Climate-Specific Ammonia Mitigation Strategies for Humid Tropical Regions in Poultry Production: A Systematic Review and Critical Synthesis

Ammonia (NH₃) emissions from intensive poultry production have emerged as a primary concern across environmental, veterinary, and occupational health disciplines. Agricultural activities account for approximately 88% of global anthropogenic NH₃ emissions (European Commission, Joint Research Centre, 2022), with chicken farming alone contributing an estimated 5.5 ± 1.2 Tg N yr⁻¹, representing approximately 13% of agriculture-derived NH₃ emissions globally (Jiang et al., 2021). These emissions exert significant downstream consequences including ecosystem acidification, eutrophication of water bodies, formation of atmospheric particulate matter, and direct adverse effects on animal welfare and human health (Lahav et al., 2008;Vilela et al., 2020).

The challenge of NH₃ mitigation is substantially compounded in humid tropical regions. The AMCLIM-Poultry model- a process-based framework incorporating meteorological variables-demonstrates that the fraction of excreted nitrogen emitted as NH₃ can be up to three times greater in humid tropical housed systems compared to cold or dry climate counterparts (Jiang et al., 2021).

High ambient temperatures accelerate the decomposition of uric acid to total ammoniacal nitrogen (TAN), while elevated relative humidity reduces the evaporation rate of moisture from litter, maintaining conditions that favor ammonia volatilization (Vilela et al., 2020; Sousa et al., 2023). These biophysical dynamics interact with the predominantly open-sided or curtain-sided housing typologies prevalent in tropical poultry systems, which complicate both emission quantification and control (Osorio Saraz et al., 2013).

Global poultry production has intensified substantially over recent decades (Sarker et al., 2026), particularly within the humid tropics. Brazil, the world’s largest broiler meat exporter, exemplifies this intensification, with open-sided housing designs now accommodating densities of 10–16 birds m⁻² (Oliveira et al., 2021). Nations across Southeast Asia, sub-Saharan Africa, and Latin America increasingly adopt intensive production models, yet management protocols largely mirror those developed for temperate, closed-housing systems, which are fundamentally ill-suited for humid tropical conditions (Sousa et al., 2018; Nwagwu et al., 2010).

NH₃ concentrations exceeding 25 ppm within poultry houses are associated with significant reductions in growth performance, feed conversion efficiency, egg production, respiratory health, and immune competence (Bist et al., 2023; Patterson and Adrizal, 2005). At the environmental level, deposited NH₃-derived nitrogen contributes to biodiversity loss in sensitive ecosystems and participates in secondary aerosol formation affecting regional air quality (Nurhidayah et al., 2024). Despite the documented severity of these impacts, the scientific literature addressing NH₃ mitigation in humid tropical poultry contexts remains fragmented, methodologically heterogeneous, and limited in its translation to practical management frameworks.

This systematic review addresses this gap by synthesizing current evidence on climate-specific NH₃ mitigation strategies for humid tropical poultry production systems. It evaluates the efficacy and adaptability of litter amendments, ventilation engineering approaches, measurement methodologies, and predictive modelling tools under tropical climatic conditions. By integrating empirical findings with theoretical frameworks, this review aims to inform the development of standardized, climate-adapted mitigation protocols with direct applicability to tropical poultry producers, policymakers, and researchers.

A systematic literature search was conducted following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. The primary research question- “Climate-specific ammonia mitigation strategies for humid tropical regions in poultry production”- was expanded into multiple targeted queries to ensure comprehensive coverage across disciplinary domains. Searches were conducted across Web of Science, Scopus, PubMed, Google Scholar, and CAB Abstracts from January 2005 through May 2026.

The transformed search queries encompassed: (1) climate-specific NH₃ mitigation in tropical poultry facilities; (2) innovative ammonia reduction technologies for humid tropical climates; (3) litter treatment and management techniques for tropical poultry; (4) alternative litter materials and their ammonia emission profiles; and (5) ventilation design and environmental control in open tropical poultry housing. The initial database search retrieved 101 candidate papers meeting pre-defined keyword criteria.

Studies were eligible for inclusion if they: (1) reported original empirical data or validated modelling approaches relating to NH₃ emissions or mitigation in poultry systems; (2) involved tropical, subtropical, or humidity-relevant climatic conditions; (3) were published in peer-reviewed journals or proceedings; and (4) were available in English, Portuguese, or Spanish with accessible abstracts. Studies exclusively pertaining to temperate, closed-housing systems without climate-specific adaptation were excluded, as were opinion pieces and grey literature without methodological transparency.

Backward citation chaining was applied to the core paper set to identify foundational studies informing current evidence. Forward citation chaining traced newer works that cited core papers, capturing methodological advances and emerging debates. This process identified an additional 124 papers, yielding a total pool of 225 candidate papers. Each paper was subjected to a relevance scoring protocol incorporating thematic alignment, methodological rigor, climate-specificity, and recency. Fifty papers were classified as highly relevant and form the analytical core of this review.

To address methodological transparency, each candidate paper was evaluated against a four-criterion relevance scoring protocol, with each criterion scored on a 0–3 scale (0 = absent/not applicable, 1 = weak, 2 = moderate, 3 = strong): (1) thematic alignment with climate-specific NH₃ mitigation; (2) methodological rigor, assessed using an adapted checklist drawn from the JBI (Joanna Briggs Institute) Critical Appraisal Checklists for quasi-experimental and cross-sectional studies, covering clarity of study aims, adequacy of experimental controls, appropriateness of NH₃ measurement methods, and statistical reporting; (3) climate-specificity, reflecting explicit tropical, subtropical, or humidity-relevant conditions; and (4) recency, favouring studies published within the preceding ten years while retaining seminal older works. Papers scoring ≥9 of a possible 12 points were classified as highly relevant and retained for detailed synthesis; the fifty studies meeting this threshold form the analytical core of this review. Two reviewers independently scored a 20% random subsample to check scoring consistency (concordance = 88%), with disagreements resolved by discussion.

Given the substantial heterogeneity in study designs (laboratory incubation, field trials, and modelling investigations), NH₃ measurement methodologies (photoacoustic spectroscopy, colorimetric methods, and CFD-derived estimates), and outcome metrics (percentage emission reduction, absolute concentration, emission factor) across the 50 included studies, a formal statistical meta-analysis with pooled effect-size estimation was judged not feasible or appropriate. This narrative synthesis approach was therefore a deliberate, a priori methodological decision rather than an ad hoc omission, consistent with recommended practice for reviews spanning highly heterogeneous experimental and observational designs. Where sufficient homogeneity existed within a mitigation category (e.g., alum-based amendments), reported effect ranges are nonetheless presented comparatively in Table 1 Table 2 Table 3 to support semi-quantitative cross-study comparison in lieu of formal statistical pooling (Figure 1).

Figure 1 Chronological distribution of peer-reviewed publications on NH₃ mitigation in humid tropical poultry production systems (2005–2025). Bar heights indicate annual publication counts; the overlaid line represents the cumulative total. Research phases are annotated to reflect distinct periods of disciplinary focus.

The 50 highly relevant studies span experimental trials (n = 32), modelling investigations (n = 10), and systematic or narrative reviews (n = 8). Geographic distribution favored Brazil (n = 18), Nigeria (n = 5), Sri Lanka (n = 4), Turkey (n = 2), and the United States (n = 7), with the remaining studies drawing from Southeast Asian, European, and multi-regional contexts. This geographic concentration reflects the intensification of tropical poultry production in Latin America and West Africa but indicates a relative

under-representation of South and Southeast Asian systems. Only four of the fifty highly relevant studies originated from South or Southeast Asia (all from Sri Lanka), despite these regions collectively housing some of the largest and fastest-growing poultry industries operating under humid tropical conditions; this imbalance is carried forward as an explicit limitation and priority research need throughout this review.

Thematically, the literature clusters around eight major domains: litter materials and amendments (18/50 papers), climate-specific emission dynamics and modelling (14/50), ventilation and environmental control (12/50), measurement and monitoring methodologies (11/50), NH₃ capture technologies (6/50), poultry welfare and productivity impacts (8/50), litter moisture and pH dynamics (9/50), and regional operational variability (7/50). Figure 5 illustrates this thematic distribution, underscoring the dominant focus on litter management as the primary mitigation lever (Figure 2).

Figure 2 Thematic distribution of the 50 highly relevant studies across major research domains in humid tropical poultry NH₃ mitigation. Values indicate the number of papers addressing each theme.

NH₃ generation in poultry litter proceeds through

Table 1: Comparative performance of litter amendments for NH₃ Mitigation in humid tropical poultry systems

Table 2: Alternative litter substrates evaluated in humid tropical poultry production

the microbial decomposition of uric acid-the primary nitrogenous excretion product of poultry- via urease-catalyzed hydrolysis to TAN, followed by pH- and temperature-dependent partitioning into gaseous NH₃ (Vilela et al., 2020). In humid tropical environments, this biochemical cascade is substantially accelerated. The AMCLIM-Poultry model, calibrated against global meteorological datasets, quantifies NH₃ emission fractions from housed chicken systems across climate zones, revealing that emission rates in humid tropical regions can be up to three times greater than those in temperate or cold climates (Jiang et al., 2021).

Figure 3 Estimated relative NH₃ emission factor as a function of air temperature and relative humidity in poultry litter systems. Values represent relative emission intensities normalized to baseline cool-dry conditions (20°C, 40% RH = 0.6). The dashed white rectangle demarcates the typical humid tropical operating range. Data are derived from the AMCLIM-Poultry model framework (Jiang et al., 2021) and empirical field data (Sousa et al., 2023; Nwagwu et al., 2010).

For manure land-spreading operations in semi-dry tropical zones, this multiplication factor extends to

Table 3: Ventilation & Environmental control strategies: NH₃ Reduction effectiveness

fivefold relative to cold, wet climates. Field studies corroborate these modelling projections. Nwagwu et al. (2010) documented NH₃ concentrations exceeding safe thresholds in Nigerian tropical poultry pens during the rainy season, attributing peak concentrations to synergistic interactions between elevated ambient temperature, high relative humidity, and inadequate natural ventilation. In Brazilian broiler facilities, Sousa et al. (2023) validated predictive models demonstrating that NH₃ generation potential from wood shaving and coffee husk litter increases non-linearly with temperature between 25-40°C.

The interactive effects of temperature and relative humidity on NH₃ volatilization are illustrated in Figure 6. Under conditions typical of humid tropical poultry facilities-ambient temperatures of 28-38°C and relative humidity of 70–90%-estimated NH₃ emission factors are 1.8 to 3.5 times greater than baseline temperate conditions, underscoring the climate specificity of mitigation requirements (Figure 3, 4).

Figure 4 Climate-dependent relative NH₃ volatilization rates in poultry production systems based on the AMCLIM-Poultry model (Jiang et al., 2021). All values are normalized to the cold dry climate baseline (1.0×). The annotation highlights the threefold amplification characteristic of humid tropical housed systems relative to baseline conditions.

Agriculture accounts for an estimated 88% of global anthropogenic NH₃ emissions (European Commission, Joint Research Centre, 2022), with agricultural emissions increasing by approximately 78% between 1980 and 2018 (Liu et al., 2022). Within the agricultural sector, poultry farming contributes approximately 13% of agriculture-derived NH₃ emissions globally, estimated at 5.5 ± 1.2 Tg N yr⁻¹ based on 2010 data (Jiang et al., 2021). Both pigs and poultry systems individually account for more than 15% of estimated total livestock emissions (Jiang et al., 2024). These figures contextualize the environmental significance of NH₃ mitigation in tropical poultry systems, where expanding production intensification is projected to increase emission burdens substantially in coming decades (Beaudor et al., 2025) (Figure 5).

Figure 5 Global ammonia emission source distribution. Panel (A) presents the proportional contribution of major source categories to total global NH₃ emissions (European Commission, Joint Research Centre, 2022). Panel (B) disaggregates agricultural emissions by livestock species, highlighting the 13% contribution of poultry farming (Jiang et al., 2021).

Aluminium sulphate (alum) is the most extensively investigated litter amendment for NH₃ mitigation. Its mechanism of action involves acidification of litter pH, which shifts the ammonium-ammonia equilibrium toward the ionized, non-volatile ammonium form (NH₄⁺), thereby suppressing gaseous NH₃ release (Choi and Moore, 2008).

Laboratory studies report NH₃ volatilization reductions of 77–96% relative to untreated litter when alum is applied at recommended rates (Choi and Moore, 2008). Field trials corroborate substantial efficacy; Eid et al. (2022) demonstrated significant reductions

in litter NH₃ concentrations and improvements in laying hen productive performance following alum application.

The economic accessibility of alum has been constrained by rising procurement costs, motivating investigation of alternative formulations. Anderson et al. (2020, 2021) evaluated alum mud litter amendment (AMLA)-a by-product of aluminium ore processing combining alum mud, bauxite, and sulphuric acid-and reported NH₃ emission reductions of 27–52%, comparable to standard alum applied at equivalent rates. Crucially, AMLA demonstrated no adverse effects on broiler production performance and showed favorable litter chemistry outcomes, presenting a cost-competitive alternative for tropical poultry producers.

Biochar has attracted considerable research attention as a sustainable, multifunctional litter amendment. Its high surface area, cation exchange capacity, and surface acidity provide multiple mechanisms for NH₃ adsorption and retention within litter matrices (Pereira et al., 2024). During broiler housing, Pereira et al. (2024) documented NH₃ emission reductions of 40–60% with biochar application, while simultaneously reducing global warming potential emissions by approximately 31%. Notably, biochar demonstrated comparable efficacy to alum (60% vs. 58% reduction respectively), without the environmental trade-offs associated with acid-based amendments.

Graves et al. (2024) investigated engineered Miscanthus biochar as a broiler litter amendment, reporting approximately 20% reduction in NH₃ volatilization. Production parameters including pyrolysis temperature and feedstock composition were identified as critical determinants of biochar efficacy, with higher pyrolysis temperatures generally producing biochar with superior ammonia adsorption capacity. The potential for biochar to be produced locally from agricultural residues-including rice husks and crop waste abundant in tropical regions-positions it as a particularly promising, cost-effective amendment for low-resource tropical poultry systems (Table 1).

(FCR = Feed Conversion Ratio. GHG trade-off refers to the effect of the amendment or treated litter on greenhouse gas emissions during storage and land application. Cost is indicative and regionally context-dependent.)

Zeolites are microporous aluminosilicate minerals with high cation exchange capacity and surface affinity for ammonium ions, functioning as NH₃ absorbers within litter matrices. In a field trial conducted in humid tropical Nigeria, zeolite litter treatment significantly reduced NH₃ emissions and microbial loads while enhancing broiler performance indicators including body weight and feed conversion ratio (Ezenwosu et al., 2022). Mohammed and ElBably (2020) demonstrated that clinoptilolite zeolite at 10–20 g kg⁻¹ effectively reduced NH₃ concentrations and pathogenic microbial loads in duck litter managed under tropical conditions.

Lara (2023) documented zeolite’s ability to reduce NH₃ volatilization and alter litter pH and moisture in avian production systems, with timing and dosage identified as critical efficacy determinants. Recent comparative analyses using biochar (13–17% w/w) and zeolite (8–11% w/w) indicate NH₃ reductions of 41-46% and 20–33%, respectively, confirming biochar’s superior efficacy while acknowledging zeolite’s advantage in avoiding greenhouse gas co-benefits (Bist et al., 2023).

Conventional litter substrates such as wood shavings may be economically inaccessible or ecologically unsustainable in many tropical regions, motivating investigation of locally abundant alternatives. Linhares et al. (2022) monitored NH₃ concentrations from coir-husk litter in Brazilian poultry houses using diode laser photoacoustic spectroscopy, demonstrating lower NH₃ emissions relative to wood shavings while offering environmental sustainability benefits through utilization of coconut processing waste.

Thennakoon et al. (2024) evaluated Salvinia-an aquatic fern prevalent in tropical water bodies-as a broiler litter material, reporting lower NH₃ emission rates compared to paddy husk while maintaining comparable growth performance and reducing hock burn incidence. Carvalho et al. (2025) investigated quartzite tailings and kamafugite powders as litter additives under closed tropical broiler conditions, demonstrating moderate NH₃ reductions while preserving favorable litter pH and humidity profiles. Tan et al. (2019) demonstrated that mixed litter

formulations incorporating tea leaves outperformed single-substrate litters in NH₃ inhibition, particularly under varying moisture levels. These findings collectively support local waste valorization as an economically viable strategy for NH₃ reduction in tropical poultry systems (Table 2) (Figure 6).

(NH₃ performance is relative to conventional wood shavings under comparable tropical conditions. ‘Context-dependent’ indicates outcomes varied significantly by season, stocking density, or measurement method.)

Figure 6 Comparative NH₃ emission reduction efficiency of principal litter amendments tested in poultry production systems. Bars represent mean reduction percentages with error bars indicating the observed range across studies. The vertical dotted line marks the 25-ppm critical threshold above which adverse effects on bird performance and welfare are documented. Sources: Choi and Moore (2008); Pereira et al. (2024); Anderson et al. (2020, 2021); Bist et al. (2023); Eid et al. (2022); Atapattu et al. (2017); Graves et al. (2024).

Ventilation and environmental control strategies

Most poultry houses across humid tropical regions employ open-sided or curtain-sided designs with natural ventilation, relying on prevailing wind patterns and stack effects for air exchange. While this approach minimizes capital costs, it introduces substantial variability in indoor airflow patterns, creating localized zones of NH₃ accumulation near litter surfaces and bird breathing zones (Osorio Saraz et al., 2013;Manno et al., 2011). Jongbo (2020) documented inadequate air velocity for cooling in battery-caged tropical poultry pens, attributing environmental NH₃ accumulation to insufficient air exchange, and recommended computational fluid dynamics (CFD) analysis for airflow optimization.

Polat (2015) demonstrated that building design parameters-including orientation, eave height, inlet configuration, and curtain positioning-exert dominant influence over indoor air quality in humid climates. Natural ventilation efficacy was shown to vary seasonally, with NH₃ concentrations elevated during wet-season low-wind-speed periods corresponding to peak ambient humidity. These findings highlight the necessity of climate-responsive building design rather than static standardized configurations.

Mechanical ventilation systems offer superior control over indoor microclimatic conditions but require higher capital and operational investment. Manno et al. (2011) demonstrated that tropical broiler facilities with higher-technology ventilation systems paradoxically recorded elevated NH₃ concentrations due to higher stocking densities facilitated by environmental control capability, highlighting the interaction between ventilation technology and management decisions.

Gfeller et al. (2024) reported the most dramatic ventilation-based mitigation documented in the reviewed literature: floor heating combined with heat exchangers in tropical broiler housing reduced indoor NH₃ by a factor of approximately nine, attributing this to maintenance of drier litter conditions through reduced litter moisture accumulation.

Shah et al. (2022) evaluated citric acid-treated burlap exhaust filters applied to broiler house ventilation systems, achieving NH₃ reductions of 17–42% while simultaneously reducing thermal stratification. The study reported filter clogging as a practical limitation, partially addressed by mechanical shakers. Imam et al. (2022) designed an exhaust fan system incorporating activated charcoal adsorption in closed tropical broiler houses, maintaining NH₃ concentrations within 25-30 ppm-below the critical welfare threshold-under operational conditions.

(Estimated NH₃ reduction effectiveness for various ventilation and environmental control strategies in tropical broiler production. The floor heating + heat exchanger system (Gfeller et al., 2024) represents the highest single-strategy reduction but requires significant capital investment.)

Mou et al. (2024) conducted a field investigation in commercial broiler houses examining the effects of maintaining moderate relative humidity (50–60%) with enhanced air movement over litter surfaces. Results demonstrated that this ventilation strategy effectively controlled litter moisture content and significantly reduced NH₃-related footpad dermatitis, providing practical evidence for humidity-targeted ventilation management in tropical systems without requiring high-capital mechanical solutions.

CFD modelling has provided valuable theoretical foundations for airflow optimization in tropical poultry facilities. Saraz et al. (2015) applied CFD simulation to predict NH₃ concentration profiles and emission fluxes in naturally ventilated open houses, demonstrating the utility of these models for building design optimization and ventilation management. The model revealed that strategic inlet and baffle configuration could substantially reduce NH₃ hotspots near litter surfaces, offering practical guidance for building design in tropical contexts.

Beyond passive litter amendments and ventilation, several active NH₃ capture systems have been investigated. Lahav et al. (2008) pioneered a dedicated air capturing system incorporating an acidic bubble column reactor positioned proximal to litter surfaces within tropical broiler houses, achieving near-100% NH₃ absorption efficiency. The system operates independently of building ventilation, capturing NH₃-laden air from the bird microenvironment and converting it to ammonium sulphate, a marketable nitrogen fertilizer. While pilot-scale results were highly promising, full economic evaluation remains pending.

Lee et al. (2020) investigated UV-A light combined with photocatalytic treatment under simulated poultry barn conditions, achieving NH₃ reductions of 2.6–18.7% depending on humidity levels and catalyst configurations. Humidity was identified as a critical modulator of photocatalytic efficacy, with higher relative humidity reducing treatment efficiency, a significant practical constraint for humid tropical applications. Scale-up and economic evaluation remain prerequisite for field implementation (Table 3).

The AMCLIM-Poultry model represents the most comprehensive climate-dependent NH₃ emission model developed for global chicken farming (Jiang et al., 2021). Applied at 0.5° global resolution using FAO livestock density data and ECMWF ERA5 meteorological inputs, the model simulates uric acid decomposition, TAN formation, and NH₃ partitioning at hourly to daily resolution. Key outputs include the demonstration that ambient temperature and relative humidity are dominant drivers of emission variability across climate zones, with humid tropical regions exhibiting emission fractions far exceeding global average values. The model’s climate sensitivity provides a mechanistic basis for designing region-specific mitigation interventions.

Sousa et al. (2023) developed and validated predictive models for NH₃ generation and emission potential in Brazilian broiler facilities, analyzing litter samples from 30 commercial aviaries across varying temperature regimes (25–40°C) and litter reuse cycles. Models demonstrated high predictive accuracy and practical utility for emission estimation in facilities where direct measurement is logistically or economically infeasible. The models accommodate two primary litter types (wood shavings and coffee husks) across up to four and six reuse cycles, respectively.

Saraz et al. (2015) applied CFD modelling to naturally ventilated tropical poultry houses, predicting spatial NH₃ concentration profiles as functions of ventilation rate, wind velocity and direction, and litter emission fluxes. The model provides decision-support capability for optimizing building orientation, inlet positioning, and ventilation management in open tropical housing. Miles et al. (2016) characterized spatial variability in NH₃ flux within humid subtropical broiler houses, revealing significant within-house concentration gradients attributable to litter moisture heterogeneity and ventilation non-uniformity, informing targeted litter management interventions.

Despite significant advances, current modelling approaches exhibit notable limitations for humid tropical applications. The AMCLIM-Poultry model, while globally calibrated, has not been comprehensively validated against field measurements from open-sided tropical facilities, where ventilation variability fundamentally differs from the closed-housing assumptions underlying model parameterization (Jiang et al., 2021; Sousa et al., 2018). Climate variable integration in most models is restricted to temperature and relative humidity, without explicit representation of rainfall effects on litter wetting in open facilities, or the thermal buffering effects of building design on indoor microclimate (Osorio Saraz et al., 2013). Decision-support translation-converting model outputs into actionable management recommendations for field practitioners-remains underexplored across the reviewed model literature (Table 4).

The reviewed literature demonstrates robust convergence across several key findings. First, humid tropical climate conditions substantially amplify NH₃ emission rates relative to temperate baselines, with temperature and relative humidity identified as dominant drivers across empirical, field, and modelling studies (Jiang et al., 2021; Sousa et al., 2023; Nurhidayah et al., 2024). Second, litter pH is a critical mediator of NH₃ volatilization, and amendments that lower litter pH below 7.0 consistently achieve significant emission reductions across diverse study designs (Choi and Moore, 2008; Toledo et al., 2020; Eid et al., 2022). Third, the interactive effects of litter moisture and temperature create a compounding emission burden in tropical systems that necessitates integrated rather than single-factor mitigation approaches (Mou et al., 2024; Gfeller et al., 2024).

Divergence in the literature is most pronounced in assessments of amendment efficacy under field versus laboratory conditions. The substantial NH₃ reductions reported in controlled laboratory incubation studies (77–96% for alum; Choi and Moore, 2008) frequently exceed field-level reductions (27–52%; Anderson et al., 2020), reflecting the buffering effects of building aerodynamics, variable litter moisture, and management heterogeneity on apparent amendment performance.

Table 4: Summary of Climate-Dependent NH3 emission models applied to tropical poultry systems

Similarly, the positive broiler performance effects associated with litter amendments (e.g., zeolite and alum) reported in several studies (Ezenwosu et al., 2022; Ali et al., 2023) have not been universally replicated, with some investigations reporting no significant performance differences despite demonstrated NH₃ reductions (Atapattu et al., 2017; Manno et al., 2011). This variability likely reflects differences in baseline NH₃ concentrations, production density, and bird genetics across study populations.

Geographic specificity of findings also limits cross-study synthesis. The dominant representation of Brazilian and Nigerian production systems within the tropical literature means that findings may inadequately reflect the distinct challenges of South Asian (India, Bangladesh, Vietnam) and Southeast Asian (Indonesia, Philippines, Thailand) poultry systems, which differ substantially in housing design, production scale, litter material availability, and regulatory context.

Several critical gaps emerge from the synthesis. The absence of standardized NH₃ measurement protocols adapted for open tropical housing represents the most fundamental barrier to evidence synthesis, preventing valid cross-study comparisons and emission inventory development (Sousa et al., 2018; Osorio Saraz et al., 2013).

Long-term field validation of both advanced mitigation technologies and novel amendment strategies is lacking; the majority of promising findings derive from short-duration trials of two to eight weeks under controlled conditions, with unknown translation to commercial production across multiple seasons and flocks. Comprehensive economic analyses integrating procurement costs, labor requirements, application logistics, and performance benefits are critically underrepresented, severely limiting adoption decisions by tropical producers with constrained capital.

Finally, multi-factorial experimental designs that simultaneously vary climate conditions, litter types, amendment rates, ventilation configurations, and bird density are conspicuously absent, preventing mechanistic understanding of the interaction effects that dominate real-world production environments (Table 5).

This review reinforces and extends several theoretical frameworks relevant to NH₃ emission dynamics. The climate-biochemical coupling model-integrating ambient temperature, relative humidity, and litter physicochemistry as co-determinants of NH₃ volatilization-emerges as the most empirically supported theoretical framework for understanding tropical emission amplification (Jiang et al., 2021; Vilela et al., 2020). The paradigm shift introduced by Lahav et al. (2008)-separating NH₃ capture from building ventilation through localized active capture systems-represents a theoretically elegant solution to the fundamental tension between air exchange requirements and NH₃ containment in tropical open housing. The emerging evidence base for biochar as a sustainable, multifunctional amendment contributes to theoretical understanding of the physicochemical interactions between carbonaceous substrates and the ammonium-ammonia equilibrium in complex biological matrices (Pereira et al., 2024; Graves et al., 2024).

Table 5: Critical Synthesis Summary: Convergence, Divergence, and Priority Gaps

For poultry producers operating in humid tropical contexts, the evidence supports a hierarchical mitigation strategy. As a primary intervention, litter pH management through acidic or adsorbent amendments (alum, biochar, zeolite) should be implemented at validated application rates, with selection informed by local cost, availability, and production context. In tropical regions with abundant agricultural biomass waste (rice husk, coconut shells, crop residues), locally produced biochar represents a particularly cost-effective and circular-economy-aligned option. As a secondary intervention, ventilation optimization-including maintenance of moderate relative humidity (50–60%), enhanced litter surface airflow, and where economically feasible, heat exchanger integration-substantially reduces litter moisture accumulation and NH₃ burden. CFD modelling provides a cost-effective tool for pre-emptive building design optimization. As a tertiary intervention in high-density closed-housing systems, active capture technologies (bubble column reactors, activated charcoal exhaust treatment) offer the highest efficacy potential but require further economic validation before widespread recommendation.

For researchers, the priority agenda is clear: standardized measurement protocol development for open tropical facilities, multi-season field trials of integrated mitigation strategies, and comprehensive socio-economic impact assessments that incorporate producer decision-making contexts. For policymakers, the evidence base supports the development of tropical-specific emission inventories and climate-responsive regulatory frameworks that incentivize adoption of proven mitigation technologies while supporting smallholder producers’ participation in sustainable intensification pathways. Given the current concentration of evidence in Brazil and Nigeria, funding bodies and regional research networks should prioritize field trials and validation studies in South and Southeast Asian poultry-producing countries (e.g., Bangladesh, India, Vietnam, Indonesia) to close this critical evidence gap.

Several limitations of this systematic review warrant acknowledgement. First, the geographic concentration of the underlying literature in Brazil and Nigeria limits the generalizability of findings to the full diversity of humid tropical poultry production contexts, particularly South and Southeast Asian systems which exhibit distinct housing designs, litter material availability, and production scales. Second, the heterogeneity of experimental designs, NH₃ measurement methodologies, and outcome metrics across included studies precluded formal meta-analysis, necessitating a narrative synthesis approach with attendant susceptibility to qualitative interpretation biases. Third, the English-language dominance of the search strategy may have excluded relevant studies published in Indonesian, Vietnamese, Thai, or other regional languages. Fourth, the rapid pace of innovation in litter amendment chemistry and ventilation engineering means that some commercially available products may not yet be represented in the peer-reviewed literature captured by this review.

This systematic review demonstrates that ammonia mitigation strategies developed for temperate poultry production cannot be directly transferred to humid tropical production systems because climatic conditions substantially alter NH₃ generation and volatilization dynamics. Elevated temperature, high relative humidity, and persistent litter moisture create conditions that intensify ammonia emissions, making climate-specific management essential for environmental sustainability, poultry welfare, and worker health.

Among the reviewed mitigation approaches, litter amendments- particularly alum, biochar, and zeolite- provide the most practical and evidence-supported interventions for reducing NH₃ emissions while often improving litter quality and production performance. Ventilation optimization, humidity control, and advanced engineering technologies further enhance mitigation effectiveness, whereas process-based and computational models offer valuable decision-support tools for predicting emissions and designing climate-adapted management strategies. The evidence consistently indicates that integrated approaches combining litter management, environmental control, and predictive modelling outperform single-intervention strategies.

Despite these advances, significant research gaps remain. The lack of standardized NH₃ measurement protocols for open tropical poultry houses, limited long-term commercial field trials, insufficient economic evaluations, and the under-representation of South and Southeast Asian production systems restrict the broader application of current findings. Future investigations should emphasize multi-season validation studies, cost–benefit analyses, and region-specific mitigation frameworks suitable for resource-constrained tropical poultry industries.

Overall, this review provides a comprehensive synthesis of current knowledge and establishes a foundation for developing evidence-based, climate-adapted ammonia mitigation strategies capable of improving environmental performance, animal welfare, and the long-term sustainability of poultry production in humid tropical regions.

The authors would like to express their sincere gratitude to the Bhuiyan Center for Interdisciplinary Research & Innovation (BCIRI) for its support and encouragement during the preparation of this manuscript.

This review is the first to comprehensively synthesize ammonia (NH₃) mitigation strategies specifically for humid tropical poultry production systems rather than extrapolating evidence from temperate regions. It integrates findings on litter amendments, ventilation engineering, ammonia capture technologies, and climate-dependent emission modelling within a single climate-focused framework. The review further identifies critical methodological gaps, including the lack of standardized measurement protocols, limited long-term field validation, and the under-representation of South and Southeast Asian poultry systems. By proposing an integrated, climate-adapted mitigation framework and highlighting regional research priorities, this review provides a practical evidence base to support sustainable poultry production, environmental protection, and future policy development in humid tropical regions.

Md. Raufur Rahman Akanda: Conceptualization, Methodology, Writing- Original Draft, Data Curation, Statistical Analysis, Investigation, Visualization.

Md. Mahmudul Islam, Md. Mahmudur Rahman, Fahad Ahmed, Debasree Saha, and Md. Al-Amin Sarker: Writing Original Draft, Data Curation.

Md. Rimon Bhuiyan: Conceptualization, Methodology, Project Administration, Validation, Visualization, Writing Review & Editing, Supervision.

Not applicable

Not applicable.

The authors declare that no generative AI and AI assisted technology was used in the creation of this manuscript.

The authors declare no competing interests.