Enhancing Sustainability in Sugarcane Production Through Effective Nitrogen Management: A Comprehensive Review

Abstract

The nitrogen (N) requirement of sugarcane (Saccharum spp.) is very high due to the extensive growth of biomass. N fertilisers are applied excessively to ensure the optimum growth of the sugarcane crop. Improper N management causes a decrease in nitrogen utilisation efficiency (NUE) and contributes to N losses via leaching and gaseous emissions in the form of ammonia (NH₃) and nitrous oxide (N₂O), leading to unintended negative consequences. Asynchronous timing between the sugarcane N demand and supply by the N sources exacerbates these losses. Therefore, proper N management strategies need to be implemented to mitigate losses and enhance NUE. This review provides an overview of global sugarcane cultivation and discusses the N requirements for sugarcane crops. Additionally, it summarises the various strategies utilised in N management for sugarcane cultivation and evaluates their effectiveness. Furthermore, it identifies research gaps and outlines future research directions.

1. Introduction

Sugarcane (Saccharum spp. L.), a significant crop renowned for its diverse applications, is cultivated extensively on a global scale for purposes ranging from sugar production to biofuel (ethanol) generation, as well as the utilisation of byproducts like bagasse and molasses for energy production and the distillation of alcoholic beverages [1]. Leading sugarcane-producing nations such as Brazil, India, China, Thailand, and Pakistan play crucial roles in the industry, with Brazil notably at the forefront [2]. The economic ramifications of sugarcane cultivation are profound, offering livelihoods to millions and making substantial contributions to agricultural sectors worldwide. Nonetheless, sustainability concerns have surfaced due to issues surrounding land use alterations, water consumption, and environmental impacts linked to cultivation practices, particularly concerning N fertiliser applications [3].

Sugarcane has a significant N requirement owing to its faster growth and biomass accumulation. This high demand for N necessitates the use of fertilisers to ensure optimal growth and yield, making soil management practices crucial in sugarcane cultivation [4]. Sugarcane is initially planted as a new crop, harvested at maturity for sugar production, and then regrows from the base (ratoons) for multiple subsequent crops, enhancing yield and sustainability. N fertiliser recommendations for both plant and ratoon crops vary among countries, ranging from 40 to 500 kg N ha⁻¹. However, the NUE of sugarcane ranged between 30% and 50%, showcasing that a large portion is lost to the environment [5,6]. These losses are mainly through leaching, NH₃ volatilisation, and N₂O [7,8].

The average N losses from sugarcane cultivation ranged between 20 and 60% [9]. Poor management practices, including excess N applications, untimely application, excess irrigation, surface application, and use of conventional application methods, are some of the factors that exacerbate N losses [7,10]. Some of these factors can lead to a lack of synchronisation between the N demand of plants and their supply. Nevertheless, even under optimal management practices, N losses cannot be entirely eliminated, as they are a natural occurrence within the soil-atmosphere nexus [11,12].

Various strategies are employed to mitigate N losses in sugarcane systems, which can be broadly classified into good management practices, the use of crop simulation models, the use of precision agricultural tools, the use of enhanced-efficiency fertilisers, the use of biotechnology and genetic engineering and conventional practices (Figure 1). Adhering to good management practices and traditional techniques is crucial for reducing N losses, given that these methods are straightforward and cost-effective. On the other hand, implementing other strategies, which range from moderately to highly expensive and require technical expertise, can significantly enhance NUE despite the associated costs and complexities.

Limited research has been devoted to sustainable N management in sugarcane systems. For instance, Skocaj et al. [3] delineated N management guidelines specific to the Australian sugarcane production system, while de Castro et al. [8] explored optimal N fertiliser management practices within the context of green cane trash blanket (GCTB) adoption in Brazil. However, these studies primarily focused on regional practices, lacking a comprehensive global perspective. Moreover, they did not delve into the utilisation of precision agriculture tools, biotechnology, genetic engineering interventions, or simulation models. In order to fill these gaps, this literature review aims to consolidate knowledge of N loss pathways and their environmental impacts, along with an overview of various strategies employed in N management globally for sustainable sugarcane production. The review also identifies research gaps and suggests future directions for sustainable N management in sugarcane cultivation.

2. Overview of Global Sugarcane Cultivation and Production

Sugarcane grows best in warm, humid areas with plenty of rain, which is common in tropical and subtropical regions. The overview of the global production of sugarcane is shown in Figure 2 [2]. Brazil is the largest sugarcane producer in the world, which contributes 25% to global sugarcane production in the year 2023/2024, following India with a contribution of 19% to global production [13].

The top ten sugarcane-producing countries in the world in 2022 were Brazil, India, China, Thailand, Pakistan, Mexico, Colombia, Indonesia, the USA and Australia. Their corresponding productions were 37.77, 22.91, 5.42, 4.80, 4.59, 2.88, 1.83, 1.69, 1.64 and 1.50% (a total of 85%) of the world’s total cane production [2]. The total extent of sugarcane cultivation was 21,575,278 ha, where 85.67% of the land extent has been occupied by the top ten producing countries [2]. Brazil and India occupied nearly 45.75% and 23.98% coverage of the land extent, respectively. Sugarcane yield (kg ha⁻¹) obtained from these top 10 major sugarcane-producing countries placed 24th, 16th, 20th, 46th, 35th, 34th, 13th, 37th, 17th and 15th rank, respectively, from the overall 103 producing countries [2].

Sugarcane production increased steadily across the top seven producing countries from 1973 to 2013 (the last 41 years). This growth was primarily due to the expansion of cultivation areas, while yield improvements also contributed significantly. Cultivated land extent grew remarkably in Brazil (500%), China (237%), Thailand (286%), and Pakistan (57%), whereas it marginally increased in India (94%), Mexico (52%), and Colombia (61%) [14]. Yield gains were generally uncertain, with Thailand leading at 70%, followed by Pakistan (58%) and China (59%). In contrast, the USA had only a 31% rise in cultivated land extent and a slight yield decrease of 7% [14].

3. N Requirement of Sugarcane

A sufficient level of N application is essential for sugarcane to maintain the optimum level of yield and maintain the quality of the yield [15]. The sugarcane plant growth curve and N demand curve follow a sigmoid pattern (Figure 3). Insufficient N application results in slower development, lower sugarcane yield, delayed maturity, less sugar accumulation, and lower-quality juice [15,16]. N deficiency in sugarcane plants can make them more vulnerable to pests and diseases, as it weakens the plants and makes them more susceptible to attacks. Therefore, farmers can ensure the vigour and health of sugarcane plants, making them more resilient against potential threats, by providing the correct amount of N needed for their growth and development [17].

During the early developmental stages, the percentage of N fertiliser was around 60%, gradually decreasing to 20% as the plants approached the harvesting stage [18]. Excessive N application is also not beneficial for sugarcane production for various reasons. Firstly, high N application increases vegetative growth and biomass production, which can reduce juice quality or sugar content [19]. Secondly, elevated N levels can delay the ripening process by prolonging the vegetative growth phase and inhibiting the accumulation of sugars in the stalks [20,21]. Thirdly, excessive N fertilisation promotes vigorous vegetative growth and weakens stalk structure, increasing the risk of lodging or bending of the plants [21]. Hence, optimal N application is advantageous not only for economic and environmental reasons but also for maximising sugarcane production efficiency.

The N requirement of sugarcane varies depending on multiple factors. Ratoon crops are more sensitive to N than plant crops, necessitating higher N application rates [22]. The well-established root systems in ratoon crops enable higher N uptake, approximately 25% more than in plant crops [23]. For example, in Australia, N recommendations for plant and ratoon crops are 140–180 and 160–220 kg N ha⁻¹, respectively [24]. Another contributing factor is cropping age, with older crops typically requiring less N fertilisation due to increased exposure to the summer season. In the summer season, more mineralisation takes place, decreasing the fertilisation requirement [21]. Moreover, if sugarcane is not cultivated in the summer, the adoption of N-fixing crops in the summer fallow period can minimise the N requirement for the next cropping cycle [11,25]. Two different harvesting practices are adopted: burnt harvesting and GCTB. In burnt harvesting, the field is deliberately burned to remove old leaves, minimising disturbances during harvesting. On the other hand, the GCTB method involves threshing the green leaves and mulching them on the surface. Compared to burnt harvesting practices, mechanical harvesting followed by the GCTB method can lead to an increase in N content in the soil over time after the adoption of this practice. Nearly 60% of N is provided by the GCTB method to the soil, thus more favourable than the burnt harvest [26]. Additionally, climatic and soil conditions also determine the N requirement. Therefore, the optimal N requirement for sugarcane varies from one system to another, depending on various the above-mentioned factors.

4. N Fertiliser Recommendations

Urea, a white crystalline solid with a N content of 46%, is commonly used in agriculture as both a fertiliser and an animal feed additive [27,28,29]. An average crop of sugarcane removes 208 kg of N at a yield of 100 tons ha⁻¹ [30]. In Brazil, N fertiliser is applied at planting at rates ranging from 40 to 80 kg ha⁻¹, typically incorporated into the planting furrow (Table 1). For ratoon crops, the N application rates are considerably higher, reaching 100 to 150 kg ha⁻¹, either surface-applied or incorporated [31]. The N fertiliser requirement for cane crops in India depends on the topography and external factors of the country (Table 1). In northern India, about 150–180 kg ha⁻¹ of N is required, while in southern India, it ranges between 250 and 350 kg ha⁻¹ [30]. The annual requirement of N application rate in China for sugarcane usually reaches up to 500–700 kg ha⁻¹ [32]. N rate recommendations for sugarcane in Louisiana, USA, vary by crop age (plant cane or ratoon) and soil texture (light or heavy) (Table 1). In the 1950s, rates ranged from 45 kg N ha⁻¹ for plant cane on light-textured soils to 112 kg N ha⁻¹ for ratoon cane on heavy soils. By the 1980s, these recommendations increased to 90 kg N ha⁻¹ for plant cane on light soils and 180 kg N ha⁻¹ for ratoon cane on heavy soils [33].

As per the recommendation of the Sugarcane Research Institute of Sri Lanka, the application of urea (kg ha⁻¹) is advised based on the soil type and irrigation type (Table 1). The sugarcane cultivated soil types in Sri Lanka are: Uda Walawe, Sevangala, Hingurana reddish-brown earth, Hingurana non–calcic brown soils, Hingurana alluvial soils, Kantale, Kilinochchi, Badulla and Low Country Intermediate Zone (Mahiyanganaya, Padiyathalawa, Maha Oya) soils [34]. Urea requirement for cane is 300 kg ha⁻¹ (at planting = 50 kg ha⁻¹, 45 Diammonium Phosphate (DAP) = 100 kg ha⁻¹, 90 DAP = 150 kg ha⁻¹) and for ratoon is 325 kg ha⁻¹ (after stubble shaving = 50 kg ha⁻¹, 45 DAP = 125 kg ha⁻¹, 90 DAP = 150 kg ha⁻¹) for all soil types except Badulla. The urea amount required for sugarcane in Badulla region is 100 kg ha⁻¹ (at planting = 25 kg ha⁻¹, 45 DAP = 25 kg ha⁻¹, 90 DAP = 50 kg ha⁻¹) while for ratoon is 125 kg ha⁻¹ (after stubble shaving = 50 kg ha⁻¹, 45 DAP = 25 kg ha⁻¹, 90 DAP = 50 kg ha⁻¹). Under rainfed sugarcane cultivation, the urea requirement for canes in these soil types is 250 kg ha⁻¹ (at planting = 75 kg ha⁻¹, between 6 and 12 weeks after stubble shaving = 175 kg ha⁻¹) while for ratoon is 275 kg ha⁻¹ (at planting = 125 kg ha⁻¹, between 6 and 12 weeks after stubble shaving = 150 kg ha⁻¹) [34]. Sugarcane recommended in South Africa depends on the topography, as inland, coastal lowland, Natal midlands and lowlands, whereas the range of N recommended for plant cane and ratoons are 80–200 and 100–140 kg ha⁻¹ [35] (Table 1). Different N recommendation is based on several factors, including soil type, sugarcane cultivar, climatic conditions, crop age, and management practices [3].

Table 1. N fertiliser requirement for the plant and ratoon crop (N/A: Not available).

Country	Major N Source	N Fertiliser Recommendations		Reference
		Plant Crop (kgN ha−1)	Ratoon Crop (kgN ha−1)
Brazil	Urea	40–80	100–150	Otto et al. [31]
India	Urea, Ammonium Sulphate	135–250	200	Shukla et al. [30]
Thailand		200–300	N/A	Yanai et al. [36]
Australia	Urea, Controlled-release N	120–160	140–180	Bell et al. [37]
South Africa	Urea, Ammonium Nitrate	80–200	100–140	DOAFF [35]
China	Urea	>500	>500	Zeng et al. [19]
Mexico	Urea, Ammonium Nitrate	67–112	90–135	Gravois [38]
United States	Urea, Ammonium Nitrate, N Solutions	45–90	112–180	Viator et al. [33]
Pakistan	Urea	173–222	173–222	SCRI [39]
Colombia	Urea, Ammonium Nitrate	67–112	90–135	Gravois [38]
Sri Lanka	Urea	250–300	275–325	SRI [34]

Table 1. N fertiliser requirement for the plant and ratoon crop (N/A: Not available).

Country	Major N Source	N Fertiliser Recommendations		Reference
		Plant Crop (kgN ha−1)	Ratoon Crop (kgN ha−1)
Brazil	Urea	40–80	100–150	Otto et al. [31]
India	Urea, Ammonium Sulphate	135–250	200	Shukla et al. [30]
Thailand		200–300	N/A	Yanai et al. [36]
Australia	Urea, Controlled-release N	120–160	140–180	Bell et al. [37]
South Africa	Urea, Ammonium Nitrate	80–200	100–140	DOAFF [35]
China	Urea	>500	>500	Zeng et al. [19]
Mexico	Urea, Ammonium Nitrate	67–112	90–135	Gravois [38]
United States	Urea, Ammonium Nitrate, N Solutions	45–90	112–180	Viator et al. [33]
Pakistan	Urea	173–222	173–222	SCRI [39]
Colombia	Urea, Ammonium Nitrate	67–112	90–135	Gravois [38]
Sri Lanka	Urea	250–300	275–325	SRI [34]

5. Challenges in N Management Within Sugarcane Farming

5.1. N Losses to the Environment

The N losses from agricultural land are a major problem for different agricultural systems, including sugarcane production [40]. This leads to economic loss due to the waste of expensive resources, poor N utilisation efficiency (NUE), soil degradation, pollution of ground and surface water sources and greenhouse gas (GHG) emission [41]. Like other agricultural systems, in the sugarcane system also the N is predominantly lost via leaching and a small portion through gaseous losses such as NH₃, nitric oxide (NO) and N₂O [42]. However, unique management practices such as burnt harvesting and GCTB may alter the N cycle. Further, the ratoon crop continues to grow from the base of the stem, which absorbs the N from the soil after harvesting the mature stem. Therefore, the N carryover effect is minimal compared to other crops.

In Australia, there has been a shift in N application practices from product maximisation to profit optimisation, with a primary focus on minimising N losses [3]. Studies suggested that a maximum of 60% of the applied N is assimilated by the sugarcane cultivated in Australia [43,44]. The remaining N is either retained in the soil through organic matter or assimilated by microbes, or it may be lost to the environment. The NUE in sugarcane systems varies across different regions, with values reported as 24–41% in Australia, 30–42% in the USA, 27–36% in South Africa, and 6.1–34% in Guadeloupe [21]. The lower NUE in sugarcane cultivation is often attributed to high losses via nitrate leaching. In the Australian context, average nitrate leaching losses range from 1 to 70 kg N ha⁻¹, accounting for nearly 1–40% of the recommended fertiliser application [40]. In Brazilian sugarcane production, the average nitrate leaching and runoff losses are around 0.1–8.5% and 0.08–5.8%, respectively [45]. Nitrate leaching is identified as the primary contributor to N losses in Chinese sugarcane systems, accounting for 10–45% of the applied N [9,19]. As such, controlling nitrate leaching losses is crucial to enhancing NUE in global sugarcane production systems.

Crop residues under GCTB retain soil moisture, which minimises NH₃ volatilisation [46]. Furthermore, N fertilisers are generally applied through sub-surface application for sugarcane, which minimises volatilisation losses. The average loss of NH₃ through volatilisation from sugarcane cultivation varies from as little as 1% to as high as 31.2% of the applied N, and it is influenced by factors such as the type of N fertiliser utilised, management practices and environmental conditions [47,48]. In green harvested sugarcane, the trash left on the soil surface, called GCTB, can accelerate N loss, with estimates ranging between 30 and 70% through NH₃ losses. Following the application of N fertiliser to soil that had sugarcane residue, the urease enzyme present in the residue hydrolyses urea, leading to an increase in NH₄⁺ levels in the soil compared to soil without residue [49]. Due to the limited capacity of sugarcane residues to retain NH₄⁺ ions, they are susceptible to loss through NH₃ volatilisation [50].

Under hot and humid conditions, the combination of sub-surface application of nitrogen fertilisers and high levels of residue can promote the denitrification process in soil. This can lead to high emissions of N₂O [51]. High soil moisture resulting from residue cover and the high carbon-to-nitrogen (C: N) ratio of sugarcane residues can exacerbate N₂O emissions [52]. The average N₂O emissions from sugarcane cultivation exhibit significant variation depending on N fertiliser application rates and environmental factors. Studies suggest that cumulative N₂O emissions can range from 0.3 to 4.1 kg N per hectare, with emission factors (EFs) falling between 0.7% and 2.4% of the applied N [53]. The moisture retained by the GCBT in the soil after N fertiliser application enhances the denitrification process [54]. This increased the N₂O emissions at all rates of N fertiliser application [55]. However, a study utilising ¹⁵N labelling demonstrated that trash retention (sugarcane residue mulching) increased N₂O emissions by 102% compared to trash removal (sugarcane residue removal), although the difference was not statistically significant [51]. Hence, it is essential to identify the reasons for the high N losses in the sugarcane system and implement corrective measures to ensure its sustainability in terms of N management.

5.2. Causes for N Losses

It is crucial to identify the causes of N losses in the sugarcane system and implement mitigative measures. Factors contributing to N losses include excessive N application, untimely application, irrigation following fertiliser application, surface application of N fertiliser, application of N over crop residue, broadcast application, single application, and early application before the crop establishes a substantial canopy [19,56,57,58].

The application of N beyond the recommended level in intensive sugarcane systems resulted in a 26% loss of the applied N. This can be attributed to the soil reaching its maximum threshold for retaining mineral N [59]. A very high application rate in China (400–800 kg N ha⁻¹) leads to N losses above 50% [60]. Studies showed that the application of urea on the surface or residue increased the NH₃ volatilisation by nearly 38% [61]. N recovery by plants decreased for fertiliser application on the residue compared to soil incorporation by 75% for both plant and ratoon crops [62]. According to Prasertsak et al. [61], N runoff losses increased by 60% for surface application than subsurface application. Continuous water application through drip irrigation can elevate soil moisture levels, leading to increased soil mineralisation. Hence, the practice of fertigation or fertiliser application for a field irrigated with drip irrigation should be approached cautiously to prevent over-application and potential N losses [63]. Therefore, the adoption of good management practices and improved sustainable N management strategies can help reduce N losses in sugarcane systems.

5.3. Environmental and Health Consequences of N Losses

The environmental and health consequences of N losses in sugarcane systems can have far-reaching impacts on ecosystems and human well-being. The significant environmental threats include groundwater and surface water contamination, greenhouse gas emissions, soil degradation, and eutrophication [41,64]. Numerous studies have highlighted the environmental impacts of N losses. The Burdekin region, known for sugarcane production in the dry tropics of Australia, exemplifies this issue. High N application rates in irrigated sugarcane production in this region threaten the ecosystem of the Great Barrier Reef (GBR), renowned internationally for its environmental and social significance. The N loss from sugarcane production in the Burdekin region leads to groundwater pollution, endangering drinking water sources [65] and causing a decline in water quality in coastal wetlands that support freshwater ecosystems [66].

Sugarcane expansion in Brazil increased soil degradation, groundwater contamination and GHG emissions [67]. Vinasse dark-coloured, liquid residue left over after the fermentation and distillation processes used to produce ethanol from sugarcane. Studies showed that vinasse with N application increased the N₂O emission by 5 times compared to bare land in south-central Brazil [58]. A similar study reported that application of organo-mineral formulation-based concentrated vinasse increased N₂O emission by 91% in the wet season [47]. The application of vinasse may increase soil microbial activity and enhance denitrification processes, which could contribute to the observed higher N₂O emissions [68]. The cultivation of sugarcane on sloping lands contributes significantly to river contamination with reactive N species, accounting for approximately 14% of the contamination [69]. Moreover, studies have demonstrated that this N is a primary contributor to the eutrophication of water bodies [9]. A significant amount of NH₃ is emitted from Brazilian sugarcane plantations, constituting approximately 19% of the emissions [9].

N application rates in China are notably high, ranging from 400 to 800 kg N ha⁻¹, which is 3–10 times higher than in Brazil. The substantial N losses from sugarcane agricultural systems pose a significant threat to agroecological systems [60]. Research indicates that approximately 40% of groundwater sources in Guangxi, China’s largest sugarcane-producing region, are contaminated with N species [70]. The N runoff and leaching losses are recognised as significant contributors to river pollution in the Guangxi region due to sugarcane cultivation [71,72]. All these examples from major sources of evidence demonstrate that N losses from sugarcane production systems pose a significant threat to environmental well-being.

Nitrogen losses from agricultural practices can have significant health impacts on humans. Excessive nitrogen in the environment can lead to the contamination of water sources, particularly through nitrate leaching into groundwater [41]. High nitrate levels in drinking water are a concern as they can pose health risks, especially to vulnerable populations such as infants and pregnant women. Nitrate exposure has been linked to methaemoglobinaemia or “blue baby syndrome,” a condition that reduces the blood’s ability to carry oxygen and can be fatal if not treated promptly [73]. Nitrogen losses create harmful algal blooms, contaminating water sources with toxins that endanger human health through contaminated water and seafood consumption [74].

6. Sustainable N Management Practices

6.1. Split N Application

Split application of N fertiliser is a technique that involves the application of required N fertilisers by splitting them into portions for the crop at the appropriate time [75,76]. This split N fertiliser application technique improves the plant N intake, thus improving the crop yield [77,78], nutritional quality and reducing N losses [79,80]. In a Brazilian study, split N application was tested for the first ratoon (regrowth after initial harvest) and second ratoon stages (subsequent regrowth after 2nd harvest) into three harvest periods: autumn, winter and spring. The results showed an increase in yields by 3 to 7 Mg ha⁻¹ and higher N sensitivity in autumn compared with spring and winter when the N fertilisers were applied 50% soon after harvest and 50% at the beginning of the rainy season [76] (Table 2). In another Brazilian study, three different N application rates such as 40, 80 and 120 kg ha⁻¹ were applied for sugarcane variety SP81 3250 as split applications [81] (Table 2). These N applications were applied in splits of 75%, 13%, 7% and 5% for the plant cane, first, second and third ratoons, respectively. The maximum N recovery by sugarcane was 39% for 40 kg ha⁻¹ followed by 35% for 80 kg ha⁻¹. The highest application rate (120 kg ha⁻¹) decreased the N recovery to 27%. However, sucrose levels were not significantly different between treatments.

In India, the practice of split N application is largely contingent on the irrigation systems in place. The total N application levels differ based on the method of irrigation. In coastal and flow-irrigated areas, the recommended N application level is 275 kg per hectare, while in lift-irrigated regions with limited water availability, the application rate is 225 kg per hectare (Table 2). For jaggery production regions, the recommended N application level is 175 kg per hectare. These prescribed N application levels are typically applied in three equal splits at intervals of 30, 60, and 90 days [82].

Over a two-year period, an investigation was conducted to evaluate the impact of administering 100% of the recommended N dosage in four split applications: at planting, 30 days after planting (DAP), 60 DAP, and 90 DAP [83,84]. This regimen notably enhanced the shoot population by the 120th DAP, the stalk population by the 240th DAP, and the count of millable cane at the time of harvest (Table 2). Consequently, this methodology led to a heightened cane yield of 85.4 metric tons per hectare, surpassing the outcomes achieved through the traditional method of applying 100% of the prescribed N in two separate doses at 45 DAP and 90 DAP.

Table 2. Split application of N for sugarcane (N/A: Not available).

Country	No. of Splits	Split Levels	Main Finding/s	Key Limiting Factor	References
Brazil	2	50% of the recommendation	Increase in yield	Low soil organic content (SOC) limits N supply	Tenelli et al. [76]
	4	75%, 13%, 7% and 5% of the recommendation	Increase in sucrose level	Low SOC and mineralisation conditions	Franco et al. [81]
India	3	30, 60 and 90 days after planting	Enhance the quality and quantity of sugarcane for jaggery production	N/A	TNAU [82]
	4	100% (at planting, 30, 60 and 90 DAP)	Improved shoot population at 120 DAP, stalk population at 240 DAP and millable cane population at harvest	N/A	Lakshmi et al. [84]
	7	Application rate was 18.99 and 1.64% higher than the recommended level	23.9% increase in millable stalk count, 10.7% increase in internode length, 82.9% increase in cane-to-top ratio	N/A	Bhilala et al. [83]
	5	Normal farmer application, 4, 6, 8 and 10 splits	6 splits N application showed an increase in yield (6 splits > 8 splits > 10 splits > 4 splits > farmer’s practice under drip irrigation)	Flood and furrow irrigation limits the effectiveness of split application	Singh et al. [77]
Pakistan	2	252 kg N ha−1 application rate in 2 equal splits	Higher N rates (336 kg ha−1) also enhanced crop growth rate and leaf area, but had lower NUE.	High temperature limits the growth	Ghaffar et al. [85]
Iran	2 or 3	92 kg N ha−1 and an application pattern of 30-30-40%	Increase the juice purity to 90% application	N/A	Koochekzadeh et al. [86]

Table 2. Split application of N for sugarcane (N/A: Not available).

Country	No. of Splits	Split Levels	Main Finding/s	Key Limiting Factor	References
Brazil	2	50% of the recommendation	Increase in yield	Low soil organic content (SOC) limits N supply	Tenelli et al. [76]
	4	75%, 13%, 7% and 5% of the recommendation	Increase in sucrose level	Low SOC and mineralisation conditions	Franco et al. [81]
India	3	30, 60 and 90 days after planting	Enhance the quality and quantity of sugarcane for jaggery production	N/A	TNAU [82]
	4	100% (at planting, 30, 60 and 90 DAP)	Improved shoot population at 120 DAP, stalk population at 240 DAP and millable cane population at harvest	N/A	Lakshmi et al. [84]
	7	Application rate was 18.99 and 1.64% higher than the recommended level	23.9% increase in millable stalk count, 10.7% increase in internode length, 82.9% increase in cane-to-top ratio	N/A	Bhilala et al. [83]
	5	Normal farmer application, 4, 6, 8 and 10 splits	6 splits N application showed an increase in yield (6 splits > 8 splits > 10 splits > 4 splits > farmer’s practice under drip irrigation)	Flood and furrow irrigation limits the effectiveness of split application	Singh et al. [77]
Pakistan	2	252 kg N ha−1 application rate in 2 equal splits	Higher N rates (336 kg ha−1) also enhanced crop growth rate and leaf area, but had lower NUE.	High temperature limits the growth	Ghaffar et al. [85]
Iran	2 or 3	92 kg N ha−1 and an application pattern of 30-30-40%	Increase the juice purity to 90% application	N/A	Koochekzadeh et al. [86]

In a field trial conducted in India during the spring season of 2020–2021, it was observed that the application of N in seven splits resulted in the highest counts of millable stalks (mature stalks which are at harvesting stage) (144.90 × 10³ ha⁻¹), increased internode length (9.31 cm), and elevated cane-to-top ratio (3.86), showcasing enhancements of 23.9%, 10.7%, and 82.9%, correspondingly (Table 2). Split N application strategy also exhibited a statistically significant improvement in cane and sugar yields, with increments of 18.99% and 21.64%, respectively, compared to the conventional single N application method [83]. However, this study reported that quality parameters such as brix, sucrose, and commercial cane sugar were not significantly impacted by the split N application [83,84].

A similar two-year Indian study investigated the impact of a single application, 4, 6, 8 and 10 split N application with three irrigation methods, such as flood, furrow and drip, on the performance of sugarcane [77]. The study reported that 6 split N applications performed better than other treatments, with top values in tiller count (165.6 × 10³ ha⁻¹), millable canes (116.3 × 10³ ha⁻¹), cane yield (154.72 t ha⁻¹), and commercial cane yield (23.39 t ha⁻¹) (Table 2). All the tested split N applications performed better than a single application for the above parameters. A study found that N dose and timing of split N application significantly affected most growth parameters in sugarcane, with the highest cane and sugar yields at 252 kg N ha⁻¹ in two equal splits.

A few studies reported that split applications did not affect the sugarcane yield and other agronomic parameters. For example, an Iranian study showed that application rates and split application of N fertiliser did not significantly influence sugarcane characteristics [86]. However, this study reported that an interaction between the N application rate and the application pattern (AP) affected juice purity, with the combination of 92 kg N ha⁻¹ and an AP of 30-30-40% producing the purest juice at 90%. Similarly, Kingston et al. [80] reported that split nitrogen applications provided comparable final N uptake and yield of sugarcane compared to a single application of N.

All these findings emphasise that split N application in sugarcane farming enhances growth, yield, and N use efficiency while reducing environmental losses. Applying N in phases aligned with crop growth stages outperforms conventional methods in productivity and economic returns. However, its effectiveness varies with soil, irrigation, and local conditions, requiring tailored strategies for optimal results.

6.2. Retention of Crop Residue

Residue retention improves yields, soil health, and nutrient availability. Research indicates that there is no substantial decrease in yield when urea is applied on the soil surface as opposed to beneath it, underscoring the importance of residue decomposition in mitigating the requirements for fertilisers [87]. Residue retention has increased yields by 10 t ha⁻¹ annually compared to burnt systems and enhances soil moisture, organic matter, and nutrient cycling [88]. These practices ensure the potential of residue management for sustainable sugarcane cultivation.

6.3. Subsurface Fertiliser Application

Subsurface application of N fertiliser is recommended for sugarcane as surface application leads to NH₃ volatilisation and reduced yields. Methods like stool splitting (split a mature sugarcane plant into sections that contain one or more buds or nodes) effectively place fertiliser within the soil, reducing N loss in GCTB systems [3]. In areas where subsurface application is impractical, strategies such as applying urea in bands, incorporating it with water, or waiting for sufficient canopy growth can minimise volatilisation losses [89]. However, while subsurface application reduces NH₃ loss, it may increase denitrification and leaching, requiring careful management to mitigate residual nitrate accumulation in the soil [90].

In most of the sugarcane systems, fertiliser is applied as a subsurface application. Subsurface application of liquid N fertilisation has shown improved sugarcane yields compared to broadcasting methods, offering an alternative for efficient N delivery [85]. Nutrient availability also influences root activity and distribution. Fertigation systems are effective in sub-surface fertiliser application. Under full sub-surface fertigation, the uptake of mineral N from the top 1 m of soil varied with application rates [90]. In a similar study, subsurface fertigation was reported to increase the mean yield of sugarcane by 17 t ha⁻¹ compared to surface fertigation [91]. In another study, subsurface drip irrigation showed that sugarcane grew faster and generated more tillers than conventional fertilisation. Furthermore, it increased the yield by 22.8% compared to conventional fertilisation [92]. This could be due to the N retention ability being high, and losses are low in sub-surface N application.

6.4. Application Closer to the Root Zone

Applying N closer to the root zone is known as banding in the sugar cane industry [62]. It improves nutrient uptake efficiency, reduces N loss through leaching and runoff, enhances root development and ensures targeted delivery of nutrients, leading to better crop growth and higher yields [93]. Incorporating the N fertiliser beneath the soil and closer to the root zone increased nitrogen recovery by plants by 79% and decreased N₂O emission by 22% [62]. Therefore, nitrogen use efficiency (NUE) can be increased by enhancing the yield and minimising losses through this method.

6.5. Timing of Fertiliser Application

The timing of N fertiliser application is important to match the crop’s demand for N during different growth stages, ensuring efficient nutrient use and optimal yields. In plant cane, N is often applied in split doses, which is a basal application at planting to support root establishment and early shoot growth, followed by top-dressing during the tillering stage (30–60 days after planting) to promote tiller development and canopy formation. The final application occurs during the grand growth stage (90–120 days after planting) to support rapid stalk elongation and biomass accumulation. Late-season N application is avoided to prevent reduced sucrose content and delayed harvest [94].

For ratoon crops, fertilisation is best timed when the plants are actively growing and about 0.5 m high, as newly developed root systems can efficiently utilise N at this stage. Synchronising applications with rainfall or irrigation further enhances NUE and minimises losses [95]. Studies show that sugarcane absorbs only 25–30% of its total nutrient demand during the tillering phase, emphasising the need for an early, high concentration of N to support tiller formation, which directly impacts yield potential [96]. For short-duration cultivars under assured irrigation, applying 150 kg N ha⁻¹ at planting has proven effective, but higher doses may be split across three applications under subtropical conditions. Early fertilisation, particularly at planting, results in maximum yields and better juice quality, highlighting the critical early-stage N requirement [3]. Effective timing of N fertiliser applications, tailored to the crop’s growth stage and environmental conditions, is essential for maximising productivity while maintaining sustainability.

6.6. N Budgeting

Nitrogen budgeting plays a crucial role in improving nitrogen management strategies in sugarcane cultivation. By accurately estimating the nitrogen inputs and outputs within the system, farmers can optimise nitrogen use efficiency, reduce environmental impacts, and enhance crop productivity [97]. It involves balancing N inputs like fertilisers, atmospheric sources, and biological fixation and outputs like crop uptake, harvest removal, leaching, runoff, and volatilisation [98]. This information helps farmers make informed decisions on nitrogen application rates, timing, and placement to match crop requirements, minimise losses, and sustain soil fertility.

6.7. Optimum N Application Rate

Optimum N application helps to maximise sugarcane yield and quality as it is a precise amount of N fertiliser required while minimising environmental impacts, nutrient losses, and economic costs [85]. The optimum N application rate for sustainable sugarcane cultivation varies globally depending on factors such as soil characteristics, crop variety, and management practices [15]. In many countries, including Brazil, N fertiliser recommendations are based on the concept of expected yield [99].

This approach shows that supplying N to maximise crop growth during the initial stages, while the soil fulfils the major N demand through the mineralisation of soil organic matter. N rate in Brazilian sugarcane fields ranges from 60 to 100 kg ha⁻¹, which is significantly lower than the rates in other countries, such as 150 to 400 kg ha⁻¹ in India and 100 to 755 kg ha⁻¹ in China [94]. Split N applications and the use of precision agriculture further enhance NUE, reducing environmental impacts and input costs. For example, soil and plant testing in Brazil has reduced fertiliser use by 10–15% without compromising yield [100]. These strategies, tailored to local conditions, underscore the importance of sustainable N management to achieve high productivity and environmental conservation.

6.8. Use of Slow-Release or Controlled-Release N Fertilisers

Slow-release N fertilisers (SRNFs) or controlled-release N fertilisers (CRNFs) are proven methods of controlling N losses [101]. It is well-documented that SRNFs have significantly minimised the N losses while increasing the yield in sugarcane. A study conducted by Rathnappriya et al. [102] reported that the application of coated SRNF significantly (p < 0.05) increased the dry matter (DM) yield in plant cane by 0.5 ton ha⁻¹ compared to urea, but DM yield was comparable for SRNF and urea in the ratoon crop. In the first season (plant cane), nearly 16 kg N ha⁻¹ of nitrate leaching loss was controlled by SRNF than by urea. However, in the ratoon crop, only 3 kg Nha⁻¹ of nitrate leaching loss was controlled by SRNF compared to urea. Application of SRNF (Osmocote^®, Bloomington, IN, USA) in sugarcane seedlings significantly (p < 0.05) increased the shoot and root DM more than conventional N fertiliser [103]. The evidence suggests that the efficiency of SRNFs depends on the type of sugarcane being cultivated, whether it is a seedling crop or a ratoon crop.

A few studies reported non-significant or negative effects of SRNFs on either DM yield or N losses. For example, a study tested nano-N chelated (NNC) SRNFs on sugarcane and found that both DM yield and nitrate leaching losses were similar for NNC and urea, even at different application rates between 80 and 161 kg N ha⁻¹ [29]. Unexpectedly, the application of polymer and sulphur-coated urea (PSCU) significantly (p < 0.05) increased the release of N₂O by 35% and 46%, in the first and second ratoon cane, respectively [104]. The reason for this could be the lock-off effect of the fertiliser within the coatings. This prevents the availability of N for plant uptake and allows microbes to denitrify it to N₂O [42].

The effectiveness of SRNFs is contingent upon various factors, encompassing climatic conditions like rainfall and temperature and soil parameters including soil moisture, soil temperature, and microbial activity. In addition to external factors, internal elements such as the type of SRNF, release pattern, and application period also play a crucial role in determining the success of SRNFs [105]. Hence, the application of SRNFs should be conducted judiciously and appropriately to mitigate potential adverse effects and optimise their benefits.

6.9. Use of Urease Inhibitors

Urease inhibitors are used to minimise or control the hydrolysis of urea molecules to ammonium ions by inhibiting the urease enzyme [106]. A variety of chemical substances, including natural and synthetic organic molecules and metallic ions, have been identified as inhibiting urease enzyme activity. The application of urease inhibitors is reported to control the N losses in sugarcane cultivation [107].

A study focused on the optimisation of urease inhibitor usage to reduce NH₃ emission following urea application over crop residues, focusing on the GCTB systems in Brazil [107]. The findings indicated that with the increase in N-(n-butyl) thiophosphoric triamide (NBPT) concentration, the timing of the peak loss rate was delayed, leading to a decrease in cumulative NH₃ loss. This relationship exhibited linearity up to a concentration of 1000 mg kg⁻¹ of NBPT. However, further increases in NBPT concentration did not significantly reduce NH₃ volatilisation. A similar study reported that NBPT decreased NH₃ emission significantly (p < 0.05), but had no impact on the DM yield of green sugarcane cultivation in Brazil [108]. Nevertheless, this study proposed that application rates of NBPT exceeding 530 mg kg⁻¹ of urea do not influence NH₃ loss, a finding that contradicts the results of a prior study. This could be due to the interaction effect of other climatic and soil factors.

Gallucci et al. [109] used boric acid as a urease inhibitor with the application of N-enriched vinasse in a sugarcane field. The NH₃ volatilisation was significantly (p < 0.05) controlled by boric acid + urea treatment compared to urea alone. However, sugarcane DM yield and leaf N content were not significantly different between the treatments. According to the findings of Otto et al. [110], boric acid and phosphoric acid-treated urea, mono-ammonium phosphate (MAP) and NBPT did not significantly reduce the NH₃ loss compared to urea and a mixture of urea and NH₄NO₃ in the laboratory study. Nevertheless, in the field experiment, boric acid and MAP decreased the cumulative NH₃ loss by around 50%.

6.10. Use of Nitrification Inhibitors

Nitrification inhibitors delay the nitrification process, resulting in the retention of NH₄⁺ ions in the soil. As a result, plants are able to take up these ions, leading to a reduction in N₂O emissions [101]. N₂O is a major greenhouse gas in Brazil, with agriculture being the primary contributor to its emissions [111]. The new trend of applying vinasse to sugarcane fields aims to increase the organic content of the soil and utilise it as a nutrient source for the sugarcane crop. Vinasse application increased the N₂O emission from 1.1% to 3% which demands new ways of controlling N₂O emission [112]. Towards this end, a study tested dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) in ratoon crops for two cycles [104]. Compared to urea application alone, DCD and DMPP application with urea significantly (p < 0.05) decreased N₂O emission by 95% and 98%, respectively, in the first year. The corresponding significant (p < 0.05) reduction in the second year was 81% and 100%. In a comparable study, the application of DCD with urea significantly (p < 0.05) reduced NH₃ emissions by 14–16% when compared to urea alone. This effect was more pronounced at higher urea application rates, particularly in the range of 100–150 kg N ha⁻¹ [113]. These results showed that DCD and DMPP can be used in ratoon crops for controlling gaseous losses.

In the first step of nitrification, ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB) are involved, and in the second step, nitrate-oxidising bacteria participate in the nitrification of ammonium [114]. Li et al. [115] showed that the application of DMPP with urea suppresses the growth of AOB in highly acidic sugarcane soil. However, the incorporation of biochar with them decreased the effectiveness of DMPP as it is absorbed by the biochar. Nevertheless, this study found that biochar application along with urea inhibited the AOB growth.

A simulation study with DayCent modelling software (https://www.nrel.colostate.edu/projects/daycent/, accessed on 12 December 2024) evaluated the effectiveness of using soybean fallows and DMPP in Australian sugarcane cropping systems [116]. According to the findings, DMPP-coated urea abated overall N losses by 41% and N₂O emissions by 30% without compromising the yield. An Australian study showed that DMPP-coated urea was not effective in controlling N₂O emission and DM yield of sugarcane compared to urea at 100 and 140 kg N ha⁻¹ application levels [117]. However, this study pointed out that the application of DMPP with urea decreases fertiliser N application from the normal recommended rates (160–180 kg N ha⁻¹).

A study evaluated the effectiveness of DMPP and herbicides (atrazine and glyphosate) in controlling N losses in sugarcane soils collected from Australia [118]. DMPP significantly (p < 0.05) decreased N₂O emission until 14 days, whereas atrazine and glyphosate controlled up to 7 days only. The study suggested that DMPP, atrazine, and glyphosate can decrease soil nitrification and denitrification rates by inhibiting microbial gene abundances in AOA and AOB, with DMPP being most effective in reducing N₂O emissions in sugarcane cropping soil.

6.11. Incorporating Biochar

Several organic amendments, including biochar, lignite and charcoal, are being used to improve the soil conditions and yield in different agricultural systems. Studies showed that biochar application decreased the N leaching losses from sugarcane fields [119,120]. A study found that biochar derived from sugarcane bagasse reduced NH₄-N and NO₃-N losses by 33–167% and 35%, respectively, in sub-tropical regions [121]. Eykelbosh et al. [122] found that biochar application was as effective as vinasse application in controlling NO₃-N leaching losses. However, due to its stability, biochar may offer more consistent control over time compared to vinasse. The active sites in biochar trap reactive N species, thereby reducing leaching losses [123]. An incubation study on highly acidic sugarcane soil reported that biochar inhibits the nitrification process, also contributing to loss control [115]. Biochar decreases N₂O emissions by facilitating denitrification, acting as an electron shuttle for soil denitrifying microorganisms, reducing N₂O to N₂ [124]. A few studies reported that the application of biochar decreased the N₂O emission. According to Abbruzzini et al. [125], sugarcane straw biochar reduced N₂O emissions by 24% and 34% in sandy and clayey soil, respectively. Butphu et al. [126] found that biochar increased the sugarcane yield and N uptake by 41% and 70%, thus increasing the NUE by 118%. Further studies are needed to enhance our understanding of the impact of biochar on mitigating N losses in sugarcane systems.

6.12. Precision Agriculture Tools

Precision agricultural tools have transformed the approach to N management in sugarcane production. These advanced tools leverage state-of-the-art technology, canopy reflectance sensors, chlorophyll meters and electrical meters to offer farmers real-time field information, enabling them to apply N more precisely and efficiently [127,128]. Crop canopy reflected sensors are simple tools to measure the N status of sugarcane crops [129]. This sensor measures the amount of red and near-infrared wave reflection of a leaf surface at a particular wavelength to measure the chlorophyll content in the leaf. GreenSeeker (Zagreb, Croatia), Crop Circle (London, UK), Yara-N sensor and N-Sensor^TM (Yara International, Oslo, Norway) are among the well-known brands of canopy reflection sensors used in agriculture. In a comparative study, the Normalized Difference Vegetation Index (NDVI) calculated from the Crop Circle ACS-430 sensor using a red-edge waveband (NDRE) exhibited the highest accuracy and sensitivity on sugarcane when compared to the Crop Circle ACS-210 and GreenSeeker sensors [129]. N-Sensors^TM is used to measure the leaf N content in sugarcane to support variable rate application of N for sugarcane [130]. This sensor showed consistent performance irrespective of sugarcane variety, soil type and growth stages of sugarcane. Other optical sensors, such as visible spectrometers and hyperspectral sensors, are also used in the N management of sugarcane. In sugarcane, 717 nm was found to be the most influential wavelength in measuring the N content for the Visible micro spectrometer [131]. Studies reported that 530–570 nm, 680–750 nm, and 750–1300 nm hyperspectral ranges are most suitable for predicting the N status in sugarcane [132].

Apart from these, chlorophyll meters (SPAD) are widely used in the N measurement of sugarcane. Chlorophyll meters operate by assessing the ratio of transmitted red and near-infrared light through a plant leaf [133]. Several studies used chlorophyll meters, such as measuring the leaf N content under drought conditions [134] and measuring leaf N supply under low temperatures [135]. A few attempts were made to measure the leaf N content using image processing [136,137]. All these methods are non-destructive and give informed decisions about the N status of the crop in real-time. This avoids expensive and time-consuming chemical tests to identify the N status of the cane. Therefore, the farmer can take immediate precautions to avoid over- or under-application of N fertiliser. Additionally, these sensors are used to measure the crop variability in sugarcane fields and facilitate variable rate application of N fertiliser [129,130]. Disadvantages of these sensors include limited precision, susceptibility to interference factors, calibration requirements, limited depth of analysis, cost and accessibility barriers, challenges in data interpretation, and a focus on plant nutrient status over other growth factors [129,138,139].

6.13. Legume Inter or Rotational Cropping

Intercropping with legumes has attracted much attention worldwide, regarded as a sustainable alternative to chemical N fertilisers [140]. The incorporation of legume cover crops in cash crops like sugarcane is one of the major practices followed in all types of farming techniques [141,142]. Sugarcane cultivation with green manure legumes resulted in a 27–43% increase in spring sugarcane yield and contributed 41–71 kg N ha⁻¹ through biological N fixation [143]. In sugarcane cultivation, legume cover crops like Crotalaria spectabilis [25,76], Crotalaria junceae L. [144], soybean [145], cowpea [145], Sesbania aculeata and Melilotus alba [146,147] are integrated for elevating the biological N fixation in the soil. Although the main purpose of this work was understanding how to improve N management in sugarcane production, the reduction in mineral fertiliser use is simultaneously of great importance considering the increasing prices of fertilisers, the GHG emitted during the production of these inputs, etc. [4,76,141,142] and also these incorporations of legume crops during the renovation period of sugarcane will lead to the reduction in pest infestation, weed suppression and soil erosions [140]. Also, these legume cover crops are selected in order to establish an association with bacteria and fix atmospheric N into the soil with their special abilities [144,148].

About 3.34% average sugarcane yield is reduced globally due to legume intercropping in the sugarcane farming system [149], while sugarcane shows positive responses when being rotated with a legume, with stalk yield improvements ranging from 15 to 25% in Australia [144,150] and up to 30% in Brazil [144,151]. Tenelli et al. [76] evaluated the impact of legume cover crops (Crotalaria spectabilis) on N sustainability in sugarcane production, focusing on N fertiliser reduction. In both sandy and clayey soils, treatments with different N fertiliser rates (60, 120, and 180 kg N ha⁻¹) and control were applied after plant-cane harvest. Results showed that the cover crop increased soil N storage and microbial biomass carbon. Sugarcane ratoon yields responded more positively to N fertilisation under bare fallow, but cover crops enhanced yields by 9% in sandy soils and 15% in clayey soils compared to bare fallow. The cover crop also replaced 9 and 15 kg N ha⁻¹ annually in sandy and clayey soils, respectively.

The study conducted in South Brazil showed that incorporating sun hemp (Crotalaria junceae L.) during the sugarcane renewal period can replace 60 kg ha⁻¹ of N at planting, enhance stalk yield by 20 Mg ha⁻¹ over two ratoon cycles and improve NUE by reducing the N required per unit of stalk harvested by 12.5%. Additionally, sucrose content and sugar yield increased under the rotation system, which increased the NUE. However, soil inorganic N dynamics and plant N content were minimally affected by the rotation. This approach demonstrates a practical strategy to enhance bioenergy crop sustainability [144]. These pieces of evidence prove that legume cover crops and intercropping in sugarcane farming enhance soil health, boost N fixation, reduce fertiliser reliance, and support higher yields.

6.14. Application of Biofertilisers

Biofertilisers are sustainable, eco-friendly agricultural inputs derived from renewable sources, offering a low-cost, non-bulky alternative that complements chemical fertilisers like urea, ammonium sulphate and ammonium nitrate [142]. This biofertiliser enriches the soil with organic matter (manures, composts and waste matters), provides an advantage over mineral fertilisers and has gained consideration as a sustainable alternative for use in sugarcane and other crops [152,153].

Several studies have indicated that biofertilisers produced comparable results to those obtained with urea fertilisers. For example, a biofertiliser with a 40% reduced N load, formulated with a 50:50 mix of N from recycled waste and mineral fertiliser, demonstrated superior performance over traditional mineral fertilisers [152]. When combined with plant growth-promoting rhizobacteria (PGPR), this biofertiliser also enhanced bacterial and fungal associations with sugarcane, potentially contributing to improved crop yield [152]. Also, biofertilisers enriched with effective PGPR can enhance nutrient use efficiency by mineralising, solubilising, mobilising and supplying N to plants [152].

A systemic biofertiliser containing Pseudomonas fluorescens, Azospirillum brasilense, and Bacillus subtilis was tested on two sugarcane varieties (NCO-310 and Mex 57–473) across three locations in Mexico [154]. The biofertiliser, which enables bacterial entry through the stomata, was applied at three doses per hectare throughout the annual production cycle: once per year at Potrero Nuevo and Champotón, and for six years at Ameca. Results showed a significant (p < 0.05) increase in sugarcane yield at each location: 2.5 times increase (73.7 tons ha⁻¹) at Potrero Nuevo, 1.9 times increase (77.7 tons ha⁻¹) at Ameca, and 1.4 times increase (23.8 tons ha⁻¹) at Champotón. The biomass increase was primarily due to enhanced tillering (new shoots or side shoots emerge from the base of the main sugarcane stalk) rather than changes in stalk height or diameter. Additionally, the biofertiliser improved sugar quality, with an increase in °Brix (2.6°) and sucrose content (0.7%) [154].

de Mendonça et al. [155] compared the effects of biofertiliser (from cattle shed wastewater) and urea at various N doses (0, 16, 48, 64, 80, and 96 kg ha⁻¹) on two sugarcane varieties (RB 867515 and SP 803280). The results showed that 80 kg ha⁻¹ application of biofertiliser increased Brix values above 21% for cultivar RB 867515, yielding 147.5 tons ha^−1, and SP 803280, yielding 152.25 tons ha⁻¹. There were no significant yield differences between biofertiliser and urea for both cultivars (p ≤ 0.05). The biofertiliser also improved crude protein content in cultivar RB 867515. Doses of 64–96 kg N ha⁻¹ resulted in similar growth and biomass yields, suggesting that biofertiliser can replace urea as a N source, with 80 kg N ha⁻¹ being optimal for sugarcane cultivation.

6.15. Site-Specific N Application

Site-specific N application ensures the crops receive an even amount of N fertiliser by applying different levels of N to the soil according to the N variability in soil, thus minimising the risk of over-application and subsequent losses. A few studies reported that site-specific applications are effective in improving NUE and controlling N losses. For instance, a four-year field study demonstrated that variable rate application of lime, N, K and P improved soil nutrient distribution, while maintaining sugarcane stable yields (~80 Mg ha⁻¹ year⁻¹). Even with similar yields compared to conventional N application, site-specific fertiliser application delivered significant economic and environmental benefits, including reduced production costs, enhanced soil fertility and lower climate impacts through decreased N usage [156,157,158]. This approach is especially critical under tropical conditions, where soil fertility is an important limitation and sugarcane benefits from nutrient availability in both surface and deep soil layers [159]. Precision agriculture, particularly N site-specific management, enhances sustainability and profitability in sugarcane cultivation [158].

Foliar analysis has been utilised in sugarcane cultivation for the planning and evaluation of fertiliser management programmes and harvest scheduling. A study conducted in the USA assessed the effectiveness of soil testing and foliar analysis interpreted using the Critical Nutrient Level approach and the Diagnosis and Recommendation Integrated System (DRIS) as tools for guiding sugarcane fertilisation. The findings indicated that, for the first time, N application was recommended on Torry muck soil (euic, hyperthermic Typic Medisaprist) based on foliar analysis incorporating both soil testing and DRIS methodologies [160].

Amaral et al. [129] employed canopy reflectance sensor readings for variable-rate N application in sugarcane. The normalised sensor data effectively predicted yield variations across fields. An algorithm was created to apply lower N rates in high-yielding areas, offering a favourable approach for optimised N management. According to the authors, further field validation and research on biomass variability are needed to improve the algorithm’s accuracy and efficiency for site-specific N management. Precision agriculture and canopy sensors optimise N management in sugarcane by addressing spatial variability. These site-specific methods improve NUE, soil fertility and sustainability while providing economic and environmental benefits.

6.16. Biotechnology and Genetic Engineering Approach

Plant breeding and genetic engineering technologies are adopted in the sugarcane industry to increase the NUE. Choosing a highly N-responsive sugarcane variety (a variety that responds well to N addition in terms of growth, yield, or other agronomic traits) can also enhance NUE. Several studies aimed at finding better genotypes which yield better sugar content at lower application levels. A study conducted in China examined the optimal sugarcane variety for low and moderate N application rates, identifying ROC22 and GT42 varieties as suitable for low and moderate N application levels, respectively [78]. Three different varieties tested in studies found that compared to C0775 and SL7130, SL8306 showed higher biomass accumulation under Sri Lankan conditions [161]. An Ethiopian study found that D42/58 has a yield of 9.1% sucrose, higher than the NCo-334 variety [15]. In general, most of the sugarcane varieties are low responsive to the N and therefore, more hybrid varieties need to be developed for better NUE that suits their conditions [162].

Only a limited studies reported the genetic engineering approach to improve the NUE of sugarcane. Under low N stress, plants sense the signal of N deficiency and initiate a cascade of molecular responses to cope with the stress [163]. This involves the amplification and transmission of the N deficiency signal to downstream molecules. Investigating the molecular response mechanisms of crops under low N stress and elucidating the biological functions of microRNAs (miRNAs) can establish a scientific basis for enhancing crop NUE. A study revealed a negative correlation between specific miRNAs, such as miR168 and miR396, and their target genes under low N conditions [164]. Notably, miR156 showed significant upregulation in the roots of ROC22 subjected to low N treatment. Through experiments involving the overexpression of sugarcane miR156 in Arabidopsis, it was observed that this miRNA promoted root growth and increased the expression of key genes and enzymes involved in nitrogen assimilation under low nitrogen stress [164]. These findings suggest that sugarcane miR156 may play a crucial role in enhancing nitrogen assimilation capacity, thereby potentially improving sugarcane NUE.

Transgenic sugarcane engineered with genes to enhance NUE shows promise for sustainable agriculture [165]. By introducing specific genes involved in nitrogen uptake, assimilation, and remobilisation pathways, transgenic sugarcane can optimise nitrogen use, leading to increased yield with reduced nitrogen input. The alanine aminotransferase (AlaAT) gene is crucial in plant alanine metabolism. AlaAT facilitates amino acid interconversion, particularly between alanine and α-ketoglutarate, aiding nitrogen metabolism, amino group transfer, and stress response in plants [166]. A study showed that transgenic sugarcane with AlaAT improved the NUE compared to wildtype at low N concentrations [162]. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), a pivotal enzyme in photosynthesis, catalyses the carboxylation of RuBP with CO₂ to produce 3-PGA, initiating the Calvin cycle. This process is crucial for converting CO₂ into organic compounds, supporting plant growth. Transgenic overproduction of Rubisco shows promise for enhancing crop productivity and NUE. A 20% increase in the Rubisco level increased the canopy photosynthesis by 14% in sugarcane [167], thus could increase NUE.

6.17. Symbiotic Nitrogen Fixation

Sugarcane forms a symbiotic relationship with nitrogen-fixing bacteria (NFB), particularly species of the genus Azospirillum and Gluconacetobacter, that contribute to its nitrogen nutrition [168]. These beneficial bacteria colonise the roots of sugarcane, promoting growth and enhancing nitrogen uptake efficiency. Through the process of biological nitrogen fixation, the bacteria convert atmospheric N into ammonia, which can be utilised by the plant as a N source. This symbiotic interaction not only reduces the reliance on synthetic N fertilisers but also improves nutrient availability for sugarcane, ultimately enhancing crop productivity and sustainability. In a study, Singh et al. [169] showed that Bacillus species, more prominently Bacillus megaterium and Bacillus mycoides strains, fixed N in sugarcane. Infection of Acetobacter species proved to be a way of increasing N fixation in sugarcane [170]. Studies reported that NFB can contribute nearly 47% of the N requirement of the sugarcane [141].

Intercropping sugarcane with leguminous crops increases nitrogen fixation in sugarcane. Intercropping soybeans with Zhongzhe1 and Zhongzhe9 sugarcane varieties increased the nitrogen-fixing bacteria (NFB) community and nitrogen content by 18% [171].

Leguminous cover crops (LCC) can replace nearly 9–15 kg ha⁻¹ yr⁻¹ of synthetic N fertiliser [76]. Sunn hem cover crop with sugarcane significantly increased the soil N content and favoured the ratoon crop growth, NUE and yield [25]. Legume fallow crops were conventionally used for improving soil N in many countries, including Australia and Brazil. Soyabean rotation crop retains the soil N up to the fourth ratoon crop [140].

7. Adopting Simulation Models for N Management

Simulation models are valuable tools in agriculture for supporting sustainable N management by offering advantages by optimising the N application level, predicting the N losses, and predicting the NUE. Simulation enables the forecasting of outcomes, aiding in long-term planning for N management. Additionally, simulations allow for extended long-term projections, which can be challenging to achieve experimentally over extended periods [172]. Several simulation models, such as SUCROWS I, AUSCANE, CANEGRO, QCANE, CENTURY-Sugarcane and APSIM-Sugarcane, have been developed for simulating the yield and sugar content of sugarcane (

Table 3. Simulation models used for N management in sugarcane.

Simulation Model	Prediction	Key Finding	Challenge	Reference
APSIM-SWIM	NO3− leaching	The prediction was reasonable	Preferential flow minimises the accuracy	Stewart et al. [173]
CANEGRO	NO3− leaching	Prediction accuracy ranged between 0.95 and 0.98	-	van der Laan et al. [174]
APSIM	N2O emission	A close relationship between observed and predicted values	Lower concentrations of N2O highly impact the results	Thorburn et al. [175]
DNDC	N2O emission	The IPCC method underestimates the emission compared to the DNDC model	Data availability	de Oliveira et al. [55]

). Of them, AUSCANE, CENTURY-Sugarcane and APSIM-Sugarcane have the ability to simulate the N limitation to growth [44].

Due to the costly and time-consuming nature of monitoring N leaching losses, simulation models are employed as a cost-effective alternative for this purpose. In a study conducted in Australia, a combination of the Agricultural Production Systems Simulator (APSIM) and soil-water and N transformation modules (SWIM) was utilised, referred to as APSIM-SWIM, to simulate nitrate leaching in sugarcane systems [173]. The study demonstrated reasonable predictions of nitrate leaching at a depth of 1.5 m. However, the accuracy of predictions was challenged by preferential flow (Table 3). In a comparable study, N leaching losses in the plant crop, first ratoon crop, and second ratoon crop were forecasted with R² values of 0.95, 0.98, and 0.98, respectively, using CANEGRO [174]. These results suggest that commonly used models such as APSIM and CANEGRO can be effectively employed to forecast N leaching losses, enabling adjustments in N fertilisation to minimise these losses (Table 3).

A few studies employed simulation models to predict the N₂O emissions from agricultural soils. These models incorporate the factors that mainly influence N₂O emission, including soil moisture, organic matter content, plant N uptake, N management practices and other climatic factors [44]. APSIM with the denitrification sub-model was used to model the N₂O emission in Australian sugarcane production systems [175]. This study found a close relationship between measured (2.9 kgN ha⁻¹) and predicted 2.1 kg N ha⁻¹) cumulative N₂O emission (Table 3). This long-term simulation study, spanning 40–60 years, revealed that N₂O emissions from Australian sugarcane production systems typically equated to 3–5% of N fertiliser usage. The authors suggested that high emissions could be associated with the reside management. Therefore, further long-term simulation studies are warranted in different regions to confirm this hypothesis. A study used a fusion of GIS data with the Denitrification Decomposition (DNDC) model and the International Panel of Climate Change (IPCC) for predicting N₂Os in sugarcane systems in Brazil under different loads of residue [55]. The study found that predictions differed significantly for the N₂O emissions when using the IPCC and DNDC methods. According to the authors, this discrepancy could be due to the spatial variations not being well captured by the IPCC model.

The limitations of the modelling approaches include the requirement of long-term data for testing and validation of the model, multiple factors involved in N dynamics in soil and plants [176], the need for testing and validation across different soil and climatic conditions [177], errors in the experimental data affecting the accuracy of the model’s predictions, the absence of a single model that incorporates all management practices in sugarcane along with N dynamics, significant time consumption for the learning curve, and complexity of the models reducing interest among users [178], assumption made for unavailable data made uncertainty in results [55] and the model’s inability to capture the complex interactions between crops, pests, and diseases [179].

8. Conclusions and Perspectives

Poor nitrogen (N) management can hinder plant N assimilation and increase losses. To enhance nitrogen utilisation (NUE) efficiencies in sugarcane farming, various methods have been embraced, such as sustainable management practices, good agricultural practices, modelling approaches, and precision agricultural tools. For sustainable nitrogen management in sugarcane, it is essential to strictly adhere to good nitrogen management practices, as these methods are fundamental. Additionally, less costly methods such as split application, site-specific application, biofertiliser, and biochar applications can be adopted. However, somewhat more expensive methods, such as enhanced efficiency fertilisers (EEFs: slow-release nitrogen fertilisers (SRFs) and inhibitors) and precision agriculture tools, can be employed to achieve better results when there is a need to strictly control nitrogen losses and improve NUE. For optimal effectiveness, integrated approaches combining conventional practices with modern techniques need to be adopted into a holistic smart N management system for sugarcane.

This review has identified several research gaps in N management in sugarcane cultivation and proposes ways to enhance understanding.

• While limited studies have utilised simulation models to aid N management in sugarcane cultivation, such simulation studies have significant potential to serve as supportive tools in N management, as evidenced by their application in other plantation crops. Therefore, there is a need for further simulation studies to be conducted to bolster decision-making processes regarding N management.
• The utilisation of EEFs, including SRFs as well as urease and nitrification inhibitors, remains relatively uncommon within sugarcane agricultural systems. While these methodologies are widely embraced in various other cropping systems, there exists a necessity for further investigations employing recently developed environmentally sustainable EEFs to deepen comprehension of their efficacy within the context of sugarcane cultivation.
• Genetic engineering is in its infancy in the sugarcane industry, with a focus on improving NUE. Only a few studies have reported transgenic sugarcane aimed at enhancing NUE, and no varieties have been released by any country. In the future, genetic improvement through genetic engineering should be a priority.
• Bacteria in a symbiotic relationship with sugarcane have been identified. However, an inoculum containing highly efficient endophytic nitrogen-fixing symbionts has not been developed or released for sugarcane.
• The impact of climate change on the nitrogen cycle in sugarcane systems needs to be thoroughly investigated and understood as a response to increasing extreme weather events.
• Most studies focus on a single or a couple of approaches in improving NUE. Research using an integrated approach to increase NUE in sugarcane production needs to be conducted.

Additional research should be directed towards assessing the NUE of newly developed enhanced hybrid sugarcane varieties. Furthermore, the categorisation of these varieties based on their responsiveness to N inputs would be advantageous.