The productivity of cereals such as rice, wheat, maize, sorghum, millets, barley, etc., is heavily influenced by nitrogen. Yet factors such as low plant population, excessive fertiliser application, and poor methods and timing of fertiliser use have led to nitrogen (N) losses of up to 70%, contributing to reduced cereal yields.
Agriculture, one of the largest sectors in the world—especially in India—has struggled to achieve significant milestones in feeding the rising population. The global demand for food is increasing rapidly due to population growth at an alarming rate. To meet this demand, synthetic fertilisers have been used dramatically over the last 50 years to attain higher production of agricultural products, mainly cereals (2).
Cereals are the major sources of dietary energy and nutrients in both developed and developing countries (used as staple foods in daily diets) as they contribute 30.4% of total dietary energy supply with 64.1% manganese, 51% carbohydrates, 48.5% dietary fibre, 6-15% protein etc. (3). However, shrinking farmland due to decreasing arable land, declining ground water level, increasing crop intensity and a shift toward high-value cash crops have widened the gap between potential and actual yields, resulting in low cereal production worldwide (4). The productivity of cereals such as rice, wheat, maize, sorghum, millets, barley, etc., is heavily influenced by nitrogen. Yet factors such as low plant population, excessive fertiliser application, and poor methods and timing of fertiliser use have led to nitrogen (N) losses of up to 70%, contributing to reduced cereal yields (5). Thus, the excessive, imbalanced and inefficient use of fertilisers, mostly nitrogenous, has become a serious global challenge (1).
In many Asian and Latin American countries, urea and nitrate are the most commonly used inorganic nitrogen fertilisers for widely cultivated cereals (7). The Haber-Bosch process made it possible to produce nitrogenous fertilisers at scale, enabling food production for nearly half of the world’s population (8). Plant absorbs nitrogen in the form of NH4+, NO2‒, and NO3‒, which together account for less than 5% of total soil nitrogen. Uptake before flowering enables the synthesis of amino acids, which further contribute to enzymes, proteins, and the photosynthetic machinery (8,10). Nearly 6% of nitrogen in cereal seeds is stored as protein reserves (10).
However, excessive nitrogen in soils from continuous fertiliser application can negatively impact soil and environmental health. Millions of tons of nitrogen fertilisers are applied globally every year, leading to environmental pollution and rising fertiliser costs (6). When nitrogen present in the form of nitrate is not absorbed by plants, it takes other nutrients like magnesium (Mg) and calcium (Ca) with it and leaches out, resulting in the eutrophication and acidification of soil (11,9). The eutrophication of marine and freshwater ecosystems is a major example of such nitrogen-related impacts (8).
A study by Donner and Kucharik showed that increasing nitrogen fertiliser application by 30% raised corn yield by only 4%, yet caused nitrate leaching to rise sharply by 53%. In contrast, reducing nitrogen application by 30% lowered yield by 10% but significantly reduced nitrate losses through leaching and runoff by 37% (9). This demonstrates the challenge of achieving high productivity while preserving environmental quality at the same time (8). Since improving nitrogen use efficiency (NUE) can increase yields and profits while reducing environmental harm, significant efforts are required to enhance NUE in cereals to address global ecological and environmental burdens caused by nitrogen (12,13).
Understanding of Nitrogen Use Efficiency (NUE) in relation to cereals
Cereals like rice, wheat, maize, millets, barley, and sorghum are an essential part of our staple diet and human nutrition, but their basic fundamental element is nitrogen, without which they cannot achieve proper growth or produce quality yield (14). Nitrogen is considered one of the main limiting nutrients because of its role in protein synthesis, phytohormonal regulation, photosynthesis, and overall plant growth and development (5).
Being low in availability in soil, nitrate (NO₃⁻) is typically the predominant source of nitrogen fertiliser, except in the case of rice, where ammonium ions (NH₄⁺) serve as the major inorganic nitrogen source. Its availability varies with pedoclimatic factors such as temperature, pH, and soil type (10). Therefore, it should be administered in the recommended amount; otherwise, limiting inorganic nitrogen can result in reduced food production and, consequently, hunger, while a surfeit of it can lead to environmental problems by contaminating air and water (16). Hence, nitrogen use efficiency (NUE) is a critical parameter governing the uptake and utilisation of nitrogen, and it varies with nitrogen doses, application methods, and other agronomic factors. It is defined as the percentage of applied nitrogen fertiliser that is absorbed and used by different crop components (14).
Agronomically, NUE refers to the output–input ratio, or the uptake, utilisation, or photosynthetic efficiency of cereals, expressed as grain yield per unit of nitrogen applied (7). It consists of two main components: nitrogen uptake efficiency (NupE) and nitrogen utilisation efficiency (NutE). NupE refers to the plant’s ability to absorb nitrogen from the soil, mainly in the form of ammonium and nitrate ions, while NutE refers to the plant’s capacity to use absorbed nitrogen to produce grain yield (8). These can be calculated as follows (5)
NupE = N contents in plant/total N applied
NutE = Total yield / N contents in plant
And, NUE = NupE * NutE
The uptake and utilisation of Nitrogen undergoes two important phases, where one is the vegetative phase, where young leaves and roots act as a sink, and the other is the post-flowering development phase, where roots and shoots act as sources. These phases include sub-processes such as reduction, assimilation, translocation, and remobilisation (8,17). The root cell membrane contains two important transport systems: a low-affinity transport system (LATS), which becomes active under high nitrate conditions, and a high-affinity transport system (HATS), which functions under low nitrate availability (10).
Nitrogen, a structural part of amino acids, nucleic acids, chlorophyll, phyto-hormones and ATP, undergoes metabolic processing influenced by enzymes such as glutamine synthetase, glutamine oxoglutarate aminotransferase, nitrate reductase, nitrite reductase and asparagine synthetase enzyme (5). In the cytosol, nitrate is first reduced to nitrite by nitrate reductase; nitrite is then transported to plastids and chloroplasts, where nitrite reductase converts it to ammonium (10). Ammonium is subsequently assimilated through glutamine synthetase (GS) activity. In cereals such as wheat and rice, enhanced GS activity in leaves has been associated with increased grain dry matter, higher Nitrogen utilisation efficiency, and improved Nitrogen harvest index (10).
The question then arises: why is NUE so low in cereals? Low NUE is largely attributed to substantial nitrogen losses after fertiliser application. When nitrogen fertilisers such as ammonium nitrate, ammonium sulfate, or urea are applied, they undergo losses through ammonium volatilisation (conversion of NH₄⁺ to NH₃), nitrous oxide emissions (during nitrification and denitrification), and nitrate leaching due to the high mobility of NO₃⁻ in soil (18). For instance, studies have reported nitrogen losses of 52–73% in corn and 21–41% in winter wheat, with surface runoff alone accounting for 1–13% of total applied nitrogen (19). In other words, when large amounts of nitrogen fertilisers are supplied to cereals to produce maximum grain yields, the NUE often remains below 50%, as the remaining nitrogen escapes into the environment through various mechanisms—despite advances in soil management, fertilisation techniques, irrigation practices, and improved cultivars (8).
The nitrogen that escapes is lost through leaching (via dissolution in water), immobilisation, clay fixation, denitrification by anaerobic bacteria, or ammonia volatilisation (13). Improper fertiliser application—such as failure to incorporate fertilisers into the soil—further increases surface runoff, with nitrate leaching contributing to nitrogen losses of up to 40%, influenced by temperature, surface residue, and soil pH (19). That’s why, even after a drastic increase in the usage of nitrogenous fertiliser, the world cereals production is very less as per requirement, as crops recover on an average of 33% of total applied N fertiliser only (13). Therefore, improving NUE is essential for achieving sustainable global cereal production and reducing atmospheric pollution, and must be prioritised urgently (5).
Ways To Enhance Nitrogen Use Efficiencies of Cereals
Efficient Application Rate
To fulfil the demand of a large population for high crop production, farmers often apply excessive amounts of synthetic nitrogenous fertilisers, which lead to several problems such as lodging due to shoot overgrowth and weak stems, deterioration of grain quality, increased insect and pest infestations, and ultimately low nitrogen use efficiency (NUE) (5). Reducing the rate of nitrogen fertiliser application to enhance NUE may cause delayed leaf senescence; however, studies have shown that delayed senescence under low nitrogen conditions can maintain higher photosynthetic capacity, thereby increasing grain yield (5).
Moreover, NUE in response to N fertilisation rate, particularly under crop rotation, was observed to be 32-38% and 31-40% during the years 2011-2012 and 2012-2013, respectively (20). Countries like Australia have reported no yield loss when the N fertiliser application rate is minimised by 20% but with the condition that the plant density should be high (5).
Real-Time Foliar Application and Deep Placement of N Fertiliser
It has been reported that the seasonal application of nitrogenous fertilisers to cereals, in the form of topdressing or deep placement, results in more efficient fertiliser uptake and utilisation compared to early incorporation or late application. Early application of excess nitrogen lowers NUE and produces sub-optimal grain yield, whereas late, high-rate nitrogen application increases grain protein content but reduces NUE (19). The nitrogen losses caused by nitrification and denitrification in flooded rice systems can be reduced by deep placement of fertilisers, as this method prevents the conversion of NH4 to NO3, leading to a 65% reduction in nitrogen loss and a 50% increase in rice yield (18).
Grain protein content in cereals was found to increase by 4.4% when a urea solution (11–56 kg/ha) was applied as a foliar spray at the flowering stage, while nitrogen recovery ranged from 55–80% when supplied during anthesis, but only 30–55% when applied at planting (19). In another study, 94% suppression of NH₃ and N₂O losses was observed using deep placement. At the same time, its advanced version, the Closed-Slot Injection Method, proved even more effective in reducing nitrogen losses, as seen in maize, where NH₃ emissions decreased by 75% (18). In barley, foliar application was found to be more efficient than broadcasting, as grain protein content increased when 50 kg N/ha was sprayed at the awn-emergence stage (19).
Drip Fertigation
Drip fertigation with optimum nitrogen levels has been found very effective in improving NUE alongside good crop health and potential yield as it supplies a specific or precise quantity of water and soluble fertilisers directly to the root zone of the crop, thereby increasing yield per unit of nitrogen and water supplied (5). For arid regions, drip fertigation combined with high plant population may play a significant role in reducing unnecessary nitrogen use while maintaining sustainable yield and improved NUE (5).
Cultivars with high harvest index and low forage yield
A plant’s ability to convert absorbed nitrogen into economic yield and the yield produced per unit of nitrogen depend on its physiological efficiency and nitrogen utilisation efficiency, respectively, while nitrogen allocation to yield relates to the plant’s total nitrogen content (21). Studies have shown that NUE increases and nitrogen losses decrease in cereal varieties with a high harvest index and low forage yield—for example, certain varieties of wheat and rice (19). Thus, the nitrogen harvest index plays a crucial role in determining the amount of total nitrogen returned to the soil as plant residue, which then acts as a nutrient source depending on the timing of fertiliser application relative to crop nitrogen demand (21).
Enhanced Fertilizers
When soil nitogen (N) content is deficit, the crop parameters like nitrogen content, biomass and yield depends largely upon applied fertilizers and influenced by the factors such as agronomic efficiency (illustrates the improvement in productivity of a crop by supplied N), recovery efficiency of fertilizer (addresses the apparent enhancement in crop N uptake in response to the N application), partial N balance (illustrates the ration of N removal to N use) and N balance intensity (describes the difference between N removed and used) (21). Therefore, fertilisers should be used in enhanced forms, which include the fertilizer coated with low permeable materials attached to either urease or nitrification inhibitor such as phenylphosphorodiamidate, 3,4-dimethyl pyrazole phosphate (DMPP), N-(n-butyl) thiophosphoric triamide (NBPT) and dicyandiamide (DCD) that regulates the process of urea hydrolysis and nitrification as a result of which there is reduction in N loss and increase in N uptake by crop s specially cereals (18). DMPP and DCD with urea are found to be effective in reducing N2O emission, while the addition of NBPT is found to reduce NH3 volatilisation from agricultural soils (18).
Intercropping Method
Countries like China, India, Southern Asia, Latin America, and Africa, are using the method of intercropping as sustainable practices which involves efficient land and nutrient use, high yield, etc. (22). Under varying nitrogen supply, crops grown under intercropping, particularly under interspecific combinations show better growth and improved nutrient use efficiency due to enhanced resource sharing (5). According to a study, at high nitrogen input, there is an undesirable outcome for biological N fixation. However, in legume-nonlegume mixtures, e.g., maize and soybean, higher soil nitrogen increases microbial nitrogen fixation and stimulates the transfer of fixed nitrogen to the non-legume crop, especially under low nitrogen input (5).
Shaping Microbial Activities Mechanism
Microbial systems such as arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing microbes play a critical role in regulating nitrogen response in soils by enhancing nitrogen uptake efficiency, nitrogen cycling, and overall NUE (21). Under low soil nitrogen, enhancing AMF associations can expand root surface area, improve nitrogen mineralisation, and facilitate uptake. In high nitrogen soils, efficient fungal partners capable of proliferating and acquiring NH₄⁺ become more relevant (21). Another strategy involves promoting nitrogen-fixing cereal associations—rice, maize, wheat, and sorghum, which harbour diverse fungi and bacteria, including diazotrophs, derived from seeds, soil, or irrigation water, which enhance plant growth by supplying nitrogen to the host crop (23). Genetically modified nitrogen-fixing bacteria, such as ammonium-excreting Azospirillum and mutants of Azospirillum, Kosakonia, Pseudomonas, and Azotobacter, have been shown to enhance nitrogen supply and stimulate wheat growth (23).
Site-Specific Nitrogen Fertiliser Management
Crop response to applied nitrogen varies with soil conditions and depends on crop nutritional needs, soil nitrogen-supplying capacity, calibrated nitrogen dosages, and the crop’s ability to utilise available nitrogen efficiently (18). Improper timing and excessive nitrogen application reduce NUE and recovery efficiency. Although high grain yields have been achieved through semi-dwarf cereal varieties, farmers often apply excess nitrogen, increasing the risk of lodging and pest/disease susceptibility (24). To address this, Site-Specific Nitrogen Management (SSNM) was developed to optimise nitrogen use in cereals based on crop N demand (24). SSNM considers yield potential, soil nitrogen supply, nitrogen response, and NUE calculations to determine the appropriate nitrogen rate, involving four key steps:
(a) Based on 85% yield potential, set an achievable grain yield target,
(b) Estimate total nitrogen supply without fertiliser,
(c) Estimate N response, and
(d) Estimate total nitrogen requirement based on NUE (24).
Many other strategies under this have also been developed for variable-rate N management during the early growth and development stage of the plants, including the real-time fluoro-sensing technique (emerging technology), where 15 kg N/ha has been reported to increase the NUE without any wheat grain yield reduction and, proximal sensor-based technique, which reported an increase in maize yield with improvement in NUE (18). Compared to conventional farmer practices, SSNM has been shown to reduce N fertiliser use by 32% while increasing cereal grain yield—especially in rice—by about 5% (24). Therefore, site-specific nitrogen application enhances both the quality and quantity of yield, improves NUE, and contributes positively to economic and ecological sustainability (5).
Nitrogen use efficiency remains a central challenge for sustainable cereal production, especially under rising environmental concerns and fertiliser costs. The integration of improved application techniques, enhanced fertilisers, intercropping systems, and site-specific management demonstrates substantial potential to minimise nitrogen losses while maximising yield. Together, these strategies form a pathway toward productive, resource-efficient, and environmentally responsible agriculture.
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