The reviewed studies dealt with concentrations of gases considered harmful to the environment and health, with a particular focus on NH3. They analysed EFs and emission rates within livestock buildings and proposed sustainable solutions to limit the resulting pollution.
3.1. Investigation of Potential Factors Influencing Emissions, Examination of Concentrations within the Barn, and Estimation of Ammonia Dispersals
Various studies [
12,
18,
25] have focused on the quantitative assessment of NH
3 emissions from livestock housing, as well as from manure storage and its final application to the soil. In this review, the phase of NH
3 production that takes place inside the livestock building was particularly considered and analysed (
Table 1).
Table 1 shows the results of the studies analysed in this review paper, and in some cases the values of the NH
3 emission ranges taken from the original data reported, expressed in "LU" (Livestock unit) or "HPU" (Heat production unit), were converted into values expressed in g AU
-1 d
-1 (where animal unit AU is equivalent to 500 kg body mass).
It is well-documented in the scientific literature that various factors must be considered when evaluating the dynamics that promote ammonia emissions. These factors include, for instance, the type of diet administered to the animal, milk production, cattle breed, and the methodology and instrument used to measure gas concentrations [
27]. Nonetheless, in this study, particular attention has been given to building-related factors, which, according to the current state of the art, are the most influential. These include barn ventilation type, the category of barn and the technological features of the structure, flooring type, and the strategy for cleaning the barn floor [
7,
23,
28,
29,
30,
31].
In order to be able to identify under which conditions the lowest emissions occur, a comparative analysis was carried out between: NV barns with solid flooring and scraper cleaning system, NV barns with slatted flooring and scraper cleaning system, NV barns with solid flooring and flushing cleaning system, and NV barns with slatted flooring and flushing cleaning system (
Figure 3).
It is important to point out that the results presented take into account different studies that certainly have differences among them, in terms of experimental methodology, location of the study site, different intervals and frequencies of floor cleaning, and different methodologies and tools for surveying barn emissions [
30,
31]. Therefore, comparisons among these values should be carefully evaluated. In order to analyse the experiments that compared the same flooring and manure handling conditions (i.e., NV-slatted-scraped and NV-solid-scraped), emission intervals were highlighted in
Figure 1 with the same pattern for the same experiment.
From the results of these analyses, it can be observed that generally NH3 emissions are lower when there is slatted floor, compared to when there is a continuous type flooring in the barn. When the scraper is used to clean the floor, there seems to be no substantial difference between solid and slatted floor; however, when the flushed system is used, the emissions are lower with the slatted floor.
It is important for future research to deal with the standardisation of results, through the use of databases or precise standards of experimental test execution, along with statistical analyses to corroborate the conclusions drawn from the analyses.
To achieve a thorough understanding of the results, several factors that have a significant influence on the estimation of the NH
3 emissions must be taken into account. The main determinant is the decomposition of urea excreted in animal urine, which can lead to soil acidification and alter the balance of terrestrial and aquatic ecosystems [
16]. Among the publications reviewed, a significant group observed phenomena that directly affect EFs and determined emission rates.
The purpose of the studies by Sanchis et al. [
7] and Qu et al. [
5] was to quantitatively define the effect of temperature, wind speed, relative humidity, and ventilation rate on NH
3 release in the barn, conducting a meta-analysis. Different climatic conditions (i.e., warm, transition and cold) and barns (i.e., naturally-ventilated (NV), mechanically-ventilated (MV) were studied. The analysis of EFs and rates showed a significant effect of temperature, while neither wind speed nor relative humidity had a statistically significant effect on NH
3 emissions.
Both publications claim that NH
3 emissions increase with increasing air temperature. These results differ from those by D'Urso et al. [
32], who suggested that relative humidity is a conditioning factor in addition to air temperature. A more humid environment can contribute to NH
3 formation through processes such as urea hydrolysis in urine. Flight speed can reduce local concentrations but contributes to gas dispersion and dilution in the environment.
Similar assessments were found in the study by Wu et al. [
23], who measured NH
3 concentrations in two semi-open dairy barns. A multiple linear regression model was used to describe the relationships between NH
3 emission and influencing factors such as wind direction, wind speed, outside air temperature, inside air temperature, and inside air speed. It was found that an increase in wind speed has a positive effect on NH
3 emissions, as increased ventilation promotes dispersion into the environment. An increase in external air temperature also affects the increase in emissions, as higher temperatures favour biochemical processes that contribute to the emission increase. Despite the individual intensifications of wind speed and outside air temperature, together they can have a negative effect on NH
3 emissions.
These results are confirmed by two studies on different types of livestock buildings, i.e., NV and MV structures [
20,
33]. Tabase et al. [
33] analysed internal climatic conditions in buildings, both MV and NV, focusing on variables such as temperature, humidity, and air exchange rate. This study demonstrates that high relative humidity contributes to increased NH
3 volatilization in poorly ventilated structures, and diurnal variations in air exchange rates reduce gas concentrations in buildings. Additionally, this study makes assessments regarding manure management, stating that this practice significantly influences emissions. Specifically, it was found that floor cleaning with robotic scrapers reduces NH
3 emissions more than manual cleaning. In the study by Wang et al. [
20], the distribution of NH
3 concentration was evaluated by using low cost and accurate instruments in a NV barn, finding dependence on environmental factors such as wind speed and prevailing wind direction, sunlight, and the cows' activity in the barn.
Other studies, considered in this review [
19,
22,
34], detected NH
3 concentrations and estimated emission rates (using different methodologies such as the CO
2 mass balance method or regression models). They again concluded that gas emissions are correlated with temperature and relative humidity. Based on these studies, it has been possible to highlight an amplification of NH
3 emissions during the day, correlated with stronger winds and lower humidity, and that a lower internal temperature significantly reduces emissions according to an exponential function [
34]. A further outcome was the significant seasonal variation in NH
3 emissions, which are higher in spring and summer than in winter, with peaks linked to animal activity during meals and milking routines [
22].
The effects of animal activity and air temperature inside the barn were analysed in the study by Ngwabie et al. [
35] who carried out the measurements of in a NV building that had a free-stall system with a solid concrete floor mechanically cleaned every hour. This publication claims that NH
3 emissions increased as the indoor temperature of the building increased, since the temperature of the manure is critical for emissions. Higher indoor temperatures probably lead to higher NH
3 emissions because of the relationship between air and manure temperatures and the relationship between air temperature and ventilation rate through natural convection.
Wind and relative humidity are also conditioning factors; in fact, wind can affect the natural ventilation of the building, contributing to the dispersion of gases emitted by animals and manure; relative humidity can influence the formation of ammonia and other gaseous compounds within the building, as high humidity conditions can promote certain chemical reactions and emission processes. In addition, it was observed that NH3 emissions showed significant variations throughout the day, with two peaks probably related to feeding routines, which supports the thesis that cattle activity is an influential element in gas production.
Similar findings were obtained in the studies carried out by Maasikmets et al. [
25] and Bougouin et al. [
4], who employed modeling to quantify the main relationships between EF and factors influencing. Maasikmets et al. [
25] stated that NH
3 emissions from livestock production result from a complex intertwining of various factors related to animal and manure management, the type of facility housing the animals, and environmental conditions. According to previous studies it was observed that the rate of NH
3 emission depends on temperature, with lower emissions occurring during cooler periods, such as autumn. Two farms were selected for the experiment: the first with fixed housing and a solid manure system removed three times a day, and the second with free stall housing and a liquid manure system removed several times a day through channels and pumps. Gases were measured at the floor surface and, considering temperature and relative humidity, the EFs for NH
3 was highest in the fixed housing barn, where cattle were kept on straw beds mixed with manure. The reasons for this difference could also include the feeding management.
Bougouin et al. [
4], showed a positive correlation between outdoor temperature and NH
3 emissions, with a tendency for emissions to increase as outdoor temperature increases. In particular, NH
3 emissions were found to be higher during periods of higher temperatures, such as in summer. But unlike previous studies in this trial, it was found that wind speed and relative humidity had no significant effect on NH
3 emissions.
It has also been observed that barns with continuous floors have higher NH3 emissions than those with slatted floors. In barns with continuous floors, this is due to stagnation of urine and faeces mix, whereas in slatted barns, urine can flow through the cracks and separate from faeces, thereby reducing NH3 production.
Studies conducted by some authors [
24,
36] investigated the management practices of barns. However, they use different approaches and methodologies to achieve their results. In detail, Gilhespy et al. [
24] examined the relationship between emissions and time cattle spend in barns, quantifying NH
3 losses from livestock housing over a measurement period of about one year. It was found that NH
3 loss was not directly proportional to the amount of time cattle spent in the polytunnel utilised in the experiment. Emissions were higher when calculated over 24 hours of measurement than when calculated over 48 hours, because emissions continued after the animals were moved from the polytunnel, and emission rates decreased over the subsequent 24 hours. As a result, emissions as a percentage of TAN emitted decreased with increasing occupancy. To significantly reduce airborne NH
3, the research concluded that cattle should not occupy pens for more than six hours. Rzeźnik et al. [
36] instead, considered and compared different manure removal systems, building types and resting areas. The evaluation of the results showed the dependence of emissions on these factors. It was found that barns with gravity ventilation systems and partially controlled side openings by curtains had lower NH
3 emissions than barns with windows that could be opened. Furthermore, barns with slatted floor manure removal systems had lower NH
3 emissions than barns with scraper or chain manure removal systems.
Assessing NH
3 emissions in livestock housing systems, by focusing on the main types of open housing buildings with natural ventilation, was the aim of the studies by Pereira et al. [
37] and Zhang et al. [
38]. Differences between the three buildings studied included variations in stall layout, floor type, ventilation opening placement, manure management practices and overall size. It was found that NH
3 emissions varied considerably depending on the type of building as well as the management practices adopted; the differences observed could be attributed to various structural, climatic and management factors specific to each building. These differences were also observed in the review by Rzeźnik et al. [
6], which examined research articles containing emissions data published between 1997 and 2015. The study shows that livestock housing systems can have a significant impact on gas emissions. It mentions systems with litter and systems without litter (with fully or partially slatted floors). It was found that litter-free housing is characterised by lower NH
3 emissions, but also by lower animal welfare standards compared to litter systems.
Ventilation also influences emissions, as it helps removing moisture and heat from livestock buildings, thereby affecting air quality. The study by Rong et al. [
39] analysed this influential factor, studying the impacts of climate parameters on gaseous emissions, air exchange rate and concentrations in a dairy building with hybrid ventilation. The results showed that the hybrid ventilation system collected 64-83% of NH
3 emissions and 10-50% of CH
4 emissions, thus helping to reduce emissions compared to NV buildings. In addition, it was found that NH
3 emissions through natural ventilation were about 60% lower than the values reported in the literature.
Recently, scientific studies have focused on investigating EFs and creating EF databases. Sommer et al. [
8] discussed the development of new EFs for NH
3 derived from manure management. Data from 276 studies, mainly from peer-reviewed scientific journals, were used to calculate new EFs. The EMEP/EEA Air Pollutant Emission Inventory Guide provides EFs to support the compilation of emission inventories. New EFs have been developed for different phases of manure management, showing a wide variability and highlighting the influence of many factors such as management practices, animal species, climate and housing conditions. The adoption of these new EFs will influence national NH
3 emission estimates, as variations in EFs will affect the estimation methods used in national inventories.
Hassouna et al. [
40] have developed a global database (i.e. the DATAMAN Housing and Storage databases) of EFs for NH
3, CH
4 and N
2O from livestock housing and manure storage. These EFs were provided to improve the accuracy of national inventories, conduct environmental assessments, and identify mitigation techniques and influencing parameters. The DATAMAN database was developed by collecting relevant information for different animal categories, manure types, livestock buildings, outdoor storage and climatic conditions from published studies, conference proceedings and existing databases published between 1995 and 2021. The data were collected and screened for suitability for inclusion in the database. The data were then converted into EFs using a methodology specifically developed for this purpose. The conclusions of the document highlighted the lack of studies in some important livestock production areas and the need to standardise future study approaches and harmonies data from existing studies. In accordance with this statement, Çinar et al. [
41] conducted a study with the aim of improving knowledge and reducing uncertainty on the effects of influential factors, analysing the emission rates of NH
3 and GHG collected in the DATAMAN database. This study agreed with the previous one on the importance of collecting data to better understand variations in emissions and increase the accuracy of emission models. This research found that NH
3 emissions are affected by environmental factors such as relative humidity and temperature, as well as by the characteristics of the livestock housing system, showing NH
3 emission rates ranging from 0.036 to 146.7 g N LU
-1 d
-1.
After analysing the results of the aforementioned studies, this review aims at evaluating the correlation between specific factors influencing ammonia emissions within naturally ventilated buildings for dairy cattle housing, and the highest concentration values reported in the literature. The first investigated correlation is between the highest recorded NH
3 concentration values in the barn and the number of animals housed within the barn for various building-related conditions (
Figure 4) (
Figure 5) (
Figure 6).
Observing the data, a potential correlation between the number of animals and the peak of recorded values emerged. Specifically, it was noted that the highest the number of animals the highest is the maximum concentration of NH3.
Equally, when the number of animals decreases, a similar trend was observed in the maximum values of NH3 concentration.
The second investigated correlation concerned the milk yield and the highest values of recorded NH
3 concentrations in the barn (
Figure 7) (
Figure 8) (
Figure 9).
The data analysis revealed a possible correlation between the number of animals and the maximum recorded values. Notably, when milk yield peaks, the highest concentration value tends to increase. Likewise, as milk yield decreases, the maximum value of concentrations declines.
The last correlation studied was between the highest NH
3 concentration values recorded in the barn and the average temperature measured inside the building (
Figure 10) (
Figure 11) (
Figure 12).
Observing these graphs, although in
Figure 10 it appears to be no direct dependency between the maximum recorded value range and the internal temperature, however, examining
Figure 11 and
Figure 12, it was evident that as the temperature increases, the concentrations also rise, which confirms the assertions of many of the studies described [
5,
7,
19,
22,
25,
34,
35,
38,
41].
These results indicate that ammonia emissions are not uniquely dependent on a specific factor, but rather are the outcome of the interaction of all surrounding conditions. Among these conditions, in addition to those examined in the graphs, factors such as the breed of cattle also play a role [
42]. Only 17.6% of the selected articles specified the breed of cattle present inside the barn during the experimental measurements; among these the indicated breeds were: Norwegian red (nrf), Swedish Holstein, Bos taurus, Charolais x Friesian, and Simmental beef cattle.
Another extremely influential factor is the technique used for measuring the concentrations in the barn, along with the choice of instruments, the number of repetitions performed, and the season in which the experimentation was conducted [
30] [
43]. In particular, several techniques have been used to measure ammonia concentrations. Passive flow samplers were employed by 17.6% of the studies, the photo-acoustic technique by 47%, and electrochemistry by 5.9%. Considering the influence of the type of instrument used, it is important to highlight their detection accuracy. In fact, the study conducted by Calvet et al. [
44] provided typical precision values (which may vary for different concentration ranges); these are: 5 - 10 % of magnitude for the technique with passive flow samplers, 2.5% for the photo-acoustic system, and 8% for electrochemical technology.
Regarding the season in which the measurements were conducted, among the articles selected for this review, experiments were most frequently performed during spring and summer, and much less often in winter (
Figure 13).
These influential elements (such as the diet administered to the animals) have been thoroughly investigated in specific reviews [
4] on livestock emissions. In the following section, the focus of this study is oriented towards the mitigation strategies for NH
3 emission reduction, encompassing both characterization of the influences due to the building itself (e.g., type of ventilation, flooring, and technique for cleaning livestock effluents) and other animal-related influencing factors (e.g., nutrition, and PLF).
3.2. Mitigation Strategy Analysis
Among the studies reviewed (
Table 2), a large group focused on exploring strategies, technologies and practices to reduce the negative impacts caused by NH
3 emissions. Mendes et al. [
17] used a modelling approach to evaluate potential factors to reduce NH
3 emissions in a dairy farm equipped with at least one mitigation technique, comparing it with a barn where no mitigation technique was applied to assess its benefits. The developed model was based on a specific algorithm aimed at identifying the most appropriate abatement management technique among those considered: floor scraping, floor washing with water, different types of flooring, and internal manure acidification. The most effective strategies, in order of efficiency, were found to be manure acidification, floor scraping, and floor washing. The most valid combinations, in order of efficiency, were floor scraping combined with manure acidification (up to 44-49% reduction of emissions), solid flooring combined with scraping and washing (between 21-27% reduction) and floor scraping combined with washing and floor scraping only (17-22% reduction). Acidification, which was considered the most effective strategy in this study, was also investigated by Fangueiro et al. [
11]. Acidification involves lowering the pH of the slurry by adding acids or other substances. Several research studies and experiments have shown that lowering the pH changes the composition of the slurry. When slurry is acidified, NH
3 is converted to ammonium, which is less volatile than NH
3, thus reducing emissions to the environment. In addition, acidification can affect the overall production of ammoniacal nitrogen in the manure, further reducing emissions. Although considered a complex practice, acidification can pose threats such as increased solubility of certain mineral elements in manure, thus increasing the risk of leaching. However, if correctly applied, acidification can be an effective strategy to reduce NH
3 emissions and improve manure environmental management.
Bobrowski et al. [
45] proposed to evaluate the seasonal mitigating effects of a urease inhibitor under practical conditions and provide information on two theoretical application scenarios to estimate an annual scenario. The inhibitor, called "inhibitor K", is described as a ready-to-use pyrrolidone-based chemical formulation. It was manually applied on the surfaces of two dairy sheds for three days during the summer, winter and transition period. The work shows that the use of urease inhibitors in NV dairy barns, in combination with appropriate application, can significantly reduce NH
3 emissions in dairy sheds in all seasons, representing a potential solution to mitigate the environmental impact of such emissions. Another research carried out by Bobrowski et al. [
46] suggested that urease inhibitors may be a mitigation solution with lower investment requirements, also in a MV dairy housing system. However, they observe that further research is needed to assess the direct reduction in NH
3 concentrations.
Further valuable insights into mitigation strategies can be derived from a meta-analysis of 126 published studies on manure management at all stages by Hou et al. [
12]. Various strategies were considered, including reducing the protein content of animal diets, manure acidification, covering manure with straw or artificial film, compacting and covering solid manure, and manure application by broadcasting, incorporation and injection.
The results showed that managing animal diets and manure acidification were the most effective strategies for reducing harmful gas emissions throughout the livestock waste management chain. Furthermore, manure incorporation and covering with straw or artificial film were proposed in this meta-analysis as effective strategies for reducing NH
3 emissions. However, Baldini et al. [
16] emphasised the need to regularly renew the bedding of the cubicles to avoid anaerobic conditions in the deeper levels of the straw. In this study, greenhouse gas emissions measured in NV barns with different types of flooring and manure handling systems were studied and defined, focusing on the potential for reducing emissions through the choice of flooring type and manure handling strategies. From the described analyses, it was found that the EFs were higher in barns with solid floors equipped with scrapers, and lower in barns with perforated floors or washing systems for manure removal. These results were attributed to the contact surface between urine and urease in faces, which influenced the percentage of urea effectively converted to NH
3. In addition, scrapers usually leave a thin layer of slurry, which increases NH
3 volatilization by increasing the surface area over which urine is spread and reducing the thickness of urine pools. The study highlights strategies and practices used in barns that have been shown to reduce emissions of harmful gases: frequent and complete removal of manure from floors, including perforated floors; tilting of floors for faster separation of liquid and solid parts; and use of rubber mats instead of concrete floors, coupled with scrapers to increase cleaning efficiency.
However, the study conducted by Chiumenti et al. [
47] stated that rubber coated floors showed higher emission rates than concrete floors, both before and after the operation of the robotic scraper.
Snoek et al. [
48] evaluated the characteristics of fresh urine pools (i.e., area, depth and resulting volume) in commercial dairy barns and studied how variables such as flooring type, season and manure scraping method influenced these characteristics. Four types of barns were considered for the experiment: the first was the reference situation with slatted floor and manure pit; the second had a fully closed and grooved floor; the third had a fully closed V-shaped asphalt floor; the last had a slatted floor like the first type. It was found that manure scraping can significantly influence the area and depth of urine pools. The type of flooring and the season also influenced the size of the pools. In this study largest and statistically significant puddle areas occurred in the V-shaped, solid asphalt floor. Upon comparing the slatted floor to solid floor configurations, it becomes apparent that the drainage of excreted urine is notably more effective on slatted floors, facilitating its flow into the underlying manure storage system. Slatted and grooved floors could be considered more effective in reducing ammonia emissions than asphalt V-shaped floors. A valid mitigation strategy proposed by the authors was to reduce urine pool sizes and consequently NH
3 emissions by the use of manure scraping practices combined with "Preclean" floor that showed a potential for reduction of urine pools of about 50 - 70% less than just scraping.
According to Xu et al. [
18], low nitrogen feeding is the most effective mitigation system and, together with proper manure management, offers the best prospects for reducing total NH
3 emissions and minimising impacts. To reach this conclusion, emission trends over time were analysed to identify factors contributing to emission increases and to identify areas with the highest emissions.
The study by Tullo et al. [
10] examined the environmental impacts of livestock practices and discussed the benefits of PLF as a potential system to mitigate environmental risks. PLF is a technology that uses sensors and algorithms to monitor and manage animal production more efficiently and sustainably, aiming to make livestock farming more economically, socially and environmentally sustainable through observation, interpretation of behaviour and individual animal control. The review presents circumstantiated examples of how PLF can contribute to reducing the environmental impact of livestock production, including studies that have used the life-cycle analysis approach to validate the potential of PLF to improve animal welfare, enhance technical performance, and minimise environmental impact. Through a review of the scientific literature, the study concludes that precision farming offers significant benefits in terms of monitoring and control, reducing harmful emissions and improving animal welfare, thereby contributing to making livestock farming more economically, socially and environmentally sustainable.