The taxonomy of mushrooms currently encompasses a vast array of fungi, comprising approximately 12,000 species, among which more than 2000 species are distinguished as edible and possess therapeutic attributes, resulting in their widespread consumption ^[1][2][3][1,2,3]. In 2018, the global mushroom market achieved a notable volume of around 13 million tons, with projections indicating a substantial growth to 21 million tons by 2026 ^[4][5][6][7][4,5,6,7].

As the mushroom industry advances, it yields a consequential by-product known as spent mushroom substrate (SMS). Comprising residual fungal mycelium, lignocellulosic biomass, and enzymes, SMS has garnered significant attention as a substantial waste product ^[7][8][9][7,8,9]. The composition of raw SMS can vary, with contents of up to 48.7% cellulose, 34% hemicellulose, and 39.8% lignin, contingent upon the source of the mushroom cultivation medium. SMS also serves as a source of essential vitamins and minerals, including iron, magnesium, zinc, and calcium. Moreover, it is remarkably rich in bioactive compounds, encompassing polysaccharides, polypeptides, and phenolics. SMS additionally harbors valuable enzymes; research has indicated that SMS from Lentinula edodes, Agaricus bisporus, and Pleurotus eryngii possesses enzymes such as β-glucanase, xylanase, laccase, and phytase [7].

However, as the edible mushroom sector thrives, SMS accumulates at a remarkable rate, approximately 5 kg for every kilogram of freshly harvested mushrooms, culminating in a staggering 60 million tons over a decade ^[5][7][5,7]. Regrettably, SMS frequently encounters disposal as agricultural waste, incurring substantial costs ranging from 10 to 50 EUR/ton in Europe, thus accumulating a considerable financial burden that could potentially surpass EUR 150 million annually for the industry [8]. The absence of sustainable disposal strategies emerges as a significant hindrance to the continued expansion of the mushroom industry [10]. Arguably, as illustrated in Figure 1, SMS enzyme extraction and bioactive compound potential emerge as promising avenues ^[11][12][11,12]. The incorporation of SMS as animal feed has been practiced for several decades, with P. ostreatus and L. edodes being used in ruminants [13]. Beyond ruminants, SMS integration into livestock, poultry, and fish diets enhances microbial balance and promotes growth [7]. SMS’s potential as a feed source is evident in various species [14], including insects (Beetle: Protaetia Brevitarsis; Mealworm: Tenebrio molitor) ^[15][16][15,16], rabbits (Cuban Brown: Oryctolagus cuniculus) [17], pigs (Sus scrofa domesticus) [18], and ruminants (Sika deer: Cervus nippon, Dairy cows: Bos taurus coreanae, Bos taurus) ^[19][20][21][19,20,21]. Similarly, to mushrooms’ fruiting bodies, whose biological activity is far more studied, SMS can exhibit nutraceutical properties worth studying, such as antioxidant capacity ^[22][23][24][22,23,24], antibacterial and antifungal activity ^[25][26][27][25,26,27], antiaging activity [28], and other health benefits [29].

In the realm of agriculture, spent mushroom substrate (SMS) emerges as a compelling sustainable alternative to conventional chemical fertilizers and soil amendments, offering solutions to both energy consumption and environmental concerns ^[7][30][7,30]. Remarkably, even after undergoing multiple cycles of mushroom production, SMS retains its robust nutrient content and organic matter, rendering it an ideal choice for biofertilization and soil enhancement ^[10][31][10,31]. The distinctive qualities of SMS, including excellent air permeability, water and nutrient retention capabilities, and a loose texture, foster an improved ecological environment for soil microorganisms and enhance the physical structure of the soil, thereby minimizing plant stress and bolstering nutrient bioavailability ^[7][31][7,31]. Furthermore, SMS exhibits the capacity to fine-tune soil pH levels and mitigate soil contamination by various pollutants such as heavy metals, pesticides, and polycyclic aromatic hydrocarbons ^[31][32][31,32].

Beyond its applications in agriculture, SMS holds substantial promise in the realm of renewable energy production from agro-industrial biomass ^[30][33][30,33]. It seamlessly integrates into various bioenergy production methods, bolstering the generation of biomass fuels, biofuels, bioethanol, and biogas ^[30][34][30,34]. Co-digestion of SMS with other lignocellulosic biomass sources has displayed significant potential, particularly in the enhancement of methane and hydrogen production ^[30][34][30,34]. This collaborative approach not only advances renewable energy objectives but also contributes to cost reduction in operational processes [35]. Expanding its role, SMS emerges as a bioremediation contender, targeting environmental contamination through living organisms ^[36][37][36,37]. SMS from various mushroom species effectively removes pollutants like H₂S, volatile compounds, and heavy metals, even degrading harmful compounds like polycyclic aromatic hydrocarbons and pesticides. This underscores SMS’s potential for remediating polluted soils and waters ^[32][36][38][32,36,38].

2. Alternatives to Conventional Fertilizers

Soil organic matter plays a central role in driving soil processes and functions and is a key resource base for agriculture. As demonstrated in Figure 2, it contributes to an improvement in soil structure, water retention, and quality. The incorporation of residual spent mushroom substrate that remains after the mushroom harvesting process, along with the poultry manure obtained from waste disposal has gained substantial endorsement for its advantageous role in agricultural recycling. This practice promotes the establishment of entirely natural nutrient and organic carbon cycles, aligning with the pursuit of enduring sustainability. Both organic materials hold significant value as constituents of composts. Their inherent alkaline characteristics contribute to the amelioration of soil acidity while simultaneously augmenting the accessibility of nutrients ^[39][41]. Furthermore, the utilization of poultry manure and litter extends to serving as a source of chemical energy for the generation of electricity and heat, whereas SMS finds application as a substrate for other mushroom-forming fungi and becomes integral to the production of biofuels and enzymes ^[40][42]. As previously stated, the field of study primarily revolved around fertilizers, particularly the co-composting of spent mushroom substrate (SMS) and poultry manure. The central objective was to explore their potential as substitutes for conventional inorganic fertilizers. Leading this investigation, Maynard ^[41][42][43,44] was one of the first to study this matter in 1993 and 1994. The core focus was to determine whether composted animal manures, specifically SMS and poultry manure, could effectively serve as a nutrient source for intensive vegetable production systems, while also assessing their potential impact on groundwater quality. Maynard’s pioneering studies unveiled intriguing insights. He ventured into a comprehensive analysis of compost compositions. The SMS compost consisted of horse manure and bedding amended with cottonseed, gypsum, chicken manure (CM), and cocoa bean shells. In contrast, the chicken manure compost consisted of chicken manure (43%), horse manure, SMS, and sawdust. Following yearly applications of these composts as the sole source of nutrients, yields of seven crops from these amended plots were compared to yields from control plots fertilized with NPK fertilizer. The CM compost-amended plots had similar or better results compared to the fertilizer used as a control in all three years, with one exception ^[41][43]. Nitrate concentrations in ground water from beneath all compost-amended plots were assessed and the results showed they remained below 10 ppm during the study, while concentrations beneath the fertilized control climbed to 14.7 ppm. Spent mushroom substrate (SMS) has emerged as a focal point in agricultural practices, particularly in the realms of waste management and organic fertilization. SMS boasts a rich composition of organic and mineral matter, with a noteworthy emphasis on essential elements like nitrogen, phosphorus, and potassium ^[43][49]. Moreover, its abundance in organic material renders it a highly sought-after soil amendment. SMS lends itself effectively to composting when combined with other organic waste materials, including chicken feathers and manure. Ali et al. ^[43][49] conducted an enlightening investigation into the response of various onion cultivars to different organic manures, encompassing farmyard manure, poultry manure, and spent mushroom compost. Their findings underscored the significant enhancements in growth and yield achieved through SMS application, ultimately concluding that SMS holds great promise as a potent organic manure option for bolstering onion production. Similarly, Hussain et al. ^[44][50] delved into the effects of organic and inorganic regimes on cauliflower growth, production, and quality characteristics. Their results illuminated the substantial improvements realized across these parameters with SMS application, highlighting its efficacy as an organic fertilizer for promoting robust cauliflower growth and enhancing its nutritional quality, particularly when complemented by the inclusion of poultry waste as a nutrient source. In their research, Rao et al. ^[45][51] employed pelleted organo-mineral fertilizers derived from composted pig slurry solids, animal waste, and spent mushroom substrate for enhancing amenity grasslands, which they aptly described as “golf courses.” The study’s outcomes revealed that these pelleted fertilizers exerted a positive influence on grass growth, shedding light on the promising role of SMS in sustainable nutrient management and its potential to bolster the productivity of grasslands. Shifting our focus to the dynamics of soil nutrients, Li et al. ^[46][52] conducted an insightful in situ field incubation study in apple orchard soil. Their investigation centered on the impact of composted manure and chemical fertilizers, including SMS derived from poultry waste. The results unveiled that the combined application of composted manure and chemical fertilizers, featuring SMS, significantly influenced soil nutrient availability. These findings strongly suggest that the inclusion of SMS in composted manure can enhance nutrient cycling and promote sustainable orchard management. Exploring innovative uses of waste materials, two studies ventured into the potential of composted media crafted from waxed corrugated cardboard as a soil amendment and growing medium for container-grown woody ornamentals. These investigations showcased that these composted media, which incorporated poultry waste-derived SMS, exhibited favorable physical and chemical characteristics, rendering them suitable for stimulating plant growth and serving as eco-friendly alternatives to conventional growing media ^[47][48][53,54]. Collectively, the studies presented herein underscore the substantial potential of integrating spent mushroom substrate (SMS) as a valuable asset in sustainable agriculture. Through inquiries into its integration into composting processes and its impact on diverse crops, these studies illuminate SMS’s capacity to enhance soil quality, stimulate plant growth, and elevate yields. These highlighted benefits, combined with its rich organic and mineral composition, position SMS as a promising organic fertilizer and waste management solution. This opens up exciting avenues for its incorporation into a wide spectrum of agricultural practices, fostering both enhanced productivity and environmental responsibility. Shifting our focus to co-composting practices, Xie et al. ^[49][55] delved into the biochemical and microbiological properties during the co-composting of spent mushroom substrate and chicken feathers. Their study effectively demonstrated the prowess of the co-composting process in breaking down waste material, proving that SMS is a feasible composting feedstock due to its abundant fungus. These findings underscore the considerable potential of co-composting SMS and poultry waste as a viable waste management strategy, yielding nutrient-rich compost while addressing environmental concerns. Kulcu et al. ^[50][56] delved into the intricate process of co-composting, involving spent mushroom substrate, carnation wastes, poultry, and cattle manure. Their research yielded compelling evidence that the co-composting of these materials resulted in the creation of stable compost possessing enhanced physical and chemical characteristics. This discovery underscores the practicality of co-composting as a waste management strategy, simultaneously generating high-quality compost tailor-made for agricultural applications. In a thorough and comprehensive assessment, Jia et al. ^[51][57] embarked on an exploration of the co-composting realm, specifically focusing on the synergy between spent mushroom substrate and poultry manure, with the addition of garden waste. Their meticulous study meticulously scrutinized the physicochemical attributes, humification process, and spectral characteristics of dissolved organic matter throughout the co-composting journey. The findings unveiled that the introduction of garden waste significantly bolstered the composting process, culminating in compost possessing favorable attributes and heightened organic matter stability. These collective findings highlight co-composting’s potential, serving both as an effective waste management strategy and a means of generating nutrient-rich compost ideally suited for sustainable agricultural practices. Lastly, with a focus on the microbial intricacies inherent in composting, Lin et al. ^[52][58] undertook a comprehensive investigation into the dynamic shifts within the microbial community during the large-scale co-composting of swine and poultry manure with spent mushroom substrate. Their results illuminated a dynamic transformation in the composition of the microbial community, accompanied by an increase in microbial diversity throughout the co-composting process. This dynamic shift in microbial composition can be attributed, in part, to the alterations in physiochemical properties resulting from the addition of manure. Such insights into microbial dynamics are pivotal for comprehending the underlying mechanisms and ensuring the production of high-quality compost. These findings highlight the importance of bacterial and fungal dynamics when considering an effective waste management strategy.

3. Renewable Energy and Gas Emissions

Spent mushroom substrate (SMS) and poultry waste stand as formidable allies in the battle against greenhouse gas emissions and environmental degradation. These organic resources hold immense promise, finding effective utilization avenues in composting processes, biogas production, and precision nutrient management strategies ^[53][54][55][59,60,61]. By redirecting these valuable waste streams away from landfills and harnessing their inherent attributes, we can take substantial strides towards sustainable waste management, climate change mitigation, and the promotion of a more robust ecosystem. The utilization of SMS and poultry waste unfolds an opportunity to tackle waste management challenges while concurrently reaping the rewards of environmental enhancement through resource recycling and the generation of renewable energy ^[53][54][55][59,60,61]. This is especially pertinent considering that these lignocellulosic agro-wastes represent accessible raw biomass that can be harnessed to produce biofuels and bioenergy. The process is thoughtfully elucidated in Figure 3. Recent studies have made significant strides in exploring the myriad possibilities presented by spent mushroom substrate (SMS) and poultry waste within the realm of agricultural practices, effectively highlighting the intrinsic value of these resources as versatile organic assets. In one notable endeavor, Zhang et al. ^[53][59] delved into the realm of co-composting, evaluating the potential of chicken manure (CM) in conjunction with SMS, tobacco powder, and vinasse/mushroom bran. This study placed a strong emphasis on harnessing SMS and poultry waste to elevate compost quality while simultaneously mitigating greenhouse gas emissions. Expanding on this theme, another study by Zhang et al. ^[54][60] ventured into the benefits reaped from the combined utilization of SMS and layer manure in co-composting, with a specific focus on the poultry industry. Notably, the incorporation of bamboo biochar into this process emerged as a strategic enhancement, significantly improving nitrogen preservation, and mitigating nutrient loss. This pioneering research underscores the remarkable potential of SMS and poultry waste in optimizing nutrient management practices within poultry farming. In a different research domain, Gao et al. ^[55][61] took on the challenge of anaerobic co-digestion, where SMS was coupled with various forms of livestock manure, including poultry waste, for the purpose of biogas production. This study brought to light the substantial enhancement in biogas production achieved through the co-digestion of SMS and poultry waste, effectively spotlighting the potential of these organic resources as drivers of renewable energy generation within the poultry industry. Turning to the domain of energy recovery, Shu et al. ^[56][62] undertook a study focused on the two-stage anaerobic digestion of SMS and CM, with the aim of optimizing methane production. The outcomes demonstrated a notable increase in methane production, showcasing the substantial potential of SMS and poultry waste in facilitating energy recovery within poultry farming operations. Furthermore, Nyinoh and Utume [37] explored the vital realm of bioremediation, concentrating their efforts on the remediation of oil-contaminated soil employing Pleurotus ostreatus spent substrate. This study underscored the potential of SMS and poultry waste in addressing environmental contamination issues, including those entwined with poultry farming. In collective harmony, these studies underscore the versatile potential of SMS and poultry waste across a spectrum of agricultural applications, including biogas production, precision nutrient management, soil enhancement, and bioremediation, with a specific focus on the dynamic context of the poultry industry. Leveraging these organic resources can significantly contribute to the promotion of sustainable agricultural practices and the efficient management of waste in poultry farming, yielding both ecological and operational benefits.

4. Animal Nutrition

Spent mushroom substrate holds promising potential as a valuable ingredient in animal diets. SMS, derived from the cultivation of mushrooms, offers a rich source of nutrients and bioactive compounds that can benefit livestock nutrition and health. Studies have demonstrated the effectiveness of incorporating SMS in animal feed formulations. Notably, compounds like ellagitannins, lignans, isoflavones, and flavanones, which play a role in gut microbiota, are likely to provide health benefits in the gastro-intestinal tract. Additionally, the metabolites produced by gut microbiota from these compounds might contribute to the broader systemic effects associated with their parent compounds, as observed in Figure 4 ^[57][63]. However, the evidence of the biological activity and the mechanisms underlying the effects of these metabolites in vivo is still unclear, meaning incorporating SMS in poultry diets needs more studies, meaning fewer studies were found. One such study by Noopan et al. ^[58][64] explored the impact of using SMS derived from Cordyceps militaris on broilers’ growth performance and blood metabolite levels. The researchers found that incorporating SMS into the broilers’ diet positively influenced their growth performance, indicating its potential as a beneficial feed supplement. In another study by Chuang et al. ^[59][65], waste mushroom compost was evaluated as a broiler feed supplement. The researchers assessed its effects on broilers’ fat metabolism and antioxidant capacity. The results demonstrated that incorporating waste mushroom compost into the diet had favorable effects on fat metabolism and enhanced the antioxidant capacity of the broilers. This suggests that SMS can serve as a valuable feed supplement for promoting better health and metabolic function in broilers. Foluke et al. ^[60][66] investigated the feasibility of utilizing SMS as a replacement for wheat bran in the diet of broilers. Their study aimed to determine the potential of SMS as an alternative dietary ingredient that could provide similar or improved nutritional benefits for broilers. The findings revealed that SMS could be a suitable replacement for wheat bran in the broilers’ diet, indicating its versatility and potential as a feed ingredient. In a study by Azevedo et al. ^[61][67], the researchers explored the use of spent substrate derived from Pleurotus sajor-caju in the diet of broiler chickens. They examined its effects on the performance of the chickens, focusing on various parameters. The results showed that incorporating the spent substrate into the diet of broiler chickens positively influenced their performance, suggesting that SMS could be a valuable component for optimizing broiler growth and development. Lastly, Machado et al. ^[62][68] focused on using spent mushroom substrate derived from Agaricus blazei in the diet of broiler chicks. Their study aimed to determine the potential benefits of incorporating this specific SMS into the diet of young broilers. The researchers found that the inclusion of Agaricus blazei spent substrate positively affected the growth and development of broiler chicks, further supporting the potential of SMS as a valuable dietary component for poultry. These studies highlight the promising effects of utilizing spent mushroom substrate (SMS) in poultry diets, specifically broilers, chickens, and chicks. The incorporation of SMS as a feed supplement has been shown to positively impact growth performance, blood metabolite levels, fat metabolism, antioxidant capacity, and overall performance in these poultry species. These findings emphasize the potential of SMS as an alternative and beneficial feed ingredient in the poultry industry, offering a sustainable and economically viable approach to enhancing poultry nutrition and productivity. These studies collectively contribute to a body of evidence that highlights the beneficial effects of SMS on animal health and performance. While each study provides insights into specific aspects of animal nutrition, their consistency in observing positive outcomes strengthens the overall conclusions drawn. Furthermore, the safety implications are indirectly supported by the absence of reported negative effects in these studies, However, more research is needed into the matter due to the lack of studies.