Verrillo et al. Chem. Biol. Technol. Agric. (2021) 8:28 https://doi.org/10.1186/s40538-021-00226-7 RESEARCH Open Access Humic substances from green compost increase bioactivity and antibacterial properties of essential oils in Basil leaves Mariavittoria Verrillo1,2* , Vincenza Cozzolino1,2, Riccardo Spaccini1,2* and Alessandro Piccolo1,2 Abstract Background: The Essential oils (EOs) are important bioactive secondary metabolites of aromatic plants, such as Basil, and fnd extensive utilization in pharmaceutical sector as therapeutic agents. A more eco-friendly industrial produc- tion of EOs requires new sustainable methods to improve yield and quality of these bioactive compounds in aromatic plants. Here, we evaluated the biostimulation efects of natural organic materials, such as humic substances (HS) extracted from a humifed green compost made with artichoke biomasses. Results: The molecular characteristics of HS from green compost were determined by 13C-CPMAS NMR spectroscopy and ofine pyrolysis–gas chromatography/mass spectrometry and combined to the bioactivity of the EOs extracts to investigate the structural–activity relationship. Basil plants were grown under HS treatments at diferent concentra- tions and the yield, the chemical composition and the antioxidant and antimicrobial properties were determined for EOs extracted by steam distillation method from Basil leaves. HS showed a positive efect on total extraction yield of the most bioactive EO components, with a progressive enhancement of abundance for eugenol, eucalyptol and geranyl acetate with increasing HS concentrations. Humic biostimulation infuenced the EOs antioxidant proprieties, as well as their antimicrobial activity against some common Gram-negative bacterial strains such as Klebsiella pneumo- niae. The HS biostimulation of Basil plants was associated to their structural hydrophobic characteristics that may have favoured the interactions with the Basil rhizome and to specifc bioactive molecular components, such as a lignin- derived phenols and polar saccharidic and aminoacidic compounds. Conclusions: Our results support the potential use of humic substances from green compost as promising efective biostimulants to improve the production and bioactive properties of EOs in aromatic plants. Keywords: Humic substances, Green compost, Basil, Essential oils, Steam distillation, Antioxidant, Antimicrobial Background from secondary metabolism that determine the typical Aromatic plants are extensively used to provide valuable aroma and favor of plant extracts [3]. Sweet Basil (Oci- natural products for either food or pharmaceutical appli- mum basilicum L.) of the Lamiaceae family is one of the cations [1, 2]. Te aromatic features result from the spe- most difused aromatic plants involved in the production cifc synthesis of bioactive organic components derived of fragrances for the nutraceutical sector. Furthermore, it fnds a large utilization as medicinal plant in the treat- ment of headaches, coughs, diarrhea, constipation, warts, *Correspondence: [email protected]; [email protected] worms, kidney malfunctions, in the control the blood 1 Centro Interdipartimentale di Ricerca per la Risonanza Magnetica glucose and diabetic disease [4]. Tis fast-growing annual Nucleare per l’Ambiente, l’Agroalimentare, ed i Nuovi Materiali (CERMANU), Università di Napoli Federico II, Via Università 100, plant represents a source of bioactive Essential Oils (EOs) 80055 Portici, Italy extracted from the leaves and the fowering tops [5]. Full list of author information is available at the end of the article Te EOs are mainly composed by a mixture of diferent © The Author(s) 2021. 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The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Verrillo et al. Chem. Biol. Technol. Agric. (2021) 8:28 Page 2 of 14 secondary metabolites, such as terpenes and phenylpro- stress, concur to reduce energetic inputs, and improve panoids compounds [6, 7]. Te characteristics of these the biosynthesis of bioactive secondary metabolites in aromatic compounds in Basil plant are related to the spe- aromatic plants [22, 23]. Tese positive efects have been cifc genotype which determine the amount and compo- explained by the heterogeneous supramolecular nature of sition of EOs [6, 7]. HS incorporating hormone-like molecules, which exert To enhance extraction yields, several methodologies their activity on plants physiological and metabolic pro- are applied for the isolation of EOs [8, 9]. Te applied cesses [24]. Te bioactivity or reactivity of humic extracts pressure and temperature of extraction afect the qual- closely depends on their origin and molecular composi- ity of essential oils, besides their quantitative output. tion [25–27]. Te ascertained involvement of various Te conventional techniques of essential oils isolation structural activity relationships between specifc humic from medicinal and aromatic plants relies on the solubi- or humic-like materials and plant biochemical pathways, lization properties of organic solvents. However, the cur- call for a comprehensive understanding of the HS molec- rent requirements to reduce the environmental impact ular features as a basic requirement for their application of extraction processes and improve products recover- as plant biostimulants [26, 28]. ing, have promoted the development of new eco-friendly No literature information is currently available on the approaches to match both high yield and quality of EOs use of humic extracts from green compost for the pro- [10]. In this respect, water is increasingly regarded as a duction of bioactive EOs from Basil plants as supporting useful green solvent for EOs extraction [11]. In fact, the therapy agents against pathogens. Te innovative objec- eco-compatible steam distillation method may reduce the tive of this work was to evaluate the biostimulation efect occurrence of secondary chemical reactions and unde- of humic substances extracted from artichoke composted sirable by-products, thus combining the maintenance of biomasses on the synthesis of EOs in Ocinum basilicum high levels of yield and products quality with reduction of plant. Te isolation of EOs extracts was performed with economic and environmental costs [12]. water distillation methodology thereby analyzing yield, Te bioactive properties of EOs have been related to molecular composition, antioxidant and antimicrobial anti-bacterial [13], anti-fungal [14] and antioxidant activ- activities of EOs against some human common bacterial ities [15]. Tese features are key factors for postharvest strains, such as Pseudomonas Aeuruginosa and Staphy- protection of fresh commodities [16] and for their use in loccocus aureus. pharmaceutical [17], cosmetic and food industries [18]. Te stability of both quality and quantity of EOs active Methods components is required for the industrial exploitation of Composting process and extraction of humic substances medical aromatic plants, such as Basil. In fact, the syn- Te bulk green compost was obtained in the compost- thesis of these bioactive compounds is afected by dif- ing facility of the Experimental Farm of University of ferent factors, such as soil management practices, inputs Napoli Federico II at Castel-Volturno (CE). Te vegeta- (e.g., fertilizers, water, and pesticides) and biotic and abi- ble artichoke residues were mixed with maize straw at otic stresses, that may have an impact on the biosynthesis 70/30 w/w ratio, and structured on static piles placed and accumulation of secondary metabolites [19, 20]. on perforated rubber tubes through which air was insuf- A topical challenge of modern agriculture is to guar- fated by a rotative pump. Te composting process lasted antee adequate crop yields and productivity, while 100 days including thermophilic and mesophilic phases concomitantly matching the stringent environmental for about half of the cycle, followed by the fnal curing regulations devised for agro- and forestry ecosystems. period. Te fnal compost pile was randomly sampled to Terefore, less intensive and sustainable cultivation obtain a fnal amount of 1 kg of bulk compost which was methods are currently promoted for the cultivation of air dried, sieved at 2 mm and stored a 4 °C for the subse- aromatic plants to improve biomass yields with large and quent analyses. steady amount of bioactive substances while lowering Humic substances (HS) were isolated as by a conven- input rates of both mineral fertilizers and agrochemicals tional extraction procedure described elsewhere [29]. [20]. Te use