Effects of Nitrogen Fertilizer Application on N₂O Emissions from Rice Cultivation: A Review

Abstract

Rice is a major dietary component for more than half of the world’s population, and its cultivation requires a careful balance of nutrients to ensure high yields and sustainable practices. Soil-derived N₂O fluxes represent a major environmental challenge with global implications. While agriculture is a necessary activity to feed a growing population, it must evolve to minimize its ecological footprint. This review provides an update on the effects of nitrogen fertilizer application, such as ammonium nitrate (NH₄NO₃), urea (CO(NH₂)₂), ammonium sulfate ((NH₄)₂SO₄), and calcium ammonium nitrate (CAN), on N₂O emissions from rice cultivation. The role of various nitrification inhibitors (e.g., dicyandiamide (DCD), 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin) and 3,4-dimethylpyrazole phosphate (DMPP)) in minimizing the release of N₂O from soils to the atmosphere was also discussed. Here, we described N₂O production by nitrification and denitrification processes in the paddy rice field, and then summarized the strategies, such as optimized fertilizer use, improved drainage and water management, and the use of organic amendments, that can enhance crop productivity while promoting sustainable reductions in N₂O emissions.

1. Introduction

Greenhouse gas emissions, such as methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O), are a major driver of climate change, influencing global temperatures, weather patterns, and environmental health. These gases trap heat in the Earth’s atmosphere, causing the “greenhouse effect” and contributing to global warming.

As a staple crop, rice supports the nutritional needs of over half the world’s population. Rice paddies are unique agro-ecosystems often characterized by periodic flooding, which creates both anaerobic (oxygen-deprived) and aerobic (oxygen-rich) conditions in the soil. These varying conditions influence the production and emission of CH₄, CO₂, and N₂O. CH₄ is produced under anaerobic conditions by methanogenic microorganisms that decompose organic matter in flooded soils. The use of organic fertilizers (e.g., manure, or straw) increases the CH₄ production. Microbial decomposition and organic matter oxidation in the soil contribute to CO₂ emissions. CO₂ flux is higher in aerobic conditions, such as during dry periods or alternate wetting and drying (AWD) [1,2]. Soil type, temperature, and organic carbon availability influence CO₂ emissions [3,4]. The Intergovernmental Panel on Climate Change (IPCC) has reported that agriculture is responsible for about 60% of global N₂O emissions, with rice cultivation being a significant source in Asia, the region that produces most of the world’s rice [5,6].

Understanding the impact of N₂O emissions from rice fields is essential in tackling climate change and ensuring sustainable food production. Synthetic fertilizers, animal manure, and crop residues left on the soil are the main contributors to N₂O emissions, contributing to climate change [7,8]. Excessive use of nitrogen fertilizers in agriculture frequently leads to significant accumulation of nitrogen (N) in the soil. This excess N can be lost through leaching into nearby water bodies or emitted into the atmosphere as N₂O, particularly during periods of rainfall or soil disturbance. These losses diminish the effectiveness of fertilizers and play a major role in environmental problems, such as water contamination and increased greenhouse gas emissions.

Rice straw is rich in carbon but relatively low in N. Its decomposition creates a temporary imbalance in the carbon-to-nitrogen (C:N) ratio, which can influence N transformations in the soil. Rice straw application involves incorporating crop residues into the soil after harvesting, either by plowing them in or allowing them to decompose on the field surface. This practice has agronomic benefits, such as enhancing oil organic matter, improving soil structure and water retention, and providing a slow-release source of nutrients. While this practice supports sustainable agriculture and reduces waste, it also influences the emission of greenhouse gas, especially N₂O, a very potent gas with a global warming potential nearly 300 times that of CO₂ [9,10,11].

Soil texture affects the transport of water, gases, heat, and nutrients, thereby influencing the structure and variety of microbial populations, which are essential for supporting healthy plant development [12,13]. Research has shown that N₂O emissions are typically highest at intermediate levels of soil moisture, often expressed as 60–80% water-filled pore space. Efficient soil management and optimized fertilizer application (e.g., the right amount and type of fertilizer, and timing application) can reduce N₂O emissions, contributing to climate-smart agriculture and environmental sustainability [14,15,16]. Li et al. [17] reported that factors related to soil N contents, such as nitrate (NO₃⁻), ammonium (NH₄⁺), and total N, have a greater impact on the variability in denitrification than climatic and soil properties, including mean annual temperature, soil moisture, and soil pH. Liu et al. [18] evaluated N₂O emission rates under different soil moisture conditions, corresponding to 20%, 40%, 60%, 80%, and 100% of the soil water holding capacity (WHC). The results indicated that N₂O uptake was significantly higher at 80–100% WHC compared to 20–60% WHC, probably due to reduced oxygen availability and the inhibitory effect of high NO₃⁻ concentrations (>50 mg N kg⁻¹ of soil NO₃⁻N content). Xu et al. [19] compared treatments with and without rice cultivation to assess the impact of rice planting on N₂O emissions in central China. The findings revealed that seasonal N₂O emissions were notably higher in treatments without rice cultivation, measuring 0.88 ± 0.08 kg N ha⁻¹ during the initial season and 1.69 ± 0.24 kg N ha⁻¹ in the following season. In contrast, treatment with rice cultivation emitted lower amounts (0.51 ± 0.10 and 1.04 ± 0.18 kg N ha⁻¹, respectively). These results suggest that rice planting plays a significant role in reducing N₂O emissions from agricultural fields. Wu et al. [20] demonstrated that the increase in N₂O emissions caused by rice soil drainage can be attributed to changes in soil pH. It has been indicated that drainage decreased soil pH due to increase in the redox that facilitates oxidation processes and proton generation. A negative correlation was observed between soil pH and N₂O emissions, indicating that the decline in pH may have contributed to the increase in N₂O emissions following the drainage of rice soils.

The aim of this publication is to examine the effects of using nitrogen fertilizers, such as ammonium nitrate (NH₄NO₃), urea (CO(NH₂)₂), ammonium sulfate ((NH₄)₂SO₄), and calcium ammonium nitrate (CAN), on the release of N₂O in rice cultivation. This future research will investigate the influence of crop treatments on N₂O emissions. Furthermore, it reviews the emissions of N₂O, along with their environmental and climatic impacts. Special attention is given to comparing different mitigation strategies (optimized fertilizer use, improved drainage and water management, and the use of organic amendments), and evaluating their potential to reduce N₂O emissions.

2. N₂O Production in Rice Cultivation

N₂O in rice cultivation is mainly produced through microbial processes in the soil, especially nitrification and denitrification. These microbial processes utilize inorganic nitrogen compounds and play a crucial role in maintaining the balance of N in soil and water environments. Soils cultivated with rice exhibit high levels of nitrification and denitrification as a result of frequent changes between wet and dry conditions.

2.1. Nitrification

Nitrification is a two-step biological oxidation process that converts NH₄⁺ into NO₃⁻ under aerobic conditions by soil microorganisms. Microorganisms responsible for nitrification can be classified into two main groups: (i) ammonia-oxidizing bacteria (AOB), such as Nitrosomonas, Nitrosococcus, and Nitrosospira, and ammonia-oxidizing archaea (AOA) from Thaumarchaeota, which convert ammonia (NH₃) to nitrite (NO₂⁻), using enzymes (e.g., ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO)), and (ii) nitrite-oxidizing bacteria (NOB), such as Nitrobacter, Nitrococcus, Nitrolancetus, Nitrospira, and Nitrispina), which further oxidize NO₂⁻ to NO₃⁻, using nitrite oxidoreductase as an enzyme. The reactions that occur in the nitrification process are shown in Figure 1.

It has been reported that AOA and AOB play distinct roles in soil nitrification, their activity being influenced by several factors, such as pH, fertilization regime, and temperature [21,22,23]. Ouyang et al. [21] reported that the optimum temperature was 41 °C for AOA, while for AOB it was 31 °C. Due to fundamental differences in protein synthesis and nutrient utilization between archaea and bacteria, AOA and AOB respond differently to various C and N sources under different fertilization regimes. While several studies have reported that AOB are the main contributors to nitrification in agricultural soils, other research has highlighted that AOA are dominant in rice soils, especially in acidic ones [24,25,26]. Furthermore, AOA populations tend to be more sensitive and diverse than AOB in soils receiving N input [27]. In general, AOA may dominate NH₃ oxidation in ecosystems with low NH₄⁺ concentration, while AOB become more competitive in environments with higher NH₄⁺ concentration [28,29]. Wu et al. [30] employed quantitative polymerase chain reaction (qPCR) to examine the functional gene abundance and community composition of AOA and AOB, as well as the ammonia-denitrifying bacteria (nirS- and nirK-type). Nitrous oxide (N₂O) production was investigated under high CO₂ conditions, following five years of CO₂ fumigation in three different treatments: CK (current level of CO₂ in the atmosphere), T1 (CK with an annual increase of 40 ppm up to a total of 200 ppm), and T2 (CK with an additional 200 ppm of CO₂). During the grain filling and milk ripening stages, N₂O emissions were reduced by 173% and 41%, respectively (p < 0.05). In contrast, under the T2 treatment, N₂O emissions increased by 279% and 172%, respectively, compared with T1 treatment during the tillering and milk ripening stages. During the elongation stage, the potential for N₂O production was greater under the CK treatment compared to T1 and T2. Notably, during the drainage phase of the mild season, the T1 treatment showed significantly lower N₂O production potential and AOA amoA gene abundance than the CK treatment. Although elevated CO₂ had no significant effect (p > 0.05) on the structure or diversity of nirK- and nirS-type denitrifying bacterial communities, the abundance of nirK-type bacteria had a significant influence on N₂O flux. In addition, soil N levels had a notable impact on the composition of nirK- and nirS-type denitrifying communities. These findings suggested that elevated CO₂ levels attenuated N₂O emissions, primarily through their effects on the abundance of AOA amoA and nirK genes.

Previous studies have shown that nitrification depends on certain environmental conditions, such as oxygen, temperature, pH, and moisture [31]. Nitrification is essential in converting N into a form that plants can absorb. While NH₄⁺ can be taken up by plants, NO₃⁻ is more mobile in the soil and often the dominant form of N used by crops [32,33]. However, excessive or poor nitrification can lead to environmental problems, such as leaching, N₂O formation, and soil acidification [34]. NH₃ exists in equilibrium with NH₄⁺ in soils and aqueous solutions in a pH-dependent manner. Plants can also emit NH₃ into the atmosphere, particularly those with high N content, with emissions peaking between flowering and maturing stages in rice. Once NH₄⁺ is converted to NO₃⁻, the negatively charged NO₃⁻ ion, which binds weakly to most soil particles, readily leaches through the soil profile into groundwater, causing environmental damage. Peng et al. [35] demonstrated that soil application of a biocontrol agent (Streptomyces naraensis) can enhance rice seedling growth and significantly decrease N₂O emission rates.

2.2. Denitrification

Denitrification is a microbial process carried out primarily by facultative anaerobic bacteria, such as Pseudomonas, Paracoccus, and Bacillus species. This process involves the stepwise reduction of NO₃⁻ to nitrogen gas (N₂), facilitated by four key metalloenzymes. Nitrate reductases, including both periplasmic and membrane-bound types (encoded by the narG and napA genes), catalyze the reduction of NO₃⁻ to NO₂⁻. This is followed by the reduction of NO₂⁻ to nitric oxide (NO) via cytochrome cd₁ and copper-containing nitrite reductases (encoded by the nirS and the nirK genes, respectively). NO is then reduced to N₂O by nitric oxide reductase (norB), and finally, nitrous oxide reductase (nosZ) converts N₂O to dinitrogen gas (N₂) [36]. The denitrification process is illustrated in Figure 2.

Denitrification primarily occurs in anaerobic environments, such as waterlogged soils, wetlands and rice paddies, sediments of lakes and rivers, and wastewater treatment systems. Several studies have shown that denitrification is influenced by factors such as low oxygen levels (suppressed in aerobic conditions), available NO₃⁻, organic carbon, soil temperature and pH [37,38,39,40]. Pan et al. [40] found that soil denitrification rates were positively influenced by water-filled pore space (WFPS) (p < 0.01), NO₃⁻ concentration (p < 0.05), and soil temperature (p < 0.01), while higher oxygen levels in the soil significantly suppressed these rates (p < 0.01). N₂ emissions were shown to increase with latitude (p < 0.05), WFPS (p < 0.01), and soil mineral N content (p < 0.05), but decreased when oxygen availability was high (p < 0.05). It was found that N₂O/(N₂O + N₂) ratio increased with soil oxygen content (p < 0.01) but decreased with increasing C levels (p < 0.05), C/N ratio (p < 0.01), soil pH (p < 0.05) and WFPS (p < 0.01) Their findings also indicated that strategies such as optimizing N application, using NH₄⁺-based fertilizers instead of NO₃⁻-based fertilizers, incorporating biochar, and applying nitrification inhibitors could effectively reduce denitrification rates and associated N₂ emissions.

The impact of pH on N₂O emissions after waste incorporation is influenced by how efficiently and completely nitrification and denitrification occur during the mineralization process [41]. Maintaining soil pH at the optimal level to maintain high crop productivity in agricultural systems is usually achieved by applying materials that neutralize soil acidity [42]. It was reported that, in flooded (anaerobic) rice paddies, rice straw decomposition consumes oxygen, promoting denitrification. When oxygen levels are low but not completely absent, denitrification may be incomplete, leading to increased N₂O emissions [43]. Qui et al. [44] demonstrated that moderate temperatures (20–30 °C) and near-neutral pH favor the activity of denitrifying microbes.

Xiang et al. [45] elucidated the mechanism of N₂O emission resulting from denitrification in rice field soils. They demonstrated that denitrification operates in a highly modular manner, involving microbial communities (e.g., Actinobacteria, Planctomycetes, Desulfobacterota, Cyanobacteria, Acidobacteria, Bacteroidetes, and Myxococcus) working together to accomplish the entire process, resulting in an estimated N₂O emission of 13.67 ± 5.44 g N₂O·m^–2·year^–1 in surface paddy soils. Wang et al. [46] indicated that procyanidins, plant-derived biological denitrification inhibitors, can suppress soil denitrification, thereby decreasing N₂O emissions and N loss from the soil.

Tan et al. [47] investigated the influence of temperature and soil moisture on nitrification and denitrification dynamics in a lowland paddy field. The rates of both processes were evaluated at three different stages of rice growth (early-vegetative, early-tillering and panicle initiation), under four temperature conditions (20 °C, 25 °C, 30 °C, and 35 °C, respectively), and at three soil depths (cultivated layer, plow pan and illuvial horizon). The highest nitrification activity was observed in the cultivated layer during the early tillering stage, with rates increasing with increasing temperature. Denitrification was also most active during early tillering, showing slightly higher rates in cultivated and plow pan layers compared to the illuvial horizon, and also showed a modest increase with increasing temperatures. Overall, nitrification rates ranged between 12.3 and 23.2 mg·N·m⁻³·h⁻¹, while denitrification rates ranged from 3.6 to 5.8 mg·N·m⁻³·h⁻¹. Liu et al. [48] examined the roles of autotrophic and heterotrophic nitrification and denitrification in contributing to N₂O emissions from two types of rice soils (one acidic and one neutral). The results indicated that low soil pH inhibits soil nitrification and ammonia oxidizer activity compared to neutral rice soil.

Nitrification inhibitors target the enzyme ammonia monooxygenase (AMO), which catalyzes the first step of nitrification. By inhibiting the microorganisms (e.g., Nitrosomonas, and Nitrobacter), the inhibitors help retain N in NH₄⁺ form for longer periods of time, reducing the availability of NO₃⁻, and thus promoting more sustainable agriculture practices (Figure 3) [49].

In agriculture, various nitrification inhibitors, such as dicyandiamide (DCD), 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin), and 3,4-dimethylpyrazole phosphate (DMPP), have been developed to slow down the activity of AOB in soils, thereby reducing N₂O emissions by 30% to 70% [50,51,52,53]. Each has varying effectiveness depending on soil type, temperature, moisture, and microbial activity [54].

Dicyandiamide (DCD) delays the nitrification process, thereby limiting the substrate available for denitrification. The application of DCD has resulted in increased rice yields due to better N availability and reduced N losses. This is especially beneficial in low-input systems or where nitrogen fertilizer costs are high. DCS may also contribute to improved water quality by decreasing NO₃⁻ runoff into surface and groundwater, which is a key factor in preventing eutrophication. However, its effectiveness is limited by environmental factors, such as waterlogged soil conditions, temperature, and short persistence in the field. In agricultural soils at 25 °C, the concentration of DCD can drop to half of its original level in just 20 days after application [55,56]. Under anaerobic conditions, its inhibitory effect on nitrification may be significantly reduced, especially if the soil remains saturated for extended periods.

It has been reported that nitrapyrin enhances N availability to rice plants during critical growth stages by stabilizing NH₄⁺ in the soil for a longer period of time. This can lead to improved crop uptake, reduced fertilizer loss, and potentially higher yields. Nitrapyrin can be used alongside AWD irrigation, slow-release fertilizers, or site-specific nutrient management tools, making it adaptable to evolving climate-smart agriculture strategies [57]. Most existing research on nitrapyrin has been conducted in temperate or upland cropping systems. More field trials are needed in tropical rice-growing regions, which experience different soil conditions, temperatures, and management practices [58]. Mi et al. [59] indicated that the high soil temperatures may reduce the effectiveness of nitrapyrin in rice cultivation.

In rice cultivation, 3,4-dimethylpyrazole phosphate (DMPP) represents a scientifically proven and practical tool for mitigating N₂O emissions. By inhibiting nitrification and maintaining N in the ammonium form, DMPP not only reduces environmental impact but also improves N efficiency and supports sustainable rice production. As global agriculture moves toward climate-resilient practices, integrating DMPP into rice nutrient management facilities offers a promising pathway to reducing the sector’s greenhouse gas emissions [60,61]. Wang et al. [62] examined the impact of DMPP on N₂O emissions and related biological indicators under three different water management regimes: continuous flooding (CF), mild AWD (Mi-AWD), and moderate AWD (Mo-AWD). The results indicated that DMPP significantly reduces N₂O emissions under oxygen-rich AWD conditions, rather than in anaerobic CF systems, indicating that the use of nitrification inhibitors can help resolve the trade-offs between water-saving irrigation and N₂O mitigation. Lei et al. [63] demonstrated that nitrification inhibitors reduce N₂O emissions by an average of 58.1% while increasing soil NH₄⁺ concentrations by 71.4%. DMPP application together with organic and inorganic fertilizers was more effective in reducing N₂O emissions in alkaline soils compared to acidic soils. Due to nitrification inhibitors, the reduction in N₂O emissions was significantly associated with changes in AOB populations (p < 0.01). In addition, a notable relationship was found between changes in NH₄⁺ concentration and the abundance of the AOB amoA gene after nitrification inhibitor treatment (p = 0.014). These findings highlighted the dominant role of AOB in the nitrification process and suggested that AOB abundance can serve as a reliable indicator of effectiveness of nitrification inhibitor.

3. Mitigation Strategies

Some effective strategies to reduce N₂O emissions are optimized fertilizer use, improved drainage and water management, use of nitrification inhibitors, and organic amendments.

3.1. Optimized Fertilizer Use

The global N₂O budget for the period 2007–2016 highlights agriculture as the principal anthropogenic contributor to N₂O emissions, mainly driven by the use of nitrogen fertilizers [64]. Forecasts suggest that N₂O emissions could increase by 35% to 60% by 2030, largely due to increased reliance on synthetic fertilizers [65].

In agriculture, nitrogen fertilizers are widely used to enhance crop yields and maintain soil fertility. These fertilizers provide plants with readily available N, enabling faster growth and improved productivity [66]. Literature data have shown that applying the right type and amount of nitrogen fertilizer, and at the right time, can reduce excess N in the soil and thus limit N₂O production [67,68,69,70]. Liu et al. [70] reported that the optimal nitrogen fertilizer application rate is between 270 and 330 kg/ha (applied in four separate doses) to optimize yield under the conventional nitrogen fertilizer management.

Several types of nitrogen fertilizers were utilized in agriculture, such as ammonium nitrate (NH₄NO₃), urea (CO(NH₂)₂), ammonium sulfate ((NH₄)₂SO₄), and calcium ammonium nitrate (CAN).

Ammonium nitrate (NH₄NO₃) is a valuable nitrogen fertilizer in rice cultivation due to its high N content, containing both NO₃⁻ and NH₄⁺ forms. It is fast-acting and widely used for field crops, especially in temperate regions. Pélissier et al. [71] showed that NH₄NO₃ can contribute to increasing rice yields by providing an adequate and timely amount of N (2.5 mM of NH₄NO₃) during critical growth stages. Fan et al. [72] investigated the effects of different NH₄NO₃ and NO₃ mixture treatments (100:0, 75:25, 50:50, 25:75, and 0:100) on the growth, N uptake, and yield of different rice cultivars. The findings revealed that rice root biomass, tiller number, and yield increased by 69.5%, 42.5%, and 46.8%, respectively, in the 75:25 NH₄-NO₃ treatment compared to the 100:0 NH₄-NO₃ mixed treatment. It was concluded that the NH₄-NO₃ ratio of 75:25 enhanced the activity of N-metabolizing enzymes and stimulated uptake by upregulating genes associated with N metabolism, leading to increased tiller production and improved rice yields. Yang et al. [73] analyzed the effect of NH₄NO₃ (1.43 mM, 30 min) in rice roots. It has been found that the concentration of inorganic N in soil can fluctuate dramatically, typically ranging from a few hundred micromolars to over 20 mM, being influenced by factors such as soil composition, microbial activity, and the application of fertilizers. In contrast, the concentration of inorganic N inside the plant cell tends to be more stable compared to the concentration of inorganic N in soil. NO₃⁻ concentrations were generally found between 1 and 6 mM in the cytosol, and can range from 5 to 75 mM in the vacuole.

Urea (CO(NH₂)₂) is the most commonly used nitrogen fertilizer globally. It contains about 46% N, making it highly concentrated. Before being absorbed by plants, urea must first be converted into NH₄⁺ and then into NO₃⁻ in the soil. To determine the effects of CO(NH₂)₂ on N₂O emissions, experimental plots can be set up with varying urea application rates (e.g., 0, 50, 100, and 150 kg N/ha) [74,75]. Treatments may also include split applications (e.g., basal vs. top-dress), incorporation methods (e.g., surface vs. deep placement), and timing relative to irrigation events [76]. Ke et al. [67] studied the effects of two fertilization techniques (broadcast and deep placement via mechanical side-dressing fertilization) along with three types of controlled-released nitrogen fertilizers (CRNFs): sulphur-coated urea (SCU), polymer-coated urea (PCU), and a bulk blended formulation (BBF). Conventional high-yield fertilization (urea applied in four split doses totaling 216 kg N ha⁻¹) and 0–N treatments were included as controls. The results indicated that NH₄⁺-N concentrations in both percolation and surface water varied depending on the CRNFs used, regardless of the fertilization method applied. It was observed that the deep placement of SCU had the highest N leaching of 6.65 and 5.34 kg N ha⁻¹. The studies showed that BBF, particularly when applied through deep placement, offers a viable and efficient alternative to a conventional high-yield fertilization practice.

It has been reported that the higher rates of CO(NH₂)₂ generally lead to increased N₂O emissions, but the relationship is not always linear due to other interacting factors, such as soil properties and water management [77,78]. Ju et al. [79] investigated N₂O fluxes associated with CO(NH₂)₂ treatments during rice cultivation phases over a three-year period, using chamber-based sampling and gas chromatography. In addition, they analyzed how changes in soil temperature and moisture influenced N₂O emissions during these periods. The final results demonstrated that changes between anaerobic and aerobic soil conditions caused by drainage, in combination with N fertilization, play a crucial role in influencing the variability of N₂O emission during the rice cultivation period. Kim et al. [80] investigated the impact of rice cultivation on N₂O emissions by applying the various rates of CO(NH₂)₂ (0, 45, 90 and 180 kg N ha⁻¹, respectively) in a typical temperate rice field. They compared N₂O fluxes between soils with and without rice cultivation. It was concluded that seasonal N₂O emissions exhibited different patterns depending on N application and planting conditions. In non-planted soils, N₂O fluxes ranged from 0.31 to 0.34 kg N₂O ha⁻¹ under zero N input, and increased significantly with higher fertilization rates, averaging 0.0024 kg N₂O per kg N applied over three years. In contrast, rice-planted soils also showed increased N₂O emissions with nitrogen fertilization, but notable negative N₂O fluxes were observed across all fertilization levels. Consequently, the reduction in N₂O emissions in planted soils could be related to a decrease in denitrification potential in the rice soil environment.

Islam et al. [81] investigated the effects of urea deep placement (UDP) with AWD irrigation on N₂O emissions and rice yields compared to conventional broadcast application. It was observed that drying conditions could have enhanced microbial nitrification, while subsequent re-flooding promoted denitrification, both contributing to increased N₂O emissions. In flooded soils, the use of UDP significantly reduces floodwater NH₄⁺-N levels, allowing most N to remain in the form of NH₄⁺ within the reduced soil zone for prolonged period. This minimizes N losses through ammonia volatilization, nitrification, denitrification, and leaching. Consequently, UDP provides a sustained supply of available N to rice plants throughout the growing season. Webb et al. [82] examined whether polymer-coated urea could lower soil-derived N₂O emissions in aerobic rice systems in the semi-arid temperate regions of Australia, compared to conventional urea fertilizer. The experiment was conducted over one season, under different nitrogen fertilizer managements at a rate of 220 kg N ha⁻¹. The results showed an 84% reduction in cumulative N₂O-N emissions compared with urea (4.36 ± 1.07 kg N ha⁻¹ and 27.9 ± 5.70 kg N ha⁻¹, respectively), suggesting that polymer-coated urea may be more effectively adapted to the soil conditions typical of aerobic rice cultivation in the region.

Ammonium sulfate ((NH₄)₂SO₄) is an inorganic fertilizer used in rice cultivation because it provides essential nutrients (N and sulfur (S)) for crops. The use of (NH₄)₂SO₄ can lead to improved N use efficiency, correction of S deficiency, and regulation of soil pH. Despite its benefits, the use of (NH₄)₂SO₄ in rice fields must be carefully managed due to several environmental concerns, such as ammonia volatilization, stimulation of N₂O emissions, and soil acidification. To mitigate these problems, several management practices were recommended, such as applying ammonium sulfate under flooded conditions to minimize volatilization, using site-specific nutrient management to match fertilizer input with crop needs, and incorporating slow-release formulations or combining with organic amendments to improve N retention and reduce emissions [83]. Ndille et al. [84] evaluated the effect of different levels of (NH₄)₂SO₄ (0 kg ha⁻¹, 40 kg ha⁻¹, 60 kg ha⁻¹, and 80 kg ha⁻¹, on the growth and the yield of rice. The results indicated that applying high levels of fertilizer can significantly improve rice yields. In another study, it was demonstrated that the addition of a large amount of (NH₄)₂SO₄ fertilizer (50 kg ha⁻¹) leads to plant growth and grain production, regardless of whether the application is made at the vegetative stage or at the panicle initiation stage [85].

Calcium ammonium nitrate (CAN) is a granular fertilizer that combines NH₄NO₃ with calcium carbonate. It is less volatile and suitable for acidic soils, helping to reduce soil acidification [86]. This fertilizer offers a balanced and efficient nutrient source for crops. While CO(NH₂)₂ and (NH₄)₂SO₄ are more commonly used in flooded rice systems, CAN provides distinct agronomic advantages, particularly in certain soil and water management conditions. CAN fertilizer can be used in upland rice systems, intermittently flooded paddies, or as a top-dressing fertilizer during drier stages of the crop cycle. When integrated with appropriate agronomic strategies, CAN contribute to sustainable rice production and improved yields.

Yao et al. [87] showed that yield-related N₂O emissions can decrease with increasing N use efficiency or N₂O emissions for production systems with rice yields >6.8 Mg ha⁻¹. This highlights that N use efficiency indicators can be used to balance high crop production with N₂O emission mitigation.

The main benefits of nitrogen fertilizers to agricultural systems and environmental considerations are presented in

Table 1. Benefits of nitrogen fertilizers to agricultural systems and environmental considerations.

Benefits	Environmental Considerations
Nitrogen is vital for vegetative growth and directly impacts the quantity and quality of agricultural produce.	Runoff from fields can carry nitrates into water bodies, causing eutrophication and harming aquatic ecosystems.
Adequate nitrogen supply accelerates plant growth, helping crops reach maturity more quickly.	In flooded conditions, nitrate can be lost as nitrogen gas through microbial processes, reducing fertilizer efficiency.
Some nitrogen fertilizers can improve the microbial activity and nutrient balance in the soil.	Overuse of nitrogen can lead to soil acidification and loss of beneficial microorganisms.
Applying nitrogen in smaller doses reduces the risk of excessive nitrification and nitrate leaching.

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3.2. Improved Drainage and Water Management

Improved drainage and water management practices in rice cultivation offer viable solutions to enhance productivity, conserve water, and reduce environmental impacts. Improved drainage involves the ability to remove excess water from the field at key times during the crop cycle. This offers several advantages, such as improved root oxygenation which promotes healthier pant growth, improved nutrient absorption for N-based fertilizers, and a lower risk of soil salinization [88].

Water management is essential for ensuring the sustainability of rice cultivation in a world facing increasing environmental and resource pressures. Saturated soil culture (SSC) involves shallow irrigation to achieve a depth of ~1 cm of stored water for a day or two after the stored water has drained. In SSC, the soil is maintained as close to saturation as possible, reducing hydraulic pressure, infiltration, and the penetration rate of stored water. In SSC (no standing water), penetration losses to depth are zero, whereas under continuous flooding conditions, water penetration is high (Figure 4). Water-saving irrigation methods, such as AWD and dry direct-seeded rice (DSR) systems, are gaining importance in many rice-producing countries [89], as AWD has been shown to reduce irrigation water use by up to 38% without compromising crop yield [90,91].

While AWD irrigation can reduce CH₄ emissions, it may increase N₂O emissions due to fluctuating soil conditions. Balancing these outcomes is crucial. Continuous flooding tends to suppress N₂O emissions but increases CH₄ emissions. In contrast, intermittent irrigation can reduce CH₄ but increase N₂O release, creating a trade-off between greenhouse gases.

Rector et al. [92] assessed the impact of water management practices (continuous flooding vs. intermittent flooding) and rice cultivars (pure-line vs. hybrid) on N₂O fluxes, cumulative seasonal N₂O emissions, and the global warming potential (GWP) in a direct-seeded, delayed-flood rice production system on silt-loam soil in eastern Arkansan. The results showed that N₂O fluxes differed significantly over time, depending on water management and cultivar treatments. N₂O fluxes measured at 27 and 74 days after flooding were shown to be 5 to 30 times higher than non-zero fluxes. Wu et al. [93] demonstrated that, under urea fertilization, mild AWD irrigation increased yield-scaled N₂O emissions by 17.9% compared to continuous flooding, and the incorporation of rice straw under mild AWD conditions further raised yield-scaled N₂O emissions by an additional 17.4%. Thus, combining mild AWD irrigation with rice straw incorporation may offer a sustainable agronomic approach to maintaining rice yields, lowering greenhouse gas emissions, and enhancing soil fertility. Sun et al. [94] investigated the effects of two rice varieties (a water-saving and drought-resistance rice versus a common rice variety), along with irrigation management practices on N₂O emissions from rice paddies over two growing seasons under contrasting climate conditions (one warm and dry season and a typical season). The study concluded that N₂O emissions were influenced by rice variety and irrigation practices during the two growing seasons, which experienced different climate conditions. In particular, irrigation water use was substantially higher during the warm and dry season compared to the normal season. Tariq et al. [95] tested the N₂O mitigation potential of water management regimes in two water management systems (efficient field water management (EWM) and inefficient field water management (IWM)). The study was carried out over two consecutive rice-growing seasons in northern Vietnam. It was observed that, during the spring season, the EWM system produced significantly (p < 0.05) higher N₂O emissions than the IWM system. However, no significant (p < 0.05) differences in N₂O emissions were detected between the two systems during the summer season. Additionally, in the spring, the EWM system showed a significantly greater proportion of applied N released as N₂O-N (ranging from 1.5% to 2.8%) compared to the IWM system (0.4% to 1.4%). The study suggested that EWM system is an effective for mitigating the N₂O without yield loss.

Hamound et al. [96] evaluated the irrigation system, set in three levels: R1 (30 mm–100%), (30 mm flooded and 100% saturation), R2 (30mm–90%), (30 mm flooded and 90% saturation), and R3 (30 mm–70%), (30 mm flooded and 70% saturation). It was observed that the optimal treatment combination of R3 (30 mm–70%) and S (60%) achieved the highest N use efficiency (approximately 47%). This combination promoted a significant soil swelling, which improved the absorption and utilization of N by rice plants by approximately 80% compared to the R3 (30 mm–70%) and S (40%) treatment. In contrast, allowing expansive soil to dry to 30% below saturation before re-irrigation resulted in a significant decrease in N use efficiency (decreasing by about 10%), primarily due to the cracks in the soil, which created preferential pathways for water and nutrient loss. These findings provide useful guidance for selecting appropriate water management practices and contribute valuable insights into the interactions between soil behavior, water management, and rice performance.

Wu et al. [97] analyzed the different mitigation approaches of N₂O emissions from rice paddies in China and concluded that N₂O emissions continued to increase until the N fertilizer rate reached ~150 kg ha⁻¹. Organic fertilizer application, soil tillage, and half- and full-scale straw return resulted in increases in global warming potential (GWP) by 80.3%, 33.8%, 25.2%, and 111.6%. Among water management practices, frequent drainage (DF) was found to be the most effective in reducing greenhouse gas emissions, decreasing them by 39.5% compared to continuous flooding, while mid-season drainage achieved a reduction of 18.4%. Yu et al. [98] indicated that, compared with continuous flooding, intermittent irrigation reduced CH₄ emissions while increasing N₂O emissions under elevated CO₂. They showed that the management practices, such as combining straw incorporation with intermittent irrigation, optimizing N input, and selecting high-yielding, low-emission rice varieties, can achieve both productivity and environmental objectives. Liu et al. [99] examined how mid-season drainage influenced GWP. The results showed that mid-season drainage led to a 52% reduction in CH₄ emissions but caused a 242% increase in N₂O emissions. Despite the increase in N₂O, the GWP was reduced by 47% compared to continuous flooding.

3.3. Organic Amendments

Incorporating organic materials, such as compost or biochar, can improve soil structure and nutrient retention, potentially lowering greenhouse gas emissions. These materials reduce dependency on chemical fertilizers, minimize environmental degradation, and contribute to long-term productivity by enhancing soil health [100,101,102,103]. However, their use must be tailored to local conditions for maximum effectiveness.

Compost increases the organic matter content in the soil, improving its texture, water-holding capacity, and aeration. These changes root development and microbial activity, ultimately leading to healthier rice plants. It has been reported that although it releases nutrients (such as N, phosphorus (P), and potassium (K)) more slowly than chemical fertilizers, compost contributes to a more balanced and sustained nutrient supply throughout the growing season [104].

Some studies have indicated that compost can be used in conjunction with green manures or biochar to further improve soil quality and nutrient availability [105]. Anshori et al. [106] examined the effect of different type of organic composts on N₂O production in organic rice field. The results indicated that the N₂O production is influenced by the composition of the organic compost. The lowest production of N₂O was in solid biogas waste treatment (2.80 mg kg soil⁻¹ day⁻¹). It has been reported that compost should be applied with sufficient lead time before planting to allow for nutrient mineralization and avoid competition with crops for N [107]. Wüst-Galley et al. [108] tested the use of a mineral cover layer (greenwaste compost) to reduce N₂O emissions. The experiments were realized in the Swiss Central Plateau, located in the cool temperate moist zone. Two water management strategies were applied: a high water table (WT) treatment maintained at −6 cm with mid-season drainage, and two medium WT treatments set at −11 cm and −17 cm without mid-season drainage. The low N₂O emissions suggested that wet rice cultivation on the studied organic soils in the indicated climatic region could help mitigate the effects of climate warming associated with agricultural use, offering an advantage over conventional deeply drained grassland systems.

Numerous studies have confirmed the effectiveness of biochar in reducing N₂O emissions in rice cultivation [109]. A meta-analysis of global field experiments showed that biochar reduced N₂O emissions by 30–60% on average, depending on soil type and biochar characteristics [110,111]. Biochar helps create a more sustainable and climate-friendly agricultural system by improving N retention, modifying microbial processes, and enhancing soil properties. Selvarajh et al. [112] reported that rice straw biochar is an effective material for reducing NH₃ volatilization, thereby minimizing N losses to the environment. Furthermore, its application can help reduce the need for excessive urea fertilizer, improve soil nutrient content, and promote better rice plant growth. Pramono et al. [113] investigated the effect of combined compost with biochar application on rice yields N₂O emissions in rained rice fields. The results showed that applying compost (at a rate of 3-ton ha⁻¹ in each planting season) increased grain production in lowland areas fed by rainwater. Also, the application of resulted compost increased dry grain production of the rice variety by an average of 17% and reduced the rate of N₂O emissions by 23% compared to the application of manure. Aamer et al. [114] examined the mechanisms by which rice straw-derived biochar helps reduce N₂O emissions from acidic soils. Different biochar rates (1%, 2%, and 3%) were tested to assess their effectiveness in mitigating emissions. The results indicated that the application of 3% biochar significantly reduced N₂O emissions by increasing soil pH and NO₃⁻ levels, while simultaneously reducing NH₄⁺ concentrations and nitrate reductase activity. Xu et al. [115] evaluated an incubation experiment using rice soil amended with biochar produced from Camellia oleifera fruit shell, spent mushroom substrate, and rice straw, under two water regimes (water holding capacity of 70% and 120%) to assess their impact on soil N₂O emissions. Incorporating the rice straw biochar into rice soils with high moisture levels has been shown to be more effective in mitigating N₂O emissions.

Gao et al. [116] examined the impact of repeated applications of biochar, straw, and their combination on soil N₂O emissions and crop yield, evaluating both environmental and economic outcomes. In addition, the study aimed to identify the key factors influencing N₂O emissions in different growing seasons and years. It was concluded that repeated applications of biochar significantly reduced N₂O emissions without compromising crop yield. Combining biochar with straw was shown to increase crop productivity but was less effective in mitigating N₂O emissions. Yang et al. [117] evaluated the impact of rice straw biochar and swine manure biochar on N₂O emissions from rice soil, both biochars being applied at rates of 1% and 5% (w/w). The results indicated that the cumulative N₂O emission was suppressed by 45.14–73.96% after application of biochar, and 5% swine manure biochar resulted in the lowest cumulative emission. Vat et al. [118] investigated the effectiveness of biochar and its co-application with manure or fertilizers in reducing N leaching, N₂O emissions and increasing grain yield under AWD system. The results showed that the application of biochar (as such) significantly (p < 0.05) increased rice yield by 9.49% to 14.10%, compared to treatments without biochar. Biochar combined with either manure or fertilizer also improved grain yield, but led to increased N₂O emissions. However, treatment with biochar and manure has proven to be much more advantageous than treatment with biochar and fertilizers, offering higher yields with lower emissions. The complete AWD cycle created favorable conditions in the rice soil by increasing oxygen availability in the root zone, which promoted oxidation and nutrient uptake for rice growth, although it also increased the risk of nutrient leaching.

Shen et al. [119] investigated different water management strategies, including biochar as an amendment to reduce soil N₂O emissions: AWD conditions during the mid-season period without biochar (AWD1) and with biochar (2%, AWD2), continuous flooding during the mid-season period without biochar (CF1) and with biochar (2%, CF2), and regular mid-season drainage during the mid-season period without biochar (CON1) and with biochar (2%, CON2). The addition of biochar significantly reduced the second N₂O emission peak in both the AWD2 and CON2 treatments. This reduction was mainly attributed to an increase in nosZ gene abundance and a lower ratio of denitrification and amoA genes relative to nosZ gene copies, indicating an increased potential for N₂O reduction. These results showed that AWD2 is the most effective management approach for simultaneously reducing N₂O emissions and increasing the rice productivity in tropical paddy soils.

Gwaltney et al. [120] analyzed the effects of wood biochar type (powder and coarse particle size) and application rate (0, 2.5, and 5 Mg ha⁻¹) on N₂O emissions in greenhouse-grown, imitated furrow-irrigated rice grown in a silt-loam soil. Among the coarse-sized biochar treatments, season-long N₂O emissions (41.7 kg N₂O ha⁻¹ season⁻¹) were observed in the unamended control (0 Mg ha⁻¹) (p < 0.05), followed by the 2.5 (17.1 kg N₂O ha⁻¹ season⁻¹) and 5 Mg ha⁻¹ rate (4.9 kg N₂O ha⁻¹ season⁻¹). In contrast, N₂O emissions during the season among the powder-sized biochar treatments did not show significant differences (p > 0.05). The results demonstrated that coarse biochar, produced from wood chips, can effectively reduce N₂O emissions from furrow-irrigated rice grown on silt-loam soil.

In a study reported by Wei et al. [121], two contrasting paddy soils, one acidic (pH = 5.40) and one alkaline (pH = 7.56), were used to evaluate N₂ and N₂O fluxes following the biochar application at different rates (0%, 0.5%, and 5% by weight). The results indicated that biochar significantly (p < 0.05) suppressed N₂O emissions by a factor of 0.65 to 3.64, while simultaneously increasing N₂ emissions by 5.47% to 46.14% in both soil types. Consequently, the N₂O/(N₂O + N₂) emission ratios were reduced (p < 0.05) by 1.53 to 4.65 times. These findings suggested that biochar effectively reduced N₂O emissions in paddy soils, mainly by facilitating its conversion to N₂.

4. Current Challenges and Future Research Directions

4.1. Current Challenges

Accurately measuring N₂O emissions remains a important challenge due to the spatial and temporal variability of emissions, especially in rice fields. N₂O emissions are influenced by factors such as type and timing of fertilizer application, soil moisture, temperature, organic matter content, and microbial activity. The dynamic wet–dry cycles in rice cultivation (e.g., AWD) further complicate the emission patterns, making accurate prediction and quantification difficult.

N₂O production and consumption are governed by complex microbial processes, such as nitrification and denitrification, which are influenced by eco-friendly variables including temperature, moisture, oxygen availability, and soil chemistry. A limited understanding of these processes under field conditions hampers the development of accurate predictive models.

Direct seeding of rice (DSR) is an increasingly popular method of rice cultivation in which seeds are sown directly into the field, eliminating the need for traditional nursery cultivation and transplantation. This approach significantly reduces labor costs, water consumption, and production costs, making it attractive in areas facing labor and water shortages. Although manual seeding with large-scale broadcast remains widely used, more precise techniques, such as seeding using seed drills or mechanized seeders are gaining ground. Given the growing interest in sustainable and climate-smart agriculture, research institutions are actively exploring ways to improve the efficiency and environmental performance of DSR systems [122].

Water management practices, particularly the transition from continuous flooding to intermittent irrigation or mid-season drainage, alter the redox conditions of the soil. These changes can suppress CH₄ but often enhance N₂O production. Coordinating fertilizer application with irrigation regimes remains a persistent challenge.

Many proposed mitigation practices, such as reduced fertilizer use or altered water management (AWM), may lead to unintended consequences, such as reduced crop yields or increased emissions of other greenhouse gases (e.g., CH₄). Balancing productivity with environmental sustainability remains a central challenge.

4.2. Future Research Directions

Future research should focus on optimizing N application, timing, type, and rate, through precision agriculture technologies. Decision-support tools that incorporate real-time soil and crop data can help farmers apply N more efficiently, reducing N₂O emissions without compromising yields.

Breeding rice varieties with higher N-use efficiency or introducing soil amendments, such as biochar, nitrification inhibitors, or organic composts can significantly impact N₂O emissions. Research is needed to test the long-term impacts and practicality of these strategies in different agroecological zones. Furthermore, integrating artificial intelligence and machine learning with field data can optimize N management in real time. Future studies should explore how precision nutrient management can be adapted to rice systems to reduce N₂O emissions without compromising productivity.

The choice of nitrogen fertilizer significantly influences the amount of N₂O emitted by rice fields. Among common fertilizers, NH₄NO₃ and CAN tend to lead to higher N₂O emissions, while (NH₄)₂SO₄ and CO(NH₂)₂ have variable impacts depending on soil conditions and water management. Sustainable rice cultivation requires an integrated approach that considers fertilizer type, application timing, application method, and irrigation practices to reduce N₂O emissions while maintaining high yields.

A deeper understanding of the microbial communities involved in nitrification and denitrification will help refine mitigation strategies. Metagenomic and transcriptomic approaches could identify key microbial players and how their activity responds to different fertilizers and environmental conditions.

Understanding farmer behavior, incentives, and barriers to adoption is essential. Interdisciplinary research combining agronomy, economics, and policy studies can support the development of feasible, scalable mitigation strategies. Additionally, climate-smart subsidies or carbon credit programs may encourage the adoption of N₂O-reducing practices.

Future research must adopt an integrated, interdisciplinary approach that combines advanced measurement techniques, microbial science, precision agriculture, and farmer-centered policy design. Addressing these challenges will be vital for ensuring that rice production remains sustainable in the face of climate change.

5. Conclusions

The role of rice cultivation in global N₂O emissions is a complex issue with significant implications for climate change. While rice is an essential food for over half of the world’s population, its environmental cost must be carefully managed. By adopting climate-smart agricultural practices, especially regarding fertilizers and water management, it is possible to reduce N₂O emissions and move to more sustainable rice production systems. Coordinated efforts among farmers, policymakers, and researchers are essential to mitigate the climate impact of one of the world’s most important crops.

Under anaerobic conditions prevalent in flooded rice fields, nitrification and denitrification processes are major pathways for N₂O production. When oxygen levels fluctuate, such as during mid-season drainage or intermittent irrigation, these processes are intensified, leading to increased N₂O emissions.

Rice straw application is a sustainable practice that can enrich soil fertility and reduce agricultural waste. However, it also has complex effects on N₂O emissions, depending on soil conditions, water management, and N availability.

Controlled-release fertilizers, biochar amendments, and the use of nitrification inhibitors (e.g., DCD, nitrapyrin, and DMPP) have shown promising results in reducing N₂O emissions, but their adoption remains limited due to cost and accessibility.

By understanding the biological and environmental drivers of N₂O emissions, and by implementing specific mitigation strategies, it is possible to reduce the soil contribution to greenhouse gas emissions and move to more climate-resilient agricultural systems.