Ammonia Loss in Cropping Systems and Mitigation Strategies

1. Introduction

Ammonia volatilization is a major pathway of reactive nitrogen loss in agricultural systems, occurring shortly after fertilizer application and limiting the efficiency of nitrogen use in crops. Mineral nitrogen fertilizers sustain a substantial share of global food production, with estimates indicating that nearly half of crop output depends on synthetic nitrogen inputs [1], while future demand is expected to continue increasing [2]. Despite this importance, less than half of the applied nitrogen is typically recovered by crops, reflecting significant inefficiencies in current fertilization practices [3]. Nitrogen losses occur through several pathways, including volatilization, denitrification associated with nitrous oxide (N₂O) emissions, and nitrate leaching, making their control essential for sustainable nutrient management [4]. Figure 1 illustrates these pathways within the agricultural nitrogen cycle, highlighting the main transformation processes and loss routes following fertilizer application.

Within this framework, ammonia emissions represent a particularly critical component due to their high variability and strong dependence on soil, climatic, and management conditions. Volatilization is especially relevant for urea-based fertilizers, where rapid hydrolysis can lead to substantial nitrogen losses at the soil surface. Reported emission rates vary widely across agroecosystems, highlighting the importance of understanding the underlying mechanisms that control ammonia formation and transfer rather than assuming uniform mitigation performance [5,6].

Beyond agronomic inefficiency, ammonia emissions have important environmental and societal implications. Once released, ammonia contributes to atmospheric processes that lead to soil acidification and eutrophication after deposition [7]. It is also a key precursor of secondary inorganic aerosols and fine particulate matter (PM2.5), which are associated with adverse human health effects, including respiratory and cardiovascular diseases [8,9,10]. Reducing ammonia emissions is therefore not only an agronomic priority but also a relevant strategy for improving air quality and mitigating broader environmental impacts [10].

From an agronomic perspective, ammonia losses directly reduce the nitrogen available for crop uptake, lowering nitrogen use efficiency and economic returns. Recent estimates suggest that a significant fraction of nitrogen applied as urea is lost through volatilization at global scale, representing substantial economic losses for farmers [6]. These combined environmental and economic drivers have stimulated the development of enhanced-efficiency fertilizers and related technologies aimed at reducing nitrogen losses while maintaining crop productivity [11].

Current mitigation approaches target key processes controlling ammonia emissions, including urea hydrolysis, pH-dependent chemical equilibria, ammonium retention, and nitrogen release dynamics. Urease inhibitors, nitrification inhibitors, and controlled-release fertilizers are among the most widely studied strategies, although their effectiveness varies depending on soil properties, weather conditions, and management practices [8,12,13,14]. In addition, increasing attention has been given to interactions between nitrogen loss pathways, as reducing ammonia emissions may influence N₂O emissions or nitrate leaching, highlighting the need for integrated evaluation frameworks [8,15].

A major challenge in the field lies in the discrepancy between controlled experimental results and field-scale performance. While laboratory studies provide mechanistic insights, they often overestimate mitigation potential compared to field conditions, where variability in climate, soil properties, and management practices plays a dominant role [6]. This has led to a growing emphasis on field-based evidence and context-specific assessment of mitigation strategies.

In this context, this review aims to analyse ammonia volatilization from a process-based perspective, linking the main mechanisms governing nitrogen transformations with the performance of fertilizer-based mitigation strategies under real conditions. The work seeks to identify the key factors controlling variability in mitigation outcomes and to provide a structured framework to support the selection of appropriate strategies according to dominant loss processes. By integrating mechanistic understanding with practical considerations, this study contributes to improving nitrogen use efficiency and reducing environmental impacts in cropping systems.

2. Mechanisms Governing NH₃ Volatilization in Agricultural Soils

Ammonia volatilization in fertilized soils results from the interaction between rapid biochemical transformations of nitrogen, physicochemical equilibria, and transport processes at the soil surface. In cropping systems, the highest risk is typically associated with urea and other readily hydrolysable fertilizers, as nitrogen is temporarily concentrated in localized reaction zones rather than evenly distributed. Conceptually, volatilization involves three sequential steps: formation of an ammoniacal nitrogen pool, partitioning between NH₄⁺ and NH₃, and transport of NH₃ from the soil to the atmosphere. Urea hydrolysis has been identified as the critical initial step in this process, linking volatilization to subsequent nitrogen transformations such as nitrification and denitrification [16]. This mechanistic sequence explains why mitigation performance varies across soils, climates, and management conditions. Table 1 summarizes the main control points governing NH₃ volatilization and their associated mitigation levers.

The first key driver is the enzymatic hydrolysis of urea by urease, which rapidly generates ammonium near fertilizer granules or bands. This process creates localized zones with high ammoniacal nitrogen concentrations and elevated pH, conditions that favor NH₃ formation. Although microscale pH dynamics are rarely measured in field conditions, urea is widely considered a high-risk nitrogen source due to the short but intense emission window following application. Conditions that promote the conversion of ammonium to ammonia directly enhance emissions, showing that mitigation depends on controlling both the timing and location of nitrogen transformations [17].

A second controlling factor is the equilibrium between NH₄⁺ and NH₃, which is strongly influenced by pH and temperature. As pH and temperature increase, the proportion of nitrogen present as gaseous ammonia also rises. Ammonium may be retained through adsorption or microbial uptake, but when present in soil solution or surface water, it can readily convert to NH₃ and be lost to the atmosphere [18]. This identifies two complementary mitigation targets: shifting the equilibrium toward NH₄⁺ through pH control and increasing ammonium retention within the soil matrix.

The third driver is mass transfer, which determines whether NH₃ formed at the microsite reaches the atmosphere. Emissions depend on diffusion through soil pores, transport across the boundary layer, and atmospheric turbulence. Environmental factors such as temperature, wind speed, and soil moisture influence these processes by modifying both reaction rates and transport resistance [19]. For example, wind reduces boundary-layer resistance and enhances emissions, while soil moisture can either promote hydrolysis or restrict gas diffusion depending on soil structure and timing. Temperature acts as a key accelerator of both biochemical and physicochemical processes, explaining the frequent association between warm conditions and high volatilization rates [20].

Soil properties strongly influence baseline susceptibility to volatilization. Soil pH and buffering capacity determine the NH₄⁺/NH₃ equilibrium and the extent of alkalinization following urea hydrolysis. Texture and cation exchange capacity (CEC) control the retention of ammonium through adsorption processes. Soils with high pH and low CEC tend to exhibit higher volatilization losses due to reduced capacity to retain NH₄⁺ away from the soil–air interface [8]. These properties explain why mitigation performance is highly site-dependent and why uniform emission factors are often unreliable.

Management practices further modify volatilization dynamics by influencing the spatial distribution of nitrogen and its exposure to the atmosphere. Fertilizer placement is particularly important, as surface application creates direct pathways for NH₃ loss, whereas incorporation increases diffusion distance and enhances ammonium retention. Field studies show that urease inhibitors are generally more effective under surface application conditions, where volatilization risk is highest [21]. Incorporation reduces emissions even without additives and may also limit the additional benefit of some mitigation technologies.

These interacting controls lead to the high variability observed in ammonia losses. Volatilization is governed by a nonlinear system in which small changes in pH, temperature, or placement can significantly alter emission outcomes. Global syntheses indicate that ammonia volatilization represents a substantial share of total nitrogen loss, with average losses from urea around 18%, but with reported values ranging from less than 1% to over 60% depending on conditions [5,22].

From a mitigation perspective, these mechanisms can be grouped into a limited number of intervention points. One approach is to slow urea hydrolysis, reducing the formation of ammonium and delaying the development of emission-prone conditions. Urease inhibitors operate through this mechanism, allowing urea to move into the soil before hydrolysis occurs and thereby reducing volatilization [6,23]. A second intervention targets pH at the fertilizer microsite, shifting the equilibrium toward NH₄⁺ and lowering NH₃ formation. Acidifying amendments, including sulfur-based materials, are particularly effective in alkaline soils [24].

A third strategy focuses on ammonium retention through adsorption and cation exchange. Materials such as zeolites and clays increase the residence time of NH₄⁺ in the soil, reducing its availability for volatilization [25]. This mechanism is especially relevant in soils with low CEC or under surface application conditions [26]. A fourth intervention involves controlling nitrogen release dynamics. Controlled-release fertilizers reduce peak ammonium concentrations and distribute nitrogen availability over time, lowering volatilization potential and improving synchronization with crop uptake [27].

The effectiveness of these strategies depends on how well they align with local conditions. Variability in soil properties, weather, and management explains the wide range of reported mitigation outcomes and should be interpreted as a predictable response rather than inconsistency in the literature. This reinforces the need for site-specific approaches tailored to dominant loss drivers, such as high pH soils, warm and windy conditions, or surface fertilizer application in residue-covered systems [28].

Ammonia volatilization must also be considered within the broader nitrogen cycle. Emitted ammonia contributes to atmospheric reactions leading to particulate matter formation, linking fertilizer management to air quality impacts [29]. At the same time, retaining nitrogen in the soil may increase the substrate available for nitrification and denitrification, influencing nitrous oxide emissions. This creates potential trade-offs between nitrogen loss pathways, where reducing NH₃ emissions may increase N₂O emissions or vice versa [30,31]. As a result, mitigation strategies are increasingly evaluated in terms of their multi-pathway effects rather than single-emission reductions.

3. Fertilizer-Based Strategies for Reducing NH₃ Emissions

Fertilizer amendment technologies designed to reduce ammonia volatilization operate by modifying, during the critical post-application period, one or more of the processes governing NH₃ formation and transfer in soils. These include the rate of urea hydrolysis, the acid–base conditions within the fertilizer microsite, ammonium retention through adsorption or cation exchange, and the kinetics of nitrogen release and exposure at the soil surface. The effectiveness of each approach depends on its ability to sustain these modifications under field conditions, which is influenced by formulation properties, environmental conditions, and fertilizer placement.

Figure 2 provides a conceptual classification of fertilizer amendment strategies based on their dominant mechanisms of action. These range from chemical inhibition and controlled-release systems to mineral, organic, and biological approaches. The framework emphasizes that mitigation efficiency is determined not only by the strength of the mechanism but also by its persistence and alignment with the temporal window of maximum emission risk. Accordingly, the following sections discuss amendment technologies in relation to their dominant mechanisms, while maintaining conventional functional categories for comparison across agronomic contexts.

3.1. Urease Inhibitors

Urease inhibitors are the most extensively studied fertilizer amendment for reducing ammonia volatilization from urea-based fertilization. Their effectiveness is supported by a substantial body of field data, allowing robust quantification of mitigation performance across a wide range of soils, climates, and management conditions. A large-scale synthesis of field experiments across 14 countries, including 48 studies and 256 comparisons, reported average reductions in NH₃ volatilization of 61% for N-(n-butyl) thiophosphoric triamide (NBPT) (95% CI: 57–64; n = 165), 70% for N-(2-nitrophenyl) phosphoric triamide (2-NPT) (95% CI: 63–76; n = 19), and 75% for combined NBPT + N-(n-propyl) thiophosphoric triamide (NPPT) formulations (95% CI: 58–82; n = 32), while methyl-phosphoric acid tri-isopropyl ester (MIP) showed no consistent mitigation effect at field scale (0.3%, 95% CI: −8 to 9; n = 40) [6].

The performance of urease inhibitors is governed primarily by their persistence during the period of highest volatilization risk. Rather than inhibition intensity alone, the duration of delayed hydrolysis determines cumulative NH₃ reduction. Field evidence indicates that the effective half-life of NBPT varies widely depending on soil pH, texture, and moisture, leading to substantial variability in mitigation outcomes. When inhibitor persistence overlaps with the peak emission window, significant reductions are achieved; when degradation is rapid, mitigation benefits are limited.

NBPT remains the reference compound within this class and has been widely tested across cropping systems [31]. Field studies demonstrate strong interactions with soil properties. For example, reductions of 32.3% in sandy loam and 71.4% in silty clay soils have been reported under comparable nitrogen rates, reflecting the influence of texture and cation exchange capacity on ammonium retention [32,33]. Higher mitigation efficiencies have been observed when urease inhibitors are combined with other amendments. The joint application of NBPT and dicyandiamide has been reported to reduce ammonia volatilization by up to 90%, illustrating the potential of multi-mechanism approaches under high-risk conditions [34].

Alternative urease inhibitors show variable performance. Benzoylthiourea- and benzimidazole-based compounds have been associated with reductions of approximately 10% and 22%, respectively, while commercial formulations such as Limus^® can achieve reductions of 55–60% under favorable conditions [35]. These differences reflect variations in formulation stability and soil interactions rather than differences in mode of action. Yield responses are generally modest but positive, typically in the range of 4–6%, consistent with improved nitrogen retention rather than direct stimulation of plant growth.

Urease inhibitors provide a practical and scalable option to reduce ammonia volatilization from urea-based fertilization, particularly under conditions of surface application, elevated temperatures, and limited incorporation. However, their performance in practice depends strongly on site-specific factors that control inhibitor persistence and the timing of emission processes.

3.2. Nitrification Inhibitors and Combined Strategies

Nitrification inhibitors represent a second major category of fertilizer amendments, although their direct impact on ammonia volatilization is generally limited and highly dependent on context. Global meta-analyses indicate that compounds such as dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) reduce nitrous oxide emissions by approximately 40–50%, but do not produce consistent reductions in NH₃ volatilization under field conditions [36]. In some controlled studies, increased ammonium residence time has been associated with higher NH₃ emissions; however, this effect is typically moderated in field environments by adsorption, plant uptake, and transport constraints.

A comprehensive synthesis of 366 observations from 149 studies confirmed that while DCD and DMPP significantly reduce N₂O emissions (47% and 39%, respectively), their overall effect on NH₃ volatilization is not statistically significant under field conditions [37]. Increases in NH₃ emissions observed in laboratory or incubation studies are not consistently reproduced at field scale. Field experiments further highlight the importance of environmental context. Studies under varying soil textures and moisture regimes show that the impact of nitrification inhibitors on ammonia volatilization is strongly influenced by water availability and soil physical properties. Similarly, incubation experiments demonstrate that interactions between inhibitors, soil pH, and organic matter can lead to divergent outcomes, including cases where NH₃ volatilization increases in acidic soils when inhibitor combinations are applied [38].

The primary role of nitrification inhibitors in ammonia management therefore lies in integrated strategies. When combined with urease inhibitors, they help prevent the redistribution of retained nitrogen into other loss pathways. Dual-inhibitor systems have been reported to reduce NH₃ emissions by 45–55% and N₂O emissions by 35–45% compared with untreated urea, providing more balanced control of reactive nitrogen losses [39]. These combinations may also reduce nitrate leaching and improve nitrogen uptake under favorable conditions.

The performance of nitrification inhibitors varies across soils and cropping systems. Evidence suggests that DMPP may be more effective than DCD in reducing nitrate leaching and N₂O emissions in some environments, although both compounds can be associated with increased NH₃ emissions under specific conditions, such as high soil moisture, elevated pH, or interactions with amendments like biochar [40]. These findings reinforce that nitrification inhibitors are not direct NH₃ mitigation tools, but components of broader nitrogen management strategies that integrate multiple processes to reduce overall nitrogen losses.

3.3. Fertilizers Designed for Controlled Nitrogen Release

Controlled-release and coated fertilizers, typically classified as enhanced-efficiency fertilizers (EEF), reduce ammonia volatilization by limiting the intensity and duration of ammoniacal nitrogen exposure at the soil surface. Their primary mechanism is kinetic, as they decrease peak ammonium concentrations during the early post-application period and distribute nitrogen release over time [27]. Reported average reductions in NH₃ volatilization reach 78.4% for sulfur-coated urea, 82.7% for thermoplastic resin-coated urea, and 69.4% for polyolefin-coated urea [13]. Field data compiled in the same analysis show reductions of 60.7–68.8% in maize systems and up to 77.7–83.1% in vegetable cropping systems, although values range widely depending on coating performance, soil moisture, and crop type.

Recent field studies confirm that mitigation is largely driven by suppression of the initial emission peak. In a two-year experiment, cumulative NH₃ volatilization was reduced by 80.8% with controlled-release fertilizer and by 53.2% with sulfur-coated fertilizer relative to conventional urea, alongside a clear reduction in peak emissions [41]. These results illustrate that effective mitigation depends on maintaining controlled nitrogen release under field moisture conditions during the critical volatilization window.

Advances in coating materials are moving toward engineered systems that improve resistance to physical damage, regulate water diffusion, and maintain stable release under fluctuating wet–dry cycles. Silicone-modified polymer coatings, for example, have shown reductions in ammonia volatilization of approximately 76% relative to uncoated urea, demonstrating that optimized release control can approach the performance of highly effective urease inhibitors [42]. In addition, combining release control with complementary mechanisms has shown potential to reduce sensitivity to site conditions. For instance, polymer-coated and gypsum–sulfur-coated fertilizers have been reported to reduce NH₃ volatilization, with improved performance when combined with urease inhibition functionality [43]. These findings indicate that coatings can act not only as physical barriers but also as platforms for integrating multiple mitigation functions.

Environmental considerations are increasingly relevant in the development of coated fertilizers. Studies have identified the generation of microplastics from polymer coatings as a potential concern, with evidence of biological effects in soil–plant systems [44]. Additional work has shown that such materials may adsorb heavy metals, with reported capacities of up to 54.64 mg g⁻¹ for Cd(II) and 30.77 mg g⁻¹ for Pb(II), suggesting possible secondary environmental interactions [45]. These concerns are driving the development of biodegradable coatings that maintain performance while reducing long-term persistence in soils.

3.4. Inorganic and Mineral Additives for Ammonia Mitigation

Mineral and inorganic amendments reduce ammonia volatilization primarily by enhancing ammonium retention and moderating the transient increase in pH that follows urea hydrolysis. Adsorptive materials increase the fraction of nitrogen held as exchangeable NH₄⁺, while acidifying components shift the equilibrium away from gaseous NH₃. In laboratory conditions simulating surface application, nano-clinoptilolite mixed with urea reduced cumulative NH₃ losses by 8.57–37.13% [46]. Under field conditions, stronger effects are observed when retention is combined with pH control. For example, gypsum-based treatments reduced volatilization by 58% relative to urea, while sulfur-coated urea achieved reductions of 42–74% across seasons [47].

Field studies in alkaline calcareous soils provide further insight into the magnitude and agronomic implications of these effects. In an irrigated maize system (soil pH 8.23), gypsum application reduced NH₃ losses by 58% at 150 kg N ha⁻¹ and 42% at 200 kg N ha⁻¹, while sulfur-coated urea achieved reductions of 74% and 65%, respectively [47]. These reductions were accompanied by increases in grain yield (4–17% for urea plus sulfur and 25–28% for sulfur-coated urea) and higher nitrogen uptake (7–13% and 26–31%, respectively), reflecting improved nitrogen retention. Nitrogen use efficiency also increased substantially, with reported improvements of up to 72% under sulfur-coated urea relative to conventional fertilization at higher nitrogen rates [48].

For mineral sorbents, field-scale evidence shows that increasing ammonium retention can progressively reduce volatilization losses. In a two-year paddy field study, replacing urea with nano-clinoptilolite-based fertilizer resulted in NH₃ loss reductions of 8.6%, 20.6%, 30.3%, and 37.1% as substitution rates increased from 20% to 50% [47]. This was accompanied by reductions in nitrogen runoff (23.3–43.8%) and increases in crop yield (5.1–15.3%), indicating that retention mechanisms can contribute to broader nitrogen loss control. However, diminishing returns were observed at higher substitution levels, suggesting practical limits for this approach.

These results indicate that mineral amendments act through two main pathways: acidification of the fertilizer microsite, which reduces NH₃ formation, and enhancement of ammonium retention, which limits its availability for volatilization. The strongest and most consistent mitigation is achieved when both mechanisms operate simultaneously within the same soil–fertilizer environment, particularly under conditions of surface application and high volatilization risk [48].

3.5. Waste-Derived Organic Amendments

Organic amendments derived from waste streams, particularly biochar and hydrochar, show a wide range of effects on ammonia volatilization because their properties vary significantly depending on feedstock and processing conditions. Materials grouped under the same category can differ in pH, ash content, and surface functionality, which determines whether they suppress or enhance the NH₄⁺→NH₃ shift during the post-application peak.

Field evidence illustrates this variability. In a two-year rice experiment, nitrogen-enriched biochar applied at 10–20 t·ha⁻¹ and combined with 75% of the conventional urea rate reduced cumulative NH₃ volatilization by 13.3–21.0% while maintaining yield stability [49]. However, the same study reported that conventional biochar may increase NH₃ emissions in paddy systems, highlighting the importance of controlling alkalinity and nitrogen availability.

Long-term studies reinforce this dual behavior. A 12-year field experiment showed that repeated biochar applications can increase soil pH and, under certain conditions, enhance NH₃ volatilization and reactive nitrogen losses, potentially reducing nitrogen availability and crop performance [50]. These findings indicate that the effect of biochar may evolve over time rather than remaining constant.

Recent work has focused on engineered or nutrient-modified carbon materials designed to avoid these limitations. Enriched rice husk biochar, for example, has been associated with reduced NH₃ losses and improved nutrient uptake, with reported increases exceeding 80% for N, P, and K uptake in some treatments [51]. This suggests that controlling amendment composition can significantly influence performance.

Hydrochar, produced by hydrothermal carbonization, is increasingly treated as a distinct category due to its lower ash alkalinity and different functional groups. Comparative studies show that hydrochar and pyrochar can influence NH₃ volatilization differently, particularly in flooded or saline systems where dissolved organic matter and pH dynamics are critical [52].

Interactions with other amendments can further modify outcomes. Experimental evidence indicates that combining biochar with nitrification inhibitors such as DMPP can increase NH₃ volatilization under certain moisture conditions, due to enhanced ammonium accumulation near the soil surface [53]. This demonstrates that combining mitigation strategies does not necessarily produce additive benefits and may shift nitrogen losses toward volatilization if interactions are not carefully managed.

To contextualize these materials within their technological origin, Table 2 compiles representative conversion pathways for char production from secondary raw materials, including pyrolysis, torrefaction, and hydrothermal carbonization systems. The table highlights how feedstock type and processing conditions determine key properties such as ash content and surface chemistry, which ultimately control the behavior of these amendments in relation to NH₃ volatilization.

3.6. Biological and Surface-Mediated Approaches

Biological amendments, including microbial inoculants and algae-based systems, can reduce ammonia volatilization by modifying nitrogen transformations or altering conditions at the soil or water interface. Reported average reductions of approximately 50% have been described for biofertilizers in compiled studies [13].

Microbial inoculants such as Bacillus amyloliquefaciens have shown measurable mitigation potential under alkaline conditions. Field studies report reductions in NH₃ volatilization of up to 68%, together with increases in crop yield and nitrogen recovery of around 19% [54]. These effects are associated with reduced urease activity and enhanced ammonia oxidation, indicating a shift away from the volatilization-prone ammonium pool. Other studies report more moderate reductions of around 20%, accompanied by decreases in soil pH during the emission peak and changes in microbial activity consistent with faster NH₄⁺ turnover [55].

In flooded systems, strategies that act directly on the water–air interface are particularly effective. Floating plant covers such as duckweed (Lemna minor) reduce NH₃ volatilization by lowering floodwater pH, temperature, and total ammoniacal nitrogen concentration. Field experiments report reductions of 20.0–53.7% at moderate nitrogen rates and 19.0–33.2% at higher rates, together with yield increases of approximately 9–10% [56].

Azolla-based systems show similar effects when combined with optimized nitrogen management. Reductions in NH₃ volatilization of 50.3–66.9% have been reported relative to full nitrogen application, along with improved nitrogen recovery efficiency [57]. These results indicate that biological surface strategies are most effective when they directly modify the key drivers of volatilization in flooded environments, particularly floodwater pH and ammoniacal nitrogen concentration.

A related group of approaches involves algae-based systems for nutrient recovery from wastewater, producing biomass that can be used as biofertilizer. These systems are currently more developed in terms of nutrient capture than standardized fertilizer production, and their contribution to NH₃ mitigation depends on biomass characteristics and application conditions. As such, they are best interpreted as enabling platforms whose effectiveness is linked to downstream use and system integration. Table 3 summarizes representative algae-based systems, including feedstocks, cultivation modes, and product pathways, providing an overview of their current technological maturity and application scope.