Bioactivity and antimicrobial properties of chemically characterized compost teas from different green composts

The acknowledgement of bioactive functions of compost teas promotes the research on characteristics and potential application of these heterogeneous water-soluble extracts from recycled biomasses. In this work, compost teas were isolated from on-farm composts made with agro-industrial residues of artichoke, pepper and coffee husks with the aim to evaluate the structural-activity relationship of dissolved bioactive molecules. The molecular features of compost teas were determined by ¹³C-CPMAS NMR spectroscopy, Infrared spectroscopy, and off-line pyrolysis-Gas Chromatography/Mass Spectrometry. Bioactivity of different compost teas was tested on Basil seeds germination, while the antioxidant capacity was measured by ABTS and DDPH spectrophotometric assays. The antimicrobial activity was measured against some pathogenic human bacterial strains. The seed germination experiment showed no phytotoxic effects and a significant increase of both root and epicotyls upon application of coffee husks and pepper CT samples. The same compost teas revealed the largest antioxidant activity and a clear antimicrobial effect determined by MIC (Minimal Inhibitory Concentration) against some gram-negative bacterial strains such as Pseudomonas aeruginosa, Escherichia coli, and Klebsiella pneumoniae. The bioactivity of CT-samples was related to their general hydrophobic features and to specific molecular composition. In particular, ¹³C-CPMAS NMR spectra and off-line thermochemolysis GC-MS highlighted a close correlation between radical scavenger activity and antibacterial bioactive functions with bio-available soluble aromatic compounds, such as lignin and phenols derivatives. The antioxidant and, antibacterial properties of compost teas from green composts encourages an innovative potential application of these eco-friendly products not only in agricultural applications but also in nutraceutical and pharmaceutical fields.