Organic fertilizers deriving from the sustainable valorisation of agricultural and zootechnical waste biomass: evaluation of promising products, including a newly-developed hydrochar

The ongoing global ecological crisis, characterized by widespread biodiversity loss and progressive soil degradation, poses urgent challenges to find and apply practices for sustainable agriculture. In this context, the management of agro-industrial residues, including horticultural and livestock wastes, is inefficient and conducted inappropriately, thus contributing to environmental pressures and threatening the sustainability of food production systems, while it could represent a fundamental initiative for sustainable strategies. Therefore, this work proposes a circular economy-based approach, aiming to convert agro-industrial residues into environmentally sustainable fertilizers, in line with European regulations and strategies, including Directive 2008/98/EC, Regulation (EC) 2092/91, and the Green Deal’s Farm to Fork objectives, which aim to reduce reliance on mineral fertilizers while promoting resource recovery. The research focused on the production, characterization, and agronomic evaluation of organic fertilizers derived from agricultural and livestock by-products, highlighting their potential as effective alternatives to conventional mineral amendments. All experimental activities were carried out at the University of Salerno, Department of Pharmacy, within the AgrichemLab - Laboratory of Agricultural Chemistry, where all analytical and agronomic investigations were performed. The Biogas plant C&F Energy (Capaccio, Italy) produced solid anaerobic digestate (AD) from buffalo manure and its vermicomposted form (VC), which were extensively characterized by using elemental and nutrients analysis and advanced techniques to evaluate the composition, including CPMAS-NMR, FT-IR and Pyr-GC-MS. AD contained a higher proportion of labile carbon, whereas VC exhibited a more recalcitrant and humified structure. In-vitro and greenhouse pot trials identified optimal doses at 40% (v/v) for most crops (20% for VC on lettuce) and demonstrated that both materials enhanced growth and biomass production. NMR-based metabolomics revealed marked stimulation of carbohydrate metabolism and amino acid biosynthesis, with AD and VC outperforming mineral fertilizers in modulating primary metabolism. Green compost (COM) from red chicory, escarole, spinach, and olive pruning residues, and its pelletized form (PEL), produced by OP TerrAmore (Eboli, Italy), were investigated as additional amendments. Compost was further characterized by ¹³C CPMAS-NMR, FT-IR, and Pyr-GC-MS. Pot experiments showed that both COM and PEL improved plant growth, nutrient availability, and physiological performance, with pellets offering operational advantages. Combined COM+PEL applications further enhanced soil quality and crop yield, benefitting from the synergistic action of labile and stable organic fractions. The most innovative aspect of this thesis lies in the development of hydrochars (HC) as novel bio-based fertilizers. This process is particularly advantageous, as it converts organic biomass into new materials potentially beneficial for soil and plants through hydrothermal carbonization, which involves the reaction of fresh biomass with water under subcritical conditions. The reaction was conducted with a 1-liter reactor, using the same biomass processed to obtain the COM, excepted the olive pruning residues, with two process conditions (180 °C and 10 Bar; 215 °C and 20 Bar). Both the resulting HC liquid and solid phases were isolated, characterized and tested as beneficial agents for soil and plants. In particular, structural analysis and greenhouse trials on lettuce demonstrated that HC180 significantly promoted root and shoot growth, achieving performance comparable to that of mineral fertilization. Metabolomic profiling showed increases in sugars and amino acids involved in energy production and stress response, indicating a rapid biostimulant effect driven by nutrient availability and soil–microbiome interactions. The corresponding liquid hydrochars (LHCs) were analyzed during an internship at the University of Navarra. LHC180 was rich in soluble nutrients and hormone-like molecules (auxins, cytokinins, jasmonates, salicylic acid) and markedly improved seed germination, root architecture, biomass accumulation, and nutrient-use efficiency in Lepidium sativum and Arabidopsis thaliana, while LHC215 showed limited activity. LHC180 also activated nutrient transporters, iron-homeostasis regulators, and plasma-membrane H⁺-ATPases, resembling the mode of action of humic substances and supporting its suitability for sustainable fertigation strategies. Finally, alginate-based systems (hydrogels, beads and film for a seed coating) were designed to provide slow release of water, nutrients, and humic substances extracted form AD, VC and COM. These systems were characterized for structure, water retention, ion release, and degradability. Humic-loaded coatings improved germination and early growth of watercress seeds, as confirmed by SEM and in-vitro bioassays. Iron-based beads provided effective Fe release under alkaline conditions, offering a promising approach to improve nutrient availability in challenging soils. Overall, this work demonstrates that organic fertilizers derived from agro-industrial residues, particularly hydrochar-based amendments and alginate delivery systems, represent effective, innovative, and sustainable alternatives to mineral fertilizers and synthetic biostimulants. By combining nutrient supply, biostimulant activity, and soil quality enhancement, these materials offer concrete solutions to advance sustainable agriculture and support the transition toward a circular bioeconomy.