Manure Management as a Potential Mitigation Tool to Eliminate Greenhouse Gas Emissions in Livestock Systems

Abstract

Climate change is a contemporary global challenge that requires comprehensive solutions to mitigate its adverse effects. All human activities contribute to climate change, mainly through atmospheric emissions of greenhouse gases (GHGs), such as nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). While most of these emissions are primarily due to fossil fuel use, agriculture and livestock production also contribute to a significant share of approximately 12% of global emissions. Most processes that are implemented within an animal husbandry unit are associated with greenhouse gas emissions, including manure management. This review explores the interconnection between climate change and manure management practices, highlighting the potential for sustainable approaches to mitigating GHG emissions. The key strategies for manure management, such as anaerobic digestion, nutrient management, composting, manure separation and treatment, and improved storage and handling, are discussed, as they are implemented in different livestock production systems (ruminants, poultry, and pigs). Despite the technological progress, there is still a place for further improving manure management approaches, especially in non-ruminant species leading to a higher mitigation potential and a reduction in greenhouse gases emissions. Moreover, policy support and incentives for sustainable practices are crucial for widespread adoption.

1. Introduction

Nowadays, it is unequivocal that climate change is a fact and that it greatly affects agricultural production systems [1], mainly through global warming and the increased prevalence of extreme climatic conditions, such as heat waves, droughts, and heavy rainfall [2]. Nevertheless, it is forecast that, at mid to high latitudes, it could benefit crop production or be “self-cancelled” by a simultaneous increase in temperature, alongside heavy rainfall [3], but, in general, Northern Europe, America, and Asia, as well as the Mediterranean basin and West–Central Asia, are considered the areas that will be the most affected [1,4].

Many human activities, including burning fossil fuels, deforestation, transportation, industrial processes, agriculture, and livestock production, contribute to climate change [1], mainly through atmospheric emissions of greenhouse gases (GHGs), such as nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). While most of these emissions are primarily due to fossil fuel use, agriculture and livestock production also generate a significant share. In 2020, agriculture contributed 5.87 Gt CO₂ eq., representing 12% of global emissions, with more than 65% due to livestock [2]. More precisely, in livestock production, enteric fermentation is considered the biggest contributor of emissions (about 2.8 Gt CO₂ eq.), followed by manure management combined with manure left in the pasture (1.2 Gt CO₂ eq.).

As the world population continuously increases, livestock production systems are trying to keep up by using the following four main strategies: up-scaling, intensification, specialization, and regional conglomeration [3]. Specifically, in developed countries, an increase in the number of animals is not expected in the future but rather an increase in carcass weight and intensification of livestock production operations. However, in the developing world, the animal population is expected to increase, especially for large and small ruminants. But as production systems grow, so does the production of their inevitable consequence, manure. Manure contains significant amounts of inorganic N and microbially available sources of C and water, and, if handled properly, it can be a valuable resource. On the other hand, all of these elements are ideal substrates for the microbial production of N₂O and CH₄, two greenhouse gases that contribute substantially to climate change [4].

This review focuses on the impact of manure management on climate change and the environmental implications of poor manure management practices, as well as sustainable manure management strategies. Finally, advancements in manure management technologies are discussed, along with the research gaps and areas for further investigation.

2. Impact of Manure Management on Climate Change

Manure is produced daily from farm animals at different rates and compositions depending on the type of animal, production stage, production type, and type of feeding, among others. For example, on average, a cow produces 29 kg of manure per day, a sheep 3 kg, a pig 2.5 kg, and a chicken 0.08 kg per day [5]. Manure can be categorized based on the dry matter content in its liquid, slurry, and solid forms, with the predominant nutrients being carbon (C), nitrogen (N), phosphorous (P), sulfur (S), and potassium (K). The quantity of each of these nutrients in manure is highly dependent on the diet composition and physiology of each type of animal. In general, monogastric animals excrete greater quantities of N and P and a lower quantity of C than ruminants [6].

The presence of C, N, and water in manure makes it a perfect substrate for microorganisms to produce large amounts of CH₄ and N₂O, the two major GHGs produced by manure management [7]. Methane production is the result of anaerobic decomposition of organic matter by methane-producing bacteria, and it is dependent on temperature and biomass composition and management [8,9]. On the other hand, N₂O production is due to the processes of nitrification and denitrification, primarily carried out by ammonia-oxidizing, nitrite-oxidizing, and heterotrophic bacteria [10].

In terms of the regional distribution of GHG emissions from livestock manure, according to FAO data for the year 2018, Asia, the Americas, and Africa recorded the largest amounts of manure deposited on agricultural soils, with large-percentage increases since the year 1990. Specifically, Asia deposited 40 million tons N (50% since 1990). The respective amounts for the Americas and Africa were 30 million tons N for each region, reaching increases of 20% and 100% since 1990, respectively. On the contrary, Europe and Oceania had the lowest amounts of manure deposited on agricultural soils, decreasing since 1990 by nearly 46 and 34 percent, respectively [11]. In terms of livestock species, ruminants are the champions of GHG emissions from manure, mainly left on pasture. Monogastric species (chicken and pigs) have significantly lower emissions that are mainly due to manure storage [12].

There are several examples in the literature of manure handled inappropriately resulting, thus, in substantial GHG emissions, as well as serious risks to the environment and human health. Olarinmoye et al. [13] reported that the most common manure handling methods of poultry and livestock farmers in Ogun State, Nigeria, were outright dumping in bushes, garbage sites, or open wastelands; sun-drying and burning, and/or flushing as slurry into nearby streams and rivers. In sub-Saharan Africa, manure is often stored in open piles and, after drying, is either burned as a fuel for cooking or used as a fertilizer in crops [14]. Moreover, in a study in Kenya, most farmers stored their manure in open pits for a mean period of 5 months, and only a small portion of them turned the manure during storage [15].

The same poor manure management practices are also reported for countries in Asia. A study conducted in Cambodia revealed that approximately 46% of households disposed of pig manure in the environment, while the waste from cattle was more frequently utilized as fertilizer for crops [16]. Burton and Martinez [17], found that, in contrast to Europe, livestock manure in Asia is largely sold as a product. However, the availability of land for organic waste application is limited and less accessible, resulting in river pollution. In China, concentrated animal feeding operations permit approximately 45% of the manure produced to be discharged into streams/rivers, whereas in traditional animal feeding systems, almost 90% of the manure produced is collected and used as an organic fertilizer directly on farms [4].

In Europe, manure management is better in general, mainly because of the strict laws regarding environmental protection, but this does not mean that there are no improvements to be made. For example, a study in Cyprus revealed that while most cattle manure was used as a fertilizer in crops, only half of the farmers questioned had appropriate manure storage facilities [18]. Moreover, in other parts of the continent, where legislative frameworks are not as strict, such as Russia, manure is used as a fertilizer in some cases, but most commonly it is placed in forests as a waste, which has multiple environmental implications [19].

Sustainable manure management is of grave importance both in terms of GHG emissions and in terms of nutrient management. This achievement can be accomplished with effective manure legislation and regulations, but it also requires adequate enforcement and compliance [20]. For instance, the application of subsidies for many years in northern European countries has enabled the construction of manure storage facilities [21]. Conversely, the “right to farm” legislation in the U.S. makes the development of regulations on manure management more complicate since they protect industrial-scale operations and profit in agriculture [22].

3. Sustainable Manure Management Strategies

Several methodologies, treatments, or mitigation approaches are available to eliminate the impact of manure storage, management, and use on climate change and, especially, on GHG accumulation in the atmosphere, including anaerobic digestion, diet manipulation, composting, separation, and treatment, as well as proper storage and handling. In each following subsection, we provide a summary of the management strategy, followed by its global importance and then suggestions for improvements.

3.1. Anaerobic Digestion

Anaerobic digestion is a biological process that decomposes organic matter in the absence of oxygen. It involves a mixture of different species of anaerobic microorganisms that transform organic matter into biogas and nutrient-rich digestate. This process is widely used for treating organic waste (e.g., animal manure, industrial sludge, municipal sludge, industrial and municipal wastewater, and wastewater) and is divided into the following two phases: an acidification, or “acid”, phase and a methanogenic, or “methane”, phase (Figure 1) [23]. At the same time, it generates renewable energy in the form of biogas, whose potential depends on the substrate that is used and may vary from 45 mL/g VS (e.g., leaves/straw) to 649 mL/g VS (used vegetable oil) [24]. Although Figure 1 illustrates a simplified approach to organic matter decomposition under anaerobic conditions, anaerobic digestion involves several steps depending on the final products to be produced.

Table 1. Steps of the anaerobic digestion process up to the final production of methane.

Step	Description
Feedstock Collection	Organic materials, such as agricultural residues, food waste, sewage sludge, and organic industrial waste, are collected.
Pre-Processing	Large-sized feedstocks are usually shredded or ground to increase the surface area and enhance microbial digestion.
Loading into Digester	Prepared feedstock is loaded into a sealed anaerobic digester, a bioreactor, where the anaerobic digestion process takes place.
Anaerobic Digestion	Microorganisms, mainly bacteria, break down organic matter in the absence of oxygen. The digestion process involves a series of microbial activities, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. i. Hydrolysis: the breakdown of fats, carbohydrates, protein, and long-chain polymers to fatty acids, sugars, amino acids, and short-chain polymers and dimers; ii. Acidogenesis: conversion of fatty acids, sugars, amino acids, and short-chain polymers and dimers to carbonic acids, propionic acids, alcohols, H2, CO2, and NH3; iii. Acetogenesis: production of acetic acid, H2, and CO2; iv. Methanogenesis: production of methane, CO2, and H2S through the action of methanogenic bacteria on acetic acid.
Biogas Production	Methane (CH4) and carbon dioxide (CO2) are the primary components of the biogas produced during anaerobic digestion. Biogas can be captured and used as a renewable energy source.
Digestate Production	The remaining material after digestion, called digestate, is a nutrient-rich slurry that can be used as a fertilizer.
Biogas Utilization	The produced biogas can be used for various purposes, such as generating electricity, heat, or as a vehicle fuel.

summarizes these steps leading up to the final production of methane.

Anaerobic digestion offers positive effects on the environment in addition to waste elimination. It employs methane and provides a source of renewable energy that is carbon neutral, therefore generating energy without a net increase in atmospheric CO₂. Accordingly, by capturing the methane produced during organic decomposition, anaerobic digestion assists in mitigating greenhouse gas emissions. The methane potential depends on the primary source that is used for anaerobic digestion. In general terms, methane potentials of 250, 230, 217, 210, and 173 Nm³ CH₄/t VS have been reported for horse, poultry, pig, cattle, and turkey manures [25]. More data on the methane potential of animal species are presented in

Table 2. Methane potential of manure from different animal species.

Cattle	Sheep	Goats	Pigs	Poultry	Horse	Units	Reference
Manure
51	-	-	321	295	-	Nm3 CH4/t VS	[26]
204	-	159	323	259	155	Nm3 CH4/t VS	[27]
222	150.5	-	443.6 (piglets)	173	-	Nm3 CH4/t VS	[28]
97	-	-	128	208	-	Nm3 CH4/t VS	[29]
160	-	200	325	-	-	Nm3 CH4/t VS	[30]
-	-	112	-	-	245	Nm3 CH4/t VS	[31]
Liquid slurries
261	-	-	-	-	-	Nm3 CH4/t VS	[32]
311	-	-	99	-	-	Nm3 CH4/t VS	[33]

. The noted differences in the methane potentials among the same manure families can be attributed to differences in the type of farm, duration of storage, and storage method.

Anaerobic digestion reduces, also, the odors associated with organic waste decomposition by up to 80%, creating, therefore, a more pleasant environment. According to Wilkie [23], after 3 days of storage of flushed dairy manure without or under an anaerobic digestion process, its odor level increased by 77% or decreased by 97%, respectively. Organic wastes, particularly animal and municipal sludges, can harbor infections. The anaerobic digester’s ecological conditions efficiently reduce the levels of pathogens (such as salmonella). This is caused by starvation and competition with other bacteria. Additionally, when thermophilic anaerobic digestion is used, the high temperatures help to significantly reduce pathogen levels. The presence of organic acids in anaerobic treatments can also aid in limiting the growth of infections.

In addition, biogas production and digestate can be sold or utilized onsite, creating potential revenue streams for farm units. It further serves as a sustainable and renewable energy source for the generation of electricity and heat. Biogas can also be used in modified gas boilers to produce hot water for on-site use or export. In addition, biogas can undergo upgrading and scrubbing of impurities to achieve a 98% purity in biomethane, which can then be integrated into a natural gas grid or used as fuel for cars, buses, and trains. Therefore, anaerobic digestion can provide direct financial returns through electricity generation. It can provide energy to the local grid, allowing the local utility to meet the growing demand for energy with locally sourced renewable energy. Digestate is also a by-product produced of anaerobic digestion. It is mostly liquid and contains trace elements and undegradable pollutants, including nitrogen, phosphorus, potassium, and residual organic matter. The most popular way to dispose of digestate is to use it in agriculture as a composted soil amendment or as a biofertilizer.

Anaerobic digestion may also assist in the development of a circular economy approach. Utilizing biogas as an energy source contributes to energy diversification and security by providing an alternative to fossil fuels. The digestate produced is a nutrient-rich fertilizer as noted before, enhancing nutrient recycling. In addition, the digestate is more accessible for plants in terms of absorption compared to raw manure, increasing crop productivity and yield.

A study conducted on dairy cattle in northern Spain indicated that methane emissions from manure (slurry) management (including storage and use as fertilizer) varied between 34 and 66 kg CH₄/cow/year for dairy cows [34]. The respective emissions for suckler calves ranged from 13 to 25 kg CH₄/calf/year, while the total emissions ranged from 22,223 to 107,949 kg CO₂ eq./year. Farms may potentially save between 978 and 1776 kg CO₂ eq./year if anaerobic digestion is used. The main reason for these savings is the prevention of methane emissions during slurry management. In addition, the methane required for heating water accounted for the 4–11% of the amount produced and that needed for crops and feed preparation constituted 15–43% of the overall amount. According to Marañón et al. [34], the methane generated by the anaerobic digestion of cattle slurry would be adequate to meet farms’ energy needs.

In a case study of 140 beef heifers reared in Brazil, where anaerobic digestion was considered as a manure management process, Costa Junior et al. [35] reported that 0.14 ± 0.06 kg CO₂ eq./kg animal live weight gain contributed to the direct GHG emissions, with approximately 80% coming from field application. This amount does not differ from that for conventional manure management (i.e., without anaerobic digestion) typically noted in the country (0.19 ± 0.07 kg CO₂ eq./kg animal live weight). However, it was projected that the anaerobic process would produce 2.4 MWh and 658.0 kg of N manure, which may potentially offset 0.13 ± 0.01 kg CO₂ eq./kg animal live weight.

In a study conducted by Styles et al. [36], where different farm models were examined, it was reported that considering slurry anaerobic digestion for big dairy farms may result in decreases of 14% and 67% in global warming potential (GWP) and resource depletion, respectively. However, an increase in eutrophication and acidification levels were noted (9% and 10%, respectively). On the other hand, for medium-sized dairy farms, the use of anaerobic digestion led to a minor decrease in the global warming potential, after excluding the impact of changes in land use from grass to arable land.

In a more recent study on dairy farms in Northern Ireland, a variation in GHG emissions was noted according to the implemented farming system and manure management method [37]. Specifically, when anaerobic digestion was implemented in a semi-intensive system (one-third of the daily time used for grazing during the grazing season, with the animals housed for the remaining periods), the total emissions diminished from 1750 to 1456 t CO₂ eq./yr. Considering a more intensive full-confinement farming system, the respective emissions decreased from 1900 to 1431 t CO₂ eq./yr, when manure was treated under anaerobic digestion. Scarce information exists related to a direct comparison of biogas digesters and baseline scenarios for beef/cattle manure. However, employing anaerobic fermentation for manure treatment, biogas recovery, and utilization can exhibit a high GHG mitigation potential. However, CH₄ leakage from the biogas digester, along with CH₄, N₂O, and NH₃ emissions during the digested slurry storage and land application stages, should all be considered in the assessment of mitigation effects.

Pork is a highly consumed meat product globally, and pig production is the second contributor to GHG emissions derived from the livestock sector [38]. Luckily, it has been proven since 1992 that pig manure is biodegradable and a good base for anaerobic digestion [39,40], which can contribute substantially to the mitigation of GHG emissions. In a study in Finland, Kaparaju and Rintala [41] found that the adoption of anaerobic digestion technology in sow and finishing pig farms can avoid 87.7 and 125.6 Mg of CO₂ eq. emissions per year, respectively. Moreover, Pucker et al. [42], after comparing several biogas systems that used animal manure as a substrate, concluded that all systems had lower GHG emissions than fossil fuel reference systems. Finally, a report from Mexico indicated that utilizing biogas produced from the anaerobic digestion of pig manure for electricity production, along with composting techniques, resulted in the lowest net impact on climate change of 272 kg CO₂ eq. [43].

As far as poultry manure is concerned, there are some problems with its use as the dominant substrate for anaerobic digestion, because it has a significantly higher nitro-gen content than most other substrates (cow manure, pig slurry, and food waste) [44]. Nevertheless, because of the increase in intensive poultry production, around 20,708 million tons of poultry manure are produced annually in the world [45], and it would be a great waste of valuable resources and a huge burden on the environment to discard the entire quantity simply on farmland. Only 30–40% of the chicken manure produced annually is converted into biogas by anaerobic degradation [46]. Therefore, there is ongoing research on the improvement in chicken manure as a substrate for anaerobic digestion. Some of the solutions proposed are co-digestion with other organic feedstocks [45], introducing external additives, such as microelements [47] and membrane-based ammonia separation [46].

Previous researchers used an ammonia diffusion membrane to treat egg-laying hen manure on a laboratory scale, which resulted in a biogas production increase of 1.55 times with the same digester volume, as well as to recover ammonia as ammonium sulfate, which has commercial value [46]. Molaey et al. [47] investigated the possibility of co-digesting chicken manure (CM) and maize silage (MS) and concluded that it was feasible when chicken manure composed less than 20% of the feedstock’s volatile solids because of the ammonia inhibition. In another experiment, the authors used biochar to enhance the anaerobic digestion of chicken manure and found that manure blending used improved methane production [48]. Babaee et al. [49] used a mixture of poultry manure and wheat straw and studied the effects of different temperatures and organic loading rates on the biogas yield and methane contents. They concluded that anaerobic co-digestion was better at 35 °C, with a loading rate of 3.0 kg volatile solids per m³ per day. Rodrigues-Verde [48], after experimenting with a mixture of poultry and pig manure, reported that the optimization of blending, thermo-chemical pre-treatments, and ammonia stripping procedures are the best way to utilize nitrogen-rich substrates, such as poultry manure.

Although the use of anaerobic digestion to produce biogas through manure utilization provides a promising solution for eliminating GHGs, as well as fostering a circular economy, many barriers can be highlighted that inhibit its wider use. Nevzorova and Kutcherov [50] recently and very concisely reviewed these barriers and categorized them into the following six major axes: economic, technical, market, institutional, environmental, and socio-cultural factors. These barriers also reflect, to some extent, the knowledge gaps that exist concerning the implementation of this technology. Briefly, among the technical barriers, technical infrastructural challenges (e.g., plant size, lack of resource availability, and limited number of gas-filling stations) and the need for specialized technical staff and experts are discussed. Also, the dependency on imported material, as well as the need for specific chemical characteristics of the produced biogas, is a major issue. Regarding the economic axis, the lack of available capital and subsidies, combined with the high level of investment, needed for biogas infrastructure is noted. The lower price of fossil fuels, elevated cost of biogas/biomethane, and competition with other fuels constitute the major market challenges. Regarding institutional barriers, the poor coordination between public and private sectors, high degree of bureaucracy, and lack of legislative initiatives represent the uncertain policy and investing landscapes. Socio-culture barriers mainly include the lack of public participation and consumer interest, low levels of knowledge and education, and lack of information regarding biogas technology. In addition, even though biogas production is an eco-friendly approach, environmental barriers exist including odor and noise complaints and water availability. Finally, it should be mentioned that anaerobic digestion, as a process for biogas production, still has limitations, and further improvements should be made [50]. Issues regarding the use of enzymes, bacteria, or catalysts for improving chemical processes are still under exploration. Technological improvements, including pre-treatment methods or the incorporation of high pressure and heat throughout the process, seem promising. Further, the chemical properties of the final product, including, for example, enriched biogas or biogas with less hydrogen sulfide, have been studied [51]. However, it is important to highlight that pathogens are able to survive anaerobic digestion. Although the effects of temperature on indicators such as total coliforms, Enterococci, and E. coli have been reported, a gap still exists concerning microbes, including Cryptosporidium parvum, Mycobacterium spp., and Feline calicivirus, and the elimination of any potential risk to public health. However, a recent meta-analysis of pathogen reduction through anaerobic digestion showed that the process has a positive effect on the diminishment of the pathogenic population related to viruses or Gram-positive or -negative bacteria. The reactor type and temperature, as well as pH, are affected by the noted reduction [52]. Figure 2 summarizes the potential of anaerobic digestion as an implemented method in the treatment of manure derived from farm animals.

3.2. Optimizing Nutrient Management for Sustainable Livestock Systems

Efficient nutrient management in livestock systems is crucial for mitigating environmental impacts, particularly concerning greenhouse gas emissions and nutrient excretion. Dietary manipulation emerges as a prominent strategy to reduce the environmental footprint associated with livestock production.

In efforts to address greenhouse gas emissions, Chojnacka et al. [53] highlight the significance of dietary interventions in livestock, particularly focusing on pigs and ruminants. Dietary modifications, such as incorporating enzyme products, probiotics, or herbal extracts, have been explored to mitigate harmful substance excretion in pigs [54]. Additionally, optimizing the composition of ruminant diets (e.g., optimizing the concentrate-to-forage ratio, changing the quality of forage, using additives and/or methanogenesis inhibitors, etc.) can lead to decreased enteric methane emissions and improved nutrient utilization, contributing to enhanced feed efficiency and reduced environmental pollution [55,56]. Furthermore, Nahm [57] emphasizes the importance of formulating feed rations to maintain appropriate amino acid levels, rather than solely focusing on the total protein content. This approach not only enhances feed efficiency but also results in significant reductions in manure pollutants across various livestock species [58,59,60]. Enzyme supplementation, such as phytase, has been effective in reducing phosphorus excretion in both poultry and swine, thereby decreasing environmental pollution [61,62].

Carter and Kim [63] discuss innovative dietary strategies, including phase feeding and split-sex feeding, to tailor nutrient concentrations to the specific requirements of pigs throughout various growth stages. By reducing the crude protein content and supplementing with crystalline amino acids, substantial decreases in nitrogen excretion can be achieved, leading to notable reductions in ammonia emissions and land usage for manure application [63]. Moreover, optimizing diets for rumen degradable protein in dairy cattle has shown promising results in reducing nitrogen excretion without compromising milk production [64].

Nitrous oxide (N₂O) emissions from livestock production systems pose significant environmental challenges, contributing to greenhouse gas accumulation and climate change. Understanding the complex interactions between animal management practices, nutrient cycling, and environmental conditions is crucial for devising effective strategies to mitigate N₂O emissions.

Hyde et al. [65] highlighted the primary drivers of high N₂O emissions in grazed pastures, attributing them to the nitrogen and carbon inputs from animal excreta and anaerobic conditions induced by soil compaction. Moreover, wet soil conditions post-N fertilization or grazing exacerbate N₂O emissions through denitrification processes [66]. Employing technologies, such as urease and nitrification inhibitors, offer effective mitigation by targeting urea hydrolysis and nitrification processes to control N losses from urine and N fertilizers [67].

Hristov et al. [68] emphasized the substantial GHG emissions from ruminant production systems, particularly enteric methane (CH₄) and N₂O from various sources including manure. Effective management strategies targeting animal diet optimization, N input balancing, and maintaining fiber digestibility offer avenues for reducing N₂O and CH₄ emissions from manure [69]. Oenema et al. [70] underscored the significant role of animal waste in N₂O emissions from agriculture, necessitating improvements in N-use efficiency through proper animal feeding and management. Strategic selection of animals, dietary modifications, and feed additives can help reduce N excretion and subsequent N₂O emissions from manure.

Refining animal diets to improve nutrient utilization efficiency is crucial for reducing N excretion and mitigating N₂O emissions. By carefully balancing protein quality and quantity in animal diets, it is possible to minimize N excretion without compromising animal productivity [71,72]. Additionally, incorporating phase feeding strategies tailored to different growth stages can optimize nutrient intake and reduce N excretion [73,74].

Feed processing techniques such as grinding and pelleting play vital roles in improving feed efficiency and reducing N excretion. Grinding feed to achieve optimal particle size enhances nutrient digestibility, thereby reducing N excretion in manure [75]. Similarly, pelleting feeds can enhance animal performance and reduce N excretion, contributing to overall N₂O mitigation efforts.

Mitigating N₂O emissions in livestock production systems requires a holistic approach encompassing animal management practices, nutrient management strategies, and innovative technologies. By integrating insights from diverse studies and leveraging a combination of dietary modifications, feed processing techniques, and feed additives, it is possible to achieve substantial reductions in N excretion and N₂O emissions. Continued research and collaboration across disciplines are essential for developing and implementing effective mitigation strategies to address the environmental challenges associated with N₂O emissions in livestock systems.

Precision feeding (PF) stands as a pivotal strategy in the realm of sustainable livestock management, aiming to align nutrient supply precisely with the dynamic nutrient requirements of individual animals. As emphasized by Sifri [76], PF represents a shift toward “information intensive nutrition”, wherein animals receive feed tailored to their specific needs, dictated by a multitude of factors including genetic predispositions and environmental conditions.

The necessity for precision in nutrient provision arises from the intricate interplay between animal nutrition, production efficiency, and environmental management. Carter and Kim [63] underscore the significance of optimal nutrient provision in livestock diets, highlighting the critical role it plays in maximizing animal performance while minimizing environmental impacts. Excessive nutrient excretion from animals poses a substantial risk to environmental quality, affecting water and soil integrity, as well as atmospheric emissions. Addressing this challenge necessitates a multifaceted approach, as previously articulated [77]. Precision nutrition emerges as a cornerstone of mitigating nutrient imbalances, with a focus on dynamically adjusting the nutrient supply to meet daily requirements. By adopting PF practices, such as those enabled by advanced technologies, industry can significantly enhance feed efficiency, reduce waste, and mitigate environmental pollution.

The implementation of PF, however, presents inherent challenges. Moss et al. [77] highlight the complexities associated with accurately characterizing feed ingredients and determining precise nutrient requirements, underscoring the need for meticulous management and technological infrastructure. Additionally, the adoption of PF principles, as advocated by Halas and Dukhta [78], offers a holistic approach to address various challenges spanning animal health, welfare, and environmental impacts. Efforts to advance PF in livestock systems require concerted research, innovation, and collaboration across disciplines. By leveraging insights from various studies focusing on nutrient utilization and waste reduction [79,80,81], the industry can refine feeding practices to optimize efficiency and enhance sustainability.

Sustainable livestock management requires a multifaceted approach, integrating precision nutrition tailored to specific species and production systems. By optimizing feed composition and incorporating innovative additives, the industry can not only enhance animal welfare but also mitigate greenhouse gas emissions and nutrient excretion. This shift toward PF reflects a broader recognition of the complex interplay between animal nutrition, environmental factors, and management practices, emphasizing the need for integrated strategies to advance sustainable production systems. Through such initiatives, the livestock sector can strive for economic viability while promoting environmental sustainability and ensuring the well-being of livestock.

3.3. Composting

Composting manure is a common practice in agriculture and gardening to convert animal waste into a nutrient-rich soil conditioner. Manure from animals, such as cows, horses, chickens, and rabbits, can be valuable additions to a compost pile when properly managed. It is considered one of the most affordable and effective ways to manage manure, and it can help animals’ manure to mature more quickly [82,83]. In addition, mature compost can serve as a source of long-term crop nutrients or as an amendment to the soil. This has major implications for managing soil nutrients, livestock and agriculture production’s sustainability, and environmental development [84]. As a procedure, composting is a natural exothermic aerobic process through which the decomposition of organic matter is achieved. Specifically, aerobic microorganisms transform degradable organic matter into CO₂ and water. Composting provides various advantages for managing manure, such as controlling odors, controlling moisture and pathogens in manure, stabilizing organic matter, generating additional farm income [68]. In addition, it not only helps recycle nutrients but also reduces the risk of nutrient runoff and water pollution associated with improperly managed manure. Composting can be conducted in piles or ditches and applied on a small or large scale, and solid or liquid manure can be used. On dairy farms, composted manure solids—which are obtained by separating manure into solids and liquid—are also utilized as bedding to save production costs and improve cow comfort, provided that udder health remains unaffected [85].

3.3.1. Principles of Composting

Composting manure involves specific principles to ensure effective and safe treatment. However, proper composting of manure is of the utmost importance, as livestock manure composting contributes to GHG emissions. The moisture content in livestock waste is typically too high to allow for proper composting, since it contains urine, feces, and other materials, such as sawdust for bedding. Before composting livestock, waste must be mixed with a bulking agent to reduce its moisture content to begin to promote aerobic microbiological breakdown. However, it is frequently challenging to regulate the moisture content of animal manure to an appropriate level because of the lack of a bulking agent. This might result in inappropriate compost fermentation, which raises the organic matter’s percentage of anaerobic decomposition. As a result, more gaseous components—especially CH₄—are produced under anaerobic conditions. The amount of CH₄ emissions increases with the anaerobic proportion within the compost pile [86]. For this reason, an appropriate composting procedure must be efficient in lowering greenhouse gas emissions [10]. The principles of composting are briefly described in

Table 3. Basic principles of the composting procedure using manure.

Step	Description
Source Separation	Keep manure from different animal species separated, as their manure may contain pathogens that are not easily destroyed during composting.
Carbon-to-Nitrogen (C:N) Ratio	A balanced C:N ratio, by combining the nitrogen-rich manure with carbon-rich materials (browns), such as straw or bedding, should be achieved (25–30 parts carbon to 1 part nitrogen).
Aeration	Aerobic conditions, by turning the compost regularly, should be achieved. This helps introduce oxygen into the pile, fostering the growth of beneficial aerobic microorganisms and preventing anaerobic conditions. An aeration rate of 0.22 L Kg/min should be considered.
Moisture Management	Proper moisture levels in the compost pile should be maintained (~60%). The pile should be moist but not waterlogged. Adequate moisture supports microbial activity and decomposition.
Temperature Control	Composting generates heat, which is beneficial for destroying pathogens and weed seeds. The internal temperature of the compost pile should vary between 54 and 71 degrees Celsius. Regular turning helps distribute the heat evenly.
Avoiding Contaminants	Materials that may introduce contaminants or pose risks, such as treated wood, plastics, non-organic materials, or manure from diseased animals, should be avoided.
Biosecurity Measures	Biosecurity measures should be implemented to prevent the spread of diseases. This may include maintaining a buffer zone between composting sites and livestock areas, as well as proper sanitation practices.
Composting Time	Sufficient time for the composting process to complete should be allowed. The duration can vary depending on factors like pile size, aeration, and material composition.
Monitoring and Testing	The compost pile should be regularly monitored for temperature, moisture, and overall progress. Periodic testing for pathogens and the nutrient content can help ensure the safety and quality of the finished compost.
Final Product Handling	The finished compost should properly be stored to maintain its quality.

.

Nutrient losses and, especially, N losses can occur and are influenced by numerous factors, including temperature, C/N ratio, pH, moisture, and material consistency [87]. According to Hao et al. [88], during the composting of straw-bedded manure, up to 30% of the initial N, 53% of C, and 42% of DM can be lost. The NH₃ losses can be considerable, reaching up to 50% of the total N in the manure, depending on the composting intensity [89]. According to previous studies [90,91,92], the aeration of composting assists in lowering CH₄ emissions, but it can also result in higher NH₃ and N₂O losses. Therefore, composting of animal manure may cause significant losses in terms of N and CO₂. However, its mitigation potential toward GHG emissions is significant, as is discussed in the next section.

3.3.2. Methane Reduction and Soil Enrichment

The management of manure composting contributes to alleviating the global greenhouse effect. According to Brown et al. [93], the main advantage of composting compared to anaerobic manure storage is the reduction in CH₄ emissions. Specifically, if the same amount of organic matter is composted, this could save the equivalent of 3.1 Mg CO₂ per Mg of dry feedstocks compared to anaerobic storage systems. Raw cattle manure can emit approximately 160 (winter) to 3600 (summer) g/m³ CH₄ and release 38 to 57 g/m³ N₂O. The release rates from digested manure are almost half for methane emissions, ranging from 80 (winter) to 1200 g/m³ (summer) CH₄, and nitrous oxide, from 40 to 76 g/m³ N₂O, respectively [94]. Lopez-Real and Baptista [95] quantified methane emissions for the first 20 days of cattle manure composting. They observed that methane emissions were drastically reduced by both window turning and forced aeration. When dried grass is incorporated into dairy manure composting, a reduction in greenhouse gas emissions can be achieved. Based on research by Maeda et al. [96], the addition of dried grass in composted dairy manure decreased GHG emissions from 20.8  ±  1.3 g/ kg volatile solids (VS) to 5.4  ±  1.4 g/kg VS (74.3% mitigation) for CH₄ and from 7.4  ±  2.6 g N₂O-N/kg N initial to 2.7  ±  0.4 g N₂O-N/kg N initial (62.8% mitigation) for N₂O. According to the same authors [96], it is anticipated that the Japanese dairy industry will implement this approach and reduce its greenhouse gas emissions by 1907 Gg CO₂-eq/year. Liu et al. [97] found that windrow pile composting resulted in lower GHG emissions and nutrient loss compared to trough composting. In the same notion, reducing the moisture content or increasing the turning frequency can decrease CH₄ by 31.8% and 62.6% in dairy manure, respectively, because of the promotion of the growth of aerobic microorganisms and the decomposition of organic matter. The composting process, despite its mitigation potential, results in significant production and emission of nitrous oxide. These emissions are generally higher during thermophilic composting when the air supply is limited, as observed with heaped manure [98]. Most N₂O emissions occur during the curing and storage phase, when the ammonium nitrogen is being converted to nitrate. Despite this release of emissions from compost, Brown et al. [93] concluded that the nitrous oxide gases that are emitted during composting should be considered minimal in comparison to the advantage of methane reduction that is achieved by composting. Accordingly, composting of animal manure results in considerable N and CO₂ losses, but the benefits of reducing odor and methane emissions, compared to anaerobically stored manure, make it a recommended GHG-mitigating option [68].

The same benefits are also observed in the case of composting pig manure. Nevertheless, there is also the possibility of environmental risks since pathogens, heavy metals, and other phytotoxic contaminants may be present in the final product. Several researchers have shown that there is a risk of high concentrations of Cu, Mn, and Zn in swine manure because of the feed additives used [99,100]. The decomposition of manure involves biological actions and spontaneous chemical reactions, which may influence the concentrations and chemical characterization of elements in composts [101]. Pig manure is most suitable for biogas production, as it has a high buffer capacity and, therefore, helps to maintain stable and optimal conditions during digestion [102].

In the case of poultry manure, some physical and chemical characteristics limit its use in composting and could lower the process’ efficiency, as follows: high moisture, low porosity, and high N concentration for the organic C, which leads to a low C/N ratio and, in some cases, high pH values [103]. Therefore, there is a need for a bulking agent, and researchers have focused on determining which is more appropriate [104]. Rizzo et al. [105] studied the composting process of poultry manure mixed with other complementary organic wastes and concluded that when poultry manure was added to the mixture at a rate of 60%, the composting process was not affected, but the final product was better if even less manure content was present. In another study, when biochar was used as a bulking agent, the nitrogen losses in the mature composts were reduced, but the authors concluded that the use of sawdust would be more efficient [106].

3.4. Manure Separation and Treatment

3.4.1. Process, Principles, and Efficiency

Manure separation is a process used in livestock farms (dairy, poultry, pig, etc., farms) to separate the solid and liquid components of manure, usually using gravity or mechanical systems. As a process, it can be briefly described by the following steps: (a) manure collection—the manure, which usually consists of solid and liquid components, along with various nutrients and organic matter, is collected from livestock units; (b) pre-treatment—the raw manure may undergo pre-treatment steps, such as screening or agitation to break up the solids and facilitate separation; (c) separation—the solid and liquid components of the manure are separated using mechanical or chemical methods; (d) storage of treated manure—the liquid and solid components are stored separately for further applications or utilization.

Approaches to the separation of solid and liquid components may include gravity-based systems, mechanical separators, or chemical treatments [107]. In gravity-based systems (sedimentation), the manure is stored in large tanks, ponds, or lagoons, allowing solids to settle to the bottom over time. The liquid portion can then be pumped or drawn off from the top. The remaining thicker slurry at the bottom can be left to dry in a holding pond and then removed. With mechanical separation methods (known also as active systems), the manure is passed through screens, filters, or centrifuges that physically separate solids from liquids. Common mechanical systems encompass rotating, vibrating, and stationary, inclined screens, rollers, belts, screw presses, centrifuges, and hydro-cyclones [108]. Finally, chemical treatment employs the use of coagulants or flocculants that make small particles in manure stick together to form larger particles. This facilitates the separation process and helps target the separation of certain compounds.

Regarding separation efficiency, it is determined by the removal of solids and nutrients from the incoming manure stream. Efficient separators produce solids with high nutritional contents, particularly phosphorus. Efficiency, however, depends on various parameters, including separator type and design, manure consistency, total solids content, and flow rate. For instance, when a screening drum is employed, only 9% of the manure’s phosphorus remains with the solids, compared to an average of 30% when a screw press is used. The average mass separation efficiency of total solids, total nitrogen, organic nitrogen, inorganic nitrogen, and total phosphorous that can be achieved in manure by mechanical separation reaches approximately 45%, 18%, 20%, 15%, and 21%, respectively [109,110].

3.4.2. Advantages of Manure Separation

Manure separation offers various benefits to both livestock units and the environment [111]. The management of manure in terms of economics is improved by separation since separated solids can be transported over longer distances at a lower cost than unseparated manure. The handling volume is also greatly decreased, and the nutrient content is raised. Compared to an unseparated slurry, separated liquids require less agitation during storage and can be transported more readily with low-capacity pumps. Further, when the solid content is low enough, manure liquids can also be used in place of or to augment fresh irrigation water. The separation of solid and liquid components produces a liquid fraction (liquid manure), which is rich in nutrients and can be used as a potent fertilizer. Additionally, separation helps to mitigate environmental pollution by reducing the runoff of nutrients into surface water bodies and groundwater. Solid fractions may be composted, used as bedding material, or processed into value-added products. For example, dry manure fibers appropriate for cow bedding can be produced via well-managed solid–liquid separation. Separating manure assists in reducing the production of foul odors linked with decomposing organic matter. Additionally, the separation of the liquid fraction can decrease the ammonia content in the remaining solid fraction, leading to reduced ammonia volatilization during storage and application. Solid–liquid separation, particularly when paired with anaerobic digestion, can lower greenhouse gas (GHG) emissions from liquid manure storage, since certain volatile compounds (those that are readily degradable) in manure follow the solid stream after separation [108].

3.4.3. GHG Emissions and Solid Manure Separation

Comparing manure handling without processing to the manure treated with the use of solid–liquid equipment, GHG emissions can be reduced by up to 20% [108,111]. If manure solids are stored, methane emissions are limited because of the aerated conditions that exist during this type of storage. Liquide manure storage also reduces methane emissions since volatile solids are separated from solids. In addition, during the separation process, fibrous organic material is removed, preventing the construction of a crust on the top of the liquid phase. This way, anaerobic storage conditions are ensured, nitrate formation is prevented, and, thus, nitrous oxide emissions from liquid storage are reduced. If the crust is not prohibited, then aerobic conditions will develop, promoting the production of nitrates on the surface. Accordingly, nitrates will be converted into nitrous oxide, ensuring the increase in GHG emissions [112]. Previous studies on methane emissions from untreated cattle slurry reported 15.3 g C·ton⁻¹·d⁻¹ stored at 15 °C [113], 6.1 g C·ton⁻¹·d⁻¹ for storage at 20 °C [114], and 1.65 g C·ton⁻¹·d⁻¹ at 17 °C. Aguirre-Villegas et al. [108] indicated that solid–liquid separation can reduce GHG emissions from dairy farms by 19% compared to simple storage. Regarding ammonia emissions derived from the storage of the liquid stream after solid–liquid separation, these emissions were comparable to those in systems without separation [115]. Perazzolo et al. [116] reported that mechanical separation reduced GHG emissions (CO₂ eq.) by 40% on cattle slurries. Screw presses with high efficiency can diminish greenhouse gas emissions by over 60% [115]. Based on a meta-analysis study [111], ammonia emissions tend to increase with solid–liquid separation techniques, possibly due to the absence of consistent crust development on the slurry surface or to a reduced content of solid particles that may lower the capability of NH₄⁺ absorption and cause higher free NH₃ emissions.

In pigs, Wang et al. [117] indicated that different in-house manure collection methods have a significant impact on gas emissions, particularly for CH₄ and N₂O. For example, CH₄ emissions increased in the deep-pit mode because of the anaerobic conditions, especially during long storage periods, whereas in the separation mode, CH₄ emissions are much lower. The findings of de Vries et al. [118] showed that using a scraping under-slat separation of liquid/solid fractions of between 3 and 12 times a day, CO₂ emissions were reduced by 47% and CH₄ emissions by 90%. However, N₂O emissions increased by 250% [38].

3.5. Improved Storage and Handling

The improvement of storage and handling conditions of manure is essential for eliminating environmental impacts, reducing odors, and maximizing its potential as an alternative resource. Apart from composting and anaerobic digestion, which have already been discussed, there are more, less elaborate strategies that can be used to improve manure storage and handling, thus mitigating its environmental impact.

3.5.1. Manure Coverage and GHG Emissions

Manure coverage is the process of covering the manure’s surface with materials of a specific thickness, as opposed to the conventional practice of piling the dung up to let it air out. Covering manure presents a cost-effective solution for manure treatment and is characterized by its simplicity of operation and ease of implementation. However, the potential for eliminating emissions is affected by the material that is used for covering [119]. A different number of manure storage cover types have been reported including expanded clay, wood, straw, wood chips, oil layers, sealed plastic, semi-permeable, and natural crusts in high solids content manure storages [120,121]. The effectiveness of these types varies and depends on many aspects, such as porosity, degradability, thickness, permeability, and management. For example, straw, wood chips, or even naturally crusted manure may reduce odor, methane, and ammonia emissions, with a decrease depending on the thickness and permeability of the cover layer. However, these cover types favor nitrous oxide emissions, as they provide optimal aerobic conditions for nitrification at the cover surface and, thus, they are likely to raise nitrous oxide emissions [68,121]. Accordingly, it is unclear how efficient semi-permeable manure storage covers can be, and the outcomes vary greatly based on the material and the specific circumstances in which they are used [68].

Berg et al. [122] reported that the strongest encrustation delivers the highest N₂O emission fluxes. The application of water to the encrusted surface simulates rainfall and can reduce N₂O emissions [123]. Previous studies [121,124] indicate that although vegetable oil layers are highly effective as manure storage covers and offer a mitigating potential to diminish methane, ammonia, and nitrous oxide, they are not very practical due to their degradability, production of unpleasant odors, and challenge of keeping the oil film from mixing or being “broken” over the manure’s surface.

A potential solution could also be the sealing of the manure storage with an impermeable coverage. The rise in air pressure inside the structure lowers the fraction of gases in the gas phase and increases the fraction trapped in the liquid form of manure. However, its mitigation potential is doubtful, since during the transportation of manure the pressure inside the storage container decreases, releasing a greater fraction of gases contained in the liquid portion of the manure. Burning the captured CH₄ under the cover with a flare system or engine generator for electricity production is beneficial. Otherwise, if the trapped CH₄ escapes later, it is not beneficial to retain it.

Table 4. Parameters of permeable and impermeable types of manure coverage.

Parameter	Type of Cover
	Permeable	Impermeable
Purpose	Odor and gas reductions	Odor and gas reduction. Capture and use methane
Effectiveness	Up to 60–90% reductions in ammonia and hydrogen sulfide	Up to 95% reductions in ammonia and hydrogen sulfide
Lifespan	2 months to 10 years (depending on weather and material)	5–15 years
Manure Accumulation	Cover material needs to be removed or kept separated during agitation and pumping Breaking up natural crusts	Cover material needs to be removed or kept separate during agitation and pumping
Precipitation	Allow water in and reduce evaporation. An increase in storage capacity is required	Snow, rain, debris, and silt may accumulate on top of impermeable covers. Pumps are commonly used to remove liquid from the surface

summarizes some important aspects when considering the type of material used for manure coverage.

Chianese et al. [125] reported that the average CH₄ emissions were 6.5, 5.4, and 2.3 kg/m²/year from covered and uncovered slurries and manure in stacks from dairy farms, respectively. Lague et al. [126] noted that the average CH₄-C/(CH₄-C + CO₂-C) rations were 0.24 and 0.06 in uncovered manure and in manure covered with straw, respectively. In pigs, some studies report that emissions of odorous and volatile organic compounds can decrease by 90% with the usage of a membrane-covered composting system [127]. Ma et al. [128] found that there was a significant (p < 0.05) difference in the interstitial oxygen concentration between a laboratory-scale, aerobic, and membrane-covered composting system and a control composting system with no membrane. Specifically, the interstitial oxygen concentration was lower with the membrane cover, the utilization of oxygen was improved, leading to a decrease in methane emissions by 22% [128]. On the other hand, Bicudo et al. [129] did not observe a significant reduction in NH₃ emissions using geotextile covers in manure storage ponds, but reductions in odor and H₂S occurred only in the second year of the study.

Table 5. Effectiveness (%) of cover types in odor and gasses reductions (N/A: not applied).

Type	Material	Odor Decrease	H2S Decrease	NH3 Decrease	Lifespan
Permeable	Natural Crust	56–78%	81%	11–37%	3 months
Permeable	Straw	45–83%	86–100%	79–86%	2–6 months
Permeable	Geotexile (2.4 mm)	51–63%	59–71%	15–37%	3–5 years
Permeable	Straw + Geotexile	50–80%	60–98	8–85	N/A
Impermeable	Concrete	95–100%	N/A	N/A	20 years
Impermeable	Woodlid	75–95%	N/A	98%	N/A
Impermeable	Floating	95–99%	95%	95%	10 years
Impermeable	Air pressure	95%	95–99%	95%	10 years

summarizes the effectiveness of various cover systems in terms of odor and gasses reductions [130].

Although studies report that covering manure with organic matter offers a mitigation solution to adapt to the differences in climatic regions (temperature and precipitation), as well as differences in manure properties and covering materials, further studies should be conducted to analyze and test the potential of various combinations of these parameters and their effect on reducing greenhouse gas emissions.

3.5.2. Manure Incorporation into Soils and Mitigation Potential

Incorporating manure into soils is a common agricultural practice that helps improve soil fertility and structure and boost the organic matter content [131]. Proper incorporation techniques ensure efficient nutrient utilization, minimize nutrient runoff, and reduce environmental impacts. Therefore, its management practices can influence emissions of methane (CH₄) and nitrous oxide (N₂O), which are potent greenhouse gases. According to Hyde et al. [132], land spreading of manure offers a high mitigation potential to achieve target levels of minimized GHG emissions. Animal manure contains nitrogen (N), phosphorus (P), and other micronutrients for growing plants. Therefore, when manure is used correctly as fertilizer, farmers can frequently save money. However, since animal manure accounts for up to 10% of total global livestock GHG emissions and 7% of global agricultural emissions, its application has raised concerns. The extent of GHG emissions in response to manure application under diverse environmental conditions remains uncertain [131].

Table 6. Methods for incorporating manure into soil.

Method	Description
Ploughing	Use moldboard ploughs to bury manure deep into the soil, effectively mixing it with the soil profile.
Disc Harrowing	Disc harrows can break up clumps of manure and mix it with the soil surface, promoting faster decomposition and nutrient release.
Cultivation	Cultivators or tillage equipment can incorporate manure into the soil while minimizing soil disturbance.
Injection	Injection equipment can place manure directly into the soil at controlled depths, reducing surface runoff and odor emissions.

summarizes currently available methods for manure incorporation into soils.

A meta-analysis study [131] indicated that, compared to pig and cattle dung, poultry manure significantly increases CO₂, CH₄, and N₂O emissions. The use of poultry manure raised the overall soil greenhouse gas emissions’ GWP (global warming potential). The maximum concentrations of CO₂, CH₄, and N₂O emissions were observed in neutral soils, followed by alkaline soils and acidic soils. Soil texture, climate zone, and crop type were also noted to be significant factors in the GHG emissions’ increase.

The timing of the manure’s incorporation may also influence GHG emissions. Incorporating manure into soils during periods of active microbial activity and aerobic conditions can minimize methane emissions. Additionally, well-drained soils with good aeration are less conducive to methane production. For example, during the application stage, NH3 emissions can be reduced if manure can be applied rapidly and in narrow bands during the coolest portion of the day. Wet soils also tend to promote N₂O emissions. Therefore, the application of manure before a rain event should be avoided to prevent emissions [68].

The incorporation of organic materials like manure into soils can contribute to carbon sequestration, whereby carbon is stored in soil organic matter. This can aim to mitigate GHG emissions by removing CO₂ from the atmosphere and sequestering it in soils over the long term. For example, reduced tillage or absence of tillage, as well as the incorporation of crop residues, favors carbon sequestration in soils. However, such practices may enhance N₂O emissions. Moreover, delaying the ploughing of leys is of utmost importance in the reduction of N₂O emissions [123]. Further, shifting autumn manure application to spring and incorporating all manure within 1 day of application can reduce N₂O emissions from field-applied manure by 17% [133].

According to Carew [134], land spreading through the injection or incorporation of manure into soil is one of the most effective methods to eliminate livestock NH₃ emissions in comparison to housing and manure storage. However, the localized anaerobic conditions that are developed in the buried liquid manure and the increased degradable C pool may result in higher CH₄ emissions compared to surface-applied manure [68]. Some options to reduce CH₄ emissions are the dilution of the manure or the reduction of degradable C flux through solid separation or anaerobic degradation pre-treatments [94,135,136]. Unlike methane, most of the nitrous oxide is produced after manure application. Accordingly, during the first weeks of application, manure injections often increase N₂O emissions compared with surface-applied manure. It has been reported that maintaining the soil pH above 6.5 can mitigate nitrous oxide emissions [137]. Reducing the protein content in an animal’s diet produces manure with a decreased N content and, thus, a slower N mineralization rate. This could lead to a decrease in nitrous oxide production during manure applications in soil. Generally, it would be advisable to weigh nitrous oxide emissions and NH₃ resulting from different methods of manure application (e.g., surface applied or injected) in the soil.

3.6. Black Soldier Fly Larvae: A Sustainable Approach to Manure Management

Black soldier fly larvae (Hermetia illucens L.—BSFL) represent an innovative and sustainable approach to tackle manure disposal and mitigate greenhouse gas emissions [138]. Their ability to convert organic waste into valuable biomass and frass demonstrates their potential to address both agricultural and environmental challenges. On top of that, BSFL can decrease organic waste volume by as much as 60%, with the carbon and nitrogen being sequestered, which leads to massively impactful drops in CO₂, CH₄, and N₂O emissions during the process of bioconversion [139,140,141]. They also promote microbes that reduce methane and nitrous oxide emissions, including Methanophaga and Marinobacter [142].

With high protein (30–52%) and fat (15–50%) contents, BSFL can be used as a sustainable alternative to fishmeal and soybean in animal nutrition, while decreasing the heavy reliance on these feeds, which have high environmental footprints [138,143,144,145]. Moreover, their frass is a nutrient-rich byproduct that is high in nitrogen and phosphorus, with possible agricultural use similar to compost or some pre-processing for a biogas factory [140]. Studies have shown that BSFL are not only able to adapt to a multitude of substrates, such as animal manure, food waste, and agricultural residues, but that efficient waste conversion can be achieved under diverse conditions [140,142].

In addition to waste reduction, BSFL appears to adhere to circular economy principles, wherein waste material is effectively converted back into resource material. The capacity of larvae to contribute to nutrient recycling, while also serving as a source of protein-rich biomass for animal feed and nutrient-rich frass, highlights both their economic and environmental potential [140]. However, obstacles still exist that need to be overcome to improve the efficiency of the process in terms of optimal substrate quality, optimal larval density, and scalability [140,142]. Recent studies also emphasize the need for standardized methodologies to compare BSFL’s environmental impacts reliably [138].

Owing to their rapid manure and organic residue processing capability, coupled with their economic and environmental advantages, BSFL can play a pivotal role in providing sustainable livestock systems. The possibility of reducing GHG emissions and waste volumes, as well as contributing to nutrient recycling, which may represent a pathway toward carbon-neutral agriculture, highlights their significance [138,140,142]. Costs related to keeping an adult fly population for breeding for further usage is of utmost importance and a potential obstacle since it is more labor- and cost-intensive compared to the direct use of wild populations [143].

4. Methods Comparison

Each previously discussed method has its own merits and disadvantages.

Table 7. Comparison of manure treatment methods. The symbols +/− represent higher or lower impacts related to infrastructure, workload, cost, product value, or mitigation potential for each method. The more “+” a method receive the higher is the effect.

Method	Infrastructure	Workload	Cost	Product Value	Mitigation Potential
Anaerobic digestion	+++	+	+++	+++	+++
Nutrient management	+	+	+	−	++
Composting	+	++	+	++	++
Separation and treatment	++	++	++	++	++
Storage and handling	+	+	+	+	+
Fly larvae	+	++	++	+++	+++

presents a brief comparison of the methods depending on their need for infrastructure, necessary workload, cost, and value of the product, as well as their mitigation potential. For example, anaerobic digestion is the most expensive manure treatment method that can be applied but that also has a small workload and higher mitigation potential. Moreover, the final product, biogas, has a higher value than the other four methods.

It must be noted, though, that some of these methods can be combined, thus increasing their mitigation potential. For example, manure separation can precede composting, while both these methods can be practiced after the manure’s proper treatment and handling. Moreover, nutrient management is a rather independent strategy that can be applied irrespective of the manure management practice or in combination with any of them, thus exerting a higher mitigation potential.

The application cost of each method is always an important decision factor before their application, but it should not be the determining one. The effects of climate change are beginning to be obvious in everyday life and have the potential to be very detrimental; thus, all efforts should be made to reduce GHGs emissions, irrespective of the cost. Of course, states must play an important role in this matter by properly educating farmers and providing subsidies or incentives that can reduce the application cost and act as a boost for proper manure management. Usually, states also implement policies regarding manure treatment through authorities such as the Ministries of Agriculture and Environment and Ministries of Public Health and/or Energy [146,147]. However, the defined rules, in most of cases, usually present a way to meet targets for renewable energy and to decrease methane and nitrous oxide emissions, and, sometimes, various legislation contradict one another, or even present gaps or do not fit well with common farming practices, without, however, providing support for a holistic approach to manure management [147]. Therefore, specific attention should be given by policymakers to aspects related to proper field policies and not only practice guidance and coherency of legislation to avoid legislative gaps, as well as strengthening of the policies’ enforcement.

5. Conclusions and Perspectives

Manure management practices, including anaerobic digestion, composting, and precision feeding, can significantly eliminate GHG emissions from livestock systems and ameliorate their environmental impact (Figure 3). The optimal solution should be determined by several factors, including the species of animal, amount and type of manure produced, and proximity to specific infrastructures like biogas plants or farmland, among others. Future climate changes related to the temperature change of our planet, as presented by the projections of different climate scenarios, can also affect the potential of the various methods related to manure management. Considering the scarce information that exists on how an increase could affect the mitigation potential of the available manure management methods, future studies can shed light on this aspect.

Additionally, there is still a place for further improving manure management approaches, especially in non-ruminant species leading to a higher mitigation potential. More importantly, as the recent experience with the European Union strategy has shown, sustainable manure management should be instructed and imposed by the relative legislation, and further motivation, like subsidies or incentives, should be provided to farmers to make complying with the rules easier for them. Therefore, policy support and incentives are crucial for the wide adoption of these practices.

Additionally, alongside subsidies and incentives, implementing penalties for non-compliance can drive the more rapid adoption of best practices. Generally, it is required to use optimal approaches permitting the sustainable development of the agricultural sector [148]. Although anaerobic digestion seems to be a promising strategy for mitigating GHG emissions that derive from manure, the low ratio of C/N and ammonia’s inhibitory impact, which result in low biogas generation efficiency, could be issues upon which attention should be focused [148]. The potential of anaerobic digestion to reduce pathogen loads by 1 to 5 log units for various microorganisms [149] presents a valuable perspective for manure reuse, considering public health as well. Characterizing the process’s output and determining its future use in accordance with current legislation are also important because of the increasing regulatory limitations on biowastes and their potential reuse. Nevertheless, issues related to the consistency and thorough sanitization of all pathogens remain [149]. Therefore, the optimization of such systems for the concurrent reduction in pathogens and recovery of energy in relation to pathogen inactivation enhancement should be achieved in the future. Furthermore, the use of user-friendly simulation software or models (Gleam-i, MAELIA etc.) [150,151] can enhance the presentation of the mitigation potential of manure treatment from a farmer’s perspective. In addition, they can serve as useful tools for providing insight into potential outcomes before policies are implemented. The key idea that must be embedded is that manure is a by-product and not just waste, and, therefore, it should be treated as such, fostering circular economy principles.