization, reduce unnecessary fertilizer inputs, and support resilient vegetable-legume production systems.

2 Mechanisms of Nitrogen Uptake, Transport, and Utilization in Cowpea

2.1 Root nitrogen uptake processes and regulatory mechanisms in cowpea

Cowpea roots acquire nitrogen mainly as nitrate and ammonium, and this uptake is mediated by distinct transporter systems whose activity depends on external nitrogen availability and plant nitrogen demand. As in other higher plants, nitrate uptake operates through both low-affinity and high-affinity systems, with NRT1 family transporters predominating when external nitrate is relatively abundant and NRT2 family transporters becoming more important under nitrogen limitation (Akhtar et al., 2024). Root nitrogen acquisition is therefore not static; uptake capacity is adjusted by local substrate supply as well as systemic feedback signals reflecting whole-plant nitrogen status, and these controls are closely linked with changes in root system architecture that affect soil exploration. For cowpea, this general regulatory framework is especially relevant because fluctuating soil fertility and rainfall can alter the balance between direct root uptake and symbiotic nitrogen acquisition, making plasticity in root nitrogen capture an important component of nitrogen use efficiency.

Ammonium acquisition requires particularly tight regulation because NH₄⁺ is energetically attractive but potentially toxic when absorbed in excess, and AMT-type transporters are considered the major entry pathway for root ammonium uptake. Evidence from broader plant systems further shows that nitrate and ammonium transport are functionally integrated rather than independent: plants tend to balance NH₄⁺-to-NO₃⁻ uptake to maintain nutrient and pH homeostasis, while NRT1.1 can interact with SLAH3 to alleviate ammonium toxicity by coordinating nitrate transport and rhizosphere acidification (Rivero-Marcos, 2025). Regulation also occurs beyond transcription, since phosphorylation can switch transporter affinity or sensing behavior and can activate or inactivate both nitrate and ammonium transport systems in response to nitrogen supply. In cowpea, these mechanisms help explain why root nitrogen uptake efficiency depends not only on fertilizer rate, but also on nitrogen form, soil reaction, and the plant’s internal nitrogen status.

2.2 Interactions between biological nitrogen fixation and exogenous nitrogen supply

Cowpea has a strong capacity to obtain nitrogen through symbiosis with rhizobia, and this biological nitrogen fixation can substantially reduce dependence on synthetic nitrogen inputs while supporting productivity in low-input systems. Across grain legumes, the mean proportion of aboveground nitrogen derived from the atmosphere is high, indicating that fixation commonly supplies a major share of plant nitrogen, although its net contribution to the system also depends on belowground inputs and post-harvest nitrogen balance. In practice, however, symbiotic nitrogen fixation does not operate in isolation. Legumes manage both direct mineral nitrogen uptake and symbiotic nitrogen acquisition over the crop cycle, and the balance between these pathways shifts with soil nitrogen supply, developmental stage, and environmental conditions (Rahmat et al., 2023). This complementarity is central to cowpea nitrogen use efficiency because root uptake can support early growth or compensate when fixation is constrained, whereas effective nodulation can reduce fertilizer requirements later in the season (Figure 1).

The interaction between exogenous nitrogen and fixation is not linear. Low nitrate supply tends to support symbiotic nitrogen fixation, whereas high nitrate inhibits nodulation and nitrogen fixation, making over-fertilization counterproductive for legumes. Field evidence in cowpea also shows that the effectiveness of biological fixation varies widely across environments and management systems: Ndfa ranged from 35% to 92% at one site but only 4% to 70% at another, while the amount of N₂ fixed ranged from 1 to 71 kg·ha⁻¹, demonstrating strong environmental control over fixation performance (Mogale et al., 2023). In addition, compatibility between cowpea genotype and rhizobial strain is a major determinant of fixation efficiency, and native strains may compete with inoculant strains with variable success. Complementary management can therefore be more effective than fertilizer alone: liming and inoculation improved nodulation, plant performance, and grain yield, whereas combining mineral N with inoculation did not necessarily increase grain production further.

2.3 Nitrogen allocation and remobilization mechanisms within plants

Nitrogen use efficiency in cowpea depends not only on uptake and fixation, but also on how effectively absorbed or fixed nitrogen is allocated among source and sink organs and later remobilized to support reproduction (Chen et al., 2020). Work in fruiting cowpea showed that after flowering, nitrogen fixed during the reproductive stage supplied about 40% of fruit nitrogen, while 60% came from remobilization of nitrogen accumulated before flowering. Nitrogen exported to developing fruits originated from multiple organs, including leaves, nodulated roots, stems, petioles, and peduncles, indicating that remobilization is a whole-plant process rather than a single-organ response. This pattern is consistent with broader legume evidence showing that internal nitrogen recycling is a major component of seed nitrogen supply and therefore a central determinant of overall nitrogen utilization efficiency.

Remobilization is also developmentally regulated by sink demand. In pea, 71% of total nitrogen in mature seeds came from vegetative remobilization, and remobilization rates peaked when seed filling was most active, indicating strong control by reproductive nitrogen demand. In cowpea, labeled biologically fixed nitrogen moved rapidly to pods, with 55% located in pods within 9 days and 65% within 19 days, whereas nitrate-derived nitrogen was retained more strongly in roots and leaves, showing that nitrogen source influences allocation patterns during pod development. At the molecular level, nitrate remobilization from source leaves to sinks depends on transporters such as NRT1.7, and enhancing this source-to-sink transport improves growth and yield in several plant species. Together, these findings indicate that improving cowpea nitrogen use efficiency requires attention not only to acquisition, but also to the timing, direction, and efficiency of nitrogen redistribution within the plant. In sum, cowpea nitrogen use efficiency is governed by coordinated root uptake, symbiotic fixation, and internal remobilization. For yardlong bean, fertilization strategies are therefore most likely to succeed when they support these three processes together rather than maximizing mineral nitrogen supply alone.

3 Effects of Different Fertilization Strategies on Nitrogen Uptake and Growth of Cowpea

3.1 Effects of conventional nitrogen fertilization on cowpea growth and nitrogen acquisition

Conventional nitrogen fertilization generally improves early cowpea growth and can raise nitrogen acquisition when soil nitrogen is limiting, especially when nitrogen is supplied at moderate rates or at growth stages when symbiotic fixation is not yet fully active. Starter nitrogen has been shown to increase grain yield, fodder yield, net returns, and nitrogen use efficiency by more than 30% relative to unfertilized controls in West African savanna conditions. Similarly, early inorganic nitrogen enhances root activity, branching, peduncle formation, and canopy development, helping plants establish before effective nodulation and atmospheric nitrogen fixation dominate crop nitrogen supply (Ndor and Faringoro, 2020).

The response to conventional nitrogen, however, depends strongly on rate and timing. Nitrogen applied at sowing had little effect on grain yield or nitrogen fixation in one field study, whereas soil application at early flowering or foliar application during pod filling increased grain yield by about 300-600 kg/ha, showing that later-season nitrogen can better support reproductive demand. At the same time, higher nitrogen rates often suppress nodulation: in Nigerian field trials, 80 kg·N increased leaves, branches, vine length, and seed yield, but unfertilized plots retained the highest root nodule numbers, indicating a trade-off between direct fertilizer uptake and symbiotic nitrogen acquisition.

3.2 Effects of optimized fertilization practices on nitrogen use efficiency improvement

Nitrogen use efficiency improves when fertilization is optimized around crop demand rather than maximized by rate alone. In legumes, lower or strategically positioned nitrogen inputs can outperform conventional fertilization by improving both nitrogen capture and biological fixation; in a maize–soybean relay system, a lower nitrogen rate increased nitrogen uptake, NUE, and agronomic efficiency relative to the conventional rate, while optimized topdressing placement also increased total yield. In cowpea specifically, conventional topdressing is often unnecessary when biological inputs are strengthened: co-inoculation with Bradyrhizobium and Azospirillum increased grain yield, nitrogen accumulation, recovery efficiency, and overall NUE, with especially large gains relative to uninoculated or singly inoculated treatments (Galindo et al., 2020).

This optimization principle also appears in field recommendations for cowpea and yardlong bean. Under Brazilian no-till conditions, co-inoculation made cowpea production technically and economically viable without the need for topdressed nitrogen fertilizer, indicating that improved biological acquisition can substitute for part of the mineral nitrogen input (Shintate et al., 2020). In yardlong bean under semi-arid conditions, integrated nutrient-source optimization produced the strongest vegetative growth and yield when mineral NPK was combined with biochar, whereas sole organic or sole inorganic inputs were less effective, suggesting that improved NUE in vegetable cowpea depends on matching source combinations to soil and climate constraints rather than relying on a single fertilizer pathway.

3.3 Effects of organic fertilizer substitution and combined fertilization on nitrogen cycling

Organic fertilizer substitution and combined fertilization alter nitrogen cycling by shifting part of crop nutrition from immediately available mineral nitrogen to gradually mineralized organic pools, while also improving soil conditions that support nitrogen retention and uptake. In cowpea, integrating organic sources changed nitrogen partitioning: 50% N from FYM plus 50% N from vermicompost with PSB increased seed nitrogen content and nitrogen uptake, whereas the full recommended mineral fertilizer dose produced higher residual available soil NPK after harvest (Birla and Patel, 2022). This pattern indicates that organic–inorganic substitution can strengthen plant nitrogen capture, while sole mineral fertilization can leave a different post-harvest nutrient signature in soil.

Across studies, combined fertilization tends to improve nitrogen cycling by enhancing mineralization, soil organic matter, and nutrient synchrony. In cowpea, compost plus inorganic fertilizer produced greater pod length, pod number, seeds per plant, and crop biomass than synthetic fertilizer alone because the combined treatment improved soil physical and chemical properties and extended nutrient release through the season (Diatta et al., 2024). Evidence from yardlong bean points in the same direction: compost and vermicompost increased growth, pod yield, soil organic matter, soil pH, and post-harvest soil nitrogen, while organic farming also increased root nodulation relative to conventional fertilization, suggesting that organic substitution can reinforce both soil nitrogen storage and biological nitrogen cycling processes. Overall, conventional nitrogen fertilization supports early growth and direct nitrogen acquisition, but optimized and integrated strategies use nitrogen more efficiently by improving timing, biological fixation, and soil-mediated nitrogen cycling. For the paper’s focus on yardlong bean, the evidence supports evaluating fertilization strategies not only by yield response, but also by their effects on nitrogen uptake, nodulation, recovery efficiency, and residual soil fertility.

4 Evaluation Methods of Nitrogen Use Efficiency in Cowpea under Different Fertilization Strategies

4.1 Evaluation indicators and methodologies for nitrogen use efficiency

Nitrogen use efficiency in cowpea is evaluated most effectively with a combination of plant-based, symbiotic, and system-level indicators rather than by yield alone. At the plant level, common indicators include shoot nitrogen content, grain nitrogen content, total nitrogen uptake, and biomass production, because these variables directly capture how much applied or biologically acquired nitrogen is converted into vegetative and reproductive growth. For biological nitrogen fixation, the key phenotypic indicators are nodule number, nodule dry weight, and nodule efficiency, which show substantial genetic variation in cowpea and can therefore discriminate both treatment effects and genotype performance under different fertilization environments (Nkurunziza et al., 2025). When the objective extends beyond single plants to field nitrogen economy, partial nitrogen balance and seed yield per unit of aboveground nitrogen become useful complementary indicators, because they connect crop productivity with the net contribution of fixed nitrogen to the system.

Methodologically, nitrogen efficiency studies in cowpea combine destructive plant sampling with comparative reference approaches and, increasingly, predictive tools. The 15N natural abundance technique is widely used to estimate the proportion of nitrogen derived from the atmosphere (%Ndfa), and its application at flowering and maturity allows direct comparison of symbiotic contribution across management systems. Where isotopic methods are not used, the N-difference method remains a practical alternative for estimating fixed nitrogen by comparing cowpea with a non-fixing reference crop grown under the same amendment regime. More recently, greenhouse bioassays have paired nitrogen accumulation with relative indices of fixation efficiency, and machine learning models based on rhizobial colony traits have shown high correlations with measured total nitrogen and fixation-efficiency indices, although these tools still require field validation before routine use (Da Silva Souza et al., 2025).

4.2 Dynamic analysis of nitrogen uptake, accumulation, and translocation

Dynamic evaluation of nitrogen use efficiency requires repeated measurements across development, because nitrogen uptake and fixation change markedly with crop stage. In yardlong bean, destructive sampling across eight collection dates showed that nutrient demand is not uniform through the cycle: the period from 20 to 45 days after emergence was the most demanding phase and therefore the most informative window for evaluating uptake rate and scheduling fertilizer supply. In pulse crops more broadly, weekly sampling combined with shoot nitrogen analysis and δ15N revealed that single-time measurements at peak biomass miss important ontogenic shifts between soil nitrogen uptake and symbiotic fixation, because peak fixation can occur well before podding and its synchrony with growth differs among species (Siyeni et al., 2026).

Translocation analysis adds a second dynamic layer by tracing where nitrogen or related nutrients move within the plant as sink demand develops. In yardlong bean, uptake-curve analysis linked dry matter accumulation with nutrient extraction order and identified the reproductive period before harvest as a phase of especially high mineral demand, which is essential for interpreting nitrogen allocation to pod formation. Comparable legume work has shown that translocation factors and bioaccumulation patterns vary systematically by growth stage, with the strongest root accumulation of symbiosis-related micronutrients occurring from early vegetative growth to full flowering, the period when nitrogen fixation is being established and is most intense (Wysokiński et al., 2022). In cowpea specifically, sampling at 50% flowering and at physiological maturity has also been used to compare biomass production and %Ndfa between tillage and cropping systems, demonstrating that stage-specific measurement is necessary to capture changes in nitrogen source and return to the soil (Figure 2).

4.3 Relationships between nitrogen efficiency and yield/quality formation

Nitrogen efficiency in cowpea is positively related to yield formation when fertilization strategies increase nitrogen acquisition without excessively disrupting nodulation or source–sink balance. Inoculation with effective rhizobia increased shoot dry matter, nitrogen accumulation, and grain yield across locations, and these cumulative improvements were the basis for higher harvest productivity under low-input conditions. A similar pattern appeared in recent multi-site field work, where inoculation increased shoot nitrogen content, %Ndfa, and the amount of nitrogen fixed, and nitrogen fixed was strongly positively correlated with both biomass accumulation and seed yield (Ayalew et al., 2024). These results indicate that nitrogen efficiency should be interpreted not simply as recovery of fertilizer nitrogen, but as the coordinated conversion of total acquired nitrogen into biomass and reproductive sinks.

The relationship between nitrogen efficiency and quality formation is favorable but not always linear, because higher yield can coincide with dilution of grain nitrogen concentration. Inoculation and phosphorus fertilization increased crude protein concentration of cowpea grain, indicating that improved nitrogen acquisition can enhance nutritional quality as well as yield. However, the same study observed a negative correlation between grain yield and protein concentration across environments, showing that treatments promoting larger harvests do not always maximize concentration-based quality traits. Fertilization regime also affects forage and seed quality more broadly: intensified NPK supply increased crude protein in forage cowpea, while integrated organic–inorganic programs improved pod and seed yield traits and, in some cases, outperformed sole mineral fertilization for both productivity and economic return (Bante et al., 2026). Overall, evaluating nitrogen use efficiency in cowpea and yardlong bean requires integrating uptake, fixation, translocation, yield, and quality indicators rather than relying on a single metric. Under different fertilization strategies, the most informative assessments are those that combine stage-wise sampling with nitrogen content or fixation measurements and then relate these values to final yield and quality outcomes.

5 Effects of Fertilization Strategies on Nitrogen Cycling Processes in the Cowpea Rhizosphere

5.1 Effects of fertilization practices on soil nitrogen forms and availability

Fertilization changes the balance among ammonium, nitrate, and organic nitrogen pools in the rhizosphere rather than simply increasing total available N. A meta-analysis found that N fertilization drove a large increase in rhizosphere nitrate, whereas rhizosphere ammonium often remained unchanged, indicating that nitrate enrichment is the more consistent short-term response. In long-term field systems, moderate N application also increased available N and NH₄⁺-N while improving root growth, suggesting that optimized rates can enhance both N supply and plant access to that supply (Zhang et al., 2026).

The form of fertilization strongly affects how stable and diverse these N pools become over time. Long-term application of organic fertilizers alone or combined with synthetic fertilizers increased multiple internal pathways of mineral N production, including labile and recalcitrant organic N mineralization and release of adsorbed ammonium, showing a broader enhancement of soil N supply capacity than synthetic fertilizer alone (Elrys et al., 2024). Biochar retention also increased total N, acid-hydrolyzable N, and pH in both bulk and rhizosphere soil, and amino acid N appeared to be a major source of rhizosphere NH⁴⁺-N, emphasizing the contribution of organic N fractions to available N formation.

5.2 Microbially driven nitrogen transformation processes in the rhizosphere

Rhizosphere N cycling is driven by microbial transformations whose sensitivity to fertilization depends on both substrate supply and rhizosphere selection. Across long-term fertilization regimes, rhizosphere selection had a stronger influence than fertilizer alone on overall soil N cycling, increasing nifH, NIT-6, and narI while decreasing amoC, norC, and gdhA, which indicates selective stimulation of some fixation and nitrate-reduction functions but suppression of parts of nitrification and denitrification. More broadly, microbial ammonification, nitrification, and immobilization are all shaped by soil physicochemical conditions, so fertilization effects on N transformation cannot be separated from concurrent changes in pH, carbon inputs, and residue quality (Grzyb et al., 2021).

Management that adds organic substrates often intensifies microbial N turnover more than mineral fertilizer alone. In long-term rhizosphere soil, manure plus inorganic fertilizer increased gross N mineralization, ammonium consumption, nitrification, nitrate consumption, net mineralization, and net nitrification relative to inorganic fertilizer alone, showing that combined fertilization accelerates several linked N transformation steps at once. Related work further showed that mulching plus N application increased the abundance of nitrification- and DNRA-associated microbes and genes, which promoted nitrification and dissimilatory nitrate reduction to ammonium and thereby raised rhizosphere inorganic N content (Sun et al., 2024).

5.3 Regulation of soil enzyme activities and nitrogen supply capacity

Soil enzyme responses provide a practical indicator of how fertilization strategies regulate rhizosphere N supply capacity. A large meta-analysis showed that N fertilization increased urease activity by 18.6% on average and increased total soil N, confirming that N inputs generally stimulate hydrolase-mediated nutrient turnover even when microbial biomass declines. Another meta-analysis found that manure plus chemical fertilizer consistently increased urease, phosphatase, and invertase activity, while straw plus fertilizer also increased soil organic carbon and total N, indicating that integrated inputs support stronger enzymatic functioning than unfertilized controls (Miao et al., 2019).

The strongest improvements in rhizosphere N supply capacity usually occur when fertilization supports both enzymes and the microbial communities that produce them. In long-term rhizosphere soil, rice straw or manure combined with inorganic fertilizer increased urease, β-glucosaminidase, and arginase activities and also increased functional gene abundances linked to ureolytic and chitinolytic processes, showing coordinated enhancement of biochemical and microbial controls on N mineralization. Evidence from biochar-amended rhizosphere soil points in the same direction: biochar increased nitrogenase, urease, and nitrate reductase activities while raising NH₄⁺-N, NO₃⁻-N, dissolved organic N, and microbial biomass N, indicating a stronger overall capacity to supply and recycle plant-available nitrogen (Jiang et al., 2026). Overall, fertilization strategies affect cowpea rhizosphere N cycling through three linked pathways: shifting soil N forms, restructuring microbial transformations, and regulating enzyme-mediated mineralization. For yardlong bean, the evidence favors integrated strategies that combine mineral fertilizer with organic amendments or biochar, because they improve not only immediate N availability but also the biological processes that sustain nitrogen use efficiency over time.

6 Case Study: Effects of Different Nitrogen Management Strategies on Nitrogen Use Efficiency and Yield Formation in Cowpea

6.1 Experimental background and research design

Case-study evidence on cowpea nitrogen management is dominated by factorial field experiments that test fertilizer rate together with biological inputs under realistic production conditions. A strong example used a randomized complete block design under no-till, combining three inoculation treatments with five urea-^15N rates from 0 to 160 kg·N·ha⁻¹ and four replications, which allowed simultaneous evaluation of crop performance, fertilizer-N recovery, and residual effects. A related Brazilian trial used the same 3 × 5 factorial structure with Bradyrhizobium, co-inoculation with Azospirillum brasilense, and N topdressing from 0 to 160 kg·N·ha⁻¹, but added technical and economic indicators to connect NUE with profitability (Shintate et al., 2020).

Other studies broadened the design space by testing interactions among inoculation, starter N, liming, phosphorus, and genotype across environments. In Ghana, a factorial trial combined two inoculants with four N levels from 0 to 45 kg·N·ha⁻¹ and measured plant growth, nodulation, N fixation, and yield components. In Mozambique, a four-treatment randomized block study compared inoculation, P fertilization, inoculation plus P, and a control across three contrasting environments. Additional field designs tested liming × rhizobium × mineral N combinations in a 2 × 6 factorial across two years, while newer nutrient-efficient technology studies compared fertilizer microdosing, nano-NPK, soil-test-based fertilization, and designer seed treatments across three field seasons.

6.2 Effects of different fertilization strategies on cowpea growth, yield, and nitrogen uptake

Different fertilization strategies altered cowpea growth and N acquisition in distinct ways. Co-inoculation with Bradyrhizobium plus A. brasilense increased grain yield by 40.5% over the uninoculated control without N and also increased N use efficiency, N recovery, and N accumulation, showing that microbial inputs can strengthen both productivity and fertilizer-N capture (Galindo et al., 2020). Inoculation alone also improved cowpea performance across African environments by increasing shoot dry matter, shoot and grain N content, and final grain yield, with yields rising from 1 097 kg·N·ha⁻¹ in the control to 1 674 kg·N·ha⁻¹ under inoculation plus P.

Mineral N responses were positive but rate-dependent, and they often interacted with inoculation. In Ghana, both N fertilizer and inoculation increased plant height, branching, and grain yield, with the highest yield recorded at 45 kg·N·ha⁻¹ and BR 3 262 performing better than the comparison inoculant. However, hydroponic evidence showed that reducing N to 60% of full supply before flowering and 30% afterward increased both nodulation and yield in inoculated plants compared with full N without inoculation, whereas a stronger reduction to 0% after flowering increased nodule weight but reduced growth and yield (Savvas et al., 2018). In yardlong bean, integrated nutrient-source management produced a similar pattern: NPK plus biochar gave the best vegetative growth and pod yield, while sole organic or sole inorganic inputs were less effective.

6.3 Mechanisms of nitrogen efficiency improvement and optimization of fertilization strategies

The main mechanism behind improved nitrogen efficiency was better coordination between direct soil N uptake and symbiotic N fixation. Co-inoculation increased cowpea NUE by 216.5% relative to the uninoculated control and raised fertilizer-N recovery, indicating that beneficial microbes improved both N acquisition pathways and subsequent plant use of absorbed N. A complementary mechanism appears in elite Bradyrhizobium studies, where inoculation increased shoot N content by 40%, %Ndfa by 15%, and total N fixed by 41%, and the amount of N fixed was strongly correlated with biomass and seed yield (Ayalew et al., 2024).

Optimization also depended on avoiding excess mineral N and correcting other soil constraints that limit BNF efficiency. Excessive N fertilization tends to disrupt rhizobium-legume signaling and suppress nitrogenase activity, so the most effective strategy is usually balanced N supply rather than maximum fertilizer input (Abd‐Alla et al., 2023). This helps explain why liming plus rhizobium improved plant performance and grain yield, whereas adding supplemental mineral N to inoculated plants did not further increase grain production. The same optimization logic appears in nutrient-efficient technologies, where 50% RDF delivered through fertilizer microdosing with designer seeds gave the best growth, yield, NUE, and profitability, and in integrated organic–inorganic systems where RDF plus compost improved nutrient uptake and grain yield more than single-source fertilization. Overall, the case-study evidence shows that cowpea nitrogen efficiency and yield formation improve most when moderate mineral N, effective inoculation, and site-specific complementary practices are combined. For yardlong bean, this supports testing fertilization strategies that maximize N recovery and biological fixation together, rather than relying on high fertilizer-N input alone.

7 Development of Precision Nitrogen Management Technologies for Cowpea Production

7.1 Crop demand-based precision nitrogen application technologies

Precision nitrogen management in cowpea should be built around synchronizing fertilizer supply with crop demand rather than applying a uniform rate across space and time. Broad evidence shows that higher NUE is achieved when N is managed with the right rate, time, and placement under site-specific conditions, because soil variability, climate, and management history strongly modify crop response. Modern precision agriculture tools support this approach by enabling real-time or within-field adjustment of N inputs through soil sensing, crop monitoring, and variable-rate application, which reduces both under- and over-fertilization (Ali et al., 2025). For smallholder cowpea systems, the most practical implication is that demand-based management should prioritize low-cost, in-season decision rules that match N supply to vegetative expansion and pod-setting demand rather than relying only on basal fertilization.

Sensor-guided management has already shown promise in cowpea-based systems. In maize–cowpea intercropping, GreenSeeker-based N management produced the highest growth and yield performance and was statistically comparable to SPAD-based management, indicating that optical diagnosis can successfully guide in-season N adjustment in systems that include cowpea (Maheswari et al., 2025). Related fertigation research also shows that precision fertilization can reduce nutrient input without reducing crop growth or yield: in field trials, precision management lowered N supply by 8%-9% while maintaining biomass and grain production and improving nutrient-use efficiency and net profit. For yardlong bean, these results support testing SPAD-, NDVI-, or color-chart-guided topdressing schedules as scalable tools for improving NUE under variable field conditions (Figure 3).

7.2 Integration of fertigation and intelligent nutrient management systems

Fertigation offers a strong technical platform for precision N delivery because it allows nutrients to be spoon-fed to the root zone in small, frequent doses that better match crop uptake dynamics. In drip systems, frequent low-rate application reduces the risk of N loss compared with traditional pre-plant and side-dress strategies, but this advantage is realized only when irrigation and fertilization are actively managed against crop demand. Decision support systems, simulation-based scheduling, soil moisture sensors, and crop N monitoring tools such as optical sensing or petiole sap analysis are therefore central to intelligent nutrient management in fertigated vegetable systems. For yardlong bean, this means that fertigation should be treated as an information-guided system rather than simply a different fertilizer delivery method.

The performance gains from well-managed fertigation are substantial. Meta-analysis shows that drip fertigation increased yield by 12.0%, water productivity by 26.4%, and NUE by 34.3% relative to traditional irrigation and broadcast fertilization, largely because water and N supply were better synchronized with crop demand. A second meta-analysis found that drip fertigation increased crop yield by 9.8% while reducing NH3 volatilization, N₂O emission, and nitrate leaching, with nitrate leaching losses declining by 71.2% compared with conventional systems (Zheng et al., 2023). Even in intensive vegetable production, optimized drip fertigation sharply improved partial factor productivity of applied N and reduced NO₃⁻ and dissolved organic N leaching by about 90% without compromising yield, showing why fertigation plus intelligent scheduling is especially relevant for high-value crops such as yardlong bean.

7.3 Applications of biofertilizers and microbial regulation technologies

Biofertilizers provide a complementary precision strategy by improving root-zone N acquisition through biological regulation rather than fertilizer input alone. PGPR-based biofertilizers promote plant nutrition through mechanisms that include biological N fixation, nutrient solubilization, hormone production, and facilitation of nutrient uptake, making them a useful tool for reducing dependence on synthetic N in legume systems. In pulse crops, inoculation strategies are advancing from single-strain rhizobial products toward co-inoculation and microbiome-informed technologies, because cowpea productivity can benefit when rhizobia are combined with other growth-promoting microbes that enhance nutrient absorption and stress tolerance (Xavier et al., 2023). This shift is particularly important for yardlong bean, where biological inputs can be integrated with reduced mineral N programs to stabilize NUE under low-input or stress-prone environments.

The main opportunity is not just inoculation, but microbial optimization of the rhizosphere. Nitrogen-fixing biofertilizers already dominate the global biofertilizer market and are used to increase fixed N supply to crops, while cowpea-associated rhizobia show substantial regional diversity that can be exploited to develop locally adapted inoculants. More advanced PGPR approaches now include synthetic microbial communities and host-mediated microbiome engineering, which aim to assemble strains with synergistic nutrient-acquisition traits, although field inconsistency remains a major limitation and standardized protocols are still needed (Zuluaga et al., 2024). Experimental evidence also supports combined biofertilizers: Bradyrhizobium plus Streptomyces griseoflavus significantly improved cowpea shoot and root growth under both N-limited and N-supplemented conditions, indicating that microbial regulation can enhance plant performance even when mineral N is present. Overall, precision nitrogen management in cowpea is moving toward demand-based fertilization, sensor-guided fertigation, and microbiome-assisted nutrient regulation. For yardlong bean, the most effective strategy will likely combine in-season diagnosis, frequent low-dose N delivery, and adapted biofertilizer systems to improve yield while limiting nitrogen loss.

8 Future Research Directions and Challenges

8.1 Regulation mechanisms of nitrogen use efficiency under multifactor interactions

Future research on nitrogen use efficiency in yardlong bean should move beyond single-factor fertilizer trials toward a multifactor interaction framework that jointly evaluates nitrogen with soil properties, water supply, temperature, genotype, and crop management. Meta-regression across 2 436 observations showed that site conditions strongly modify how nutrient, crop, and soil management affect nitrogen recovery efficiency, indicating that response mechanisms are inherently context dependent. This aligns with broader vegetable-crop analysis showing that N uptake and use efficiency are shaped by many interacting variables, including soil fertility, planting distance, crop variety, application timing, and light environment, which vary in both space and time across farms (Valenzuela, 2024). For yardlong bean, this means that future studies should explicitly quantify interaction terms rather than treating fertilization rate as the sole driver of NUE.

A second challenge is to identify which interaction layers are most important for prediction and management under different production environments. Evidence from water–nitrogen studies shows that regional and climatic differences still limit the generalization of coupling effects, and that long-term links between water–N regulation and soil health remain insufficiently resolved (Qiu et al., 2025). In parallel, large-scale synthesis in wheat found that initial soil nutrient status and soil organic carbon often explained variation in yield and NUE more strongly than climate or split-input practices, suggesting that soil condition may dominate management effects when inputs are suboptimal. Future yardlong bean research should therefore combine fertilization treatments with baseline soil characterization, water-management regimes, and physiological measurements to build site-specific mechanistic models of NUE.

8.2 Applications of molecular approaches and multi-omics technologies

Molecular research on yardlong bean NUE should prioritize the integration of genomics, transcriptomics, metabolomics, and physiology to identify regulatory networks for N uptake, assimilation, remobilization, and carbon–nitrogen coordination. Reviews across crops emphasize that NUE is controlled by complex quantitative and environmental regulation rather than by a single pathway, and future gains depend on integrating plant physiology, quantitative genetics, and agronomy (Elrys et al., 2022). At the same time, whole-genome sequencing and omics-level analysis are specifically identified as priorities for resolving carbon–nitrogen metabolism and molecular responses that still limit NUE improvement. For yardlong bean, this suggests that candidate-gene screening should be linked to tissue-specific expression, nutrient flux traits, and fertilization response phenotypes under field conditions.

Multi-omics is especially promising because it can connect genes to metabolites, microbial interactions, and environment-responsive traits. Integrative studies now use QTLs, GWAS signals, transcriptomes, proteomes, and metabolomes together to predict NUE-related candidate genes and support breeding of N-efficient cultivars. More advanced systems-biology frameworks further indicate that combining multi-omics with gene regulatory networks, machine learning, and explainable AI can improve genotype-environment modeling and bridge laboratory discovery with field validation (Kundu and Tanti, 2025). In crop-specific examples, transcriptomic–metabolomic integration under low N has already identified transporter genes and carbon-nitrogen metabolic signatures associated with superior NUE, supporting the value of similar approaches in legumes and yardlong bean breeding.

8.3 Long-term field experiments and ecological benefit assessment

Long-term field experiments are essential for future yardlong bean NUE research because short-term trials cannot capture the gradual adjustment of soil N pools, crop response, and environmental losses after repeated fertilization. Long-term evidence shows that steady-state relationships among N input, yield, soil N pools, and losses can take decades to develop, so annual trials cannot reliably estimate the long-run costs and benefits of alternative N strategies (Van Grinsven et al., 2022). Long-term experiments also provide archived samples and process data that allow later study of slowly changing soil properties, pollutant trends, and model development, making them uniquely valuable for sustainable nutrient management research. For yardlong bean, establishing multi-year experiments across contrasting agroecological zones would make it possible to evaluate whether optimized fertilization improves NUE without degrading soil fertility over time.

Ecological benefit assessment should also become more comprehensive by quantifying trade-offs among yield, profit, soil quality, and nitrogen losses rather than relying only on agronomic efficiency. Recent long-term evidence shows that modest reductions in fertilizer and irrigation can maintain economic performance while improving soil quality and cutting N leaching by 35.9%, illustrating the value of multi-objective sustainability metrics. Likewise, long-term rotation and fertilization studies show that combining legume-derived organic inputs with mineral fertilizer can increase NUE and crop productivity over decades, while ecological intensification practices often substitute partly for fertilizer N at low-input levels (Vaziritabar et al., 2024). Future yardlong bean studies should therefore include nitrogen balance, leaching risk, greenhouse-gas emissions, soil biological quality, and economic return in a unified ecological assessment framework. Overall, future research on nitrogen use efficiency in yardlong bean should integrate multifactor field regulation, omics-based mechanism discovery, and long-term ecological evaluation. This combined approach is the clearest path to fertilizer strategies that improve yield while sustaining soil health and reducing nitrogen loss.

9 Conclusions

Different fertilization strategies altered cowpea nitrogen use efficiency through distinct but interacting pathways. Mineral fertilization increased biomass, yield, and forage quality when rates were optimized, but several studies also showed that maximum fertilizer input was not necessary to achieve the best agronomic response. In field evaluation of nutrient-efficient technologies, applying 50% of the recommended fertilizer dose through microdosing with designer seeds gave the best combination of growth, yield, nutrient use efficiency, and profitability, indicating that better placement and timing can substitute for part of total fertilizer input. Similarly, in maize–cowpea systems, 75% of the recommended N combined with foliar nano urea performed comparably to full N for quality traits and residual soil N, showing that strategic supplementation can reduce conventional urea use without clear performance loss. Biological and integrated strategies produced the most consistent gains in NUE because they improved both N acquisition and soil-mediated nutrient supply. Inoculation with rhizobia increased cowpea nodulation, shoot N, grain N, and yield across contrasting environments, and the combination of inoculation with phosphorus gave the highest grain yield among the tested treatments. Co-inoculation with Bradyrhizobium and Azospirillum brasilense further increased N use efficiency, N recovery, and N accumulation, with grain yield gains over both the uninoculated control and standard rhizobial inoculation. Integrated nutrient management based on FYM, residues, compost, biochar, or biofertilizers also improved growth, productivity, and residual benefits in cowpea and yardlong bean, while sole organic or sole inorganic fertilization was often less effective than well-balanced combined treatments.

Efficient nitrogen management matters for sustainable cowpea production because it affects not only yield but also soil fertility, system productivity, profitability, and environmental performance. Grain legumes contribute to sustainable intensification by reducing dependence on synthetic N fertilizers and supporting biodiversity and ecosystem services, which makes cowpea especially valuable in low-input and mixed cropping systems. In cereal–cowpea intercropping and rotation systems, cowpea can improve land-use efficiency and contribute biologically fixed N to the system, although the magnitude of this contribution depends on biomass production, density, and cropping arrangement. These functions are agronomically important because they help explain why efficient cowpea management can support subsequent crops and reduce external fertilizer requirement over time. The sustainability value of efficient N management is also evident in soil biological responses and longer-term system resilience. Organic and integrated nutrient strategies improved soil organic carbon, available nutrients, microbial populations, and enzyme activities in cowpea-based systems, even where short-term productivity advantages over chemical fertilization were modest. Organic-input combinations in cowpea also increased soil microbial biomass N, urease, and dehydrogenase activity while improving yield and economic return, linking biological soil improvement directly to production outcomes. Because synthetic N use is associated with pollution and greenhouse-gas concerns, management approaches that strengthen symbiotic fixation and reduce unnecessary fertilizer input have clear ecological value beyond immediate crop performance.

Future research should first address the fact that cowpea NUE is regulated by strong interactions among genotype, fertilizer source, companion nutrients, soil condition, and cropping system. Evidence already shows that fertilization response varies with site-specific soil and agro-climatic conditions, and that intercropping outcomes depend on spacing, density, cultivar choice, and N scheduling rather than on N rate alone. Research also needs to quantify biological N fixation more completely, because current knowledge remains limited for belowground N contributions, rotational carryover, and the effects of fertilization regimes on fixation efficiency under different environments. For yardlong bean, this means future field studies should combine N balance, fixation estimates, and yield formation across integrated nutrient treatments instead of relying only on aboveground productivity indices. A second priority is to connect molecular, microbial, and precision-management approaches with long-term field validation. Cowpea shows substantial genetic variability for nodulation and BNF-related traits, and recent studies identified genomic regions and candidate genes that could accelerate breeding for higher fixation efficiency and lower fertilizer dependence. Broader reviews also argue for integrating multi-omics, genome editing, synthetic microbial consortia, and smart precision agriculture to develop locally adapted, climate-resilient cowpea systems. At the same time, future work should evaluate dual-function inoculants and other ecological technologies that can improve fixation while mitigating environmental losses, including N₂O-reducing Bradyrhizobium strains and bioinoculant-based low-input systems. Overall, the evidence supports a clear conclusion: cowpea nitrogen use efficiency improves most under integrated, site-specific, and biologically enabled fertilization strategies. For yardlong bean, the strongest path forward is to adapt these cowpea-derived principles into long-term, field-tested nutrient programs that improve yield while sustaining soil health and reducing nitrogen loss.

Acknowledgments

We would like to thank the anonymous reviewers for their detailed review of the draft. Their specific feedback helped us correct the logical loopholes in our arguments.

Conflict of Interest Disclosure

The authors affirm 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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