Multi-Scale Control Strategies for Nitrogen Loss During Aerobic Composting of Agricultural Waste: A Review

1. Introduction

Agricultural production generates enormous quantities of organic residues, including livestock manure, crop straw, and by-products from food processing. Recent estimates suggest that agricultural waste is produced globally at the scale of billions of tons each year [1]. If these materials are not managed properly, they can occupy land, contaminate soil and water, and intensify greenhouse gas emissions. Aerobic composting converts such residues into a more stable humified product through microbial degradation and remains one of the most widely adopted biological treatment options because it combines waste reduction with resource recovery [2]. Compared with landfilling or incineration, composting is better aligned with circular agriculture, but its performance is often constrained by substantial nitrogen loss [3].

Nitrogen is a key determinant of compost quality because both the amount and the form of nitrogen retained at the end of composting directly affect the fertilizer value of the product. During aerobic composting, however, nitrogen is frequently lost through gaseous and liquid pathways, and reported losses commonly range from 20% to 70% of the initial nitrogen, with even higher losses under poorly controlled conditions [4]. NH₃ volatilization is usually the dominant pathway [5], and once emitted, NH₃ contributes to secondary particulate formation, air-quality deterioration, and acid deposition [6]. N₂O emissions are also important because N₂O has a global warming potential far greater than that of CO₂ and represents a major source of greenhouse gas emissions from composting systems [7]. In addition, nitrate-rich leachate can contaminate surface water and groundwater, increasing the risk of eutrophication and drinking-water quality deterioration [8].

Reducing nitrogen loss therefore matters at several levels. From an agronomic standpoint, higher nitrogen retention increases the fertilizer value of compost and can help reduce dependence on synthetic fertilizers. From an environmental standpoint, suppressing NH₃, N₂O, and nitrate losses lowers the atmospheric and aquatic burdens associated with composting. Economically, better nitrogen conservation improves product quality and can raise the market value of compost. A growing body of work has therefore examined mitigation options, including feedstock optimization, process control, additives, and reactor design [9,10]. Still, because nitrogen loss is controlled by tightly coupled biochemical and physicochemical processes, no single measure is sufficient under all conditions.

Against this background, this review examines nitrogen loss during aerobic composting of agricultural waste from a multiscale perspective. It first analyzes how feedstock characteristics shape nitrogen transformation and loss, then summarizes control strategies at the substrate, process, post-treatment, reactor, and policy levels. The review also compares the environmental and engineering implications of these strategies and discusses the main bottlenecks that still hinder broader deployment. By integrating mechanistic understanding with practical management options, it aims to provide a clearer basis for designing composting systems that retain more nitrogen while remaining technically and economically viable.

2. Sources and Mechanisms of Nitrogen Loss During Aerobic Composting

2.1. Feedstock Composition and Its Relationship with Nitrogen Loss

Agricultural wastes such as manure, straw, rice husks, and food-processing residues differ markedly in nitrogen form, carbon quality, moisture, buffering capacity, and structure, and these differences largely determine how much nitrogen is lost during composting. Manure typically contains high concentrations of total nitrogen, a large share of which is present as readily mineralizable compounds or NH₄⁺-N, but it often has a relatively low C/N ratio and limited structural carbon. Under these conditions, ammonification can outpace microbial assimilation, leading to NH₄⁺-N accumulation and substantial NH₃ volatilization, especially when temperature exceeds 50 °C and pH shifts into the alkaline range [11,12]. Crop straw shows the opposite pattern: it is rich in carbon but poor in nitrogen, which generally lowers the immediate risk of NH₃ loss but can slow microbial growth and prolong the composting period [13].

The initial speciation and bioavailability of nitrogen strongly influence its transformation pathway during composting. Fresh livestock manure often contains urea, uric acid, and other labile nitrogenous compounds that are rapidly hydrolyzed and can cause an early surge in NH₄⁺-N concentration, creating a pronounced risk of ammonia loss during the initial thermophilic stage [14]. By contrast, nitrogen bound in proteins, peptides, or more complex organic matrices is released more gradually, giving microbial assimilation more time to compete with ammonification. A further fraction of nitrogen is embedded in lignified or heterocyclic structures with low bioavailability; this pool contributes little to short-term loss but becomes an important precursor of stable humified nitrogen in the final compost [15].

Carbon availability governs the capacity of microorganisms to immobilize mineral nitrogen [16]. Readily degradable carbon sources such as soluble sugars, starch, and part of the hemicellulose fraction can rapidly fuel microbial growth and thereby stimulate nitrogen assimilation during the early phase of composting [17]. Cellulose decomposes more slowly and can sustain nitrogen immobilization later in the process, whereas lignin is highly recalcitrant but still matters because it can physically protect organic nitrogen from rapid breakdown and provides structural precursors for humus formation [18,19]. This is why feedstocks with balanced carbon quality often retain nitrogen more effectively than materials dominated by either highly labile nitrogen or highly recalcitrant carbon alone.

Physical structure and buffering properties also affect nitrogen loss. Bulky materials such as straw or coarse plant residues increase pile porosity, improve oxygen diffusion, and reduce the likelihood of anaerobic pockets that favor denitrification [20]. At the same time, many manures contain carbonates and organic acid salts that buffer the composting matrix toward alkaline conditions, often keeping pH between 8.0 and 9.0, which substantially increases the potential for NH₃ volatilization [21,22].

2.2. Nitrogen Transformation Pathways and Loss Routes

Nitrogen transformation during aerobic composting involves four closely linked processes: ammonification, microbial immobilization, nitrification, and humification [21,23]. Ammonification dominates early, when proteins, urea, and other organic nitrogen compounds are decomposed into NH₄⁺-N. Part of this mineral nitrogen is then assimilated by microorganisms and incorporated into biomass, a step that is essential for nitrogen retention and strongly dependent on carbon supply. As the pile cools and oxygen becomes more available, nitrification converts NH₄⁺-N to NO₂⁻-N and then to NO₃⁻-N. In later stages, humification stabilizes part of the retained nitrogen through association with humic substances, creating a more persistent organic nitrogen pool in the mature compost.

Nitrogen is lost mainly through gaseous and liquid pathways. Among these, NH₃ volatilization is usually dominant, accounting for 79%–94% of total nitrogen loss and representing 9.6%–46% of the initial total nitrogen in many composting systems [11,24,25]. This pathway is particularly important during the early and thermophilic stages, when rapid mineralization generates large amounts of NH₄⁺-N and high temperature and pH favor conversion to NH₃. Denitrification is generally the second major pathway and can account for 0.2%–9.9% of total loss, corresponding to 0.1%–5% of the initial total nitrogen [24,26]. It occurs when localized oxygen limitation allows NO₃⁻ to be reduced to N₂O and N₂. Leaching is usually less important in well-managed systems and often contributes less than 2% of the initial total nitrogen, but losses can increase sharply in uncovered windrows or under rainfall exposure because NO₃⁻ is highly mobile in the liquid phase [27,28].

2.3. Factors Governing Nitrogen Loss

Nitrogen loss is regulated by several interacting factors, among which the initial C/N ratio is fundamental [29]. When the C/N ratio is too low, typically below about 20, nitrogen is supplied more rapidly than microorganisms can assimilate it, leading to NH₄⁺-N accumulation and severe NH₃ volatilization [30]. When the C/N ratio is too high, often above 35, nitrogen becomes limiting for microbial growth, organic matter degradation slows, and the overall composting period is prolonged [31,32]. For many substrates, a C/N ratio in the range of 25–30 is considered a practical benchmark [21,30], but the actual optimum is feedstock-dependent. A recent meta-analysis suggested a lower optimum range for swine and poultry manure and a somewhat higher range for cattle and sheep manure [33].

Moisture content regulates both microbial physiology and oxygen transport [34,35]. At approximately 60%–65% moisture, microbial metabolism is generally well supported. If moisture rises above this range, pore spaces fill with water, oxygen diffusion is restricted, and anaerobic microsites can form, which promotes denitrification and the release of N₂O and N₂ [36]. If moisture falls too low, microbial activity is suppressed by water limitation and the conversion of organic matter and nitrogen slows markedly [37].

Temperature, pH, and aeration interact strongly during composting and largely determine whether mineral nitrogen is retained or lost. Thermophilic conditions accelerate organic matter degradation and are needed for sanitation, but they also enhance NH₃ transfer from the liquid phase to the gas phase and inhibit many nitrifiers, thereby increasing the risk of NH₄⁺-N accumulation and ammonia volatilization [38,39]. pH acts as a direct chemical control because the NH₄⁺/NH₃ equilibrium shifts toward NH₃ as pH rises. Once pH exceeds roughly 8.0, volatilization can increase sharply [5]. Aeration is equally important: insufficient aeration promotes anaerobic zones and denitrification, whereas excessive aeration strips heat, water, and NH₃ from the pile [40]. For this reason, nitrogen retention depends on coordinated rather than single-factor control.

Microorganisms mediate every major nitrogen transformation pathway during composting. The activity of ammonia-oxidizing and nitrite-oxidizing microorganisms determines the efficiency of nitrification, while diverse heterotrophs drive ammonification, assimilation, and humification [41]. As temperature, oxygen availability, pH, and substrate quality change, the microbial community also shifts, and those shifts can redirect nitrogen either toward stable retention or toward gaseous and liquid loss.

3. Multiscale Strategies for Controlling Nitrogen Loss

3.1. Feedstock Pretreatment and Source Control

Pretreatment of feedstocks provides the first opportunity to reduce nitrogen loss because it defines the substrate environment in which subsequent microbial and physicochemical processes occur. At this stage, the main objective is to optimize the initial matrix through physical, chemical, and biological measures so that NH₃ volatilization potential is reduced, denitrification risk is limited, and later-stage nitrogen stabilization becomes more likely.

Adjustment of the initial C/N ratio remains the most basic and most widely used source-control strategy. For nitrogen-rich materials such as livestock manure, co-composting with carbonaceous bulking agents such as straw, rice husk, sawdust, or corncob can reduce excess NH₄⁺-N formation and improve microbial immobilization. In chicken manure composting, adjusting the initial C/N ratio to 15 with wheat straw reduced nitrogen loss by 39.67% relative to the manure-only control [42]. In another study, increasing the initial C/N ratio from 18 to 22 by adding corn stover lowered total nitrogen loss from 38.62% to 32.57% [43]. Similarly, optimizing the mixing ratio of swine manure and corn stalks reduced NH₃ emissions by 64%–71% [44]. These results also show that carbon quality matters: highly available carbon can immobilize nitrogen quickly, whereas more recalcitrant carbon acts more slowly.

Bulking agents also improve the physical structure of the composting matrix. A well-aerated porous structure enhances oxygen penetration, reduces local compaction and waterlogging, and limits the formation of anaerobic microsites where denitrification can occur. It also helps drain excess water and thereby reduces leachate-driven losses of soluble nitrogen. In pig-manure composting, replacing sawdust or corn stalks with spent mushroom substrate lowered total nitrogen loss from 35.69% and 19.42% to 11.16%, corresponding to relative reductions of 68.7% and 45.6%, respectively [45].

Functional amendments can further retain nitrogen by adsorption, ion exchange, or chemical fixation. Biochar is widely studied because its high surface area, porous structure, and oxygen-containing functional groups can adsorb NH₄⁺ and NH₃ while also modifying pH and microbial habitat. During poultry-manure composting, different plant-derived biochars reduced NH₃ emissions by 1.26%–36.38% and N₂O emissions by 35.74%–52.82% [46]. In a chicken manure–corn straw system, adding 8% biochar reduced cumulative NH₃ emissions by 54.83% and increased the final total nitrogen content by 22.40% [47]. Mineral adsorbents such as zeolite and bentonite are also effective because their cation-exchange capacity allows selective retention of NH₄⁺. In chicken manure–straw composting, 10% zeolite lowered cumulative NH₃ and N₂O emissions by 28% and 55%, respectively, while increasing nitrate content by 17% [48]. In pig-manure composting, bentonite reduced NH₃ and N₂O emissions by 18.82% and 72.56%, respectively [49]. Acidifying or reactive additives such as superphosphate can also suppress NH₃ loss by lowering pH and converting free ammonia into more stable forms; in pig manure composting, superphosphate reduced cumulative NH₃ emissions by 23.8%–48.1% [50].

Chemical precipitation offers another route for nitrogen conservation. When magnesium- and phosphorus-bearing reagents are added, NH₄⁺ can be converted into struvite (MgNH₄PO₄·6H₂O), a stable slow-release fertilizer phase. In pig manure–corn straw composting, the combined addition of calcium superphosphate and Mg(OH)₂ promoted struvite formation, reduced cumulative NH₃ volatilization and total nitrogen loss by 41.78% and 13.27%, respectively, and increased both NH₄⁺-N and NO₃⁻-N in the final product [51].

Biological inoculation is increasingly used to redirect nitrogen transformation. Nitrifying consortia can accelerate the conversion of NH₄⁺ to oxidized nitrogen forms and thereby reduce gaseous loss. In cow-manure composting, inoculation with a consortium of psychrophilic and thermophilic nitrifiers increased NO₃⁻ formation by 77.87%–82.35% while reducing NH₃ and N₂O emissions by 48.89% and 20.05%, respectively [52]. Lignocellulose-degrading inoculants can also promote carbon turnover and indirectly improve nitrogen retention; one study reported 25.9% lower NH₃ emissions and 34.98% lower N₂O emissions after inoculation with lignocellulose-degrading microorganisms [53]. Acid-producing consortia provide a different mechanism. A synthetic lactic-acid-bacteria consortium reduced NH₃ emissions from livestock waste by 95.5% by lowering pH, shifting the NH₄⁺/NH₃ equilibrium toward NH₄⁺, and suppressing ureolytic bacteria [54].

3.2. Process Optimization

Process optimization aims to create conditions under which microbial assimilation, nitrification, and humification can proceed efficiently while pathways that drive NH₃, N₂O, and leachate loss are constrained. Temperature control is central to this goal. A thermophilic phase is still required for sanitation and rapid degradation, but excessively high or prolonged temperatures intensify NH₃ volatilization and suppress many nitrifiers [10,39,55]. Once hygiene requirements have been met, controlled cooling is beneficial because it helps move the system into a range more favorable for nitrification and nitrogen stabilization [10,52]. Because temperature is inseparable from aeration and moisture, it should be managed as part of an integrated control strategy rather than as an isolated variable.

Aeration is one of the most influential operating variables in aerobic composting because it controls oxygen supply, heat removal, moisture exchange, and the stripping of volatile compounds [10]. Adequate aeration is needed to avoid anaerobic microsites and limit denitrification, yet excessive continuous aeration can markedly increase NH₃ loss. In cow manure composting, two intermittent-aeration regimes reduced cumulative NH₃ emissions by 24.37% and 19.27% and total nitrogen losses by 9.07% and 6.10%, respectively, compared with continuous aeration, although N₂O emissions increased by 22.22% and 43.14% [56]. In chicken manure composting, an aeration rate of 0.1 m³·h⁻¹ achieved maturity and humification while also producing the lowest NH₃ emission among the tested treatments [40]. These findings support the use of intermittent or feedback-controlled aeration rather than fixed high-rate continuous ventilation. This approach is becoming more realistic in practice: a real-time gas sensor network recently achieved O₂ accuracy within ±1.5% and helped increase the nitrogen content of the final compost by 31.7% under optimized control [57].

Moisture control is equally important because water availability shapes both microbial activity and oxygen diffusion. In most aerobic systems, a moisture content of roughly 55%–65% provides a workable balance between microbial metabolism and air-filled porosity [10]. When moisture is too high, oxygen transfer declines and denitrification risk increases; when it is too low, decomposition slows because of water stress. In rapid kitchen-waste composting, total nitrogen loss ranged from 18.81% to 37.85% of the initial nitrogen, and NH₃ accounted for 11.09%–35.03%; based on the combined criteria of maturity and gaseous-emission control, the recommended moisture range was 60%–65% [58]. In microbial-agent-enhanced chicken manure composting, an initial moisture content of 53% produced the highest peak temperature, the longest thermophilic period, and better nitrogen retention than drier or wetter treatments [59]. Liquid-phase recirculation can also help: a condensation-return composting system reduced NH₃ emissions by 59.3% and increased the total nitrogen content of the final product by 19.4% [60].

Because pH directly governs the NH₄⁺/NH₃ equilibrium, pH management has long been considered a potential tool for nitrogen conservation. In practice, however, bulk acidification must be used cautiously because excessive acidification can delay degradation and may even increase N₂O release under some conditions. For example, acidifying manure to pH 5 prolonged degradation by 7–10 d and increased N₂O emissions by 18.6%, whereas acidification to pH 6 reduced N₂O and CH₄ emissions by 17.6% and 20%, respectively, without impairing maturity [61]. Indirect approaches are often more practical. Adding 20% apple pomace during pig-manure composting reduced cumulative NH₃ and N₂O emissions by 57% and 24%, respectively, while increasing the final total nitrogen content by 19% [62].

In windrow systems, turning is the main physical intervention used to redistribute heat, moisture, oxygen, and substrates, but the turning schedule should be matched to process conditions rather than fixed by calendar alone. Excessive turning promotes heat loss and NH₃ stripping, whereas insufficient turning increases the risk of compaction and anaerobic zones. In chicken manure–soybean straw composting, turning every 5-d reduced NH₃ emissions by 11.42%–18.95% relative to 1- or 3-d intervals and lowered total nitrogen loss compared with both more frequent and less frequent turning [63]. Model-based control offers a further step toward precision management: a kinetics-based turning strategy prolonged the period above 50 °C by 2-d and improved line efficiency by more than 10% relative to a fixed 4-d schedule [64]. Taken together, these results suggest that turning should be viewed as a dynamic control variable rather than a routine operation.

3.3. Post-Treatment Nitrogen Stabilization and Resource Recovery

Post-treatment management should stabilize the nitrogen retained after the thermophilic stage and prevent secondary losses during curing, storage, transport, and land application. Among the available options, curing is particularly important because it extends maturation under lower temperatures and lower oxygen demand, which favors both organic matter stabilization and nitrogen sequestration [65,66]. During curing, the microbial community shifts from thermophilic populations toward mesophilic decomposers, and fungi and actinomycetes become increasingly important for degrading recalcitrant lignocellulosic fractions and promoting humification [67,68]. As humification proceeds, part of the retained nitrogen is incorporated into humic substances and other relatively stable organic nitrogen pools [69]. At the same time, nitrification becomes more active as temperature declines under aerobic conditions, which helps convert residual NH₄⁺ to NO₃⁻ [10]. This does not eliminate risk, however, because excessive nitrate accumulation may interfere with humic-acid formation and can increase the risk of leaching if curing proceeds under wet conditions [70]. Deep nitrogen stabilization during curing therefore depends on adequate curing time together with careful control of aeration, moisture, and rainfall exposure.

Once curing is complete, mature compost can be upgraded into standardized commercial products that improve handling and may further reduce secondary nitrogen loss. Pelletization or granulation increases bulk density, suppresses dust formation, facilitates storage and mechanized application, and can alter the release pattern of compost-derived nitrogen in soil [71,72]. During granulation, the addition of sorptive or nitrogen-retaining amendments such as bentonite-type minerals or humic substances may further enhance nitrogen stability [24,73]. Cured compost can also be blended with mineral nutrients to produce organo-mineral fertilizers or specialized substrates with more controlled nutrient release [74]. Where ammonium-rich side streams are available, magnesium- and phosphorus-based reagents can recover nitrogen as struvite, a recyclable slow-release fertilizer. Recent work on agricultural waste streams has shown high ammoniacal-nitrogen recovery during struvite production and lower off-site nutrient-loss risk than with readily soluble fertilizers [75,76].

3.4. Reactor Innovation and high-Efficiency Composting Technology

Reactor innovation has shifted agricultural-waste composting from empirically managed open piles toward semi-closed or closed systems with improved control of heat transfer, gas exchange, and liquid collection. From the standpoint of nitrogen conservation, the main benefit of these systems is not only faster stabilization, but also tighter control over NH₃ stripping, denitrification-prone microsites, and opportunities for nitrogen recovery. Membrane-covered composting is a representative semi-closed option. Depending on membrane characteristics and operating conditions, it can substantially reduce the exchange of heat, moisture, and nitrogenous gases with the surrounding environment. Quantitative nitrogen-balance analysis showed that membrane-covered aerobic composting reduced total nitrogen loss by 33.24%–50.07% relative to uncovered composting [77]. A comparative study also reported 53.9% lower NH₃ emissions and 71.3% lower N₂O emissions under semi-permeable membrane covering [78]. At the same time, membrane systems are not uniformly beneficial: another study found a 15.43%–34.00% decrease in total nitrogen loss but a 16.91%–90.13% increase in NH₃ emissions, indicating that membrane properties and aeration strategy must be optimized together [79].

Fully or partially enclosed in-vessel systems offer even tighter operational control because aeration, temperature, exhaust gas, and condensate can be managed more precisely than in conventional windrows. In a closed aerobic composter treating laying hen manure, gas-permeable membrane capture with sulfuric acid recovered 0.61–0.65 kg total ammonia nitrogen (TAN), and the in-reactor membrane configuration achieved a recovery rate of 6.9 g TAN m⁻²·d⁻¹ compared with 1.9 g TAN m⁻²·d⁻¹ for the external module [80]. Likewise, a condensation-return composting system reduced NH₃ emissions by 59.3% and increased the total nitrogen content of the final product by 19.4% [60]. These examples show that reactor design becomes especially effective when it is coupled with ammonia capture or liquid-phase recirculation.

Another major trend is the integration of controlled aeration with intelligent ventilation design. Rather than relying on fixed, experience-based operation, modern systems increasingly use staged or intermittent aeration to balance oxygen supply, heat retention, and ammonia stripping. In reactor composting of cow manure and corn stalks, intermittent aeration reduced cumulative NH₃ emissions by 24.37% and total nitrogen loss by 9.07% relative to continuous aeration [56]. In semi-permeable membrane-covered vegetable-waste composting, a multichannel ventilation mode reduced NH₃, N₂O, and CH₄ emissions by 22.57%, 21.52%, and 32.67%, respectively, compared with single-bottom ventilation [81]. Together, these results suggest that the real value of reactor innovation lies in combining semi-/closed structures, controlled ventilation, and nitrogen capture or recirculation, thereby shifting composting from passive emission reduction toward active pathway regulation and partial nutrient recovery.

Overall, reactor innovation is most valuable when semi- or fully enclosed structures are combined with controlled ventilation, gas capture, and liquid recirculation. Under those conditions, nitrogen management shifts from simple emission reduction toward deliberate pathway regulation and partial nutrient recovery.

3.5. Policy and Governance Strategies

From a policy and governance perspective, nitrogen loss during aerobic composting of agricultural waste should be treated not merely as a process-control issue, but as a nutrient-cycling challenge that spans waste treatment, fertilizer production, and farmland application. Standard setting is a priority. A dual framework covering both compost product quality and process emissions is warranted because market acceptance remains constrained by variability in nutrient content, uncertainty in nutrient release, and concerns over contaminants in recycled nutrient products [82]. Accordingly, compost standards should move beyond conventional maturity indicators and include nitrogen-related metrics such as total nitrogen, the NH₄⁺/NO₃⁻ ratio, and, where feasible, indicators of nitrogen stability. Process-oriented standards should also include NH₃ and N₂O accounting, because composting is a source of both air pollutants and greenhouse gases. A recent critical review compiled 388 emission factors from 46 studies, highlighting the large variability in reported values and the need for more consistent accounting methods [83]. Standardized protocols would strengthen differentiated regulation of large-scale plants, support best-management-practice certification for decentralized systems, and allow more credible comparisons across technological pathways [83,84].

Regulation should combine mandatory environmental safeguards with market-based incentives. A recent review of policy impacts on nutrient circularity identified manure management and nutrient-recycling technologies as recurrent policy targets and showed that economic incentives, certification schemes, and coordinated policy frameworks are major drivers of adoption [85]. Other recent analyses likewise emphasize that supportive policies, economic feasibility, and public acceptance are all important for the wider uptake of recycled nutrient products [82]. In practical terms, this means that low-nitrogen-loss composting technologies should be incorporated into permitting and impact-assessment frameworks, while subsidies, tax incentives, green public procurement, and organic or green product certification can be used to reward compost products with demonstrably higher nitrogen retention and lower process emissions. The broader policy goal is to turn nitrogen retention from an internal plant-level efficiency concern into a regulated and economically valued performance indicator across the entire waste–fertilizer–farmland chain.

4. Challenges and Future Perspectives

Despite the progress summarized above, major challenges remain. Many control measures still involve pollution swapping rather than complete mitigation. For instance, interventions that suppress NH₃ volatilization may under some conditions increase N₂O release, shifting rather than eliminating the environmental burden. Other measures create engineering trade-offs. Large additions of bulking agents can reduce nitrogen loss at the source, yet they also lower volumetric loading and may weaken reactor productivity. Likewise, effective additives such as biochar and zeolite remain costly at scale, and their long-term field performance and possible secondary environmental risks are not yet fully resolved. High-precision control strategies improve nitrogen retention, but they often require more complex equipment, more skilled operation, and higher capital and maintenance costs.

These limitations mean that mitigation strategies cannot be judged solely by laboratory performance. They must also be tested against practical engineering criteria and assessed with consistent system boundaries. A recent review of life cycle assessment studies on composting identified 456 publications from 2010 to 2022, but only 56 were suitable for detailed analysis because of substantial inconsistencies in functional units, system boundaries, and emission-factor selection [84]. Likewise, the large variability among reported gaseous-emission factors currently limits their direct use in regulation, policy evaluation, and cross-study comparison [83]. This points to an urgent need for standardized NH₃ and N₂O accounting protocols and differentiated emission benchmarks that can support technology selection, performance assessment, and evidence-based policymaking.

Future research should therefore move beyond single-factor optimization and place greater emphasis on multiscale coupling and intelligent regulation. Particular attention should be paid to the interactions among microbial nitrogen transformation pathways, substrate properties, aeration regimes, and reactor configurations across molecular, process, and system scales. At the same time, Internet of Things technologies, online sensing, and machine-learning-assisted control create real opportunities for data-driven composting systems that can adjust key process parameters in real time. For large-scale deployment, however, such systems must be robust, affordable, and easy to operate, and they should be evaluated with combined life cycle assessment and techno-economic analysis rather than by short-term emission data alone. Ultimately, greener and higher-value composting will depend on full-chain coordination from molecular mechanisms to process design, reactor engineering, and macro-level governance.

5. Conclusions

Nitrogen loss remains one of the main constraints on the aerobic composting of agricultural waste because it lowers fertilizer value and increases environmental impacts. Across current studies, NH₃ volatilization is the dominant loss pathway, whereas nitrate leaching and denitrification-related emissions, especially N₂O, make smaller but still important contributions to the nitrogen budget. This review shows that effective nitrogen conservation requires coordinated control across multiple scales. At the substrate level, feedstock optimization and functional additives can suppress NH₃ formation and improve mineral-nitrogen retention. At the process level, aeration, temperature, moisture, pH, and turning must be managed in an integrated manner to favor microbial assimilation, nitrification, and humification. At the system level, reactor innovation, nitrogen recovery, and intelligent control create new possibilities for coupling nitrogen retention with engineering efficiency. Future work should focus on multiscale nitrogen-transformation mechanisms, low-cost intelligent regulation, and integrated evaluation frameworks that jointly consider process performance, environmental burdens, and economic feasibility.