Monitoring organic matter transformation of olive oil production residues in a full-scale composting plant by fluorescence spectroscopy

Abstract

Composting wet olive mill pomace, the main by-product of two-phase centrifugation systems, is an attractive valorization strategy in the context of regenerative agriculture. A comprehensive study of the changes in fluorescence signatures during the co-composting of this residue with olive tree pruning wastes and animal manure in a full-scale composting plant was performed. This compost showed more complex features than others at the initial stages of the process, exhibiting a singular band in the synchronous spectrum (500 nm) here attributed to polyphenol-pectin interactions. PARAFAC-derived components from Excitation-Emission matrices (EEMs) of water extracts were compared with those of fractions isolated following alkaline extraction at different maturity stages. The increase with composting of the component associated with humic-like substances (Ex 225, 365 nm/Em 476 nm) was more marked in the isolated humic acid fraction than in water extracts. Thus, the predominance of fulvic-like substances in water extracts explains inconsistencies previously reported about the relevance of the humic-like component during the composting process and the extent of humification. Finally, the correlation between PARAFAC components and several compost maturity parameters was studied. The negative correlation between the protein-like component and the germination index was explained by the protein-polyphenol interactions reflected in the emission spectra of this component. A strong positive correlation between both fulvic and humic fluorescent components and cation exchange capacity was found. In general, mature compost showed C/N ≤ 20 and no phytotoxicity (GI around 60%) although differences related to the heterogeneity of the large composting pile were important.