A long-term chemical and infrared spectroscopy study on a soil amended with municipal sewage sludge

A long-term field experiment was carried out to evaluate the effect of a se ven-year soil amendment with municipal sewage sludge on chemical fertility, soil enzyme activities and the changes in the composition of humic substan ces. The amended soil showed an increase in total and NaOH soluble organic C, available P and K. The heavy metals contained in the sludge did not affe ct soil phosphatases (alkaline phosphomonoesterase and phosphodiesterase), arylsulphatase or L-asparaginase activity, even if the concentration of tot al and available Cu, Ni and Zn increased in the arable layer of soil. The f ulvic acids FT-IR spectra showed that amended soil contains a higher concen tration of carboxylic and aliphatic groups and polysaccharides compared to unamended soil. The polysaccharide increase in the seven-year amended soil is attributable to the humification process since only a small amount of po lysaccharides were present in the sludge. The humic acids FT-IR spectra sho wed that amended soil contained a higher concentration of aliphatic and N-c ontaining groups and a lower concentration of polysaccharides than unamende d soil. The humification parameters (i.e. humification ratio, humification index and degree of humification) highlighted the differences in the humifi cation status between sludge humic-like substances and humus from soil, but were unable to show the differences between unamended and amended soils.