Structures and phenanthrene sorption behavior of plant cuticles and soil humic substances
Early decomposition-induced changes in fractionated tomato ( Lycopersicon esculentum Mill.), pepper (Capsicum anuum L.), and apple (Malus pumila Mill.) fruit skin cuticular materials were studied to evaluate the contribution of aliphatic cuticular materials to soil organic material (SOM). Aliphatic components of fresh cuticles, decomposed cuticles, and humic substances, were characterized using elemental analysis, 13C NMR, and FTIR, and underwent batch sorption experiments with phenanthrene. Bulk, dewaxed, non-saponifiable, and non-hydrolyzable fractions were isolated for analyses. Results from NMR and FTIR spectra for the fresh cuticles indicated that the cuticles of all the fruits studied were comprised of extractable lipids, polysaccharides and cutin and that pepper and apple fruit also contained non-saponifiable, non-hydrolyzable residues, likely to be cutan. Through decomposition, cutan increased in relative abundance, and cutin content decreased in relative abundance. Based on elemental analysis and NMR data, the polarity generally increased with decomposition, whereas the aliphaticity decreased with decomposition. The results of the decomposition experiment indicated that cutan from plant cuticles persisted through early decomposition and, as such, may contribute to the aliphatic nature of humic substances in SOM. The application of the cuticular fractionation method to humic substances (humic acids and humin) extracted from SOM confirmed the potential contribution of plant cuticular materials to humic materials in soil. NMR and elemental analysis revealed that the isolated fractions of the humic acid (HA) and humin (HU) were highly aliphatic, and spectroscopic data indicated that humic materials exhibited peaks and intensities similar to those observed in cuticle materials. Phenanthrene behavior in the cuticular materials demonstrated that Koc values decreased in the bulk and dewaxed fractions and increased in the saponified and acid-hydrolyzed fraction through decomposition. These decomposition-induced sorption changes made the sorption affinity of the cuticular fractions more comparable to those of the isolated fractions in the HAs and HU. Overall, all analyses performed on fractionated HA and HU exhibited similar trends to that fractionated cuticles, indicating the plant cuticular material is likely a primary contributor to the aliphatic structure in soil humic materials.