Molecular alterations of organic fractions from urban waste in the course of composting and their further transformation in amended soil

The evolution of various analytical characteristics (including the concentr ations of water-soluble fractions, free and esterified lipids, and humic-li ke substances) of solid urban waste in the course of a 7-week composting pr ocess have been monitored in two independent piles composted during differe nt seasons (Spring/Autumn). Whereas the concentration of water-soluble and mineral fractions tended to increase during composting, the opposite was ob served for total organic matter and free lipid. Unmatured compost showed co mparatively high amounts of esterified lipid. The absence of monotonic tren ds shown by this fraction, and by total humic-like substances indicate two successive stages of compost microbial reworking. Gas chromatography-mass s pectrometry (GC-MS) analysis of the lipid fractions progressively removed b y step-wise chemolysis allows the appraisal of different lipid species, whi ch differ in their resistance to biodegradation. Extended composting does n ot contribute to the accumulation of progressively aromatised humic-type ma terials similar to those present in soil. Both pyrolysis (Py)-GC-MS and spe ctroscopic techniques (Fourier transform infrared spectroscopy, FT-IR, and C-13 nuclear magnetic resonance spectroscopy, NMR) confirm that compost-der ived humic acid-like substances are not structurally comparable to soil hum ic acids. Finally, the fate of the compost organic fractions has been inves tigated in two soils amended with different doses of the final compost. In general, the most conspicuous compost-induced effects were reflected by the differences in the qualitative and quantitative composition of the soil li pid. The results indicate that compost application cannot be considered to contribute to the mid-term accumulation of stable forms of organic matter i n soil. (C) 1999 Elsevier Science B.V. All rights reserved.