Water-extractable organic matter of different composts: A comparative study of properties and allelochemical effects on horticultural plants

The water-extractable organic matter (WEOM) was obtained from a green compost (GC WEOM), a mixed compost (MC WEOM) and a coffee compost (CC WEOM) and then characterized by means of chemical and physico-chemical methods, such as pH, electrical conductivity, total organic carbon (TOC), E4/E6 ratio, fluorescence and Fourier Transform infrared (FT IR) spectroscopies and high performance liquid chromatography (HPLC). The comparative evaluation of the three WEOM samples highlighted significant differences among them. In particular, the TOC content, the E4/E6 ratio, the ε280 value and the humification index followed the same order: CC WEOM > MC WEOM > GC WEOM. The fluorescence analysis of the three WEOM samples showed the presence of a common fluorophore unit possibly associated to simple aromatic units such as phenolic-like, hydroxy-substituted benzoic and cinnamic acid derivatives. The FT IR spectra of all WEOM samples indicated the presence of aromatic phenolic structures, while the HPLC analysis showed the presence of benzoic acid derivatives such as phthalic and salicylic acids. The allelochemical potential of each WEOM sample at two concentrations was tested on tomato and lettuce germination and early growth. In general, for both species, each WEOM sample at the two doses caused a significant increase of shoot length and plant fresh weight. The germination percentage of both plants and root elongation of lettuce resulted unaffected, whereas tomato roots resulted generally shorter in the presence of any WEOM sample, particularly at the higher dose.