The direct application of vegetable residue to soil enhances fertility by promoting microbial-mediated nutrient cycling, where decomposers like Bacillus ( Table 1 ) and Trichoderma break down cellulose, while nitrogen-fixing (Nitrosomonas) and phosphate-solubilizing bacteria (Pseudomonas; Table 1 ) convert organic matter into plant-available nutrients (14). Residue composition influences nutrient release low C:N ratios (e.g., leafy greens) mineralize nitrogen rapidly, whereas high C:N materials (e.g., woody stems) may temporarily immobilize nitrogen, requiring pre-composting (15). Lignin-degrading fungi contribute to humus formation, improving soil stability, while microbial exopolysaccharides enhance aggregation, reducing reliance on synthetic fertilizers and suppressing pathogens (16). Challenges like nutrient immobilization or allelopathic effects (e.g., onion/garlic peels) can be mitigated through controlled decomposition, aligning with circular agriculture by closing nutrient loops (17).

Composting is a vital practice in sustainable agriculture, enhancing soil fertility by improving organic matter content, nutrient availability, and microbial activity, which collectively boost soil structure, water retention, and plant productivity ( Figure 1 ) (14, 18–20). The decomposition process is driven by dynamic microbial communities, including bacteria (Bacillus subtilis; Table 1 ), fungi, and actinomycetes, whose activity is optimized at a carbon-to-nitrogen (C:N) ratio of ~30:1, ensuring efficient breakdown while minimizing odor and nutrient imbalances (21). Key operational factors such as aeration (maintaining aerobic conditions), moisture (50–60%; Figure 2 ), and residue composition ( Table 2 ) critically influence composting efficiency (22). Beyond agronomic benefits, composting mitigates environmental impacts by diverting organic waste from landfills, reducing greenhouse gas emissions (23, 24).

Composting performance is critically influenced by feedstock composition, particularly the carbon-to-nitrogen (C:N) ratio and moisture content, which govern microbial metabolic activity and heat generation. High-moisture, nitrogen-rich vegetable residues often induce anaerobic conditions due to excessive water retention, slowing decomposition and promoting foul odors (14). Conversely, lignocellulosic bulking agents such as maize straw (C:N ~60–80:1, moisture <15%) or dry leaves (C:N ~40–60:1, moisture 10-20%) provide structural porosity, enhance oxygen diffusion, and balance excess nitrogen (25). Research demonstrates that blending these feedstocks in ratios achieving an initial C:N of 25–30:1 significantly accelerates microbial colonization, with thermophilic bacteria like Bacillus spp., Thermus spp. proliferating rapidly, driving composting temperatures to 55–65°C within 48 hours (26). A study by Finore et al. found that a 3:1 ratio of vegetable residue to maize straw reduced composting time by 40% compared to unmixed vegetable residue, while maintaining >55°C for 5 (+) days, ensuring pathogen inactivation (EPA Class A standards) (27). Moisture optimization (50–60%) through bulking agents prevents leachate formation and enhances lignin degradation by thermophilic fungi like Aspergillus fumigatus (28). Strategic blending thus offers a scalable solution to improve composting kinetics, reduce greenhouse gas emissions, and yield stable, nutrient-rich compost (29).

Vermicomposting enhances soil fertility and agricultural productivity by promoting nutrient cycling, microbial biodiversity, and humic substance formation through earthworm-mediated decomposition of organic matter (10, 30–32). Vermicompost increases the bioavailability of essential nutrients (N, P, K; Table 1 ) and improves soil structure by accelerating organic matter breakdown into plant-accessible forms (33). According to studies conducted by Blouin et al., application of vermicompost increases plant biomass significantly, with average increases of 26% in commercial yield and 78% in microbial biomass (34). Its humic compounds and microbial communities (Pseudomonas putida; Table 1 ) further enhance soil ecosystem functioning, microbial biomass, and long-term fertility (35–38).

Anaerobic digestion (AD) of vegetable residue is a four-stage biochemical process (hydrolysis, acidogenesis, acetogenesis, and methanogenesis; illustrated in Figure 2 ) mediated by specialized microbial communities to produce biogas (CH₄ and CO₂) (39, 40). Hydrolysis, the rate-limiting step, breaks down recalcitrant cellulose and hemicellulose via extracellular enzymes from Clostridium and Bacteroides spp., with enzymatic pretreatments often required to enhance efficiency (32, 41, 42). Acidogenic bacteria (e.g., Clostridium, Enterobacter) ferment monomers into volatile fatty acids (VFAs), while acetogens (e.g., Syntrophobacter) further convert VFAs to acetate, H₂, and CO₂, critical substrates for methanogenesis (43–46). Methanogenic archaea (e.g., Methanobacterium spp.; Table 1 ) then produce methane, with process efficiency dependent on substrate composition, microbial dynamics, and operational conditions (pH, temperature, retention time) (47–50). AD offers sustainable organic residue management and renewable energy generation, though optimization is needed for consistent biogas quality and integration into energy systems (25, 51, 52).

The thermochemical conversion of vegetable residue into biochar via pyrolysis a process involving oxygen-limited heating (100–500+ °C) yields a carbon-rich material with applications in carbon sequestration, soil enhancement, and contaminant adsorption (53, 54). Biochar properties (e.g., porosity, functional groups, stability) depend on pyrolysis conditions (temperature, heating rate, residence time), with higher temperatures favoring carbonization and lower temperatures retaining oxygenated groups for nutrient interactions (55, 56). Its use in soils improves water retention, nutrient availability, and microbial activity (Trichoderma spp.; Table 1 ) while mitigating greenhouse gases through carbon stabilization and reduced N₂O emissions (37, 53). Additionally, biochar effectively adsorbs heavy metals (e.g., Pb, Cd, As) and organic pollutants in wastewater, aligning with circular agriculture by diverting residue from landfills (75, 76).

The return of vegetable residues to the soil enhances soil fertility through improved nutrient availability and increased microbial activity, as demonstrated by studies showing that crop residue incorporation, such as tomato biomass, boosts soil organic carbon (SOC) and stimulates beneficial soil biological processes essential for nutrient cycling and crop productivity ( Figure 3 ) (69). Although residue incorporation can raise nitrous oxide (N₂O) emissions by approximately 29.7%, it also reduces nitrate leaching by 14.4%, depending on soil conditions, while compost amendments (10–20 Mg ha⁻¹) increase cation exchange capacity (CEC) by 20–40%, significantly reducing nutrient leaching losses (30–50%) without depleting plant-available nutrients (16). Moreover, residue return enhances microbial biomass carbon and enzymatic activity, leading to higher crop yields (70), with organic amendments like compost and manure increasing dehydrogenase, cellulase, and urease activities by up to 7.5, 6.8, and 17.9 times, respectively, compared to untreated soils, highlighting the vital role of organic matter in sustaining soil health and agricultural productivity (71, 72).

Incorporating organic matter into soil systems is essential for improving microbial activity, soil structure, and water retention all of which promote resilient ecosystems and sustainable farming methods (25). OM serves as a nucleus for soil aggregate formation, reducing bulk density by up to 12% and increasing porosity by 15–30%, thereby facilitating gas exchange and root proliferation (73). The stabilization of carbon varies by treatment, with lignin-rich biochar retaining 90% of carbon over centennial timescales, compared to 40–60% for compost (74). These improvements correlate strongly with aggregate stability (r² = 0.78, p < 0.001) and microbial enzymatic activity, underscoring the role of organic matter in soil structure and fertility ( Figure 3 ) (75). In the soil ecosystem, the addition of organic matter also promotes microbial activity and multiplication. Microbial communities thrive in residue-amended soils, accelerating nutrient mineralization and promoting disease-suppressive properties (55, 76).

Methane (CH₄) has a global warming potential around 25 times that of carbon dioxide (CO₂) over a 100-year period, so reducing methane emissions from landfills is essential (47). Composting diverts organic waste from landfills, cutting methane (CH₄) emissions by 38–84% (77). Anaerobic digestion of residues further mitigates 0.25–0.50 kg CH₄ per kg volatile solids (57). When converted to biochar, vegetable residues reduce N₂O emissions by 30–80% in nitrogen-rich soils by inhibiting microbial nitrification. Integrated biochar-compost systems sequester 2.8–4.2 Mg CO₂-eq/ha/yr, with additional energy recovery through 120–150 m³ CH₄/ton of residue (78). Biochar-amended soils show a 78% increase in SOC, offering long-term carbon storage. These practices align with natural climate solutions, potentially delivering 37% of cost-effective CO₂ mitigation needed by 2030 (see Figure 4 for conceptual framework) (79).

The scale of vegetable residue generation presents both a challenge and an opportunity for sustainable residue management. Post-processing residues including peels, seeds, and pomace contribute 20–30% of total agro-industrial waste, with improper disposal leading to significant environmental consequences (80–82). When landfilled, these residues decompose anaerobically, emitting 50–100 m³ of methane (CH₄) per ton, a greenhouse gas with 28–36 times the global warming potential of CO₂ (49, 83).

Anaerobic digestion of vegetable residues yields 0.25–0.45 kWh of biogas per kg, offering a renewable alternative to fossil fuels (78, 84, 85). Beyond energy recovery, agricultural residues such as potato peels have demonstrated 80–95% adsorption efficiency for heavy metals (Pb²⁺, Cu²⁺) in wastewater treatment, presenting a low-cost biosorption solution (73, 86–90). Heavy metal contamination, particularly in urban-grown crops, affects 12–40% of compostable residues, often exceeding regulatory thresholds (EU 86/278/EEC for Cd/Pb) (91). To ensure safe reuse, preprocessing steps such as biochar stabilization may be required (92). Additionally, integrating AI-driven waste classification and blockchain-based traceability systems could optimize residue sorting and supply chain transparency (93). A systems-level approach, combining technological innovation with policy incentives, could reduce landfill dependence by 30–50%, while aligning with SDG 12.3’s target to halve food waste by 2030 (see Table 3 for challenges and mitigations) (51, 94–96).

The processing and management of residue materials face significant technical challenges, particularly due to high moisture content and compositional variability, which hinder efficient pre-treatment, drying, and standardization (97–99). For instance, press water (PW) from wood fuel preparation contains 60–80% moisture, requiring energy-intensive mechanical pressing and thermal drying, yet residual moisture persists even under vacuum-drying conditions (≤5% remaining at 60°C and 0.1 bar) due to strong cellulose-water interactions (100, 101). Similarly, municipal solid residue (MSR) exhibits moisture levels of 30–50%, complicating drying processes, while its heterogeneous organic–inorganic composition (e.g., 40–60% organic matter, 20–40% inert materials) impedes standardization. Industrial residues further exacerbate variability, with pH fluctuations (4.5–8.5) and elemental disparities (e.g., C/N ratios of 15–50) destabilizing co-digestion efficiency with sewage sludge by up to 30%. These constraints demand adaptive solutions, including advanced dewatering technologies (e.g., superheated steam drying, reducing energy use by 25%) and AI-driven compositional analysis, to enhance process flexibility while maintaining output quality (102–104). Addressing these gaps is critical to improving resource recovery, reducing energy burdens (current drying consumes 15–30% of total processing energy), and achieving sustainable residue management (100, 105–107).

High capital costs and uncertain returns deter investment in residue processing technologies. Small-scale pyrolysis units require $20,000–$50,000 upfront with payback periods exceeding five years, compared to synthetic fertilizers’ immediate affordability (49). Market immaturity also plays a role where biofertilizers occupy just 1.2% of the $190 billion synthetic fertilizer market due to farmer uncertainty and delayed nutrient release (53). Successful models, however, demonstrate viable pathways. Brazil’s “ABC Program” subsidizes 60% of anaerobic digester costs for farms, while the Netherlands’ “Waste-to-Farm” initiative reduces feedstock expenses by 25% through supermarket-grower partnerships (96, 97). Scaling such approaches requires targeted subsidies, circular business models, and awareness campaigns to demonstrate long-term agronomic and economic benefits (108).

Limited knowledge, cultural preferences for chemical fertilizers, and the perception that it will be labor-intensive are the main reasons why farmers are reluctant to embrace residue valorization (44). Despite the long-term benefits to soil health, surveys show that many small-scale farmers consider composting to be more time-consuming than synthetic fertilizers (17). Furthermore, while contributing to air pollution, conventional methods like as open-field burning continue to be used because they are convenient (99). To modify attitudes, behavioral change initiatives and examples of successful case studies must be presented (109).

The valorization of vegetable residues faces significant policy fragmentation, particularly in compost safety standards and enforcement. For instance, the European Union’s stringent thresholds (EC No. 2003/2003) for heavy metals, 150 mg/kg for lead, restrict cross-border trade of organic amendments, unlike more lenient regulations in developing nations like India’s 300 mg/kg limit (110). This disparity undermines global market integration for compost products. Additionally, certification systems for residue-derived fertilizers remain underdeveloped, with only 12% of low-income countries implementing quality control programs (91). Without enforced pretreatment protocols examples like thermophilic composting for pathogen reduction, untreated residues risk contaminating soils and food systems (88). Urban-rural policy disparities further intensify adoption challenges; while municipal biogas projects in China receive subsidies of approximately thirty million Chinese yuan, smallholder farmers lack equivalent financial support (111). Risks are further increased by lax enforcement since untreated leftovers could contaminate food systems (112). To address these gaps, harmonized international standards like Codex Alimentarius guidelines and incentive-based mechanisms such as California’s “carbon credit for compost” program ($50/ton for carbon sequestration) are essential to align stakeholder interests Legislators must create uniform standards and use certification programs to reward adherence (113).

Collection and transportation are made more difficult by the decentralized character of vegetable residue generation, especially in rural locations (100). Rapid processing is required due to perishability, although storage facilities are frequently insufficient (114). Large-scale composting in urban settings is limited by space, and biomass aggregation is a problem for remote farms (97). These difficulties might be lessened by community-based composting centers and mobile pyrolysis units (115).

By accumulating dangerous germs and heavy metals, vegetable residue presents serious hazards of contamination and pathogens. The necessity for better water management techniques to reduce these dangers is highlighted by studies showing that pathogens like Salmonella Typhimurium and Listeria monocytogenes can survive in hydroponic systems and contaminate lettuce throughout its growing cycle (116). Furthermore, raw vegetables may become infected by irrigation water tainted with these infections; river waters have higher concentrations of Salmonella than other sources (117). Additionally, the use of compost made from municipal solid residue in urban agriculture raises worries regarding the buildup of heavy metals in crops. greater concentrations of these metals are seen at greater compost ratios, which calls for strict monitoring to guarantee food safety (110). The significance of putting into practice efficient mitigation techniques to deal with both chemical and microbiological contaminants in vegetable production systems is generally highlighted by these findings (109, 118).

Current studies in machine vision-based deep learning systems like MoistNet and low-energy biodrying provide scalable answers to the main problems of vegetable residue valuation in resource-constrained environments (106). High moisture content, a paucity of shredders, and restricted access to pyrolysis units are obstacles to traditional composting and biochar production, especially for smallholders (119). In contrast, MoistNet allows for real-time, non-invasive moisture assessment through RGB or hyperspectral imaging, allowing for dynamic feedstock classification for optimal processing (120). High-moisture residues are diverted to passive solar biodrying, which reduces preprocessing energy demands by up to 40%, while low-moisture residues less than 15% can be directly pyrolyzed in flame-cap kilns (121, 122). This approach minimizes reliance on mechanical shredders and centralized pyrolysis infrastructure, as automated moisture prediction (RMSE <2%) ensures consistent biochar yields even with heterogeneous feedstocks (122, 123). Integrating MoistNet with edge-computing devices such as Raspberry Pi, facilitates decentralized residue management, enabling smallholders to adopt precision agriculture techniques without capital-intensive equipment (124). Field trials have shown a 30–50% improvement in biochar consistency when MoistNet-guided protocols are applied, highlighting its potential for sustainable biomass valorization (125).

Biotechnological advancements, particularly in the fields of microbial inoculants and enzymatic pretreatment, are transforming the management of vegetable residue by enhancing decomposition efficiency and resource recovery. Engineered microbial inoculants, comprising tailored strains of bacteria and fungi, accelerate the breakdown of recalcitrant components like cellulose and lignin, reducing composting time and improving nutrient retention in the end products (126, 127).

These inoculants also suppress pathogens and enhance biogas production in anaerobic digestion systems, making them versatile tools for residue valorization. Complementing this, enzymatic pretreatment employs specific enzymes such as cellulase and ligninase to degrade complex plant structures, thereby increasing the digestibility of vegetable residue for downstream processes like biofuel production and composting (128).

Beyond traditional biotechnologies, emerging tools are revolutionizing residue valorization. AI-assisted residue sorting like hyperspectral imaging could optimize feedstock composition for microbial consortia by precisely segregating lignocellulosic fractions (129). Concurrently, CRISPR-modified Trichoderma strains demonstrate enhanced ligninolytic activity (up to 40% faster degradation than wild-type strains), addressing a key bottleneck in biochar production. These innovations synergize with existing microbial approaches to accelerate decomposition while reducing preprocessing energy costs (130).

The synergy between these technologies, where enzymatic pretreatment simplifies residue substrates and microbial inoculants further decompose them, offers a sustainable solution for converting low-value organic residue into high-value products like biofuels, compost, and animal feed (131). Despite challenges such as cost-effectiveness and regulatory hurdles, ongoing research in genetic engineering and synthetic biology promises to optimize these technologies, paving the way for their integration into circular economy frameworks (132).

Integrated Residue Management Systems (IRMS) offer a sustainable solution for managing vegetable residue by combining composting, anaerobic digestion, and biochar production to maximize resource recovery and minimize environmental impact. Vegetable residue, rich in organic matter, can be composted to produce nutrient-rich soil amendments that enhance soil health and support agricultural productivity (133, 134). Anaerobic digestion processes this residue to generate biogas, a renewable energy source, while also producing digestate that can be used as fertilizer. Additionally, converting vegetable residue into biochar through pyrolysis not only sequesters carbon but also creates a stable soil conditioner that improves water retention and nutrient availability (85).

By integrating these technologies, IRMS ensures efficient utilization of vegetable residue, reduces greenhouse gas emissions, and supports circular economy principles. Studies highlight that such systems can significantly enhance residue valorization and contribute to climate change mitigation (115).

The integration of design principles and closed-loop systems in vegetable valorization emphasizes sustainable techniques that improve resource efficiency and reduce residue (128). Advanced extraction techniques, such as subcritical and supercritical fluid technologies, allow the recovery of bioactive chemicals from vegetable residue, contributing to a circular agriculture by transforming residue into beneficial products for the food and pharmaceutical industries (135). In addition, bioponic systems use organic residue streams as nutrient sources, promoting local nutrient cycling and enhancing plant development while lowering dependency on conventional fertilizers (134). Composting in controlled environment agriculture (CEA) reinforces this closed-loop method by turning biowaste into nutrients for crop production, thus reducing environmental concerns (128).

The EU Waste Framework Directive, as well as subsidies for residue-to-resource initiatives, play critical roles in supporting sustainable practices in the bio-based economy. These policies stimulate the valorization of agricultural residues and food waste, facilitating the transition to a circular agriculture by including composting and biowaste management into urban farm systems (96, 134). However, scalability issues persist, notably with decentralized composting hubs and the certification of biochar for carbon credits, which require solid regulatory frameworks and stakeholder participation to enable effective implementation (136, 137). Integrating these approaches not only improves resource recovery but also tackles environmental problems, therefore contributing to a carbon-negative cycle in agricultural systems (138).

Metrics for analyzing circular agriculture performance in vegetable valorization include parameters such as resource efficiency, carbon footprint reduction, and economic feasibility. Nutrient recovery rates can be used to quantify resource efficiency. According to studies, only 9.6% of materials processed in the EU were secondary materials, emphasizing the need for enhanced recycling processes (139). Carbon footprint reductions are evident in food-residue reduction initiatives, where turning surplus food into processed products resulted in net revenue and variable CO₂ savings, highlighting the potential for sustainable practices (140). Circular production models, such as those used in olive oil manufacturing, have been shown to save costs and improve sustainability by reducing the use of virgin materials and residue (141). Furthermore, urban regeneration programs that prioritize recycling can significantly decrease greenhouse gas emissions, adding to the economic and environmental benefits of circular agriculture principles (142).