1. Introduction The agricultural sector is one of the largest generators of organic residues, but these materials also represent a valuable source of nutrients for soil [1,2,3]. Livestock manure, crop residues, the organic fraction of municipal solid waste (OFMSW), sewage sludge, and digestate represent the main residues and require appropriate management, while also offering opportunities for resource recovery and renewable energy production [4,5,6,7]. The European regulatory framework increasingly supports their recovery and reuse: the European Circular Economy Action Plan promotes the recovery and recirculation of resources [8], while Regulation (EU) 2019/1009 on EU fertilising products establishes rules for making compliant fertilising products available on the EU market [9]. This Special Issue was conceived in the context of this transition. The eleven contributions selected for publication address different aspects of the valorisation of organic residues, from their conversion into energy to their agricultural reuse. 2. Organic Residue Valorisation Through Anaerobic Digestion Anaerobic digestion (AD) remains a key conversion step in the circular chain [10,11,12,13], and three contributions address it with a focus on the practical aspects of process operation. Wu et al. (Contribution 1) examined an often-overlooked aspect of AD: the conditions under which inoculum is stored between reactor cycles. Working with continuously operated mesophilic reactors (35 °C), they showed that the inoculum stored at 35 °C retained a higher bacterial diversity than that stored at 15 °C, and that this diversity contributed to a stable mesophilic start-up. Furthermore, starting the reactor at 55 °C before transitioning to mesophilic conditions at 35 °C enhanced methanogenic activity, particularly favouring hydrogenotrophic methanogens, while the accumulation of propionate and butyrate associated with the enrichment of Coriobacteriaceae and Prevotellaceae reduced operational capacity. Inoculum management thus emerges not as a preliminary detail but as a design variable that significantly affects the performance of the anaerobic process. Constantinescu-Aruxandei et al. (Contribution 2) investigated an enzymatic pretreatment approach for enhancing AD of spent mushroom substrate. Rather than using harsher chemical or physical pretreatment methods, the authors demonstrated that hydrolytic enzymes extracted from spent Pleurotus substrate (SPS) and used to pretreat SPS itself increased methane production by 16% at saturation compared with untreated SPS. Beyond its effect on methane production, the pretreatment generally increased the P and K content of both liquid and solid digestates, enhancing their potential for agricultural use. A broader perspective on the valorisation of organic waste within a biorefinery framework is provided by Fermoso et al. [14], who reviewed the potential integration of valuable compound extraction and AD. This sequential approach has also been investigated for olive oil chain residues, where the recovery of valuable compounds, including protein hydrolysis and polyphenol extraction, is followed by AD of the residual biomass, illustrating the potential for further energy recovery from residual streams [15,16,17]. Co-digestion can also improve AD performance by combining substrates with different characteristics. Shrestha et al. [18] examined the effects of co-digesting food waste with animal manure, sewage sludge, and agricultural residues, highlighting its potential to improve nutrient balance, methane yield, and process stability. Similarly, Gambelli et al. [19] showed that the co-digestion of brewery spent grain pulp and Lemna minor improved biogas production and energy recovery, with the best performance associated with a more balanced supply of organic carbon and nitrogen. Montegiove et al. (Contribution 3) extended AD beyond conventional feedstocks to the increasingly common presence of bioplastic packaging in organic waste streams. Under mesophilic conditions (37 °C), co-digestion of polylactic acid (PLA) films with OFMSW resulted in a biodegradation rate approaching 100% after approximately 100 days, following a prolonged lag phase of about 60 days. The process yielded about 380 Nm3 of biogas per tonne of volatile solids, without any evidence of inhibition during the AD. Metabolomic profiling revealed a time-dependent accumulation of metabolites associated with PLA hydrolysis, particularly lactic acid and lactoyl-lactic acid, supporting PLA degradation after the prolonged lag phase. A similar study under thermophilic batch AD (52 °C) also reported limited biodegradation of PLA/polybutylene succinate (PBS) blend films, whereas the incorporation of cellulose nanocrystals into the blend increased biodegradation to 31.7%, although biogas production remained limited [20]. These findings are consistent with previous studies showing that biodegradation can vary substantially depending on the biopolymer type and the conditions of treatment. A critical review of AD as an end-of-life option for biodegradable plastics reported low degradation rates for some polymers under mesophilic conditions, while highlighting that most data on biodegradability performance come from studies based on biochemical methane potential (BMP) tests, which may not fully reflect the behaviour of continuous reactors at pilot or industrial scale [21]. A related study investigating the integrated use of mesophilic AD followed by composting showed that bioplastic degradation varied according to the polymer type, with starch-based bioplastics exhibiting higher degradation than PLA [22]. The conversion of organic substrates into renewable energy can also be pursued through technologies other than AD. Enzymatic biofuel cells based on immobilised enzymes and conductive polymer supports represent a different approach, in which the chemical energy of organic substrates can be directly converted into electricity [23,24,25,26]. 3. Stabilisation of Organic Residues Through Composting Composting is an important step in the stabilisation of organic residues, and process control is essential for obtaining a mature and stable amendment [27,28,29,30]. The stability and maturity of composts are commonly assessed using parameters such as organic matter evolution, nitrogen transformations, and humification and germination indices, although the need for standardised assessment criteria has long been recognised [31,32,33]. In this context, the choice of bulking agents and the use of additives can influence the composting process and the quality of the final product. Barthod et al. [34] reviewed the effects of additives (mineral, organic, and biological) on process parameters and the quality of the final compost, showing that additives can influence key aspects of the composting process and affect the quality of the final product. Zhao et al. (Contribution 4) examined particle size as an operational parameter in the use of spent mushroom substrate (SMS) as a conditioner for cow manure composting. After sieving SMS into coarse (≥2 mm) and fine (<2 mm) fractions, they compared different proportions of the two fractions and found that a mixture of 15% coarse and 5% fine SMS was the optimal treatment. This combination extended the thermophilic phase, and the resulting compost met the maturity criteria assessed in the study, with relative radicle growth ≥80% and ammonium nitrogen <150 mg kg−1. Available phosphorus and potassium contents also increased by 4.8% to 59.1% after composting. The results demonstrated that particle size represents an important operational factor in the conditioning performance of SMS during composting. A similar influence of particle size on composting performance was reported by Jiao et al. [35], who found that the size of branch and straw amendments affected organic matter degradation and humification during swine manure composting. 4. Agronomic Relevance of Organic Residue Valorisation The largest group of contributions concerns the agronomic use of products derived from organic residue valorisation and their effects on crops and soils. Campo-Celada et al. (Contribution 5) reported a two-year pilot rotation in which raw digestate, composted digestate, and composted sewage sludge were used as alternatives to mineral nitrogen fertilisation in forage maize and Italian ryegrass. In maize, yield and forage quality were comparable to those obtained with chemical fertilisation, while composted sewage sludge significantly increased average plant height. In Italian ryegrass, however, organic fertilisation resulted in lower yield than chemical fertilisation, suggesting that additional management strategies may be required to optimise nutrient availability for this crop. This is consistent with previous evidence showing that increased NH4+-N availability following AD does not necessarily result in greater nitrogen uptake or fertiliser savings under field conditions [36]. The different responses observed between crops further emphasise the importance of considering crop-specific characteristics and field conditions when assessing the agronomic value of digestates. Recent work with olive pomace-based digestates also highlights the importance of considering digestate characteristics in addition to their effects on nutrient availability in soil. The digestates tested increased soil carbon and nitrogen availability during a 90-day soil incubation, but also increased CO2 emissions. The study further showed that phytotoxic effects occurred at digestate concentrations above 10%. In addition, the digestates exceeded the established regulatory limits for Hg, whereas no critical issues were identified for the other heavy metals or for the analysed pathogens, underscoring the need to assess digestate quality and potential environmental risks before agricultural application [37]. The effectiveness of fertiliser use in the field also depends on its uniform distribution. Ou et al. (Contribution 6) optimised a centrifugal double-disc spreader for the distribution of fertiliser granules by coupling discrete element method simulation with computational fluid dynamics, identifying the combination of structural and operational parameters that minimised the coefficient of variation of the distribution, and validating the model against a prototype under headwind and tailwind conditions. This represents an important practical aspect of fertiliser application for achieving a uniform distribution in the field, as spreading efficiency depends on the physical characteristics of the fertiliser and the appropriate selection of spreading parameters [38,39]. Four studies address the use of biochar under different agricultural conditions, highlighting how soil properties, environmental factors and management practices can influence its effects. A quantitative meta-analysis reported a relatively small average increase in crop productivity, with larger positive effects observed in acidic and neutral soils, while also showing substantial variability among experimental conditions and limited availability of long-term studies [40]. The contributions published in the present Special Issue provide examples of how the effects of biochar depend on the agricultural system, soil conditions, and the way it is combined with other materials. Wang et al. (Contribution 7) conducted a field experiment in a tea plantation and examined the effects of biochar and biogas slurry on soil organic carbon fractions and tea quality. The study evaluated different combinations of biogas slurry and biochar, including pig-manure biochar produced at 350 °C and rice-straw biochar produced at 500 °C. The combination of 2% (by mass) 350 °C pig-manure biochar with biogas slurry replacing 100% of the nitrogen fertiliser resulted in the highest levels of readily oxidisable and dissolved organic carbon, stimulated β-glucosidase activity, and increased tea polyphenol and amino acid contents. Compared with the control receiving chemical fertilisation only, this treatment increased 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activities by 3.25% and 5.97%, respectively. Echchelh and Guilayn (Contribution 8) examined the use of biochar–compost amendments in Mediterranean drylands, showing that incorporating biochar into compost reduced carbon losses by up to 48% and nitrogen losses by up to 67%. Under water-stressed conditions, compost alone resulted in a lettuce yield corresponding to 30% of that obtained under unstressed conditions, whereas incorporating 10% biochar into the compost increased the relative lettuce yield to 88%. Henríquez-Arevalo et al. (Contribution 9) investigated the effects of biochar produced by co-pyrolysing hazel (Corylus avellana L.) pruning residues with polyethylene waste on the water retention properties of a degraded Alfisol under laboratory conditions. The resulting “plastichar”, applied at 5% w/w to a degraded Alfisol, improved plant-available water retention, particularly in micropores and mesopores. This approach combines two waste streams, although the authors highlight the need to further assess the long-term stability of the co-pyrolyzed biochar. The use of biochar was also investigated under soilless cultivation conditions. Agosti et al. (Contribution 10) partially replaced cocopeat with biochar at 2% and 5% in growth bags for soilless tomato cultivation and combined it with wood distillate, a pyrolysis by-product used as a biostimulant. The treatments improved some fruit quality attributes and were associated with changes in the microbial community of the growing substrate. In particular, the combination of 5% biochar and wood distillate increased fruit antioxidant activity and phenolic content, without significantly affecting vegetative growth or yield. Beyond digestates and biochar-based amendments, agro-industrial residues can also be converted into formulated fertilisers with controlled nutrient-release properties [41,42]. Llive et al. [43] developed calcium alginate-based capsules containing yerba mate processing waste and mineral fertilisers as a controlled-release fertiliser system, investigating encapsulation efficiency and nutrient release in water and soil. Montegiove et al. [44] developed a starch-based biopolymeric compound from olive mill wastewater and olive pomace by-products, which provided gradual nutrient release over a 90-day soil-column experiment, increasing nutrient availability and maintaining relatively low CO2 emissions compared with the other fertilisers. These studies illustrate different approaches to transforming agro-industrial residues into higher-value agricultural inputs from a circular economy perspective. Finally, Zhao et al. (Contribution 11) investigated the effect of artificial humic acid (AHA), a synthetic humic substance produced from corn stalks by hydrothermal humification, on phosphorus availability and transformation in soil. In a 60-day soil-column incubation, AHA increased Olsen-P by up to 15.4% in the 2 mm soil particle-size fraction, highlighting its potential to enhance phosphorus availability in soil. The contributions selected demonstrate that the valorisation of organic residues represents a valuable approach for recovering carbon and nutrients, with potential benefits for soil properties and for crop yield and quality. At the same time, their agronomic effectiveness depends on the characteristics of the resulting products, soil and crop conditions, and management practices. 5. Concluding Remarks The eleven contributions point to three recurring themes. First, the performance of valorised products derived from organic residues depends strongly on the conditions under which they are processed, including particle size, pretreatment conditions, pyrolysis parameters, and inoculum characteristics. In this framework, the effectiveness of bioplastic degradation also warrants further investigation, particularly to determine how the properties of biopolymers can affect the behaviour of the biological processes involved, such as AD and composting. Second, several contributions show the potential benefits of combining different inputs or approaches to improve valorisation efficiency and agronomic outcomes, including the combination of biochar with compost, biochar with biogas slurry, and biochar with wood distillate, as well as the use of enzymes derived from the residue for pretreatment. Third, the collection also illustrates important limitations after soil application, in terms of nutrient availability and crop performance. Several research gaps remain, particularly because much of the evidence comes from soil incubations and other laboratory-based experiments, while field-based validation is less represented. Longer-term field experiments are needed to better assess the agronomic performance of these valorisation pathways under practical conditions. Life-cycle and techno-economic assessments are also limited, leaving the environmental and economic performance of these valorisation pathways insufficiently quantified. Further attention is also needed to the quality and safety of agricultural inputs derived from organic residues, including the occurrence and fate of potentially persistent contaminants. In particular, the detection of microplastics in composted OFMSW [45,46] highlights the need to further assess plastic contamination in recovered organic materials intended for agricultural use. The transition towards a circular economy in agriculture requires the integration of multiple steps, from the conversion and stabilisation of organic residues to the sustainable agronomic use of products resulting from their valorisation [47,48,49]. The eleven contributions collected in this Special Issue demonstrate this interconnectedness across different feedstocks, processes, and agricultural contexts, also showing that the potential for successful valorisation depends on appropriate process control, material characteristics, and management practices. Together, these studies contribute to a more integrated view of how organic residues can be transformed into energy, soil amendments, and other agricultural inputs, providing evidence of the potential and limitations of these approaches for a circular economy in agriculture, while highlighting the need for longer-term field validation, broader assessment of environmental and economic performance, and continued evaluation of their agronomic performance and safety.
A Path for Circular Economy in Agriculture: From Organic Waste to Sustainable Energy and Soil Fertility
Montegiove, Nicolò
;Gambelli, Alberto Maria;Pezzolla, Daniela
2026
Abstract
1. Introduction The agricultural sector is one of the largest generators of organic residues, but these materials also represent a valuable source of nutrients for soil [1,2,3]. Livestock manure, crop residues, the organic fraction of municipal solid waste (OFMSW), sewage sludge, and digestate represent the main residues and require appropriate management, while also offering opportunities for resource recovery and renewable energy production [4,5,6,7]. The European regulatory framework increasingly supports their recovery and reuse: the European Circular Economy Action Plan promotes the recovery and recirculation of resources [8], while Regulation (EU) 2019/1009 on EU fertilising products establishes rules for making compliant fertilising products available on the EU market [9]. This Special Issue was conceived in the context of this transition. The eleven contributions selected for publication address different aspects of the valorisation of organic residues, from their conversion into energy to their agricultural reuse. 2. Organic Residue Valorisation Through Anaerobic Digestion Anaerobic digestion (AD) remains a key conversion step in the circular chain [10,11,12,13], and three contributions address it with a focus on the practical aspects of process operation. Wu et al. (Contribution 1) examined an often-overlooked aspect of AD: the conditions under which inoculum is stored between reactor cycles. Working with continuously operated mesophilic reactors (35 °C), they showed that the inoculum stored at 35 °C retained a higher bacterial diversity than that stored at 15 °C, and that this diversity contributed to a stable mesophilic start-up. Furthermore, starting the reactor at 55 °C before transitioning to mesophilic conditions at 35 °C enhanced methanogenic activity, particularly favouring hydrogenotrophic methanogens, while the accumulation of propionate and butyrate associated with the enrichment of Coriobacteriaceae and Prevotellaceae reduced operational capacity. Inoculum management thus emerges not as a preliminary detail but as a design variable that significantly affects the performance of the anaerobic process. Constantinescu-Aruxandei et al. (Contribution 2) investigated an enzymatic pretreatment approach for enhancing AD of spent mushroom substrate. Rather than using harsher chemical or physical pretreatment methods, the authors demonstrated that hydrolytic enzymes extracted from spent Pleurotus substrate (SPS) and used to pretreat SPS itself increased methane production by 16% at saturation compared with untreated SPS. Beyond its effect on methane production, the pretreatment generally increased the P and K content of both liquid and solid digestates, enhancing their potential for agricultural use. A broader perspective on the valorisation of organic waste within a biorefinery framework is provided by Fermoso et al. [14], who reviewed the potential integration of valuable compound extraction and AD. This sequential approach has also been investigated for olive oil chain residues, where the recovery of valuable compounds, including protein hydrolysis and polyphenol extraction, is followed by AD of the residual biomass, illustrating the potential for further energy recovery from residual streams [15,16,17]. Co-digestion can also improve AD performance by combining substrates with different characteristics. Shrestha et al. [18] examined the effects of co-digesting food waste with animal manure, sewage sludge, and agricultural residues, highlighting its potential to improve nutrient balance, methane yield, and process stability. Similarly, Gambelli et al. [19] showed that the co-digestion of brewery spent grain pulp and Lemna minor improved biogas production and energy recovery, with the best performance associated with a more balanced supply of organic carbon and nitrogen. Montegiove et al. (Contribution 3) extended AD beyond conventional feedstocks to the increasingly common presence of bioplastic packaging in organic waste streams. Under mesophilic conditions (37 °C), co-digestion of polylactic acid (PLA) films with OFMSW resulted in a biodegradation rate approaching 100% after approximately 100 days, following a prolonged lag phase of about 60 days. The process yielded about 380 Nm3 of biogas per tonne of volatile solids, without any evidence of inhibition during the AD. Metabolomic profiling revealed a time-dependent accumulation of metabolites associated with PLA hydrolysis, particularly lactic acid and lactoyl-lactic acid, supporting PLA degradation after the prolonged lag phase. A similar study under thermophilic batch AD (52 °C) also reported limited biodegradation of PLA/polybutylene succinate (PBS) blend films, whereas the incorporation of cellulose nanocrystals into the blend increased biodegradation to 31.7%, although biogas production remained limited [20]. These findings are consistent with previous studies showing that biodegradation can vary substantially depending on the biopolymer type and the conditions of treatment. A critical review of AD as an end-of-life option for biodegradable plastics reported low degradation rates for some polymers under mesophilic conditions, while highlighting that most data on biodegradability performance come from studies based on biochemical methane potential (BMP) tests, which may not fully reflect the behaviour of continuous reactors at pilot or industrial scale [21]. A related study investigating the integrated use of mesophilic AD followed by composting showed that bioplastic degradation varied according to the polymer type, with starch-based bioplastics exhibiting higher degradation than PLA [22]. The conversion of organic substrates into renewable energy can also be pursued through technologies other than AD. Enzymatic biofuel cells based on immobilised enzymes and conductive polymer supports represent a different approach, in which the chemical energy of organic substrates can be directly converted into electricity [23,24,25,26]. 3. Stabilisation of Organic Residues Through Composting Composting is an important step in the stabilisation of organic residues, and process control is essential for obtaining a mature and stable amendment [27,28,29,30]. The stability and maturity of composts are commonly assessed using parameters such as organic matter evolution, nitrogen transformations, and humification and germination indices, although the need for standardised assessment criteria has long been recognised [31,32,33]. In this context, the choice of bulking agents and the use of additives can influence the composting process and the quality of the final product. Barthod et al. [34] reviewed the effects of additives (mineral, organic, and biological) on process parameters and the quality of the final compost, showing that additives can influence key aspects of the composting process and affect the quality of the final product. Zhao et al. (Contribution 4) examined particle size as an operational parameter in the use of spent mushroom substrate (SMS) as a conditioner for cow manure composting. After sieving SMS into coarse (≥2 mm) and fine (<2 mm) fractions, they compared different proportions of the two fractions and found that a mixture of 15% coarse and 5% fine SMS was the optimal treatment. This combination extended the thermophilic phase, and the resulting compost met the maturity criteria assessed in the study, with relative radicle growth ≥80% and ammonium nitrogen <150 mg kg−1. Available phosphorus and potassium contents also increased by 4.8% to 59.1% after composting. The results demonstrated that particle size represents an important operational factor in the conditioning performance of SMS during composting. A similar influence of particle size on composting performance was reported by Jiao et al. [35], who found that the size of branch and straw amendments affected organic matter degradation and humification during swine manure composting. 4. Agronomic Relevance of Organic Residue Valorisation The largest group of contributions concerns the agronomic use of products derived from organic residue valorisation and their effects on crops and soils. Campo-Celada et al. (Contribution 5) reported a two-year pilot rotation in which raw digestate, composted digestate, and composted sewage sludge were used as alternatives to mineral nitrogen fertilisation in forage maize and Italian ryegrass. In maize, yield and forage quality were comparable to those obtained with chemical fertilisation, while composted sewage sludge significantly increased average plant height. In Italian ryegrass, however, organic fertilisation resulted in lower yield than chemical fertilisation, suggesting that additional management strategies may be required to optimise nutrient availability for this crop. This is consistent with previous evidence showing that increased NH4+-N availability following AD does not necessarily result in greater nitrogen uptake or fertiliser savings under field conditions [36]. The different responses observed between crops further emphasise the importance of considering crop-specific characteristics and field conditions when assessing the agronomic value of digestates. Recent work with olive pomace-based digestates also highlights the importance of considering digestate characteristics in addition to their effects on nutrient availability in soil. The digestates tested increased soil carbon and nitrogen availability during a 90-day soil incubation, but also increased CO2 emissions. The study further showed that phytotoxic effects occurred at digestate concentrations above 10%. In addition, the digestates exceeded the established regulatory limits for Hg, whereas no critical issues were identified for the other heavy metals or for the analysed pathogens, underscoring the need to assess digestate quality and potential environmental risks before agricultural application [37]. The effectiveness of fertiliser use in the field also depends on its uniform distribution. Ou et al. (Contribution 6) optimised a centrifugal double-disc spreader for the distribution of fertiliser granules by coupling discrete element method simulation with computational fluid dynamics, identifying the combination of structural and operational parameters that minimised the coefficient of variation of the distribution, and validating the model against a prototype under headwind and tailwind conditions. This represents an important practical aspect of fertiliser application for achieving a uniform distribution in the field, as spreading efficiency depends on the physical characteristics of the fertiliser and the appropriate selection of spreading parameters [38,39]. Four studies address the use of biochar under different agricultural conditions, highlighting how soil properties, environmental factors and management practices can influence its effects. A quantitative meta-analysis reported a relatively small average increase in crop productivity, with larger positive effects observed in acidic and neutral soils, while also showing substantial variability among experimental conditions and limited availability of long-term studies [40]. The contributions published in the present Special Issue provide examples of how the effects of biochar depend on the agricultural system, soil conditions, and the way it is combined with other materials. Wang et al. (Contribution 7) conducted a field experiment in a tea plantation and examined the effects of biochar and biogas slurry on soil organic carbon fractions and tea quality. The study evaluated different combinations of biogas slurry and biochar, including pig-manure biochar produced at 350 °C and rice-straw biochar produced at 500 °C. The combination of 2% (by mass) 350 °C pig-manure biochar with biogas slurry replacing 100% of the nitrogen fertiliser resulted in the highest levels of readily oxidisable and dissolved organic carbon, stimulated β-glucosidase activity, and increased tea polyphenol and amino acid contents. Compared with the control receiving chemical fertilisation only, this treatment increased 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activities by 3.25% and 5.97%, respectively. Echchelh and Guilayn (Contribution 8) examined the use of biochar–compost amendments in Mediterranean drylands, showing that incorporating biochar into compost reduced carbon losses by up to 48% and nitrogen losses by up to 67%. Under water-stressed conditions, compost alone resulted in a lettuce yield corresponding to 30% of that obtained under unstressed conditions, whereas incorporating 10% biochar into the compost increased the relative lettuce yield to 88%. Henríquez-Arevalo et al. (Contribution 9) investigated the effects of biochar produced by co-pyrolysing hazel (Corylus avellana L.) pruning residues with polyethylene waste on the water retention properties of a degraded Alfisol under laboratory conditions. The resulting “plastichar”, applied at 5% w/w to a degraded Alfisol, improved plant-available water retention, particularly in micropores and mesopores. This approach combines two waste streams, although the authors highlight the need to further assess the long-term stability of the co-pyrolyzed biochar. The use of biochar was also investigated under soilless cultivation conditions. Agosti et al. (Contribution 10) partially replaced cocopeat with biochar at 2% and 5% in growth bags for soilless tomato cultivation and combined it with wood distillate, a pyrolysis by-product used as a biostimulant. The treatments improved some fruit quality attributes and were associated with changes in the microbial community of the growing substrate. In particular, the combination of 5% biochar and wood distillate increased fruit antioxidant activity and phenolic content, without significantly affecting vegetative growth or yield. Beyond digestates and biochar-based amendments, agro-industrial residues can also be converted into formulated fertilisers with controlled nutrient-release properties [41,42]. Llive et al. [43] developed calcium alginate-based capsules containing yerba mate processing waste and mineral fertilisers as a controlled-release fertiliser system, investigating encapsulation efficiency and nutrient release in water and soil. Montegiove et al. [44] developed a starch-based biopolymeric compound from olive mill wastewater and olive pomace by-products, which provided gradual nutrient release over a 90-day soil-column experiment, increasing nutrient availability and maintaining relatively low CO2 emissions compared with the other fertilisers. These studies illustrate different approaches to transforming agro-industrial residues into higher-value agricultural inputs from a circular economy perspective. Finally, Zhao et al. (Contribution 11) investigated the effect of artificial humic acid (AHA), a synthetic humic substance produced from corn stalks by hydrothermal humification, on phosphorus availability and transformation in soil. In a 60-day soil-column incubation, AHA increased Olsen-P by up to 15.4% in the 2 mm soil particle-size fraction, highlighting its potential to enhance phosphorus availability in soil. The contributions selected demonstrate that the valorisation of organic residues represents a valuable approach for recovering carbon and nutrients, with potential benefits for soil properties and for crop yield and quality. At the same time, their agronomic effectiveness depends on the characteristics of the resulting products, soil and crop conditions, and management practices. 5. Concluding Remarks The eleven contributions point to three recurring themes. First, the performance of valorised products derived from organic residues depends strongly on the conditions under which they are processed, including particle size, pretreatment conditions, pyrolysis parameters, and inoculum characteristics. In this framework, the effectiveness of bioplastic degradation also warrants further investigation, particularly to determine how the properties of biopolymers can affect the behaviour of the biological processes involved, such as AD and composting. Second, several contributions show the potential benefits of combining different inputs or approaches to improve valorisation efficiency and agronomic outcomes, including the combination of biochar with compost, biochar with biogas slurry, and biochar with wood distillate, as well as the use of enzymes derived from the residue for pretreatment. Third, the collection also illustrates important limitations after soil application, in terms of nutrient availability and crop performance. Several research gaps remain, particularly because much of the evidence comes from soil incubations and other laboratory-based experiments, while field-based validation is less represented. Longer-term field experiments are needed to better assess the agronomic performance of these valorisation pathways under practical conditions. Life-cycle and techno-economic assessments are also limited, leaving the environmental and economic performance of these valorisation pathways insufficiently quantified. Further attention is also needed to the quality and safety of agricultural inputs derived from organic residues, including the occurrence and fate of potentially persistent contaminants. In particular, the detection of microplastics in composted OFMSW [45,46] highlights the need to further assess plastic contamination in recovered organic materials intended for agricultural use. The transition towards a circular economy in agriculture requires the integration of multiple steps, from the conversion and stabilisation of organic residues to the sustainable agronomic use of products resulting from their valorisation [47,48,49]. The eleven contributions collected in this Special Issue demonstrate this interconnectedness across different feedstocks, processes, and agricultural contexts, also showing that the potential for successful valorisation depends on appropriate process control, material characteristics, and management practices. Together, these studies contribute to a more integrated view of how organic residues can be transformed into energy, soil amendments, and other agricultural inputs, providing evidence of the potential and limitations of these approaches for a circular economy in agriculture, while highlighting the need for longer-term field validation, broader assessment of environmental and economic performance, and continued evaluation of their agronomic performance and safety.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
