Vermicomposting as a Valorization Solution to the Winery Sector By-Products

Abstract

Winemaking is one of the most relevant socio-economic activities in the world, particularly in countries such as Portugal, generating substantial amounts of by-products across its various phases. Managing these by-products presents significant environmental, ecological, and economic challenges. Vermicomposting, the use of earthworms to process and transform organic wastes into nutrient-rich end-products, provides a viable alternative for waste management within the wine sector. This bioprocess also produces vermicompost that enhances soil health and fosters optimal conditions for plant growth, thereby promoting resilient and sustainable viticulture practices. This review explores the various by-products produced by the vine and wine industry, highlighting the potential of vermicomposting. Remarkably, grape marc, the primary solid waste of the winemaking process, has been the most commonly vermicomposted material. In contrast, other vineyard and wine cellar by-products have been comparatively underutilized and understudied in this context. However, we hypothesize that all vine and wine industry by-products have vermicomposting potential, allowing for the production of fertilizers and soil amendments. This approach aligns with the principles of the circular economy, ensuring that all materials stemming from viticulture and wine production are valued and reused, thereby contributing to enhanced sustainability and the preservation of resources like soil.

1. Introduction

Viticulture and the wine industry rank among the world’s most important agricultural activities, having a significant economic impact that extends to employment, regional development, and exportation. The International Organization of Vine and Wine reported that, in 2022, global wine production reached approximately 260 million hectoliters, resulting in a large amount of by-products and an increasing need to find innovative ways to extract value from them [1]. Viticulture and winemaking are complex and multi-step processes encompassing grape cultivation, pruning, vine training, pest and disease control, ripening, and harvest. Subsequent stages involve crushing, pressing, fermentation, bottling, and ageing to effectively produce wine [2,3]. Importantly, viticulture is a year-round activity, as illustrated in Figure 1. Throughout this continuous cycle, the different by-products generated, namely pruning residues, leaves, and grape stems (Figure 2A–D), can be efficiently recycled, reducing the risk of waste accumulation that might otherwise find its way to landfill or incineration. It should be noted that the distillery industry also contributes significantly to the generation of by-products and residues, such as spent grape marc, lees cake, vinasses, and vinasse biosolids (Figure 2D–H) [4,5,6]. However, distillery by-products are no longer a remunerative option for the viticulture industry according to REGULATION (EU) No 1308/2013 ([7]).

In general, wine sector waste materials exhibit low pH, as well as high levels of organic matter, simple and complex polyphenols, nitrogen, phosphorus, potassium, and heavy metals [8]. Therefore, it is crucial to ensure their proper handling and disposal in order to prevent environmental issues such as odors, wastewater contamination, or soil and water source pollution. These residues have the potential to adversely affect both the chemical and biological properties of the soil, leading to potential phytotoxic effects on crops. Furthermore, polyphenol-rich by-products in animal feed have been found to be a potential problem due to their bonding with proteins, forming compounds that are unsuitable for nutritional purposes [9]. Aligning with the principles of the circular economy trend and ‘reduce, reuse, recycle’ (3R’s) strategy, innovative solutions must be explored and adopted to repurpose these products. Among the various potential approaches to valorize these by-products [10], such as animal feed, biogas production, the extraction of bioactive compounds for the pharmaceutical, cosmetic, and nutraceutical industries, and wine vinegar production or biorefinery approaches [11,12,13], vermicomposting stands out as a promising option for converting them into value-added products, such as organic fertilizers or soil amendments.

This strategy also aims to address the overreliance on chemical fertilizers in traditional agriculture, a significant concern due to their detrimental impacts on soil health, environmental sustainability, and crop yield [14]. Despite their widespread use, the correlation between fertilizer input and crop yield is not strictly linear; beyond optimal levels, further increases may not significantly improve yields and could even have adverse effects [15]. Prolonged use of chemical fertilizers has been shown to cause soil acidification, degradation, and compaction, ultimately restraining optimal plant growth and productivity [16]. Additionally, concerns about the harmful effects of chemical fertilizers on human and animal health have led to increased research into developing more sustainable alternatives with minimal environmental impact [16]. Recognizing the urgency of mitigating these issues, the European Commission has established a target of reducing non-renewable resource consumption in fertilizer production, which are still based on mineral deposits and fossil fuels [17]. Transitioning from a fossil fuel-dependent economy to a bio-based one requires the efficient extraction of nutrients from waste streams, primarily by substituting these fertilizers with bio-based alternatives.

Vermicompost, a nutrient-rich organic fertilizer resembling peat, is produced through the collaborative efforts of earthworms and microbes, serving as a potent promoter of plant growth due to its high nutritional content, aeration, porosity, and water-holding capacity [18]. This method is considered to be even more effective for organic waste degradation than composting because it passes through the earthworm’s gut, facilitating the breakdown of organic materials and promoting the growth of beneficial bacteria that aid in decomposition [19] along with reduced amounts of total and bioavailable metals content [20]. As such, vermicompost is considered to be a long-term supplier of the macro- and micronutrients required for plant growth, including nitrogen, phosphorus and potassium, zinc, magnesium, and iron in readily available forms for plants [18]. Furthermore, vermicompost has been found to improve plant growth and development, especially due to the presence of humic acids [21,22], while also suppressing plant diseases caused by antagonistic microbes and nematodes [23,24]. The vermicomposting of waste materials, particularly those from the viticulture and wine sector by-products, could prove to be highly effective in small geographical areas of wine production, facilitating the reuse and valorization of residues while ensuring the maintenance of organic matter levels crucial for soil quality and functionality [9,25,26].

This review aims to explore the potential use of vine and wine by-products as vermicomposting substrates, shed light on the value of these vermicomposts as organic fertilizers or soil amendments, and identify knowledge gaps in the current research, ultimately contributing to a more sustainable management of by-products in the viticulture and wine industry.

2. The Role of Earthworms in Organic Waste Transformation

Vermicomposting is a highly efficient process of bio-oxidation and organic waste stabilization, which involves the combined action of earthworms and microorganisms [27]. Among the various earthworm species, Eisenia fetida and Eisenia andrei are the most used in vermicomposting due to several aspects, such as remarkable resilience to a wide range of environmental conditions, including temperature and humidity, ability to decompose organic materials, high reproductive rates, short life cycles, and ease of handling [28,29,30]. As earthworms break down and consume fresh organic matter, the physical processes within their digestive tract generate smaller particles, thereby substantially enhancing the surface area accessible for microbial colonization and enzymatic activity [18]. The high microbial diversity, enzymes, and hormones found in the earthworm gut accelerate the decomposition of partially consumed materials, a considerable improvement compared to the traditional composting process [31,32]. Also, the biology of the worm’s gut makes possible the growth of fungi and bacteria that are beneficial for plant growth [18]. All processes occurring during earthworm digestion contribute to a more homogeneous organic material [33]. Furthermore, earthworm activity enhances aeration through the creation of burrows and tunnels that improve oxygen circulation, helping prevent anaerobic conditions and gradually lowering the substrate’s carbon-to-nitrogen (C:N) ratio, optimizing conditions for decomposition [34].

3. Vineyard and Wine Sector By-Products

Vine and wine by-products represent a rich source of organic materials generated throughout the various stages of viticulture and winemaking [35,36]. Knowing the characteristics and composition of these residues is essential for evaluating their potential as substrates for vermicomposting. The optimal characteristics for earthworm activity during the vermicomposting process typically involve a neutral pH range and a C:N ratio of around 25. In this section, we explore and discuss the characteristics of vine and wine by-products, such as pruning residues, leaves, grape stems, grape marc, wine lees, and vinasse.

Table 1. Summary of the chemical composition of vine and wine industry by-products.

By-Product	pH	C:N	EC (mS m−1)	Organic Matter (%)	References
Pruning residues	5.21–5.83	72.80–73.00	2.24–2.46	93.50%	[37]
Grape stalk	4.40	ND	4.44	92.00	[3]
Grape marc	3.76–7.77	14.37–27.57	0.28–1.34	90.63–95.30	[38,39,40,41,42,43]
Spent grape marc	4.82–6.40	25.00–35.00	3.00	ND	[25,44]
Lees	3.45–4.0	23.00	0.04–5.59	72.00–75.90	[3,45,46]
Vinasse	3.88	19.34	6.36	ND	[47]

EC: electrical conductivity; C:N: carbon/nitrogen ratio; ND: not determined.

provides a comprehensive overview of some of the chemical characteristics of the by-products derived from the vine and wine industry.

3.1. Pruning Residues

Pruning (Figure 2A,B) is an essential management practice of vineyards, encompassing two distinct phases: the first, called winter pruning, where post-grape harvesting is conducted (during the dormant period), serves to promote vegetative balance, regularize production and quality, and ensure the vine’s long-term health and productivity; the second phase, known as green pruning, occurs after the sprouting of new shoots in spring and is instrumental to optimize the balance of grape bunch production and enhance aeration within the vine. This process involves the complete or partial removal of leaves from the shoot’s basal area, especially where there are numerous grape bunches [48,49,50].

The winter-pruned vine branches (Figure 1 and Figure 2A), commonly referred to as vine shoots, grape vine cane, and pruning wood, represent the primary residue generated in vineyards, with an estimated annual production of 2–4 million tons per hectare of cultivation [10,50,51,52,53]. These by-products consist mainly of lignocellulosic material composed of cellulose, hemicellulose, and lignin, being also rich in polyphenols, such as phenolic acids, flavanols, flavanones, flavanols, stilbenes, and proteins [50,54,55,56,57]. Traditionally, winter pruning residues have been managed through practices such as disposal within the vineyard, on-field shredding between the rows with potential burial, or collection for burning as domestic fuel [10,58]. However, these practices present notable disadvantages. For instance, when vineyards are affected by illnesses, landfilling introduces phytosanitary concerns, while burning has an environmental impact in terms of air quality through the emission of fine particles, in addition to posing fire hazards [59,60]. Furthermore, it is important to note that burning also contributes to the release of CO₂, increasing the impacts of climate change [61]. Emerging alternatives for using this material include solid biofuels, pulp paper manufacturing, active carbon production like biochar, the cultivation of edible fungi, the development of industrial materials, and the extraction of bioactive compounds [10,50,62,63,64,65,66].

Green pruning (Figure 1 and Figure 2B) generates valuable by-products called green pruning residues, which include grapevine leaves, grapevine shoots, and fruitless young twigs [48,49]. These residues are rich in components such as quercetin 3-O-glucuronide, caftaric acid, and quercetin 3-O-glucoside and exhibit antioxidant properties [49]. While green pruning residues are generally left on vineyards, some studies have explored their potential as a source of bioactive phenolic compounds [48,49,67,68].

3.2. Leaves and Grape Stems

Grapevine leaves (Figure 1 and Figure 2C) are a by-product generated during various vineyard activities, such as pruning, defoliation, and grape harvesting. These leaves are usually left in vineyard soil surfaces or disposed in landfills, resulting in the under exploitation of this valuable resource. It is noteworthy that grapevine leaves contain several organic acids, phenolic acids, flavanols, tannins, procyanidins, anthocyanins, lipids, enzymes, vitamins, carotenoids, terpenes, and reducing or non-reducing sugars [69,70,71,72]. Moreover, leaf components have been shown to exhibit antioxidant potential [70,73]. Grapevine leaves have been used in folk medicine due to various biological activities, and they are also used as food, considered a delicacy in some Mediterranean countries, and employed in pharmaceutical and cosmetic industries as sources of bioactive compounds [74,75,76,77,78].

Grape stems (Figure 1 and Figure 2D), also known as grape stalks, are the skeleton of the grape cluster and are usually discarded during the grape destemming and crushing process [79]. Approximately 2 to 3 million tons of this by-product are produced each year of cultivation [10,80]. This lignocellulosic material is composed of lignin, cellulose, hemicellulose, acids, sugars, proteins, and tannins [35,79,81,82]. Common uses of grape stems include soil enrichment, animal feed supplementation, the absorption of toxic compounds, biomass for bio-ethanol production, substrate-activated carbon production, and the extraction of highly valued antioxidants [4,83,84,85,86].

3.3. Grape Marc

Grape marc (Figure 1 and Figure 2E), also known as grape bagasse or grape pomace, represents the primary solid by-product generated in the winemaking process. Globally, wine production generates between 10.5 and 13.1 million metric tons of grape marc annually, equating to roughly 1 kg of grape marc produced for every 6 L of manufactured wine [87]. It is the by-product resulting from the pressing stages and consists of residual grape stalk, pulp, seeds, and skins [88]. Overall, grape marc is composed of cellulose, hemicellulose, proteins, and lignin; however, it varies depending on factors such as grape variety, ripeness, and the pressing procedure [2,87,89]. Usually, this product is further processed to produce spirit drinks and alcohol; thus, it may also generate an exhausted grape marc, which can be further applied for alcohol and tartrates production—this process in turn results in the formation of a new by-product called spent grape marc (Figure 1 and Figure 2G). Moreover, additional valuable compounds can be extracted during this process, namely polyphenols (which possess antioxidant activity), pigments, and organic acids [82,90,91].

Grape marc has been used for various applications, including biofuel production, animal feed additives, and soil conditioner amendment [87,92]. However, for the later, it is essential to treat grape marc beforehand to mitigate potential negative impacts on plant growth, stemming from its phytotoxic and antimicrobial properties resulting from the high phenolic content [92]. Vermicomposting has emerged as a promising solution to mitigate the allelopathic effects, as earthworms have the ability to partially digest polyphenols, which are responsible for the adverse effects associated with grape marc [93,94].

3.4. Wine Lees

Wine lees (Figure 1 and Figure 2F) consist of the residues remaining in the wine containers following fermentation, during storage, or after authorized treatments, including those obtained through wine filtration or centrifugation [95]. These lees are mainly used in the production of alcohol and tartrates, resulting in another by-product known as solid lees cakes (Figure 2H) [96].

The composition of lees encompasses yeast, bacteria, insoluble carbohydrates, phenolic compounds, lignin, proteins, organic salts, ethanol, and organic acids, such as tartaric and acetic acids [88,96]. Several studies have explored potential applications of wine lees, including their use as a nutrient supplement, additives in the food industry, antioxidants, biochar for metal absorption, and the extraction of polysaccharides for wine composition modulation [97,98,99,100,101,102].

3.5. Vinasse

The wastewater that results from the distillation of low-quality wine, grape marc, and lees is commonly known as vinasse [9,103]. Vinasse (Figure 1 and Figure 2I) is characterized by the presence of dead yeast, grape pulp, skins, and seeds. Through proper treatment, vinasse can be transformed into another valuable by-product referred to as vinasse biosolid (Figure 2J) [25,104].

Some studies have explored the potential of these by-products derived from the wine industry as a substrate for lactic acid production [105], the recovery of tartaric acid [104], and their biocidal effects [106]. Additionally, as previously mentioned, combining vinasse biosolid with another vine waste (vine shoots) has demonstrated promising properties as a sorbent for pesticide control and as an organic soil amendment [44,107]. However, the direct application of vinasse without appropriate treatments may pose significant risks to environmental and ecosystem health, as evidenced by recent findings suggesting a high toxicological impact on both terrestrial plants and aquatic organisms, even at high dilution levels [47].

4. Vermicomposting as a Strategy to Valorize Vine and Wine By-Products

Vermicomposting emerges as a sustainable alternative for the valorization and stabilization of by-product materials generated by viticulture and the wine industry [108]. By converting these by-products into a nutrient-rich fertilizer or soil amendment, vermicomposting supports soil vineyard health and productivity. It also reduces the amount of waste and minimizes the environmental impacts associated with viticulture and wine production. Moreover, it promotes a circular economy in the sector.

The use of vermicompost as a fertilizer material offers multifaceted benefits for soil health, plant growth, and environmental sustainability [109]. By reducing reliance on chemical fertilizers and promoting more ecologically balanced agricultural practices, vermicompost contributes to the resilience and long-term viability of farming systems while minimizing their environmental footprint. Moreover, this approach offers numerous advantages, including increased soil organic matter content, improved water retention, enhanced nutrient availability and plant phytohormone production, and a significant contribution to mitigating both abiotic and biotic stresses [18].

To better understand the use of these residues in vermicomposting, a comprehensive basic literature search was conducted on PubMed in August 2023 using the following search combinations: vermicomposting/vermicompost and 1. winery wastes; 2. wine waste; 3. grape marc; 4. vineyard pruning residues; 5. grape stems; 6. leaves; 7. lees; 8. lees cake; 9. vinasses; and 10. vinasse biosolid. Review articles were excluded, and no limit was imposed on the date of publication. As illustrated in Figure 3, grape marc emerges as the predominant vine and wine by-product subject to vermicomposting, with its use documented in eleven research articles. It has been the subject of extensive study and characterization, demonstrating that vermicomposting yields a high-quality organic fertilizer with a low polyphenols content [87,93,110]. Additionally, a few studies have explored mixtures of different by-products, such as biosolid vinasse with vine shoot (three articles), lees cake with vine shoot (one article), and spent grape marc with lees cake (one article). Spent grape marc was used by itself in seven articles. A compilation of the literature’s findings, regarding vermicomposting studies on by-products from the vine and wine industry, is depicted in

Table 2. Comprehensive compilation of studies that investigated vermicomposting of vine and wine industry by-products.

By-Product(s)	Experimental Layout (Species, Duration, Reactors)	Evaluated Parameters	Main Findings	Ref.
Spent grape marc, vinasse biosolids, lees cakes, and vine shoots	Eisenia andrei (10 g/treatment); 16 wks; 1 L plastic containers	Earthworm density and biomass; enzyme activities; chemical analysis; phytotoxicity of the initial substrates and end-products	↓ C:N ratios, conductivity, phenolic compound content, phytotoxicity; ↑ pH, humic acids, and nutrient contents	[25]
Vine shoots, spent grape marc, lees cake, and biosolids vinasse	Eisenia andrei (1500); 8 mos; 2 m2 beds	Chemical analysis, TEC; TOC; humic acids; fulvic acids; total N; TP; total lignin; sorption capability	↑ pH, TEC, and HA; pesticide sorption capacity ↓ TOC, total lignin, and polyphenol contents, C:N ratio	[44]
Spent grape marc, vine shoot, and biosolid vinasses	Eisenia andrei (1500); 6 mos; 2 m2 beds	Dissolved organic carbon concentration in the leachates; diuron and imidacloprid in leachates and in soil	↓ pesticides and their metabolites leaching in soil columns	[107]
Grape marc	Eisenia andrei (220 ± 14 g f.w.); 15 d; 2 L plastic containers	Final biomass; chemical analysis	↓ abundance of bacterial and fungal phospholipid fatty acid biomarkers, microbial, protease, and cellulase activities and cellulose content	[38]
	Eisenia andrei (214 ± 26/m2); 112 d; Rectangular metal (4 × 1.5 × 1 m) vermireactor	Earthworm density; TP; chemical analysis	Polyphenol-free organic fertilizer	[39]
	Eisenia andrei (7.1 ± 4.1 g); 240 d; Vertical vermireactors (40 × 40 × 18 cm)	Chemical analysis; microbial biomass and activities; phytotoxicity assay	Stabilization detected after 240 d; ↓ phytotoxic effects	[40]
	Eisenia andrei (203 ± 28/m2); 112 d; Rectangular metal vermireactor (4 × 1.5 × 1 m; 6 m3)	Physicochemical analysis; microbiological analysis; TP	↑ concentration of macro- and micronutrients; ↓ polyphenol content and microbial activity	[41]
	Eisenia andrei (273 ± 28/m2); 91 d; Rectangular metal vermireactor (4 m× 1.5 m × 1 m)	Earthworm density and biomass; microbial identification	↑ bacterial diversity taxonomic and phylogenetic	[111]
	Eisenia andrei (297 ± 20/m2); 91 d; Vermireactor (4 m × 1.5 m × 1 m)	Earthworm density and biomass; microbial identification	↑ taxonomic and, phylogenetic α-diversity, cellulose metabolism, plant hormone synthesis, and antibiotic synthesis	[108]
	Eisenia andrei (10.000/m2); Vermireactors (6 m2)	Physicochemical analysis; basal respiration; carboxylesterase activity; sensitivity of carboxylesterase activity to pesticides; determination of pesticide residues	Pesticide-sensitive carboxylesterases; inhibited carboxylesterase activity; vermicompost degraded chlorpyrifos and enzymatically inactivated its toxic metabolite	[112]
	Eisenia andrei (97 ± 20/m2); 2 yr; Metallic container (6 m3)	Chemical analysis; activity of 12 enzymes associated with the C, N, P, and S cycles	↑ earthworm density; ↓ pH and electrical conductivity, organic matter, total C, dissolved organic N, P, K, and Cu contents; basal respiration; carboxylesterase, peroxidase, and catalase activities	[42]
	Eisenia andrei (400/group); 42 wks; PVC modules (30 cm × 70 cm)	Chemical analysis; microbial biomass, activity, and identification	↑ pH and content in macro- and micronutrients; ↓ total C and N and microbial biomass and activity in the grape marc	[113]
	Eisenia andrei (1361 ± 415 g/m2); 63 days; Metal pilot-scale vermireactors (2 m × 50 cm × 1 m)	Chemical analysis; enzyme activity	↓ basal respiration; suitable macro- and micronutrient content	[43]
	Eisenia andrei (400 individuals); 21 and 63 d; Cylinders PVC (30 cm × 70 cm)	Microbial identification; metataxonomic analysis; metagenomic analysis	While fungi and bacteria are the main agents behind biochemical decomposition, earthworms are essential to the process of plant vermicomposting	[114]
Spent grape marc	Eisenia fetida (1000 individuals); 3 mos; One wood box	Chemical analysis	C:N ratio was higher in GMC than GMV; the pH was close to neutrality for both; the total Mg and P concentrations were similar; K concentration was higher for GMC, and the Ca concentration was higher for GMV	[115]
	Eisenia andrei (50 individuals); >12 mos; Windrow vermicomposting (2.5 m × 50 m)	Chemical analysis; phospholipid fatty acid analysis	↑ N-NH4+, dissolved organic carbon, and N-NH4+/N-NO3− contents in the top layers; ↑ microbial biomass and earthworm biomass in the upper layer	[116]
	Eisenia andrei (10.923 ± 1783/m2); 42 d; Rectangular metal vermireactor (4 m × 1.5 m × 1 m)	Microbial identification	Dominance of Proteobacteria and Bacteroidetes in the bacterial microbiome; ↑ richness and taxonomic diversity of bacterial communities	[117]

d: days; wks: weeks; mos: months; ↑: increased; ↓: decreased; f.w.: fresh weight; yr.: years; GMC: spent grape composting; GMV: vermicomposting; C: carbon; N: nitrogen; S: sulfur; TEC: total extractable carbon; TOC: total organic carbon; TP: total polyphenols.

.

Nogales et al. [25] conducted a study to evaluate the capacity of the earthworm species E. andrei to compost different winery wastes, including a mix between vine shoots and lees cake, into valuable agricultural products. After a 16-week study, the resulting vermicompost exhibited a lower C:N ratio, electrical conductivity, and phytotoxicity compared to initial substrates. In contrast, it had higher levels of humic compounds, nutrient content, and pH values, making this vermicompost a promising candidate as a soil organic amendment [25]. Romero et al. [44] also conducted a comparative analysis of vermicomposts sourced from winery and alcohol industries, including a mixture of vine shoots with biosolids vinasse, for their potential use as sorbents in pesticide control. The vermicompost, originated from vine shoots with biosolids vinasse, exhibited lower sorption capacity compared to others, which was attributed to the loss of lignin and total organic carbon during the vermicomposting process [44]. In another experimental study, the vermicomposts obtained from spent grape marc and from a mixture of vine shoots and biosolid vinasse were assessed as soil amendments to minimize the leaching of imidacloprid and its metabolites in soil columns [107]. This study found that the application of vermicompost derived from these winery by-products delayed and reduced the leaching of these pesticides and their metabolites [107].

Regarding grape marc, Gómez-Brandón et al. [38] showed that E. andrei played a key role in the stabilization of grape marc over a short period (two weeks). This stabilization is achieved via their impact on organic matter decomposition, microbial biomass, and activity, including a reduction in the labile carbon and cellulose contents. Moreover, Domínguez et al. [39] conducted a 112-day assessment of industrial-scale grape marc vermicomposting, yielding a high-quality, polyphenol-free vermicompost. More recently, Gómez-Brandon et al. [43] showed the effectiveness of vermicomposting for treating and stabilizing red and white raw grape marc, resulting in reduced basal respiration and the improved content of macro- and micronutrients in the end-products. These findings are consistent with the results obtained in other experimental works conducted by various researchers [40,41,42].

Another study has explored the feasibility of vermicomposting as an extended processing method for white and red grape marc over a period of 42 weeks [113]. Using E. andrei within a continuous-feeding vermicomposting system at an industrial scale, a final vermicompost, with optimum physiochemical parameters and lower values of microbial biomass, indicative of a stabilized material, was developed [113]. In alignment with these findings, Částková and Hanč [116] found that grape marc vermicompost, produced using a continuous feeding system for more than 12  months, exhibited exceptional properties suitable for use as a fertilizer.

A study by Gómez Brandón and colleagues [111] has explored the dynamics of bacterial succession during a 91-day vermicomposting process of grape marc in a pilot-scale vermireactor. In this study, which contrasts with the aforementioned results, the final product revealed a more diverse bacterial community, both taxonomically and phylogenetically. The work of Kolbe et al. [108] reached similar results, obtaining a vermicompost from white grape marc after 91 days, rich in a stable bacterial community with functional properties beneficial for plant growth. Pérez-Losada et al. [114] assessed the taxonomic and functional diversity of bacterial and fungal communities during the vermicomposting of white grape marc. The authors concluded that, although bacteria and fungi are the main agents behind biochemical decomposition, earthworms play an essential role in vermicomposting, as evidenced by their ability to accelerate decomposition and significantly modify the physical and microbiological properties of the substrate [114]. Furthermore, Gómez Brandón et al. [117] treated distilled grape marc through vermicomposting over 42 days, and the results showed significant changes in the bacterial community, accompanied by a two-fold increase in bacterial richness and diversity, both taxonomically and phylogenetically.

Vermicompost can also be used as an amendment for slate mining wastes. In a study performed with vermicompost derived from spent grape marc, Paradelo et al. [115] concluded that these vermicomposts significantly increased the nutrient concentrations in slate processing fines and enhanced biological activity by increasing microbial biomass and enzymatic activity compared to compost from the same material [115]. Additionally, to explore the role of vermicompost as a remediation substrate, Sanchez-Hernandez et al. [112] studied the vermicomposting of grape marc in pilot-scale vermireactors and concluded that grape-marc-derived vermicompost contains pesticide-sensitive carboxylesterases, which are important enzymes for pesticide inactivation. The enzymatic bioremediating capacity of vermicompost was found to be enhanced when solid vermicompost was homogenized in water.

Presently, there is a shortage of research on the vermicomposting of wine lees and vinasse. Nonetheless, Romero et al. [44] have shown that vermicomposting a mixture of spent marc and lees cake results in a final vermicompost with the ability to sorb pesticides. Notably, this vermicompost exhibited improved characteristics when compared to control compost (without earthworms), including an increase in pH, a decrease in the C:N ratio, and a reduction in the polyphenol content.

The vermicompost obtained in the aforementioned studies (Table 2) showed several beneficial changes compared to the initial substrates, namely an increase in pH, higher levels of humic compounds and nutrient content, along with a lower C:N ratio, electrical conductivity, and phytotoxicity. These improved characteristics play a crucial role in enhancing soil fertility, structure, water retention, nutrient availability, and reduced stress on plants, ultimately supporting healthier and more productive plant growth [118]. Additionally, the use of earthworm compost promotes microbial activity, with functional properties beneficial for plant growth such as nitrogen fixation and phosphorus solubilization [119].

However, certain limitations associated with vermicomposting must be acknowledged, particularly the need for meticulous control over environmental conditions that affect the wellbeing of the earthworms. Factors such as temperature, humidity, pH levels, and the C:N ratio must be carefully monitored, as variations in these parameters can impact the viability of the organisms and, subsequently, the efficacy of the vermicomposting process. To mitigate potential issues, it is crucial to select materials with appropriate C:N ratios. This prevents adverse outcomes, such as ammonia release at low C:N ratios, which can be toxic to earthworms [120], or reduced microbial activity at higher C:N ratios due to nitrogen deficiency [121]. The integration of other materials into the vermicomposting process, such as animal slurry, can be a solution for the C/N ratios [122]. In general, the vine and wine by-products have C:N ratios close to ideal (Table 1) and can be used individually as substrates for vermicomposting. However, pruning residues, which have a high C:N ratio, should be mixed with nitrogenous materials to decrease carbon content and increase nitrogen levels, thereby improving the quality of the vermicompost [123].

Additionally, the correct amount of material must be chosen for the vermicompost container in use to avoid anaerobic conditions during the process, which would also harm the earthworms [123]. The location where these containers are stored can also influence the process, particularly temperature, which should not reach levels as high as those in traditional composting to guarantee the wellbeing of the earthworms and their normal functioning [124].

While the literature on the impact of vermicompost derived from vine and wine by-products on soil health, plant growth, or microbial activity remains limited, noteworthy studies have demonstrated the significant potential of vermicompost. For instance, Rosado et al. [125,126] conducted a comparative analysis of the must and wine microbiota of grapevines treated with and without vermicompost derived from grape marc [125]. Their findings revealed that grapevines treated with vermicompost derived from grape marc exhibited increased grape production and enhanced organoleptic properties in the final wine [125].

In light of the ongoing emphasis on sustainable agricultural practices in the European Union [127], the integration of vermicomposting into governmental strategies has shown potential to address several environmental challenges while fostering agricultural resilience [128]. Vermicomposting aligns seamlessly with government policies and frameworks, such as the European Green Deal, that promote the transition to a circular economy and mitigate the environmental footprint of various industries, including viticulture [129]. Governments can effectively address several pressing concerns by incorporating vermicomposting into these strategies, including the management of organic waste generated by the vine and wine industry, the restoration of soil health and fertility through the use of nutrient-rich vermicomposts instead of chemical fertilizers, and an overall reduction in greenhouse gas emissions associated with conventional waste disposal methods. Future prospects in this subject, especially applied to the vine and wine industry, include the widespread implementation of product quality regulations, increased investment in research and infrastructure, and collaboration between research units, stakeholders, and environmental specialists.

5. Conclusions

This review aimed to highlight the potential of vermicomposting as an effective, simple, and environmentally friendly solution for valorizing vine and wine industry by-products, transforming them into valuable fertilizers and/or soil amendments and promoting sustainable agricultural practices. The comprehensive analysis of existing research, notably focusing on grape marc, highlights the positive outcomes of vermicomposting in terms of reduced polyphenol content, improved nutrient profiles, and enhanced microbial activity. Regarding other by-products, there is still limited experimental research exploring their use as substrates for vermicomposting, namely grape leaves or stems. From a circular economy perspective, all residues generated by the vine and wine industry can be successfully vermicomposted, resulting in a final product that represents an economically viable and sustainable solution for enhancing soil health and plant growth. In our opinion, further research is needed on this subject, as the potential of these by-products is demonstrated through their chemical and biological characteristics. However, there are only a few studies evaluating the impact of vermicompost derived from these by-products on soil health and plants growth. Nevertheless, vineyard owners play a crucial role in driving this paradigm shift by adhering and following key European policies and initiatives, namely the European Green Deal. Acknowledging the importance of investing in more sustainable practices, such as integrating vermicomposting systems, they will contribute to promoting environmentally responsible viticulture with long-term benefits, including higher grape yields and the production of better wines.