Composition changes of soil humus after massive application of urban waste compost: a comparison between FT-IR spectroscopy and humification parameters

A laboratory trial and a six-year field experiment were carried out to evaluate the changes in the composition of humic substances in a soil amended for six years with urban waste compost (UWC), and to compare the results obtained by FT-IR spectroscopy with empirical humification parameters. The fulvic acid FT-IR spectra showed that amended soils contain a higher concentration of carboxylic and aliphatic groups compared to unamended soil (UNAS). The six-year amended soil (SYAS) also showed a higher content of polysaccharides on FT-IR spectra of both humic and fulvic acids. The polysaccharide increase in the six-year amended soil is attributable to the humification process since polysaccharides were not present in a high quantity in the UWC or laboratory amended soil (LAAS). The humification index and the degree of humification values indicate that the urban waste compost and soils studied can be classified in decreasing order of humification as follows: UNAS > SYAS > LAAS > UWC. This behaviour is the consequence of an increasing presence of the non humified fraction in NaOH extracted organic matter. The carboxylic and aliphatic groups content from FT-IR spectra confirmed this behaviour, thus indicating the validity of the humification parameters used to evaluate the status of the organic matter from refuse and soil.